amplifier repair

This is a fast high performance well twenty years old Hifi amplifier. The repair of a defective amplifier is shown and  as with the repair step for step was proceeded. To see is, how from the repair a restoration became. Subsequently, the amplifier was measured on its characteristics as audio amplifier.

Automatic website translation from German to English done by


High Speed DC Integrated Amplifier


Error tracing and cleaning


alter verdreckter und verrosteter Kenwood Verstärker

The amplifier had accumulated a long and hard life behind, for much dust and traces of rust. The equipment serves as a spare part donor. It is actually culpably equipment in such a way comes to leave. After cleaning a few years can never harm.

Kenwood Hifi Verstärker KA-907 schon mal halb zerlegt

A second amplifier is to be repaired. It still is in a tidy condition. Broken, at the output stages the exit hangs firmly on +60 V. Without the inserted protective circuit it would have been the end for a DC coupled woofer.

zerlegte Endstufe des KA-907

Output stage plate solder side. To their in the past clearly already one soldered. To see down on the right of also the semiconductor cemetery, all defective output stage transistors. With many a low impedance connection between basis and collector. One thereby without short-circuit, large currents must have flowed.

The orginal output stage transistors are the PNP types 2SA1065 and NPN types 2SC2489. Both are very fast, them are fast bipolar power transistors (150V/10A/120W). Unfortunately all are defective. A few new 2SC2489 and a set of 2SB681 PNP and 2SD551 NPN (150V/12A/100W) are provided. At first sight these fit as replacement quite well, the amplifier however not for these types were compensated. If the original typ is still available hardly, the usual transistor comparing lists help. These lists do not beru-hit a corner-sight however always all parameters. If another transitor type is used, always exactly superior at which screw thereby one turns. In many cases is it possible, often is even improvements thereby attainable, in addition, the opposite is possible. The amplifier can reciprocate and be able even still more semiconductor broken to go.

Endstufe Oberseite der Leiterplatte

After the cleaning the printed circuit board is again clean. To see on the right of and on the left of the TO-3 versions for the output stage transistors, which are put and over the two screws the collector (housing) contacted. The put variant is very low-maintenance. White resistances are 0.47 ohms of resistances, them serve for the local inverse feedback and simplify also that join in parallel the transistors. The driver transistors apparent get along with small radiator box. Fortunately is present service the manual, without connection diagrams would be the repair a toilsome work.

To fast searches of errors in the circuit a simple circuit analyzer already meets:

  • thus the pns of transitions with for instance 600mV at basis emitters and basis collector can be examined. Rough Transitordefekte is so recognizable.

  • Diodes in passage direction show for instance 300-600mV

  • Resistances may be never many high impedance as the nominal value, otherwise it is surely broken. Often the ohm measurer shows however a lower value, is usually because of the parallel connection with other elements. Many defective of resistances show "become high impedance" up to the complete loss. The devil is however in the detail, sometimes gauckeln the resistance condensers joined in parallel the circuit analyzer wrong values forwards, it can several seconds take to the announcement stands.

  • in the inserted condition DC voltage measure. However the oscilloscope, it is clearly safer shows the cam shape, the circuit analyzer is unsuitable in this relationship.

linke Endstufe

At first sight - OK ONE - the output stage was attached now at will at the left channel. It equipped with the provided 2SB681 and 2SD551 Transitoren. Those are not the correct, but the only sentence, which are still functional. For tests is correct even if it hops go could. Volume controls on zero, entrances leave and power ON open. Often an oscillation begins not immediately with the smaller amplitude to reciprocate but only with larger - thus a small signal put on and volume slowly untwist and at the oscilloscope observe which happened, if the sine begins easily - danger! - can have gone to volume back - if the scope then becomes nevertheless suddenly completely "green" is often too late it - the transistors broken. The risk of an immediate oscillation exists.

Toshiba 2SB681 und 2SD551 Bipolar Transsitoren

Japanese Toshiba power bipolar transistors in the TO-3 housing.  Installed with thermal compound and mica disks for the electrical isolation of the collector to the radiator box. When the assembling of the insulating washers gladly errors are made, therefore with the ohm meter for high impedanceness to examine. The thermal compound is widerliches things at the fingers that then later also everywhere at the whole table lubricates. Lay on only as much that the surface scratches and unevenness in the insulating material, housing and radiator box are filled out. The paste is not to form an additional developing layer, which worsens the thermal resistance, really only for gaps to fill. Also fully do not tighten the screws at the beginning, so that few days later after the first enterprise still a something can be pulled tight. It lasts always something to itself the paste completely sets.

leichte Oszillation nach erstem Einschalten

After first switching on on is the amplifier stably, off approx.. it begins to reciprocate to 25 V easily. 2 kHz without load at the output.

Oszillation beginnt deutlich bei noch höherer Amplitude

Now if the amplitude is still continued to increase (10V/DIV), the oscillation begins, very clearly particularly in the negative half wave.

bei Erwärmung verstärkt sich die Oszillation

the Transitoren became warmer, by the permanently flowing idle current. The oscillation grows, slowly it off time will switch.

The causes for an oscillation are variously, probable a defect in the first part of the output stage (long plate behind the large electrolytic caps). Whereby the wrong output stage transistors this oscillation with to favour can.

Endstufe bestückt mit dem orginal 2SC2489 auf der NPN Seite

Now an attempt was started, as the last remaining 2SC2489 orginal transistors affects the output.

Nur noch die Oszillation auf der negativen Halbwelle im Leerlauf

Meanwhile times the oscilloscope exchanged one with CRT readout. The picture shows the amplifier in the no-load operation with PNP spare transistor and the orginal NPN in the positive half wave.

When putting on 4 ohms a load the amplifier becomes stable. Only strangely enough it already begins with approx.. to clippen -13 V. Still defects must be present.


Power Amp (X00-2010) part (B)


Now it applies to examine the long plate behind the large caps.

Signalaufbereitung für die Endstufe

In the long printed circuit board (X00-2010) (B) directly behind the large caps very probably is an error. The two channels are symetrically developed, quite short layout, hardly wire entanglement. The oblong, narrow, small Transitoren is Doppelfet's in the first stage (for instance in the picture center on the right of and left), in a housing that is  favourably e.g. because of the drift characteristics over temperature. Which with the years, the decreased capacitance of the elektrolyt capacitors became possibly critical. Also the small potentiometers (above) are gladly a problem, only to turn if necessarily, outside of the place where the middle contact all the years is confessed, the surface are gladly dirty, there must few times turn to become around the deposits with the middle contact to remove.

Digitalmultimeter mißt Widerstände auf zu hohen Widerstandswert

the first error already found, a 180 ohm resistance more largely than the measuring range of 2 kOhm. That is broken. The surrounding construction units, in particular semiconductors applies it now to naturally also examine. Down with the printed circuit board, sometimes more simply said than done. First a few pictures make, serves later than assistance around to know which wire again where be rank-soldered must, without studying laboriously the connection diagram. If the wires away are, still the plastic tie-clips come.

der müßte mal gereinigt werden

The amplifier became completely dirty with the years. Down one sees the four electric rectifiers capacitors. In the present condensers with same capacity became importantly smaller and also better. Nevertheless I see no reason it exchanging with those for the time being, am nearly still ok, am also not so cheap. Besides also here something Orginalzustand is lost, although is rather all the same to me otherwise actually. Measuring of the capacity would be interesting, but only if they out-solder themselves leave. Who does not have a capacity measurer for high capacities, this can judge also in such a way: Amplifiers e.g. with 4 ohms load (sufficiently strong loudspeaker goes naturally also), to untwist times correctly. Observe a sine signal and the appearing ripple voltage at the condenser with the oscilloscope as signal source. With a bad capacitor with falling capacity a high ripple will anyhow have to be seen.


Der Dreck muß runter von der Leiterplatte

Only no inhibitions, the dirt must simply down of the printed circuit board. A mild seifenloesung makes nothing at all for the printed circuit board and electronics. A cleaner is also favourably, since it dries arrears-free and well, like the table-ware also. Of course also white spirits go, are however already stronger in the effect, give gladly easily matte of arrears on the printed circuit board and are the recommendation for the second wash course. The spray "LR" printed circuit board cleaner with plastic brush in the spray button integrated, of the contact chemistry supplies good cleaning results. For a strongly verdreckte plate one needs however much of it. LR is in the best way suitable also for soldering arrears of all kinds. There are possibilities a printed circuit board to be cleaned. It can already be that water is clean-run somewhere into a construction unit, where it would when new not clean-run it a certain risk. A usual construction unit that because of washing now accelerated broken would go, soll's nevertheless calmly directly broken go, I remove it, rather with me than in the next year with you in the living room.

sinnvoll die Leiterplatte trocken zu föhnen

After washing the printed circuit board is drying to foehn, but not with the 2000 Watts, but like that that one can hold the printed circuit board thereby still comfortably with the hand. Do not adjust thus too hot. A compressed air pistol dries also well. Only at air to leave go also, the landing on water esters dry, one do not see remaining however for a very long time not receive, are not not so mad when switching on on. A heating element does good services. Landing on water esters form e.g. in the housings of the small potentiometers, these must in any case have drained before switching on on. The cleaning already requires a certain expert knowledge. A clean printed circuit board makes possible also an optical inspection. Even if it does not harm functioned, it these also under a magnifying glass to regard correctly. Fine tears in the conductive strips are to be never excluded, and mechanically damaged construction units better recognizably. Can the soldered connection one so also better observe or after-solder.

gereinigte Leiterplatte des Endverstärkers

The printed circuit board (X00-2010) (B) looks now again clean. The remaining defective construction units on this printed circuit board, Transitor Q20, one NPN 2SC2633 and two pertinent of resistances R48, 50 with ever 180 ohms. In the other channel Transitor Q19 2SC2633 was defective,  but resistances the ok. Why also always the power stage transitors went broken, very probably a defect caused the other one. By the one internally short circuit transistor Q20 for example a too high tension lies at the resistance, the high achievement makes for it from and it goes with defective. It is not always foreseeable whether a transistor in the case of defect short circuit or becomes high impedance, depends on the error situation, in addition, on the internal structure, which gives way e.g. first: the bond wires (becomes high impedance) or the silicon (can melt, with low impedance).


If already already, everything is made clean. Do you see like copper shine again beautifully? The internal printed circuit boards are all down, there are already worth it the baseplate to deseam itself. Put on the water, soap and the brush. Before however thoroughly superior where only little water drauf is. E.G. do not ask the trafos and relays. Naturally as few as possible into the potentiometers or the volume control, which leave themselves front part (potentiometer and switch) and rear part (preamplifier) well to take off here.

gereinigter Verstärker

The safety devices shine even again little. On the large plate is only rough wood at construction units drauf, for which the water does not make anything. Complete dividing and the large plate, would be so much work, which hardly counts itself, cleans the whole ten thousand wires and cables to solder on off and again - thanks no - goes here also times differently.

der alte Hifi Verstärker dient als Teilespender

spare parts for the defective Transitoren Q19 and Q20.

Verstärker an Nennlast von 4 Ohm

First no further defective construction units more found, the time to the assembly tingles/swarms in the fingers.

There see, one both to the output stage with the spare tansistors runs at both channels of the long PCB. The photo shows the output voltage at 4 ohms a load. I have here times untwisted on 36 V amplitude with a frequency of 2600 cycles per second. Is small still little more amplitude, afterwards the amplifier begins however with clipping. It brings these 36 V at 4 ohms at the scope with that orginal transitors. The radiator box will very warmly not to recommend for continuous operation with such to a sinusoidal load.

Maximum performance are:

  • 36 V * 36 V/4 ohms = 324 Watts (during the point of the sine)

  • (36 Volt/1.414) * (36 Volt/1.414)/4 ohms = 162 Watts effective achievement (RMS)

RMS of Watt thereby is meant the achievement in warmth, which would convert the sine equivalent DC voltage in the resistance in warmth. In other words: the sinus-wave voltage shown with the amplitude of 36 V would heat the load resistance on a certain temperature. DC voltage of 36 Volt/1.414 = 24.445 V would heat the load resistance exactly on the same temperature as the sinus-wave voltage.


Loudspeaker connections


die orginalen Lautsprecherklemmen sollen ersetzt werden

der schwarze Buchsenträger wird ersetzt

Plan für das Holzbrettchen

A desire the orginal loudspeaker cables connecting terminals had to be replaced with banana sockets.

The black socket carrier must be expenditure-soldered, the banana sockets can be mounted simply not on it.

As carriers now a small pretty peace of wood is to serve that then from the rear on the rear wall one screws. Thus is written "chamfers" correctly.

About the measure with the wood one can argue. Purely electrically (Ohm's resistance) it does not bring to anything, perhaps that here: a gilded banana plug and a gilded socket do not oxidize over years, the contact resistance remain constant, as long as the spring action remains (nickel plated goes naturally also). Some c-clamps loosen with the time a little, do not control there occasionally did not operate. Favourably is the banana plugs solution also, since a short-circuit actually only heavily in addition-get is. If one considers the fact that with both amplifiers the output stage transistors went broken then is the risk of short circuit not at all on the light shoulder to take times in such a way. How quickly does it happen that a few kupferadern are not under-wedged correctly and pluses and minus to touch oneself? Particularly then if one has it hasty, the light bad is etc.. Yes, that is an advantage of the banana sockets, conclusion with the Gefummel, different loudspeakers and amplifier jerk twitches exchangeably. It will look nice certainly also.

Nussbaumbrett als Träger für die Bananenbuchsen.

Bananenbuchsen zusammen mit dem Brettchen Nussbaumbrett von der Rückseite

A wood from solid nut tree heartwood does good services. Shows the untreated condition.

Inserted the combination looks board and sockets nicely. The wood was rubbed in to the protection with a hard oil, as wood preservation and at the same time little more color and gloss.

The transistors was ordered with The only source, which I found, which had at the same time the PNP and NPN types at the camp. Large selection of obsoleter Transitoren leads.

neue Elektrolytkondensatoren

In the meantime also the new condensers arrived and wait for the installation.  None are used "approve of" in addition, no "expensive" condensers. One can lead endless discussions across it which the best is. The best never gives it, each solution intends and disadvantages.

Leiterplatte für die Lautsprecheranschlüsse

The development of the loudspeaker connections requires warmth and patience. Here a hot-air dryer with narrow air duct carried good services out. During a connection easily copper up-waved, sorry.

gereinigte Leiterplatte für die Lautsprecheranschlüsse

The printed circuit board cleaned with white spirits again inserted. Wires are typically held for mass production briefly, in order to lower costs to remain electrically short as well as over. When repairing occasionally nerved.

That was an idea the plate with the sockets to connect - did not function. Thinner better flexible wires were used, above with flexible isolated lines, so that sufficient distance to the disturbing resistances can be kept. The connections of orginalen plastics loudspeaker sockets, do not sit centrically like the drillings.

Lautsprecherleiterplatte eingebaut Ansicht von oben

The printed circuit board in the inserted condition. Here it happens closely. The danger the fact that bright sockets and e.g. the wires of the resistances touch themselves somewhere are given i.e. precisely inserts the plate and checks for sufficient isolation respected and above all thereafter everywhere well.

Nußbaumbrett im eingebauten Zustand

Inserted the small Nussbaum wood looks well and fits the black housing. The socket-head cap screws can be tightened simply. The size fits exactly, so that the plus and minus symbol remain still visible.

The installation of the wood already requires something fate, patience and time, but now is it in it. Perhaps the installation for you would be also relating to crafts no problem, but someone that electrotechnically little experienced is, can fast a support here shoot, a small again-created surveyed short-circuit can the output stage damage. Check thus either fingers really completely exactly away or afterwards.


Output stages (X07-1670)


Endstufe A verdreckte Lötseite

Now it goes to the printed circuit board side of the output stages. First a cleaning with white spirits and brush. There still fluxing agent remainders and a factory-installed protective layer, which is properly dirty with dust meanwhile.

Endstufe Kanal B

The output stage of the other channel is exactly dirty, to their however was already times repaired, clearly to see at the not cleaned soldered connections, fully with fluxing agent remainders. Electrically with a low impedance output stage the not much constitutes, is however unpleasant.

verdreckte Endstufe

the component side of the right output stage is also still dirty dirty, the dust must away. Bent and bucklig inserted of resistances.

Wärmeleitpaste auf den Endstufenkühlkörper

Also this old Siff, the surplus thermal compound verdreckte meanwhile must now down. Always the hands smeared all the same where one touches. Wipe away with a rag strongly. The mica disks are naturally also deseamed and checked at the opportunity for damage. But caution the disks break easily, do not bend.

gereinigte Endstufe von KA907

Like the white spirits cleaning and drying with the hot-air foehn look the printed circuit board again correctly cleanly. Even emitter resistances shine again like tooth white from the advertisement. Thus it makes for fun on it to work. That is the left output stage, which ran already apparently, it with A and C was marked (assistance placing the transistors)

alter und neuer Elektrolykondensator im direkten Größenvergleich

We begin with the exchanging of the electrolytic capacitors. Here the expenditure-soldered C4 with 47µF/10V. Replaced by 47µF/25V.  The progress of the last decades becomes apparent also in the size, a comparable from old days would be larger.

When the assembling of the output stage transistors on the fact absolutely it respects that no short-circuit between transistor package (collector) and the radiator box develops. After each exchange of an output stage transistor this must be absolutely examined with an ohm meter or a continuity tester. An ohm meter can indicate however by condensers joined in parallel varying values, e.g. a few hundred ohms, that is normal. But on keinenfalls only few ohms - then amplifiers in danger! Also guarantee that the probe tip to the anodized radiator box has bright good contact, measuring instrument before examine.

ein Elko wurde gegen den gelben Folienkondensator getauscht

Also the Elko c1, a 1µF/50V was exchanged. With this capacity it offers on directly a more favourable foil condenser to use (the yellow is the new 1µF/63V).

defekter Widerstand R4

Oha, which is that that? Defective resistance R4 with 62 ohms in the output stage right. That was noticeable to me in the verdreckten condition, the surface was still whole already, only he was easily brownish in a place, that is often an indication for too much warmth. By cleaning and the pressing with the brush the truth came to the light. It shows the carrier body was already as weak, which left easy affecting with the brush in to break. An OK resistance does not make that for nothing at all. The broken resistance  had possibly still something of its desired value, so that the circuit still functioned, it would be however a clear Kandidant for a loss in the next time "a usual construction unit that because of washing now accelerated broken would go, soll's nevertheless calmly equivalent broken to go, I remove it, rather with me than in the next year with you in the living room". Such a confirmation comes as called.

dieser R4 wurde auch schon mal ersetzt

We throw nevertheless times a view of the output stage left. The resistance R4 with 62 ohms, soldered there, is not orginal resistor, that has another design. The replacement is a standard 0.7 Watts metal film resistor. Aha, which is also the output stage at that was already times repaired.  Exactly this R4 was already times broken. It is pure speculation why: R4 is in the event of an error overloaded and thereby into death is also pulled. A view into the circuit can clarify that.

How is the resistance replaced? In the ideal case with a larger 62 ohm layer of metal/metallic oxide resistance, e.g. a 1-2 Watt resistance. I have however at present unfortunately no larger layer of metal available with 62 ohms. One can in addition, somewhat loadable 62 ohms a themselves build oneself. For example from four pieces 0.7 Watts standard layer of metal, interconnected as in the picture, sufficient maximum energy dissipation has and is electrically and mechanically for this application like that correct.

Am Kennlinienschreiber

the arriving transistors are tested at the curve tracer whether they function and also got over transport.

Tektronix 575 Curve Tracer

with such older beautiful tube equipment a Tektronix type 575, the characteristics of Transitoren can be represented. Here a NPN transistor is attached. The transistors are all correct.

beide Widerstände R4 wurden ersetzt

At both output stages the 62 ohms of R4 of resistances are exchanged. At the output stage left even still clearly a brown mark is to be seen, that has as a cause warmth under the R4 on the plate. That is the printed circuit board with the small 62 ohm, which already already with an earlier repair once one exchanged.

defekte Diode D2 in einer Endstufe

It became so slow excitingly in the output stage left. A view into the connection diagram shows that R4 and D2 are positioned in series, an investigation of D2 in the output stage left shows these as defectively and in both directions high impedance, thus broken. By the measuring instrument display (upper contour) one can recognize "OPEN", that was an examination in flow direction, normally should in the ok case approx.. 600 mVs its. The diode replaced with one from the partial carrier.

Kandel in Wolken 1248 Meter hoch Windräder

Hier und heute wird weitergemacht

After consideration the defect of the left output stage side became clearer. Possible that a NPN output stage transistor went defectively through a high river (why also always). The collector basis distance melts to a short-circuit and thus is positioned to R4 suddenly in series with the diode D2 at +66 V, high river flows by the PNP Q2 driver  2SA2633, the whole flows partially also away into the Q19 on the long power Amp plate (X00-2010) (B) and can the destructive work fortzsetzen there. Whether that was everything also really like that, I cannot say with security.

defekter Q2 Treibertransistor

In the output stage right, the Q2 driver transistor 2SA913 is broken. Its basis emitter distance is high impedance. Probably also the right output stage died over the same mechanism as the left output stage. Always the same broken: either R4 or D2, here now even still the driver Q2 and also the Q20 + 180 ohms of resistances on the long plate. Transistor Q2 replaces by similar types.

 Wärmeleitpaste hilft mit um Transistoren kühler zu halten

The driver transistors were installed with this amplifier everything without thermal compound on the radiator box.  Something thermal compound improves (according to manufacturer) the heat transfer over up to 50%. The Treibertransitoren was expenditure-soldered, examined and provided afterwards with something paste.

gereinigte Endstufe mit Plastik 70 überzogen

After that the output stages finished and examined are them with plastics 70 protective varnish were injected. Plastics 70 is for many years a protective varnish proven in electronics, it is inexpensive and simple to use. The top sides one did not paint.

Endstufen mit Plastik 70 überlackiert

Both output stages in the painted condition. The lacquer is through solderable, i.e. later solder repairs are no problem. At the market there is many electronics lacquers for most different applications.


Power Amp (X00-2010) part (B)


die Lötseite des Power Amp im ungereinigten Zustand

Now to the long printed circuit board power Amp (X00-2010) (B) is behind the thick Elkos. The lacquer at the lower surface is dirty. It was cleaned before with soapy water, enfernt however only the rough and the dust. Here a solving cleaning agent helps e.g. white spirits.

Power Amplifier mit den ausgetauschten Elkos und alle bisher gewechselten Bauteile

At power the Amplifier (X00-2010) (B) are exchanged all electrolytic capacitors for new parts, there were also a few surprises. In the lower  half everything is which because of this amplifier was already exchanged. The parts with the white Kreuzchen are defective all.  In this printed circuit board  two Elkos C19 and C20 for each 1µF/100V were defective, it are the two small Elkos down right.

Größenvergleich zwischen altem und neuem Elko

Here still another example like the electrolytic capacitors became smaller. They did not become smaller only, but regards electrically also better. In the example the dissipation factor of 0.065 to 0.018 and the spare series resistance ESR improved reduced from 88 milli ohms on 25 mill ohms (@120 cycles per second measured with  HP 4282A).

Were all electrolytic capacitors (except C19 and C20) in power the Amp (B) correct.  The orginalen condensers did not show noticeable loss of capacity, dissipation factor and ESR after all the years in order. Leakage currents hardly measurably. Nevertheless was worthwhile itself exchanging for new condensers, which are new series in their electrical values improved. The temperature range amounts to with the new series 105°C with the old 85°C. The new ones have about 1000 hours specified life span with 105°C and may thereafter a loss of capacity of the order of magnitude about -30% show (one considers, the manufacturers spezifieren the life span often differently). Only 1000 hours with 105°C? - that is not so few, because aging obeys an exponential function vs. temperature, i.e. with 25°C lies the life span then perhaps at estimated approx. completely roughly 40,000 hours.

The life span of an electrolytic capacitor depends mainly:

  • of the quality of the condenser

  • of the local ambient temperature in the amplifier.

  • which Ripplestroeme the condenser must carry (a question circuit Design, construction unit dimensioning and the application), the more highly the alternating current by a condenser, the more internal warmth ensteht in it and co-heats it, the increased temperature again favours constantly continuing procedure of the loss at electrolyte, thus the capacity sinks. A Low ESR Elko like here the again blocked has a lower internal Ohm's resistance, after p=i*i*ESR ensteht in it less internal warmth. Straight one for applications in circuits, with which it must carry high alternating currents is a Low ESR ideally. For many applications with low alternating current a Low ESR is not necessary and a standard type in harmless environment  can thereby become very old. All types whether standard, Low ESR, 85°C, 105°C, or 125°C have all their respective pro and cons.

Changing the condensers was meaningfully, particularly in the case C19 and C20, which were totally defective. Why are these broken gone?, I do not know it.


Power Amp (X00-2010) part (A)


Now it goes to the large printed circuit board with the thick electr. caps. It is the power Amp (X00-2010) A. Er looked relatively good.  The large printed circuit board from the housing to remove I saved, that am much wire soldering and mounting, dividing am not as comfortable here as the past. The cleaning on the lower surface with white spirits can be settled also in the inserted condition, exactly the same happened it also with the exchanging of the condensers.

Elektrolytkondensator einer alten und einer neuen Serie

I can't let constantly show, what was carried out for still improved dissipation factor and ESR in the area of the electrolytics by all manufacturers around the size to reduce, in this case even with higher tension strain (needs more voltage strain more volumes, compared with the same) in addition vs. frequency.

Lötseite des Power Amp (B)

Also the power Amp (B) printed circuit board was over-painted to the conclusion with plastics 70.

Kenwood Verstärker vor dem ersten Test

Now are times again an assembly and a first intermediate test of emergencies. Everything compound and switched on - see there the amp runs now on both channels, but comes not yet correctly tested, later.

der Vorverstärker mit entfernter Gehäuseabdeckung

The phono amplifier is in the rear part quite protected between two sheet metals hidden. The printed circuit board carries also the Aux, Tape, tuner of entrances, moving coil and moving magnet inputs. The whole plate consists of many single transistors and  discrete construction units.


A cleaning of the phon amplifier is also announced.

Elkos im Vorverstärker

New condensers for the phono player amplifier.

907 Impression


Phono Amplifier (X08-1670-10)


bipolarer Koppelkondensator

phono player amplifier contains bipolar coupling electrolytics in the signal path, this by foil condensers is replaced.


Electrolytic capacitors at the phono amplifier exchanged. The upper left left corner is the Moving Coil sector, the most sensitive signals in the amplifier.

die gelben sind die neuen

thick foil condenser package with second adhesive together adhesive.

die getauschten Elkos aus dem Vorverstärker

the exchanged caps from the phono amplifier.

Reinigen mit Spiritus

the back is cleaned with white spirits.

Plattenspielerverstärker mit Plastik 70 überlackiert

finished printed circuit board with plastics 70 paints


Everywhere cable and lines where one looks, all potentials from small to high.

Netzspannung mit 230 Volt

the equipment does not have a protective grounding connection, the mains voltage runs by the equipment from the rear forward. If one of these wires were abkracht and the housing affect should, voltage is because of the housing, well to control.


tone control and first amplifiers stage (X1-1520-10) (X11-1520-813)


Vordere Seite nach abegonommener Frontplatte

To the front. Aluminum front plate down, (amazingly simply went) the front cover to hinge away. On the right of the change over switches, on the left of the actual preamplifier for all signals, the bass & treble controller, subsonic switch etc..

die Tone Control Regler

without wanting to step now possibly someone too close one, the way like that here constructionally fastened already is a little a Gefummel. Fortunately that must be opened here only rarely..

the two printed circuit boards and the housing are connected by short wires. The rear printed circuit board has some cable links. Good contrast: on the right of the noble potentiometer, left many wires.

Bass und Höhenregler

Printed circuit board with the bass and altitude controls. To see the small yellow foil condensers are instead of the Elkos.  The two thick bipolar 47µF Elkos were not replaced, they remain in it, them are still ok 47µF by foil to replace is now nevertheless somewhat too violent to me in the dimension.


manufactures printed circuit board with DC ON OFF switch, Subsonic and High filter. One considers also down right within the range of the transistors the brown discoloration of the printed circuit board by warmth.

Lötseite der DC ON OFF Subsonic High Filter Leiterplatte

like cleaning and painting also the back of the DC ON OFF printed circuit board looks again correctly new.

Rückseite der Leiterplatte mit Bass und Höhen Regler
the back the clay/tone control printed circuit board with the bass and heights automatic controllers after cleaning and painting.
Elkos aus beiden Leiterplatten

exchanged Elkos, them was electrically still all in very good condition. The small black left are the bipolar electrolytic capacitors.

die Widerstände und Transistoren heizen ganz schön kräftig ein

Resistances heated the printed circuit board. A heat nest in the left upper corner. Behind the next heater, transistors and resistances of the other plate.

Netz und Signal im Einklang

230 V of power switches and supplying and signal lines.

Now all construction units were so far exchanged, and the joy stood before the amplifier to test. Said done - which went? Nothing. In such moments annoyance does not grow nevertheless enormously, hours of the life spent in full hope and it goes. Thereupon tested which it could be, then finally found. The DC ON OFF switch has a bad bad contact also still equivalent on both channels, completely indefinable. Thereupon the switch exchanged for the functioning from the partial carrier. The soldering out requires something patience, is however simple.

DC on off Schalter

in the picture center DC on off switch with the mechanical defect is exact. With the first test that was not noticeable, can also be that up and screw the previous together the effect favoured. On the right of the spare part. The amplifier again screwed together, functions. After equal the exchange of nearly 100 condensers one has already something a mulmiges feeling whether everything goes. It is always the question in the head, I all also correctly polarized inserted, is somewhere one of the wires broken etc..

Kontaktspray 60 und Sprühwäsche WL

the contacts were well sprayed first with contact 60, cleaned afterwards with the spraying laundry wl by the replaced oxide coatings. With  using contact spray try this there to only spray where contacts are real, so that too much surrounding dirt and oxide coatings are not along-solved, only again be removed must if it at the contacts arrived.

Leiterplatten Anschlußkabel verlängert

the connecting cables of the printed circuit boards were extended something that simple measuring at the printed circuit boards became possible.

Again everything assembled


The repair work in all peace without any hecticness or time pressure were settled. I have sufficient time to taken also try the substantial elements this amplifier types to understand. The work was limited to substantial and simply feasible. One could have done still some more by the reliability to increase, as well as still the sound quality to increase, would mean however enormously much additional expenditure. Nevertheless up to now about 50 to 60 hours at work resulted. The work can be justified only with the background of the enthusiasm and the profit at experience, from economic view may one to such work only not at all struggle marriages, better only not at all to thinks - stand in no relation to the equivalent of the amplifier.

The report is to tune also a little the people thoughtfully, which the opinion are: "want favorably times fast defective equipment to buy or too ersteigern" and then try it in one afternoon repaired to get. Sometimes as loosely easy as many it does not go introducing themselves and workshops to assign can become expensive, since also must invest this much time, although they already possibly bring along experience with this types. Workshops stand with the high hourly rates in Germany, which must require them to repair under high pressure the equipment with small expenditure of time. For cosmetic operations like all Elkos is there no time exchanges, although the repairing would spend surely very gladly more time with the equipment.

Probably that that to settle can do all this.

In addition, the report is to motivate owners to seize at least times about it to think with the next defect similar measures. Electronic knowledge is actually necessary actually only for the repair, the remaining work is only questions of the fate relating to crafts.

To the conclusion still another few serious-meant references to owners:

  • please never the output stage at the exit closes briefly, also not for a short moment

  • passport on when attaching the loudspeakers cables

  • twist it well, solder before easily the ends

  • use none for thick cables, which do not fit into the sockets

  • tighten the clamps at the equipment and at the loudspeaker well, control regularly the firmness of all connections at amplifiers and loudspeakers

  • the wires may not touch themselves, also no contact to the housing

  • treat the cable laid in the room carefully

  • before the connection of the loudspeakers (particularly stranger, which did not run ever at the amplifier) should be always based with an ohm meter between pluses and minus on the cable not attached yet. The accuracy of the ohm meter is here hardly of importance. The read off measured value should lie in the order of magnitude of the nominal value of loudspeaker impedance, e.g..  4 or 8 ohms of no case loudspeaker attach, with which ohm meter indicates only very small values.

Much fun when hearing.

Messungen am Verstärker nach der Reparatur

Table with the replaced parts