kenwood TK-40 Output problem - need help with circuit debug

[Jimkarl]

Have a Vintage kenwood TK40 with no output on either channel (maybe a very minimal sound if volume turned 100% up). I found a number of caps in the power section to be bad on ESR meter, though surprisingly not the larger ones. Replaced all the bad ones but only seeing 40V dc on the left side of D2/D3 in the schematic (attached). Schematic says should be 46v so a bit of difference. Right side does measure 31v (ac) so I believe the transformer is good and something is pulling the voltages down.

Did an audio trace and I have audio coming to the power amp section (inputs 1 & 2 on the left hand side of the schematic. On the right channel, I traced audio on the other side of the Cap C7. On the left there was no signal there at C10 so I checked the cap and it was shorted open. Replaced it and now have audio on the other side of that cap but audio is now totally weak and distorted, so something else in the circuit is affecting it. While the right channel doesn't have the same issue, is also has no speaker out either, so the signal isn't making it's way out.

Basically all the voltages on that board are low - substantially lower then what they should be - so I'm not sure if that is because the power supply section has a problem, or something on the board is "leaking" voltage somewhere. I'm no expert in electronics - definitely still in learning mode. Suggestions on what to try next/where to look?

 

[Harald Kapp]

What's the power consumption without load? The low voltages lead me to supsect an overloaded power supply which in turn could be the result of one or more shorted transistors in the audio stage(s).

Continue your audio tracing:
Do you have a signal at the emitters of Q1 and Q8?
Do you have audio at the base of Q3, Q4 8Q5, Q6 respectively)?
etc.

Good candidates for a defect are Q2...Q5 to the right of the dashed box. These drive the speakers and are subject to the highest stress.

 

[Jimkarl]

Sorry for the delayed response - I started to recapture a variety of voltage points (attached and shown in red) but I pushed Q1 2SC669 too far over and the base lead came off (right from inside the base so there's no repairing it. Google also fails to shows any equivalent for this transistor. The 2SC669A has a substantially lower upper limit (30v instead of 100), the Ved max is slightly lower (5v vs 6) and the Ptot (something to do with temperature/power dissapations is 1WF vs 10WC) so although I could probably get by with the 30v upper, I'm unclear if the latter two spec items are adequate for replacement. Anyway - I'm dead in the water until I can replace this piece. Thoughts on a suitable replacement?

Also on a related note - I found the TH2 (SDT35 thermistor is missing from the right channel amp board. Looks broken off and the Left side one doesn't look great either. I can't find any specs or alternate replacement from google. Looks like I could get them from sears parts direct but the shipping brings it to $13 so I was hoping to find a alternative that I could either get cheaper or from some place I'm more likely to order other things from.

 

[Harald Kapp]

Sorry, I'm not familiar with these transistor types and can offer no replacement. Google says NET373 may be an equivalent.

What about the audio tracing?

The missing and broken thermistors are probably not the main issue. They should be active in a thermal overload only. Therefore it is a good idea to replace them provided you can find out the specs. Otherwise the amp should operate without them aas long as you don't drive it into thermal overload.

 

[Jimkarl]

Harald - Someone told me a while ago it wasn't safe to use an audio tester to deep into the power amp section so I haven't not tried that yet. Was that incorrect? I don't have a (working) oscilloscope, some I'm just using and old Radio shack audio tester which is a small handheld amp/speaker. How far into that circuit is it really safe to connect?

FYI - I also captured some more output transistor voltages (attached)

 

[Harald Kapp]

That depends on the audio tester. A little amp+speaker can be overloaded by the high voltages within a power amplifier. If you add a voltage divider to the input (potentiometer), you can scale down high voltages from the amplifier under isnpection and still trace the audio. The position of the potentiometer (i.e. the ratio of voltage division) is an indicator of the signal strength.
An oscilloscope would be ideal, but this method is useful, too.

From the values you measured:

• (V(R28right)-V(R28left))/R28 = 1.5V/100Ω=15mA doesn't look like an overloaded supply (following the trace to the left of R28 10.3mA+2.4mA are labeled which adds up nicely).
• What is the voltage at the emittter of Q1? The schematic says 14V, is that o.k.?
• The voltage at the collector of Q4 should be 0.6V, it is 0V. This indicates the transistor is fully off. What is the base voltage of Q4?´it should be around 18V.
• Look also at the base voltage of Q2, it should be around 1V.

Generally: Since both channels exhibit the same behavior, concentrate on one channel to find the issue. By the way: apart from the low audio level, is it a clear signal or do you hear lots of hum (mains frequency)? In that case measure the AC componnet of the 42.5V supply voltage (use the multimeter in AC range).

 

[Jimkarl]

With the base lead now broken on (power supply) Q1 - is the emitter value is currently 0. I would think I need to replace that first? I don't recall what it was before - at least 10v I'm sure, might have been higher.

Base voltage (amp board) Q4 is 22.8
Base voltage of (amp board) Q2 is .8

Also checked and updated some other values in the attached Rev C, though please note that each time I measure I get some fairly wildly different readings so don't be surprised if I contradict myself when talking about any given voltage. Like those output caps voltages which have gone down drastically from a day ago and that was pre-me breaking Q1....

 

[Harald Kapp]

I would think I need to replace that first?

I think so, too.

 

[Jimkarl]

I think so, too.

Q1 Part on order!