Vintage Kenwood Receiver Volume Issues

[Arty30]

Hello All. I am working on a 69' Kenwood/Trio TK-140X Stereo Receiver. I have done a re-cap since a few tested caps were marginal and some distortion issues were present. The issue that I'm having is that the distortion in the left channel is still present, and seems to be caused by the gain being way too high, which is causing peaking/distortion. If it turn the balance almost all the way right it is balanced and no longer distorted. I have ruled out the main amp which leaves the "tone amp" and "Pre amp" which might not amplify the AUX signal I'm using, its hard to follow the switch diagrams. my question to you is what could make an amp have too high gain? I can think of several ways for too low, but not too high. I'm planning on doing more tests with my signal tracer to narrow down the tone or pre amp, but I'm interested to see if anyone has any insights, Thanks.

I've linked a schematic.

Arty.

[wasedadoc]

Feedback loop gone open circuit?

If you have a scope and sig gen it should be easy to compare corresponding points in left and right channels.

[andy3055]

It could be a bad ground on the signal path.

[David_AVD]

Replace the 2SC458 transistors in the tone amp section as they are known causes of all sorts of problems. You can use KSC1845 in their place, but check the pinout. The face with the text is opposite between the 2 types.

Also check for resistors that have gone high in value.

[floobydust]

I would take DCV measurements on the transistors and write them down. Don't slip with the probe. The schematic lists many voltages and you can compare readings between the two channels to narrow down a stage having problems.

I suggest to do a recap of the unit, for something of this age it was out in 1969, I outright replace all the electrolytic capacitors. They'll have dried out, going low value or worse if they short. Hmm I see ads in Playboy for it lol.

Not sure why but the transistors also fail with age, go noisy.

[Arty30]

Thank you for all the suggestions, since it was mentioned, I have already gone and replaced the electrolyitcs. I will look into those transistors, I think those 458s are sprinkled pretty heavily throughout. I have definitely had transistors go out before, my pioneer sx-737 ended up having all replaced in stages due to issues cropping up with use.

[Arty30]

can confirm, schematic lists about 17 instances of the 2SC458LG of varying gain ratings (B,C or D).

[Arty30]

Having seen plenty of evidence now that the 2sc458 especially the 2sc458LG variant are well known to become unreliable with age. should I go ahead and replace them, or just find the fault (assuming its a transistor) and replace just that one? It wouldn't matter either way to me, just curious what fellow hobbyists would do.

[floobydust]

Since you're already in there, I second replacing them, as they are known to not really last. It just saves problems down the road.

[squadchannel]

Having seen plenty of evidence now that the 2sc458 especially the 2sc458LG variant are well known to become unreliable with age. should I go ahead and replace them, or just find the fault (assuming its a transistor) and replace just that one? It wouldn't matter either way to me, just curious what fellow hobbyists would do.

If it were me, I would replace the capacitor first, because if the opportunity is already 50 years old, the capacitor is most likely dead.

My capacitor replacement timing is 20 years. I replace any equipment if it is over 20 years old.

I looked up C458 on a Japanese website. It seems that the legs of the transistor corrode.
This symptom also occurs in Sony radios of the time. (Skysensor ICF-5900) The legs of the transistor will corrode and even the bonding wire inside the epoxy package will corrode. Replacement is recommended.

Capacitors first, then check the condition again, and if it is still failing, start debugging from there.

[Arty30]

Thanks, like I mentioned, I have already replaced the capacitors except for the main filters and output couplers due to their size. I think I will go ahead and replace the transistors on the tine amp first and go from there.

[squadchannel]

oops. missed the mention of it.

If the problem occurs on all inputs and radio, then the MAIN AMP block is suspect, but first it is going through the NF TONE block.

First, bypass the NF TONE block and make sure the NF TONE block is not the problem.
You need to isolate the problem area.

If you have an oscilloscope, you can also check to see if the signal is passing properly through each transistor. compare the left and right channels from the first stage to the last stage to find what is wrong.

[Arty30]

This unit is one that has a helpful pre-out/main-in jumpers, which I have already tested separately and found to be functioning properly.

[floobydust]

Another approach I have used is to inject a 1kHz test tone and measure/compare ACV between channels and stages, to locate a signal problem.
But you have to know your multimeter, if it is AC or DC-coupled on ACV. Cheaper DMM's get it wrong if you have say 30VDC with 0.5VAC signal riding on it, the DC saturates the ADC front-end and it gives bullshit readings. Not a problem for a scope, any DC bias.

[Arty30]

Its been a while, between being busy, getting covid etc, I have finally solved the problem. I went and replaced the four 2sc458s on the tone amp, however this turned out to be premature. doing voltage measurements, I found that two of the four were not biased on the base. surprisingly the culprit was two 220K resistors that were open circuit, I have never seen resistors go open on their own. weirdly they were small grey dipped resistors, the style that sort of look like a dog bone, and were unique to this board only. the last small annoying fault I'm chasing is the stereo lamp is not functioning. if i tune into a string station is does not illuminate. it is the MPX board that is responsible for this, I will post a screenshot of the part of schematic. There are three transistors that appear to be cascaded, Q7 is off, so Q8 is on, thus Q9 is off and the lamp is out. This doesn't change if I tune in or out of a station, and Im not great with these circuits to tell where the "control" signal is coming from. sorry for the essay, but if anyone has any input I would be grateful, Thanks.

Arthur

[wasedadoc]

Does the tuner actually switch to stereo mode and ouput stereo audio when you tune a strong station? If not, that is a different fault from the indicator not working.

[Arty30]

Should this be something I can hear? not sure the best way to test that.

[Arty30]

I think you might be right, I can't tell any change in the sound on any "Mode" selection. it has Left, Right, Stereo, Rev, and Mono. I don't know what most of these modes mean or how they work, I have a hard time reading the schematic diagram for rotary selectors, but I think that I should hear a difference in Left and Right. this would be a problem with the MPX board I'm guessing?

[Arty30]

I tested the AUX input and the mode selector works as expected, so the FM tuner is outputting mono audio no matter what setting? seems odd.

[David_AVD]

The receiver looks to be a little unusual in that the Mode switch does affect the tuner's FM stereo mode.

If the Mode switch is in Stereo or Reverse, stereo operation is enabled for the FM tuner. In the other modes stereo is disabled.

Using headphones you should easily be able to tell if any music is really being decoded in stereo.

[Arty30]

I don't have the schematic as I'm writing this, but does the FM tuner have a separate mono output or does the MPX section simply output the same mono signal on both L and R outputs? presumably the latter since in "Stereo" mode, I still get audio output. I did a quick test of the several diodes on board, they were good, but it may be good to note that this MPX board does exclusively use the often distrusted  2SC458LG transistors, although so far the one's I've checked tested ok.

[David_AVD]

Audio is output from the FM tuner on the left and right lines in both modes.

Did you confirm that the audio output sounds like mono or stereo? Until you do that don't go replacing or adjusting anything.

[Arty30]

I definitely won't go twisting things in any tuner circuits before I know how. The only thing I have to go off of is that in AUX  mode with my phone I can hear the audio change when I go from mono to stereo, this is not the case when listening to the radio. Is there a better way to be sure?

[floobydust]

Many things can cause the receiver to seem stuck in FM mono.
Make sure the FM Stereo lamp is not burnt out, it also acts as a pullup resistor.
I would measure DCV on some of the transistors QC4-9 that will tell you what's going on there.
You'd have to check to 38kHz MPX carrier signal strength if you have a scope.

There are two trimpots VRc1 GATE and VRc2 STEREO DEFEAT if you can mark their position and turn them to see if it can switch modes to stereo at all.
Most of it is based on signal strength and if the FM signal appears weak, it will stay in mono.

[Arty30]

Thanks, the stereo lamp was burnt out, but I have replaced it, could this be a sign of anything? I measured B-E voltage on Q7-9 closest to the lamp and it seems they are cascaded and currently they are permanently in the state where Q9 is off because Q8 is on because Q7 is off, so I don't suspect that section yet but I'll check the others and possibly play with those pots.