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JVC JA-S55 with some out of spec voltage

Corktowner

Corktowner

Active Member
Hello again everyone. Quarantine here in Michigan has got me back on the JVC integrated that I am trying to repair. So far I have recapped the amp, which worked for a while, and then I had to replace on of the STK modules, which I did and which worked for a while, but now I am chasing a problem with the left channel that has me stumped.

The unusual readings I have are high negative voltages in the section just before the STKs

With R301 removed to isolate the circuit here is what I have found:

X311 C: -10.21. B&E voltages are correct
X313 (X311 feeds in). B: -10.21. E: -11.05. C voltage is correct
X315 C: -13.43. B&E voltages are correct.

X301, X303, X305, X307, X309 check good.

I have one extra of the STKs, and I got these results with both modules. I have swapped X311, 313 and 315 with their R channel counterparts but the problem stays on the L channel.

Any assistance of where to look next would be appreciated. I don’t know if perhaps both of the STKs are bad and I am getting some voltage flowing up from downstream of the signal path...I’m not even sure that is possible but at this point I am out of ideas.

Thanks as always!
 
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Looks like bad STK modules. The emitter voltage of X313 goes thru the bias circuit then back into the collector of X315.

Craig
 
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llwhtt said:
Looks like bad STK modules. The emitter voltage of X313 goes thru the bias circuit then back into the collector of X315. If that bias circuit is open you'll get the readings you're getting.

Craig
Click to expand...

Craig, thank you. From reading all of the STK replacement thread I saw the internal schematic - looks like I can check the bias circuit of the modules between pins 1 and 10. There are two diodes and two resistors; should I check on diode mode or just for resistance or will it not matter?

I’ll report back later what those results are.
 
Depending on the resistor values diode mode may not read anything. You won't be able to read TR1 either way. BDent has real STK0060s in stock if you need them.
 
I did a rethink on this, disconnect pins 2 and 9 of the STK module and then see what's on pins 206 and 208? If it's back to normal it has to be a bad STK.
 
Ok, I have disconnected Pin 2 and 9 from the STK on the bad channel and confirmed it with the DMM.
New Readings:
Pin 206: -13.53
Pin 208: -10.99

X311 C: -10.3 B&E voltages are correct
X313 (X311 feeds in). B: -10.3. E: -10.9. C voltage is correct
X315 C: -13.5. B&E voltages are correct.

I checked the voltages around pins 2 and 9 as well:
Pin 2: -11.9
Pin 2 surround: +48.9 (positive power cap side)
Pin 9: -12.47
Pin 9 surround: -49.0 (negative power cap side)

On the STK I pulled out, which was one of the two replacements I bought a few years ago, on diode check between pins 1 and 0 I got 1.6. Measured open on a normal ohm check. The original STK measured open between those pins on diode check.,

I also pulled the STK from the good channel. It measured 1.55 between 1 and 0 on diode check. I will swap STKs tomorrow and see what the results are. If the problem follows the STK then we’ll know. If not I’ll be calling on everyone’s expertise again.
 
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OK, I have swapped the STKs between the good and bad channel, and then I swapped them back to confirm.

The problem stayed on the left channel. It did not move with the STK units. So the problem is in the left power circuit somewhere. As I said before I have R301 removed so the problem isn’t coming from the upstream tone circuits.

Pin 3 of the bad channel STK is also reading is also reading out of spec at -14.53. This is the only other unusual reading I’m getting on the bad channel besides what I have reported before.

I’m at a loss as to what to check next. Any advice would be appreciated
 
If X313 is not turned on you might get a negative voltage on the emitter. Check the voltage across R321, if 0VDC X313 is off. It should .1VDC. R335 and R337 check OK? What's the voltage on R327?
 
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Voltage drop across R321 is 0.14V. Exactly the same as R322 on the good circuit.

R327 voltage drop is 0.96V. But the drop across R328 is .06V. R327 tests correct at 11k ohm

R337 tested open (which explains why the negative voltage wasn’t getting any further and the protect relay would come on)

R335 is 0.64 ohm (should be 0.47)
 
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With R337 open there's probably massive DC offset on the output that's why I asked for the voltage on R327, the feedback resistor. Put your meter leads together to see what the Ohms are, then subtract that from your R335 reading. DMMs aren't too spiffy at very low Ohms.
 
Ok, I got 0.19 ohs from my dmm. It’s an old Fluke benchtop - 8050A. So that makes R335 in spec.

So, it seems I need a 0.47ohm, 5 watt resistor. I don’t see the MPC74s available anywhere, which is the stock resistor. Any suggestions on what to use?

Also, thank you so much for the help on this. I was really lost at this level of diagnosis.
 
I've never had much luck finding those "plate" resistors, just use a .47 Ohm 5W wirewound.
 
Ok, I ordered the correct parts from Mouser, they arrived and I let them sit a few days because corona virus. The new resistors had thicker wire, 0.9mm, than the old 0.6mm resistors, but after some sanding I got the replacement in. Voila, the strange negative voltages disappeared and things are starting to look much better, but of course not yet perfect.

DC offset on the left channel is still -1.0V, with the adjustable resistor on full. I rechecked voltages in the circuit just before the STKs and found that the collector of X311 was only reading 0.9V, but the good circuit X312 collector read 1.7V. The manual says 1.9 but my rail voltages are a little low on this unit. Should I replace X311 and try again, or should I be looking at something else in the circuit.

X311: B +41.3V, E +41.9V, C +0.9V
X312: B +41.3V, E +41.9V, C +1.7V
 
X313: B +0.9V, E +0.25V, C +42.3V
X314: B +1.78V, E +1.06V, C +42.3V

Also, I pulled X311 because I had some replacement transistors already, but in doing so I found I had already replaced X311 (probably chasing the old problem). I still had the original X311 transistor, and I put that back in the circuit, but there was no change.
 
X315: B -41.1V, C -2.1V, E -41.7
X316: B -41.1V, C -1.3, E -41.7

IC301 #1: -2.1V #0 +0.25V
IC302 #1: -1.3V #0 +1.05V.

Interesting that the absolute values of IC301 and 302 add up to the same number.
 
They should be fairly equal but opposite polarity. The difference you see is probably DC offset. With all of the B-E voltages looking good the only thing left is DC offset.
 
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