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JVC JA-S55 with some out of spec voltage

llwhtt said:
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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In looking for the DC offset issue I have found what seems to be s similar circumstance on the volume board. I removed R201 in order to isolate the board from anything upstream in the circuit, and here is what I found:

Test point U: -42.3V
Test point V: +42.5V

X203 E: 41.95V
X204 E: 13.45V

I checked R219 and it was OK, and then I swapped with R220 and the result didn’t change. So something else is “contaminating” the circuit with over voltage. Any idea where I should look?
 
They all check out OK. I also pulled R215 and checked that as well.

X203 E voltage is now +37.9 with the 5 resistors out of the circuit. R201, 205, 213, 215, 217
 
The full B+ voltage appears on both sides of R219 because there is no current flow in that part of the circuit. Check R221 that's input for the B- which is needed to complete the circuit.
 
R221 measures 830 ohm, so its also OK as well. I’ll start putting some of these back to get the current flowing again.
 
OK, at some point in the past I had replaced the FET X201. I put back the old FET pair and now the circuit is measuring correctly again. I guess I didn’t get the correct replacements for those - hard to figure out some of these old transistors.

I still have the high DC offset on the left channel, though. Back on the hunt. Thanks and please stay tuned.
 
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Still DC offset at the amplifier output? R301 installed? if not install it. Ground TP222, the input to the power amp, what's the offset now? If still bad replace X301 and X303 with a matched pair.
 
R301 is installed. DC offset is about -0.83V at the speaker connections - it’s jumping around a little bit. The adjustment resistor is turned full.

I grounded TP222 to the case with a long screwdriver. The voltage measured at the speaker connection jumps to +1.76 before the protection circuit is triggered. I was able to repeat this several times to verify.

So the DC offset is coming in from upstream of the power circuit?
 
OK, compare that with TP224. The right channel is OK? Where was R311 set when you tried grounding TP222? Try putting R311 in the middle and then try grounding TP222. If the protection doesn't kick in measure and adjust your DC offset at the speaker terminal. The power amp section is DC coupled meaning that ANY DC at the input will be amplified by the power amp. If you can ground the input AND adjust for 0VDC the power amp section should be OK. Does the Tone Switch make any difference?
 
llwhtt said:
OK, compare that with TP224. The right channel is OK? Where was R311 set when you tried grounding TP222? Try putting R311 in the middle and then try grounding TP222. If the protection doesn't kick in measure and adjust your DC offset at the speaker terminal. The power amp section is DC coupled meaning that ANY DC at the input will be amplified by the power amp. If you can ground the input AND adjust for 0VDC the power amp section should be OK. Does the Tone Switch make any difference?
Click to expand...

Sorry for the delay - had some things come up which took away my electronics time.

TP224 is showing -35mV. I had previously went through the procedure for the equilazer amp center voltage on page 8 of the service manual.

I soldered a small piece of wire between TP222 and TP223 to ground it out. For the LH channel, turning R311 full ccw lowered the DC offset at the speaker terminal to 0.42V, which is still much too high. The RH channel I could adjust to less than 1mV.

So, am correctly judging that there are problems in both the pre-amp and power amp section on the left hand channel?
 
With the ground still in place between TP222 and TP223 I checked some voltages on the left hand circuit. The Collector of X311 is too high, at 2.9V instead of 1.9V. I assume this is also causing the output (emitter) of X313 to be too high also. I measured it at 2.3 (1.3 is spec).

Also, the X315 collector is reading only -60mV. It should be -1.3V. All of the corresponding measurements on the RH channel are correct.

Are these measurements reason to replace transistors X311 and X315?
 
X311 B: +40.88. E: 41.53
X313 B: +3.0. E: +2.34

Yes, Changing R311 while measuring both the B & E voltages of X313 causes those voltages to change. I can’t quite get them in spec, though
 
Thanks for your help in getting this far, Ilwhtt!

Mouser is showing the KSC1845 out of stock. I have some KSC2316YTA that I had previously ordered. They are NPN with the correct voltage and current rating. Let me know if you agree that they should work.

Last question - should I assume that transistors sold and packaged together are matching? Or is there a more specific way to test? I checked the transistor selection thread by Racoon1400, but there wasn’t much in that for matching.
 
I know, I have 100 of them on B/O from Mouser. The KCS3216 looks OK UNTIL you get to the hFE spec it's a bit low for this application. What are the original transistors?
 
Originals are 2SC1775A. hfe1 is 400min, 1200max. hfe2 is 160min.

For KSC2316, hfe1 is 60 min, no max spec. hfe2 is 80min, 240max.
 
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