JC120 +15 supply issues

[Mr_bibbles]

I have an 80's JC-120 on the bench- Channel 2 has no signal.

https://music-electronics-forum.com/...1&d=1208815304

Tracked down an issue at q5, a 2sk117, only getting about 2 volts instead of 15 for it's supply.

There IS a 15v supply- I measured it at r18, and again at the entry of q27 , but it seems like it lowers down to 2 volts at the exit of q27, an 2sd313 transistor.

2sd313:

B: 15v
C: 37v
E: 2v

Since there's SOME voltage coming out, should I assume something on the 15v rail is pulling it down to 2? Is there a better way to approach this problem?

Thanks as always.

[DrGonz78]

It appears that the zener diode is zeening away at 15v. It would seem that something downstream is pulling the voltages down. Check the many different IC's on the board to see if any of them are getting warm. Check the non-power rail legs of those IC's to see if we have voltages where they are not supposed to be.

[Mr_bibbles]

Thanks Gonz! That's what I had thought but I wanted to be sure before I started process. I'll get back with a record of the labor for anyone else who might run into a problem like this.

[Mr_bibbles]

IC 1:
1. 14.5v
2. 14.5v
3 14.25v
4. 0v
5. 14.5v
6. 14.5v
7. 14.5v
8 28v

All of the other IC's have about 2 volts on pin 1, Bad IC1, 4558??

[Mr_bibbles]

Update; I removed the 4558 and measured the other IC's, still the same low voltage at around +1.5-2 Volts where there should be +15.

There is no IC installed but I do have +15 where pin 3 of IC1 would be... Full measurements:

Pin 1 .73v
Pin 2 .73v
pin 3 +15v
pin 4 .5mv
pin 5 .+15
pin 6 .8v
pin 7 .8v
pin 8 +30v

Seems like the feed is healthy, but perhaps there is another transistor rather than IC pulling down voltage??

[Mr_bibbles]

Update; I removed the 4558 and measured the other IC's, still the same low voltage at around +1.5-2 Volts where there should be +15.

There is no IC installed but I do have +15 where pin 3 of IC1 would be... Full measurements:

Pin 1 .73v
Pin 2 .73v
pin 3 +15v
pin 4 .5mv
pin 5 .+15
pin 6 .8v
pin 7 .8v
pin 8 +30v

Seems like the feed is healthy, but perhaps there is another transistor rather than IC pulling down voltage??

Sorry for the multiple posts- Just installed a new 4558 and the measurements are pretty much the same as the old one....

[52 Bill]

This is a single sided power supply for the opamps, so there should be 30 vdc on pin 8 and ground 0 volts on pin 4. So the reading you get at IC1 are correct, with about 15 volts bias on all of the inputs and outputs.

If the 15 vdc supply from Q27 is low, then as Gonz suggests, either the pass transistor is bad or there is something dragging down the voltage. Meter test the transistor, and then start looking for something shorting the 15 volts to ground.

[DrGonz78]

If the 15 vdc supply from Q27 is low, then as Gonz suggests, either the pass transistor is bad or there is something dragging down the voltage. Meter test the transistor, and then start looking for something shorting the 15 volts to ground.

Would it be a valid test to jumper the 15v from the base of Q27 to force voltage onto the emitter leg connection? Or simply disconnect the emitter leg from circuit and check what voltage is there. But yes first test the transistor with your meter in diode mode. Also, C16 or some other cap might be leaking to ground maybe?

I would assume the reverb circuit is not working correctly since there is a 15v feed for the reverb driver transistors before the reverb tank. But good to know the IC's are not the culprits.

[Mr_bibbles]

This is a single sided power supply for the opamps, so there should be 30 vdc on pin 8 and ground 0 volts on pin 4. So the reading you get at IC1 are correct, with about 15 volts bias on all of the inputs and outputs.

If the 15 vdc supply from Q27 is low, then as Gonz suggests, either the pass transistor is bad or there is something dragging down the voltage. Meter test the transistor, and then start looking for something shorting the 15 volts to ground.

Thank you for explaining the single sided supply design, that's extremely helpful! Back to the pass Transistor-

Metered it with nothing connected, diode tests came out ok. I disconnected the emitter from the circuit to measure the +15 out, and it only measured +4v. When I connect the leg, I get only +2 volts. Is it possible the transistor can measure OK, but not be putting out full voltage?

Another question- why not just have a zener for the 15v supply? What's the benefit of having a Pass Transistor here? Thanks guys!

[Mr_bibbles]

Hey, Saw this after I did the tests, good to see I'm not far off. Surprisingly, the reverb circuit IS working well. As Bill said, the Single supply for IC1 (reverb) all looks good, though it's not clear from the schematic how they're isolated from the rest of q27's supply.

[52 Bill]

I assume that you mean if you shake the tank the springs you can hear them through through the speakers? The reverb return circuit is done with opamps, the drive circuit is part of the 15 vdc circuit.

How did you isolate the pass transistor from the circuit to test the supply with no load?

The pass transistor can handle more current than a low wattage Zener diode.

[Mr_bibbles]

Thanks for explaining the pass transistor! And yes, I can hear the springs.

I'm not sure if this is a valid test, but I disconnected only the emitter from the circuit by removing a jumper wire, so only two legs were in circuit. I've started disconnecting each 15+ supply on the circuit one by one to see what's pulling down voltage, but if anyone knows a different way to definitively test q27...

[DrGonz78]

I'm not sure if this is a valid test, but I disconnected only the emitter from the circuit by removing a jumper wire, so only two legs were in circuit. I've started disconnecting each 15+ supply on the circuit one by one to see what's pulling down voltage, but if anyone knows a different way to definitively test q27...

Well if you only had 4v measured on the emitter leg of Q27 after disconnecting that leg from the circuit, then my guess would be that something was at fault with Q27. I would think without load on the rail the voltage measured should be 15v or so.

Edit: I'll be curious to know what the fault is on this one. It is a bit elusive.

[Mr_bibbles]

Thanks, I'll definitely post results, I haven't seen anything like this on the forum or elsewhere. I have a TIP31C, can I use that as a suitable pass transistor? it seems pretty equivalent, but I know some old heads are into this nitty gritty....

[g1]

As long as you account for any difference in pinout, Tip31C will be fine there.