Roland KC-500 voltage problem

[soundinvst]

Here is the spot I mentioned. It's only on the top; no damage to the trace. I just can't see where it came from unless it was from IC2.

[nickb]

J5 is basically a node connected to the neg. lead of C58. I agree it probably could use replacing. C13 and R13 are in channel 2 circuit. I had I time finding them on the schematic. I don't know what heated the board in that area, unless it was a prior failure and unrelated to the present problems.

Thx for the piccie - that helps a lot. I have no idea what caused that. It does not look like an overheated track and there is no component there. I think I'd be inclined to ignore it.

The dim-bulb just gives a flicker when I first power up, with or without the preamp plugged in.

That's good. The bulb limiter does help protect the power amp but it won''t do much for the sensitive components of the preamp.

And, I get a speaker bump. The preamp is still bringing the voltage low on Q204, but I hear grounding noise through the speaker when I touch the base with my probe, like when you touch the plug of a guitar cable that's plugged in.

That's not right. Do you really mean Q204? That's the switch for the anti turn on thump FET. It's base will start off at 0v and then ramp negative on power on to turn on the FET Q203. You should get hum pick up when touching CN201 pin 4 which is the power amp input. In any case that's all a side issue. We're are trying to focus on the preamp for now. Concentrate on one thing at a time.

So I think you are saying that you plugged the power to the preamp in again. I guess nothing smoked or you would have said. Before you plug it in again do try fix the 4 ohm problem. Did you check using the meter as instructed? What was the result? What is the condition of the caps C57 and C58?

As far as plugging the preamp in the resistors R201 and R202 do provide a measure of current limiting to about 100mA so I would say that it is OK to plug in but you will need to watch that those resistors don't over heat.

The voltage on the ICs is on pins 4 and 8 and pin 1 and 7 is ground, correct?

Yes. pin 4 = -15V, pin 8 =+15. The output pins 1 and 7 are 0 VDC.

[soundinvst]

The emitter of Q204 is just and easy place to check the neg. voltage going to the preamp because it is tied to the header pin 1 via JP207. I pulled the pre just after getting the the supply right to see if there was any change. It has not been plugged up since I started checking resistance.

I have been trying to check the resistance along the -15v rail to ground and I still get very low resistance (well under 10 ohms) all the way to jumper 93 which is just after it passed through pin 4 of IC9. I think there is a parallel line that runs up close to one of the pots. The +15v rail is reading about 85 ohms to ground. By the way, I pulled out C57 and C58 before I started so they are no longer a factor.

[nickb]

The emitter of Q204 is just and easy place to check the neg. voltage going to the preamp because it is tied to the header pin 1 via JP207. I pulled the pre just after getting the the supply right to see if there was any change. It has not been plugged up since I started checking resistance.

Ah! OK - gotcha!

I have been trying to check the resistance along the -15v rail to ground and I still get very low resistance (well under 10 ohms) all the way to jumper 93 which is just after it passed through pin 4 of IC9. I think there is a parallel line that runs up close to one of the pots. The +15v rail is reading about 85 ohms to ground. By the way, I pulled out C57 and C58 before I started so they are no longer a factor.

What is your strategy here? Are you just running along the -15V track hoping to see a difference (won't work without special equipment) or are you removing jumpers and / or cutting tracks to isolate the short?

[g1]

Your mention of 85 ohms from +15 to ground implies the fault is not just on the -15 rail. 85 ohms is also much too low.
Most probable would be an IC (rather than something only connected to one rail, such as a decoupling cap).
There are a few methods to try and find the faults, first is as nickb mentioned, isolating sections of the preamp.
Another method would be running at low power with a variac and checking for overheating IC's or other components.
Yet another method would be to desolder the power supply pins on the IC's (likely pins 4 & 8 for most types) until you find one or more that are causing the low resistance. But this can be difficult with double sided board as you can't see whether the pin is truly detached from the circuit.

[soundinvst]

I'm pretty green and have not used resistance as a troubleshooting tool, so I guess I was just hoping to find an obvious change in resistance as a starting point. As far as strategy goes, given my limited resources (dim-bulb and a DMM) I guess I'll check the 4 or 5 diodes on the preamp board and then check Q5 and Q6 that are sitting on the LV lines.

After that I guess I'll power it up through the limiter and check IC voltages. It would be so much easier if they were in sockets and could just pop 'em in and out. And, if frogs had wings...

[nickb]

I'm pretty green and have not used resistance as a troubleshooting tool, so I guess I was just hoping to find an obvious change in resistance as a starting point. As far as strategy goes, given my limited resources (dim-bulb and a DMM) I guess I'll check the 4 or 5 diodes on the preamp board and then check Q5 and Q6 that are sitting on the LV lines.

After that I guess I'll power it up through the limiter and check IC voltages. It would be so much easier if they were in sockets and could just pop 'em in and out. And, if frogs had wings...

The resistance method will not work for you. You need specialized gear to make that work. Forget it. Also, there is little point in checking IC voltages until you have fixed the high current problem as they will not be working normally.

As mentioned posts #20, #17 and #14 the way forward for you is to connect it up and look for overheating components. Please read those posts as I'm too lazy to retype

For current limiting we are relying on the series resistors in the power supply so watch they don't overheat. I think this is safe since you have already plugged it in a couple of times without that happening.

If that does not work out then it gets harder so try this first.

EDIT: On reflection, checking IC voltage might help to indicate a bad chip by comparison with others, but it not guaranteed as the chips are operating outside of their recommended range

[Enzo]

Let it run a little while, then feel on each IC to see if any are getting hot. Hot ICs are usually bad, and might be the source of your loading the supply.

[soundinvst]

I found the source of the smoked area if the board. It was C12 and C13. I didn't see that these spots were holes until I got it under a magnifier. Replacing them raised the +15v rail resistance to ground from around 85 ohms up to 375 ohms. The -15v rail is still around 4 ohms. I pulled the ICs so they are no longer a factor.

[nickb]

I found the source of the smoked area if the board. It was C12 and C13. I didn't see that these spots were holes until I got it under a magnifier. Replacing them raised the +15v rail resistance to ground from around 85 ohms up to 375 ohms. The -15v rail is still around 4 ohms. I pulled the ICs so they are no longer a factor.
[ATTACH=CONFIG]35935[/ATTACH]

Well done on spotting those The worry is that there are plenty more like that also connected to the power rails and it's possible excess voltage was the root cause. You'll just have to slog it out trying one at a time until you've got then all. Now, there may be a DC path that I've missed, but I think with all the IC's out you should measure many K ohms on the rails with the meter as there would just be leakage paths left.