Sunn SL 260 negative voltage supply issues

[jstnmlls]

Hey y'all
I've included a schematic at the bottom
Got a Sunn SL 260 2*12 combo that will not output more that half a volt on the negative supply for the pre amp. I'm at my wit's end here. Every transistor tests fine, the output transistors are brand new and are stable. The negative voltage to the power amp and various transistors is fine. I'm not super clear on which components regulate the voltage for the preamp, but everything looks kosher up until pin 4 on the first opamp in the power supply which reads .5 volts no matter what ac voltage it sees from my variac. Any advice would be sick. Thanks!SL 160-260 Schematics.pdf

[Enzo]

So you have both +45 and -45v?

HAve you swapped out IC202? Note at bottom CMOS SUPPLY ADJUST. Check that trimmer and the resistors in line with it, they reference IC202.

And isolate the problem. Either the supply isn't working or it is being loaded down, so if possible disconnect the -15v from the other board.

[Dave H]

The -15V comes from the emitter of series pass transistor Q213 at the bottom left of the schematic. Also check Q211 as it's part of the -15V regulator circuit.

[jstnmlls]

So you have both +45 and -45v?

HAve you swapped out IC202? Note at bottom CMOS SUPPLY ADJUST. Check that trimmer and the resistors in line with it, they reference IC202.

And isolate the problem. Either the supply isn't working or it is being loaded down, so if possible disconnect the -15v from the other board.

Yes I've replaced ic202 with two different known working 4558 chips.
Ok so the cmos adjust pot has .5 or .6 volts on all pins and won't adjust the voltage on the wiper at all. So you'd think the pot is bad, except it's not. Every 10k pot I replace it with has the same readings. The voltage feeding the pot on the outboard side of the 470k resistor is 16v like it should be. Another thing to note is that I have full voltage on the output transistors, but on the collector of the tip30 and 2n4250 there is .8 volts when it's supposed to be 16v. I've replaced both of these transistors twice. Same readings. Nearly every PNP transistor in this amp is obsolete BUT I found some 4250's online... Who knows if they are actually real. either way this should be working.

I'm about to yeet this thing into the garbage, y'all.

[Enzo]

16v on the 470k? I sure hope you mean -16v. And by outboard side do you mean the end that connects to C205.

All pins of trimmer with same dinky voltage? Actually that doesn't suggest a bad pot. The voltages on the pot come from the -45v rail through a voltage divider. Out of the -45v, the 47k and 470k should drop about 40v. Leaving about 5v on the pot. The 10k of the pot is tiny compared to the 470k, so across the pot itself I expect about 0.8v. That is all. That narrow range sits about -4v from ground. So consider the 47k, 470k, 10k and 51k. -45 at start, -41 at the left end of 470k, -4 at the trimmer. You have -45 at the outputs, but do you have it at D201 and the 47k resistor? You don't have it there , it won't magically appear downstream.

Also, polarity is critical here. YOu say you have 0.5v on the trimmer. Is that true? Or is it -0.5v? -0.5v at least tells us it is dealing with negative. If the voltage is positive, then it is leaking through the op amp because the op amp has no supply. Totally different problems.

TIP30 has 0.8v instead of 16v? I hope you mean -0.8v and -16v. If the collector of the TIP30 has only 0.8v, how can we expect -16 to come out the emitter? it isn't the transistors fault no voltage gets to it. On the schematic it suggests the V- fuse is open. but if you have -45 elsewhere, then it is likely OK. SO find out why that -45v is not getting to those transistors. Is the collector resistor open on the TIP30? The -45v from that fuse has to travel circuit traces to get anywhere. A cracked trace would remove said voltage. There is a lot more to the circuit than the transistors.


DOn't throw it out, learn something. Troubleshooting is a systematic approach, and learning that skill applies to ANYTHING you need to repair.

[jstnmlls]

16v on the 470k? I sure hope you mean -16v. And by outboard side do you mean the end that connects to C205.

All pins of trimmer with same dinky voltage? Actually that doesn't suggest a bad pot. The voltages on the pot come from the -45v rail through a voltage divider. Out of the -45v, the 47k and 470k should drop about 40v. Leaving about 5v on the pot. The 10k of the pot is tiny compared to the 470k, so across the pot itself I expect about 0.8v. That is all. That narrow range sits about -4v from ground. So consider the 47k, 470k, 10k and 51k. -45 at start, -41 at the left end of 470k, -4 at the trimmer. You have -45 at the outputs, but do you have it at D201 and the 47k resistor? You don't have it there , it won't magically appear downstream.

Also, polarity is critical here. YOu say you have 0.5v on the trimmer. Is that true? Or is it -0.5v? -0.5v at least tells us it is dealing with negative. If the voltage is positive, then it is leaking through the op amp because the op amp has no supply. Totally different problems.

TIP30 has 0.8v instead of 16v? I hope you mean -0.8v and -16v. If the collector of the TIP30 has only 0.8v, how can we expect -16 to come out the emitter? it isn't the transistors fault no voltage gets to it. On the schematic it suggests the V- fuse is open. but if you have -45 elsewhere, then it is likely OK. SO find out why that -45v is not getting to those transistors. Is the collector resistor open on the TIP30? The -45v from that fuse has to travel circuit traces to get anywhere. A cracked trace would remove said voltage. There is a lot more to the circuit than the transistors.


DOn't throw it out, learn something. Troubleshooting is a systematic approach, and learning that skill applies to ANYTHING you need to repair.

Yeah I've been tryna learn something here for a few months on this guy. I'm just frustrated. I'm not a real engineer but have a fair amount of experience troubleshooting pretty complicated tube equipment and I'm trying to learn SS stuff a bit better.

All the voltages were negative sorry I didn't type that in correctly. This the first time I've ever really tried to talk about this stuff with anyone and I'm not expressing myself very well. Polarity IS important.

I disconnected the preamp power supply rails from the circuit to see if the supply was working without a load. It was! However I have found a considerable positive voltage on the negative rail. I'll have to find where that's coming from.

[Enzo]

Go back to the source. The -45v has to get to the TIP30 and elsewhere. You have -45v, find the point at which it disappears. If you have to follow every trace from -45v, so be it.

Maybe this will help. Op amps and other things with split power supply balance their outputs. Imagine putting a black dot on a rubber band. Hold it up in front of your face and stretch the band either side. The black dot stays in the center. Your hands may be a foot apart or only 6 inches, but as long as you stretch it evenly, the dot will center. Now leg go one side. The whole thing slaps over to the other side. So imagine the hand that let go was the negative side. Now your black dot and even the "negative end" of the band are all over at some positive point.

When your negative supply disappeared, the rest of the stuff is still working but bunched up towards positive. SO the negative corner of the op amp can float up to a positive voltage.

And really, don't focus on the solid stateness, focus on it being a circuit. If you had B+ at your 5U4 in a tube amp, but there was no B+ for any of the 12AX7s, does it matter that it is tubes? it is the exact same problem we have here. VOltage at the main supply is not reaching secondary circuits.

[Dave H]

The negative voltage to the power amp and various transistors is fine. I'm not super clear on which components regulate the voltage for the preamp, but everything looks kosher up until pin 4 on the first opamp in the power supply which reads .5 volts

Does this mean the -45V rail is OK? The +/-15V regulators are bottom left on the schematic. Pin 4 of the op-amps is -15V. Is it the -15V rail that reads 0.5V (or -0.5V)? You have to check the supplies in the correct order. Nothing works unless the +/-45V rails are OK. The +/-4V rails won't work unless the -15V rail is OK and the -15V rail won't work unless the +15V rail is OK so check +/-45V then +/-15V then +/-4V. It's no use trying to fix the 4V supplies if a 15V supply is missing. As you said pin 4 of the first op-amp is 0.5V I suspect either a -15V supply fault or there's a short on the supply. Are any op-amps getting too hot? Giving up is not permitted on MEF.

[Enzo]

His -45 supply is not reaching the -15v regulator transistor.

[Dave H]

His -45 supply is not reaching the -15v regulator transistor.

Thanks Enzo. Where does he say that? I missed it.

[Enzo]

Post #4

I have full voltage on the output transistors, but on the collector of the tip30 and 2n4250 there is .8 volts when it's supposed to be 16v

[Dave H]

Post #4

Thanks, I see it now. I must have been half asleep. It was way past midnight in the UK.

[Tassieviking]

Manual: SUNN SL260.pdf