Sunn Alpha 112r oscillation under load

[TASBrendan]

Hi all-
I have a Sunn Alpha 112r in with an oscillation that happens only when there's a speaker or load on the output. Scoping the output shows a sine wave. I've narrowed it down to Q105, Q106, Q107 where removing any one of them cuts the problem. I tried removing IC107 and Q108 with no change. I've inserted a plug into the Power Amp In, and also lifted a leg of R143 to break connection with the preamp side. I'm using a current limiter bulb and read no current across R170 in all situations. There's 3mA current at R172 when there's no load and no sine wave happening, and about 130mA when the sine wave is present.

Any help where to look next is greatly appreciated! Thanks!

[Helmholtz]

Is there DCV at the speaker output without a load?
If yes, do NOT connect a speaker.
Please post a scope pic of the 122Hz output with a dummy load connected.

[TASBrendan]

Thanks Hemholtz - there's no DC at the speaker terminals without load.
Well, I'm thinking it's the filter caps- On the scope I was reading the sine wave at the -45v rail, but not the +45v rail - that rail had some minor ripple. I decided to swap the caps in place to see if the problem was then replicated on the +45v rail. The sine wave followed the cap switch to the +45v rail with the load in place, but without the load, the sine wave is now being massively amplified - the same when there's no filter caps in place. So I think they're both shot and I aught to have just started there!

[TASBrendan]

Here's some pics, for fun at this point....
Top trace (ch2) is probing the filter caps, one photo each. Bottom trace (ch1) is the output on both photos. Since switching the filter caps, the output wave amplitude has decreased. Both probes are x10

[TASBrendan]

Well, I replaced the main filter caps with no change. I must have done something wrong when I swapped the caps and thought that the problem followed to the other rail...over excited that I found the problem. Some more info - the sine wave is present at the -15v junction of R175 and the 15v Zener. So same thing going on as the photos above - top trace picture 1, large sine wave is on the -VDC rail at the filter cap. Top trace picture 2, small filtered power at +VDC rail. Removing any of the transistors 105, 106, 107 cuts the sine wave, yet they all test good on my transistor tester - no shorts, good Beta, no leakage. Should I be looking for a faulty ground in the power supply?

[TASBrendan]

Also, I have spare diode bridges so I swapped out the old one, just in case but no change

[TASBrendan]

Here's an interesting change - I lifted the leg of R175 from the -45VDC rail and at first, the oscillation is gone and the -VDC rail is flat. But after about 1 min, the oscillation appears on the +DCV rail. The same pointed sine wave as picture 1, but with the sharper point on the positive node. Although now the sine wave has a lot of noise on it at the output or any of the transistors Q102,103,104,109. Any ideas where to go from here would be appreciated! Thanks

[Helmholtz]

I don't see any evidence of oscillation. Seems you're dealing with 120Hz power supply ripple.

[g1]

Is this issue something that was discovered while running on the limiter bulb?
Or is this the fault that happens with no bulb and speaker connected? If so, what does it sound like?

[TASBrendan]

It does seem like power supply problems. I replaced the main filter caps C126,127 with no changes. Maybe the zener or resistor network, or bad ground connection somewhere...?

The amp came to me putting out a pure 120hz tone at all times when the amp was on. This started suddenly and is constant when there's a load or speaker plugged in, but stops when there's no load or speaker. It happens whether there's a limiter bulb in place or not. I always use the limiter bulb when probing around or make changes to the circuit- it doesn't light up as a short, but it does show a significant amount of current happening when there's a load in place and the sine wave is present.

The only significant change is what I was talking about in my last post - Lifting the 330ohm resistor out of circuit on the fuse side causes the sine wave to occur at the positive rail, but only after a minute - the signal on that side is drawing more current and the sine wave is fuzzy/noisy looking on the scope.

[g1]

Like Helmholtz said, it's 120Hz hum (ripple). It will save time to call it that rather than oscillation or a sine wave.
Excess ripple is usually the result of high current somewhere. What you said about the bulb verifies that.
Disconnecting only one of those 330R supply resistors will cause an imbalance so you can't draw any conclusions from that. And the power amp requires the +/-15V for Q101,Q106 and IC107. The amp is all DC coupled, so you can't remove parts to troubleshoot, you have to sub working parts in. Are any of the IC's getting hot when it is loaded? Excess current usually heats up something somewhere.
From what you said about R172 in the first post, there seems to be an issue causing the minus side of the amp to over-conduct.

[loudthud]

Does the 120Hz appear at TP11 ? Note how the -45V rail goes into IC106 and that sets the rails on IC102. Check for 120Hz on pins 7 and 14 of IC102. Is the +/-4V good ? Is C120 good ?

I have a slightly different version of the schematic with a note below IC106:

[TASBrendan]

Thanks everyone! I knew I was going in circles... I did start to look around IC 106, 102, but didn't really know what I was looking for. I think C120 is ok, but I'll pull it and actually check for leakage. I think the 120Hz is at TP11 because its on anything that is coupled to the main output rail.

[g1]

Yes, I expect it to be there at TP11 as you have verified it is on the +15V rail (passed down from the main + rail).

[Helmholtz]

Don't guess. Scope TP11 and supply voltages for ripple.