Lab Series compressor

[lowell]

DM,
Thanks for help but not sure I follow. I see that your calculation is using ohm's law with the 22k/750 voltage divider and my 8.6 AC offset measurement... however, once I test that node what does that tell me? I have not had a chance to check yet... will do tomorrow.

[lowell]

Ok yes that node does measure about that. what does this infer?

[Don Moose]

Ok yes that node does measure about that. what does this infer?

I knew I shouldn't have stuck my nose back in this thread.
If it hadn't measured the expected value, it would have said that the 22k resistor was busted or something else on that node stuck to GND, and that would have pointed somewhere useful

Since that isn't the case, we're back under Enzo's care.

I'll try to sit on my hands for the rest of this, sorry.

[lowell]

Don,
ANYTHING is helpful... thanks. Enzo you have any ideas?

[Enzo]

I am no longer sure just what we are fixing. We started with compression, then shofted into no sound or something.

How about a summary of just what we are doing now?

[lowell]

Hey,
Ok so the compressor, you were right it works but is backwards from what was expected w/ the control knob.

When I was troubleshooting that, I suddenly got no sound from amp. When I checked the speaker one of the clips had broken off the speaker lug. When I reconnected it and plugged in the amp there was a horrible REALLY loud noise/hum... not unlike a bad filter cap hum. I've checked the caps visually they look ok, then I did dc leakage tests on the 2 main caps and they tested fine. I then pulled the power module and went through it only to find that everything tested fine - the output transistors, bias transistor, ballast resistors etc... side note: I cannot seem to locate 2 transistors in the amp that are listed on schematic - Q305/Q306. There is only 5ma DC on output but 8vAC on output with no input signal. I thought the rectifier diodes were shorted but it turned out that my continuity test on my meter was reading them as shorted in-circuit due to the very low resistance of the secondary on the power transformer which is a low 1ohm from CT to each side... is that normal?

[Enzo]

Power transformers will usually have low resistance, especially low voltage high current windings like this one.

On the power amp board on one corner is Q308 with a small heat sink. Q306 should be right next to it. At the far end in the corner, also with a small heat sink is Q307. Q305 is not next to it, but is over behind the third large power transistor. Not far. They are just small parts.

OK, there is no DC on the output, but you have 8VAC of noise. DO any of the controls have ANY effect on this noise? Now disconnect the speaker so you don't have to listen to it. Monitor with scope or meter instead.

You have a scope? WHat does the output look like? Is it 60Hz or 120Hz? Is it more or less a sine wave - even if rough and ragged? Or is it a ripple pattern - A sort of saw tooth?

Dry filter caps won't look any different from new ones. Leakage tests done with a meter won't tell you what they will do when they are under 70VDC. Power the unit up and verify both HV rails are about 70VDC, and neither one should show more than a tiny bit of ripple. 8VAC of ripple would be a large clue.

If those are OK, check the LV rails. Both +/-15VDC up to DC and clean of ripple?

Now divide and conquer. Is the noise signal on the preamp out? PLug something into the power amp in jack, and see if it cuts off the noise signal. If it does, connect the speaker and apply a signal there to see how it sounds. Now we know whether it is a preamp or a power amp problem.

With no DC offset, the power amp has a hard time making AC on its own, unless a ground got knocked off it. Tell me what is up with those things. I am off to dinner with the wife now, and will check in later this evening.

[lowell]

Enzo I will check those Monday... however am I gonna damage the speaker w/ this noise if I connect it?

[Enzo]

It is a solid state amp, it cannot produce a noise any louder than a maxed out signal. If the speaker handles max power out, it can handle the noise. Use the scope instead if you are concerned.

[lowell]

Enzo,
Your mentioning of a lifted ground causing ac noise set off a light bulb in my head. This noise was not there until I began troubleshooting the compressor, and that's when the speaker wire broke. I thought back and retraced my steps and realized that this was the same time that I had slightly removed/loosened the preamp board from the chassis. I just put the input jack nuts back on and the noise, gone. There are so many quirks to troubleshooting ss amps. They almost always require disassembly and that can, as in this situation, throw off any troubleshooting figures. Anyway... Thanks!

[lowell]

On another note. I just replaced the reverb tank because the old one had broken wire. The reverb is now there, however even if cranked it is BARELY noticeable. Is this the case w/ these amps? I think I will check the send/return circuits and make sure the op-amps are ok... if they are indeed op-amps... will find out.