Music Man HD 212

[lowell]

This amp has intermittent crackle at turn on that goes away after warming up. The crackle is independent of the master volume so I've been working on the PA. I've done all the following with no fix:

replaced pre and power tubes
banged on amp - has no effect
moved tubes w/ hand - no effect
replaced filter caps
disconnected PI coupling caps to power tubes
replaced power tube grid stoppers
replaced 1.5k 10w screen supply resistors
replaced bias supply filter cap and feed resistors

[lowell]

No bites eh? Well I went further and replaced the rectifiers for the HT and their respective bleeder resistors. I then sprayed all suspect components w/ feezed air to see if cooling anything would show the issue. Nothing, about to give amp back to its owner unfixed. If anyone's got any suggestions please post.

[Steve L]

Music Man Noise

Have you tried to look for the noise with a scope? That should tell you a lot.
If you did, were you able to see the noise anywhere? Did you look at the power supply with the scope?

You said you disconnected the phase inverter caps. (C-35 and C-36) I assume you still had the noise present. If not, I would suspect the LM1458 op-amp or the coupling caps themselves.

If there was still noise with C-35 and C-36 disconnected, there is not a lot left to suspect.
Any traces of carbon or other unwanted substances on the tube sockets?
I have not worked on one of those amps in a while, however I recall they used some very thin wire with thin insulation. Any small traces of arcing anywhere?

Steve

[lowell]

It's so quick after turn on. It comes for about 10 seconds and then is gone forever, until turn on again. I've pushed on just about everything with no effect so I wouldn't imagine it to be a wire. If it were arcing I'd think it would not ONLY be at turn on and for such a short time. Also I don't know how to view such noise on a scope. I'll try it though.

[lowell]

ok so scoped it and it's showing up on the grids of the power tubes. I replaced both grid wires, as one of them had deteriorated may have been arcing to chassis... however the static is still there at turn on for a bit. The static is not before the bias feed resistors so it's not the bias supply.

[Steve L]

What do you see on the cathodes?

[lowell]

I see it on cathodes too. I've ohmed out the cathode to ground and reads fine.

[Steve L]

Follow it backwards. Do you see it on the base of Q-1 and/or Q-2?
If so, do you see it on pins 1 and/or 7 of the op-amp? If so, is it on pin 2?
That is the normal signal path. Keep going back until you find the source. You need to trace it out just like an audio signal.

Also, check to see if it is on the low voltage +/- 16V supplies as well as the 16V grid supply as they come up to their quiescent levels.
Sometimes zeners can cause noise like you describe.

As I mentioned before, IC-8, C35, C36 and C42 are also what I would suspect.

Hopefully this helps. At least you will get some experience tracing out problems like this.

Steve

[lowell]

ok not sure if I really saw it on the cathodes cause now I don't, it's hard to see amongst the other junk on the scope when it's set at 50mv. So I don't think it's on the cathodes too, just the power tube grids. If I disconnected the PI coupling caps you still think it'd be at those places you just mentioned above? I'll check those, but now I'm wondering if the eyelet board where the grid wires are connected is capacitive or something.

[Steve L]

No, if the noise did go away when you disconnected those caps, I would not expect it to be there, but check those points anyway. To see the noise on the grids of the tubes and not see it beyond the 220 ohm resistors does not make sense.
When you said you ohmed out the cathode to ground at it reads fine, what did you measure?

[lowell]

The noise is not showing on the scope before the 330k bias feed resistors, and yes it's still there w/ PI coupling caps disconnected. I measured less than 1 ohm from cathode pin 8 to the 3rd prong on the power cord.

[Steve L]

OK. First, I was looking at the schematic on page 2. You apparently have the chassis on page 3. That changes things.

In any case, there should be 10 ohms from cathode to ground. Are you measuring from pin 1 instead of 8? Pin 8 is the cathode.

[lowell]

Hi Steve, someone modded this thing at some point. they removed those cathode resistors. the cathodes are all grounded. The cathodes are also tied to pins 1 and 2. Not sure if I said, but the plate treble bleed caps are also out of the circuit. Oh also, you're gonna think I'm crazy but now the static IS showing pre PI coupling cap, however the caps are still disconnected AND the static is STILL audible and present on the power tube grids.

[lowell]

ok question, could noise on another pin of an el34 cause noise to show up on a grid and then the plate amplifies it? I'm asking because I'm thinking if there's any other wiring in the PA that's causing this maybe it's being transferred to the grids somehow. Or maybe even the heaters are causing this.?

[Steve Conner]

Does it still crackle with the power tubes removed? (sounds crazy, but humour me!)

Have you tried replacing the power tube sockets? It's looking like carbon tracking on your sockets.