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]

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.

[Steve L]

I can only think of one more thing to look at.
I have seen many amps where people have spilled drinks on them. A lot of times the drink will get inside the amp. The alcohol evaporates, but the sugar and whatever else was in there will be left behind and cause problems for a long time.
Sometimes you can easily see the residue and sometimes you can't. Try cleaning the board, chassis and especially the tube sockets with a good solvent and see if the noise goes away.
If not, I am out of suggestions.

Steve

[lowell]

thank you guys. I will clean that stuff and then ask the owner if they want new wiring on the power tube sockets and new sockets. After that there's nothing left!

[lowell]

Originally Posted by 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.

Steve, no it doesn't still do it w/o power tubes. I've replaced all the sockets and wiring for the power tubes, still there. I did not say before that there is audible hum when the static is there and they both seem to go up and down in volume together, they must be of the same source. Maybe hum tips someone off on what this is? I'm bout to remove the board and clean it, am not looking forward to it though.

[lowell]

ok so I disconnected the screens on 1 pair of power tubes and the crackle/static is GONE. I'm wondering if one side of the OT is arcing. Gonna try new tubes again, just to be sure, then will cut the primary for those 2 tubes shorter, and solder in a new wire to see if the wire is the issue. I never decided to clean the board because I lifted a bunch of connections, most of them, off the board so they're floating and it didn't fix the issue.

[lowell]

The orange primary wire to those 2 tubes has some gunk on it that is also on the chassis, I pulled it up away from the chassis and problem's still there. I'm thinking the OT is the issue here. Any thoughts? I think the screen being disconnected from that side says something about this. The source is not from the board or components on the board but after that.

[lowell]

Any had an OT cause static like this?

[Mac Guy]

FWIW...I had a similar problem for about 2 years with a Fender amp...turned out that a complete re-tube cleared it. It was, in my case, the last of the 4 power tubes I changed out. I went through the entire amp one tube at a time and finally found it.

Good luck.

MG

[lowell]

still there w/ sub OT... wow, I'm at a loss.

PI coupling caps are disconnected, bias supply has new components and is floating off the fiberboard as to eliminate any possible issue w/ the board, new wiring on all tube sockets including grid wires etc. Screen supply wire is still the same but I'm not seeing the static on the screen supply. It's ONLY on the grids and plates.

[lowell]

actually now i see it on screen supply. I'll replace those wires and then possibly try a sub choke.

[lowell]

wait a minute... i just found that someone had previously wired the supplies wrong. The screens were coming off the initial side of the choke, instead of after, and the preamp supply was at the wrong node too. Wow, should've caught this long ago, but as it is wrong to do, I assumed the amp was wired correctly when it came in. Anywho I rewired it and wasn't getting any static but not sure if I fixed it yet. I'm gonna let it cool down over night and try it in the morning.

[lowell]

Ok, DAMNED static still there after all this bull@#$%. Anyone know what this little xmas tree lightbulb thingy is going from the ground switch to the fuse? Does it light up when the fuse blows or something?

[lowell]

so I was sitting there trying to get this amp to act up as much as possible by cycling the power switch on and off. I realized that the static did not come back when I did this, however a bad hum DID, and it would last about 5 seconds and go away. I thought this odd so I subbed a power switch in and all seems fine. I've let the amp cool down 3 times now and powered it up w/ no problem. I ordered the correct switch in hopes that it fixes it.

[Enzo]

While you wait, try this. Drool some Deoxit into the switch and flip it back and forth a few times. Any help?

And if you plan to replace the switch, then for now, just wire it together - short across the old switch. The amp can be without an off switch. Reassemble everything and test the amp thoroughly. That way you will know you are installing your new switch into an amp that is otherwise 100%.

[lowell]

good call. I pretty much have it rigged like that now and after 3 cool downs/restarts it's great. I'll try spraying the wash in there.

[lowell]

new switch is in and static still there. I'm quite bummed at this point. Could a power transformer cause this? My thoughts are that anything could cause this, just don't wanna swap in a PT for nothing. Anyone have any ideas on scoping this issue further? I'm not too good w/ a scope when it comes to troubleshooting power supply issues.

[lowell]

What would you do to see if the PT is the culprit here? Anything easier than putting a new one in? Meter? Scope? Need help, this guy has gig this weekend.