Mesa Boogie .50 Cal+ hum problem

[Chevy]

The Fluke 111 is an auto-ranging meter. It seems to work well, regardless of the value of the resistance being measured.... dunno. Whether I put a 1K ohm or 1M ohm resistor on the thing, or a 10M resistor on it, it reads correctly.

Hooking up the guitar cable to the input jack, with a DC voltmeter, I see .002 VDC between tip and sleeve.

I disconnected the two 100 ohm heater balance resistors, so the heater tracks were isolated from ground. Then I was reading open circuit between heater tracks and ground. Also was reading open circuit between heater tracks and each of the plate, cathode and grid of V2B and V3B. With the two 100 ohm heater balance resistors reconnected, I was reading about 52 ohms to ground from the heater tracks.

[soundguruman]

The Fluke 111 is an auto-ranging meter. It seems to work well, regardless of the value of the resistance being measured.... dunno. Whether I put a 1K ohm or 1M ohm resistor on the thing, or a 10M resistor on it, it reads correctly.

Hooking up the guitar cable to the input jack, with a DC voltmeter, I see .002 VDC between tip and sleeve.

I disconnected the two 100 ohm heater balance resistors, so the heater tracks were isolated from ground. Then I was reading open circuit between heater tracks and ground. Also was reading open circuit between heater tracks and each of the plate, cathode and grid of V2B and V3B. With the two 100 ohm heater balance resistors reconnected, I was reading about 52 ohms to ground from the heater tracks.

If the 100 ohms are out, then there should not be 52 ohms to ground on the heater tracks,
correct me but why is there 52 ohms? I must have missed something.
It should be open.
There is no CT for the heater, there is no pilot light running off the heaters.
So, I would follow those heater tracks all over the board,
ANY resistor that is laying on top of the heater tracks, lift it.
See if there is a burn spot between the (ANY) resistor and the heater track.
(probably a high voltage resistor, I think)
I think you are on to something. Those heater tracks should read open to ground.
Unless I am an idiot, and I missed something.

I am looking at MY schematic, and I do not see anything that would cause that.

Come to think of it, I am an idiot for trying to repair a Mesa over the internet.
(just a thought) But idiots, such as me, can triumph too.

Now if you please, when you find the burned resistor...
please take lots of photos, close ups, with plenty light.
We NEED documentation for future repairs...it will help a lot of people. Please.

[Chevy]

If the 100 ohms are out, then there should not be 52 ohms to ground on the heater tracks,
correct me but why is there 52 ohms? I must have missed something.
It should be open.
There is no CT for the heater, there is no pilot light running off the heaters.
So, I would follow those heater tracks all over the board,
ANY resistor that is laying on top of the heater tracks, lift it.
See if there is a burn spot between the (ANY) resistor and the heater track.
(probably a high voltage resistor, I think)
I think you are on to something. Those heater tracks should read open to ground.
Unless I am an idiot, and I missed something.

I am looking at MY schematic, and I do not see anything that would cause that.

Come to think of it, I am an idiot for trying to repair a Mesa over the internet.
(just a thought) But idiots, such as me, can triumph too.

Now if you please, when you find the burned resistor...
please take lots of photos, close ups, with plenty light.
We NEED documentation for future repairs...it will help a lot of people. Please.

Yes... you did miss something... I said with the heater balance 100 ohm R's RECONNECTED, I was reading 52 ohms from heater track to ground. Will look for any Resistor over a heater track and check out.

[grooveunit]

I'm interested in this thread because I look after a .50 Caliber+ for a guy, (he actually has a pair of them) and it just came in humming. I had previously replaced the 22k resistor in the reverb transformer supply, as it had burnt down to about 9k and was running hot enough to discolor the board. I actually thought that I was replacing the 22k dropping resistor between b and c in the hv supply. Realizing at some point that the c voltage was still way high (like 340v) instead of the 258v shown on the schem, I finally realized that the 22k (hv supply) shown on the schem is actually a 10k in the amp. Lesson learned- the schematic is not perfect.

grounding the grid on v2b killed the hum ,and the hum followed the tube when I swapped it with v1. Turned out to be a noisy tube, but I was going in circles for a bit. Had I known a bit more about these amps before I first worked on this one, I might have declined. So far, so good, though.

[soundguruman]

I posted a thread long ago about a hum problem, and never got it resolved, thought I'd give it another shot here.

The .50 Cal+ seems to generate way too much 60hz hum (or it could be 120, not sure) under normal operating conditions. I've replaced all the power supply filter caps, and checked the power supply under full power, and there's very little ripple, so power supply seems very good. I've replaced all the preamp tubes with new ones. Using a scope and a "noise grounding" test probe with a large audio coupling cap with one end grounded, I've isolated the source of the hum, it seems, to V2B or V3B (V2B is right after the tone stack, and V3B is the stage right after that... the reverb "driver"). I checked carefully under bright light for any possible black burnt streaks or areas in and around these tubes on the pcb, and see nothing bad there (thinking perhaps conductive pcb). None of the R's or caps seems to be burned or otherwise conducting to a trace underneath them.

Can this be a heater hum problem? There is no way on earth I would've designed this board the way the heater paths are... it seems the design/layout just begs for problems with electromagnetic noise. Two questions:

1) Has anyone perhaps tried to raise the DC potential of the 6.3V heater supply on one of these amps? If yes, what values of parts did you use, and how high would you raise the DC potential? (it would be a positive voltage, right?)
2) The 6L6's also run off the same heater supply; applying the new DC potential to the center of the two 100R reference resistors would raise the heater supply on the 6L6's as well... is there a problem with this?

Thanks kindly for any experience / advice on this!

Best,
Chevy

Even though this is the worst designed amp I have ever seen, It sounds good.
If it hums a lot you have a component failure or arcing on the circuit board, see previous posts.
If it works right, there is little hum...except the reverb.
The reverb tracks cross over the rectifiers, and picks up hum. That's normal for this amp.
If the reverb is off, the hum should be gone.

Even the techs at the factory have failed to fix these correctly.
They have failed to find arcing burning on the circuit board.
AND, its a hard amp to fix. Most technicians don't even want to try.

I do repair them occasionally, but it takes a lot of time and patience to make it work right.
To 'just' fix it about $200. To overhaul it about $600.
This is NOT the amp to 'learn' on. It's for advanced level only.

If you work on one, be prepared to be pushed to the limit.

[Chevy]

Partial success !

OK, thought I'd give a progress report... had to go back to square one. There was some arcing on this amp previously, right where the power resistor sits for the LDR supply. B+ was arcing to the LDR supply. I had previously repaired that by taking out the offending portion of the B+ trace and bypassing it successfully. What I did miss, though, and was cautioned by the pros on this site, was to get all the black stuff out of there. I took the pcb out and could see that there was some black burnt conduction path between the ground bus trace and the LDR supply... and when looking at the ground bus trace on the scope, very sensitive setting, I could see a little 60 cycle hump on there... so, it seems that the LDR supply was conducting directly to the ground bus trace, and inducing a hum on that. Originally I had 400mV of hum out of the 4 ohm output jack, with the GAIN at zero and the MASTER at 10. Now I have 27mV of hum with those same settings. Cool so far... some progress!

BTW... I inspected the underside of the board with a magnifying headset and bright light, and there are NO blacks spots, no burns of any sort. Board is totally clean underneath.

Now, moving on. 27mV hum, with the MASTER on full and the GAIN on zero, is still a bit much for my taste, and would like to reduce it further. But it gets worse.... with nothing plugged in, and now the GAIN on 3, MASTER on full, I get around 400mV of noise again at the speaker output. I verified the operation of the shorting contact at the input jack, and it does seem to properly ground the top of the GAIN pot when nothing is plugged in. So... here I am again with a bad hum. Weir thing is, when I engage the overdrive channel (pull out the GAIN pot), the noise drops dramatically, to pretty much very decent levels.

There is nothing anywhere I can see that it burned or arcing on the top of the board either.

[Chuck H]

Go SGM!

[soundguruman]

OK, thought I'd give a progress report... had to go back to square one. There was some arcing on this amp previously, right where the power resistor sits for the LDR supply. B+ was arcing to the LDR supply. I had previously repaired that by taking out the offending portion of the B+ trace and bypassing it successfully. What I did miss, though, and was cautioned by the pros on this site, was to get all the black stuff out of there. I took the pcb out and could see that there was some black burnt conduction path between the ground bus trace and the LDR supply... and when looking at the ground bus trace on the scope, very sensitive setting, I could see a little 60 cycle hump on there... so, it seems that the LDR supply was conducting directly to the ground bus trace, and inducing a hum on that. Originally I had 400mV of hum out of the 4 ohm output jack, with the GAIN at zero and the MASTER at 10. Now I have 27mV of hum with those same settings. Cool so far... some progress!

BTW... I inspected the underside of the board with a magnifying headset and bright light, and there are NO blacks spots, no burns of any sort. Board is totally clean underneath.

Now, moving on. 27mV hum, with the MASTER on full and the GAIN on zero, is still a bit much for my taste, and would like to reduce it further. But it gets worse.... with nothing plugged in, and now the GAIN on 3, MASTER on full, I get around 400mV of noise again at the speaker output. I verified the operation of the shorting contact at the input jack, and it does seem to properly ground the top of the GAIN pot when nothing is plugged in. So... here I am again with a bad hum. Weir thing is, when I engage the overdrive channel (pull out the GAIN pot), the noise drops dramatically, to pretty much very decent levels.

There is nothing anywhere I can see that it burned or arcing on the top of the board either.

You ARE getting it. Don't give up.
read the previous posts again. Look out for the ground buss wire, on the pots, that "looks" like it's soldered, but might not be.
I'm tell'in ya- perseverance and determination makes the impossible...possible.

OK your EQ switch:
AUTO turns on the EQ only in lead mode
IN turns on the EQ all the time
CENTER position is EQ off.
Are you fighting a blown FET in the EQ circuit?

High voltage arcing to the LDR supply:
are you fighting a shorted LED in your LDR circuit? The LED turns on and activates the photo-resistor.

If you can fix that amp, it proves you can fix anything. Cause there's nothing more evil.