Mesa Solo 50 troubleshooting

[goldenGeek]

Hello, I figured I'd put it in the general technical duscussion, feel free to move the thread if this is in the wrong place.

Anyway, I'm relatively confident with point to point amps, but now I've got a Mesa Single Rectifier (series 1 (rev e), solo 50) on the bench. I've worked little with PCB-base amps before, but this seems fairly okay because all pots and connectors are wired off board. Now to the problem - theres some irritating hum in the amp. I've replaced all tubes (with new ones) and the noise got better but the hum is still there. It's specially noticeable in the clean channel and really bad when the clean channel is turned to "pushed" mode. But even if I turn down the master volumes the hum is still present, way too loud to be okay. So I've chopsticked a bit around in the amp an found that poking the wires comming off the clean channel gain pot is awfully microphonic, almost like tapping a microphone. This does not happen on the second (overdrive) channel, which is kind of weird because theres a whole lot more gain going on there.

I must add that when I got the amp the fuse was blown and the output tubes was bad. Can this be the output transformer causing the noise? Is there a way to measure the OT without replacing it? Also, there's some burned PCB around the rectifier-diodes for V1-heaters which seems to be fixed way back. Is that where I should start replacing parts? Or should I start in the other end since the hum is there when the master is down and nothing is plugged in?

[Stevem]

Can you post up a schematic ?

[goldenGeek]

Can you post up a schematic ?

Schematics are attched now. Its the solo 50, not the rectoverb.

I've also ordered new filter capacitors and capacitors for the filtering of the DC to V1-heaters, and I've also cleaned up the burned area quite a bit with new 1n4007s. I guess it doesn't hurt to recap anyway. Oh, and I also ordered a new set of preamptubes, for another amp, but then I can swap here as well so that bad tubes can be ruled out regarding the microphonics.

[Stevem]

If the hum is still there with the master volume down then I would look to the filters in the bias supply.
Can you pin down if the hum is 60 HZ , or 120 HZ ?

[Stevem]

If yanking V1 out kills the hum, then yes those filters in its DC heater circuit could be the issue .

[goldenGeek]

If the hum is still there with the master volume down then I would look to the filters in the bias supply.
Can you pin down if the hum is 60 HZ , or 120 HZ ?

I will keep that in mind when proceding, I ordered new bias caps as well yesterday It's 50hZ (I'm in Europe), I checked against my signal generator and it very much sound like 50hZ.I didn't check with the scope, I tend to forget that I've got it...

[goldenGeek]

If yanking V1 out kills the hum, then yes those filters in its DC heater circuit could be the issue .

Pulling V1 doesn't drop the hum (but theres some other noise/hiss that goes away). Actually - (I tried this on the gain channel, it worked best there) when I dialed the output up and adjusted the master to a certain spot the hum got more quiet, if I adjusted the gain I had to readjust the master for the hum to drop again. But the hum doesnt go away. So theres probably more than one problem going on here, right?

[JerryFJA]

What's the part number on the power transformer?

[goldenGeek]

What's the part number on the power transformer?

Power transformer:
Mesa Boogie
561155
ELA606-921

Output transformer:
Mesa
562051
40-16639
166-0701

[JerryFJA]

That's the export power transformer, I just noticed you're outside the USA.

Have you tried contacting Mesa?

There are notes on the schematic that mention something about the power transformer being changed. Looks like they did something to separate audio grounds from the filament center tap on a later transformer. I don't know if your power transformer is the latest version or not.

You should be able to check out the output transformer measuring bias current of the power tubes and then swapping the tubes and measuring again. Watching the output on a scope should also pick up any issues in the output transformer.

[goldenGeek]

I've replaced all the large filtercaps (220uf, 30uf and 10uf), filter for the 6.3v DC (6800uf, replaced with 10000uf) and the bias caps 220uf/63V. I also , as previously noted, cleaned up the diode area. The hum is gone and the amp is as good as new.

[ampmike]

Great to see you got it,I have a few really nice Mesas,after I got into amp building I stopped playing them and I have heartbreaker and a tremoverb that dont work right.This thread gives me hope that I can fix these one day.They sit too long unplayed and I think the caps go.I used to cal Boogie and get a good long tech talk.Good people,Happy you got yours running smooth buddy!!!Mikey