Mesa 2:90 woes _ tubes not coming on

[Yovan]

So I've had a problem with my Mesa, the other day it was working ok but today switched in and no sound. Checked the fuses and they look fine, the tubes on the other hand were off, no glow or heat. I was slightly concerned about the fuses in the back, on the panel it specifically says use 1A and the fuses installed are 3A. I'm worried something more serious has blown down the line... anyway I'm really hoping that it can be fixed, any help or advice is greatly appreciated, thanks in advance.

[Enzo]

No heaters means they are not getting power supply. SO find out where it went.

[Yovan]

No heaters means they are not getting power supply. SO find out where it went.

I'm really not sure how to find this - I took the lid off and checked inside - there's power to the fan but that's all I'm able to discern by looking at it. Perhaps need to change the tube fuses anyway....

[Helmholtz]

Check heater voltage at tube sockets (should be around 6VAC). If missing, take out fuses and check for zero resistance. Don't go by looks.

[g1]

I don't see any internal fuses on the schematic (see attached), but I don't trust Mesa schematics either.

[Leo_Gnardo]

If your 2:90 has a comb type multipin connector on the power supply board, with a gaggle of flying leads to the tube boards, it's possible the filament pins on that connector have oxidized due to being heated from all the current they have to carry. It's a common problem on a lot of their combos & heads after they went to this type of connector. If you find that to be the case, I don't think there's much point in cleaning up the pins on the board, and sometimes the electrodes in the multipin connector are burnt beyond repair anyway. In these cases, I run a pair of heavy gauge wires directly between the boards, scraping enough of the board coating off the lands so I can get a reliable connection. Luckily the filament pins are the only ones I've found to have this kind of problem - the remainder can safely be left in place.

[nevetslab]

If your 2:90 has a comb type multipin connector on the power supply board, with a gaggle of flying leads to the tube boards, it's possible the filament pins on that connector have oxidized due to being heated from all the current they have to carry. It's a common problem on a lot of their combos & heads after they went to this type of connector. If you find that to be the case, I don't think there's much point in cleaning up the pins on the board, and sometimes the electrodes in the multipin connector are burnt beyond repair anyway. In these cases, I run a pair of heavy gauge wires directly between the boards, scraping enough of the board coating off the lands so I can get a reliable connection. Luckily the filament pins are the only ones I've found to have this kind of problem - the remainder can safely be left in place.

Just in case, I posted a number of photos on the outside and insides of the Mesa Simulclass 2:90 at this post, thread #10: https://music-electronics-forum.com/...-repair/51907-

[Yovan]

If your 2:90 has a comb type multipin connector on the power supply board, with a gaggle of flying leads to the tube boards, it's possible the filament pins on that connector have oxidized due to being heated from all the current they have to carry. It's a common problem on a lot of their combos & heads after they went to this type of connector. If you find that to be the case, I don't think there's much point in cleaning up the pins on the board, and sometimes the electrodes in the multipin connector are burnt beyond repair anyway. In these cases, I run a pair of heavy gauge wires directly between the boards, scraping enough of the board coating off the lands so I can get a reliable connection. Luckily the filament pins are the only ones I've found to have this kind of problem - the remainder can safely be left in place.

Bingo! At glance it seems at least one of the pins is singed to a crisp. So I should bypass the burnt wire and leave the others?

[Jon Snell]

Ensure you make good the two high current heater connections.
Why they use cheap and nasty connectors, I will never understand except it gives built in obsolescence.

[Leo_Gnardo]

Bingo! At glance it seems at least one of the pins is singed to a crisp. So I should bypass the burnt wire and leave the others?

Yes, replace the burnt wire, also its mate. The remainder of the connections, I've never had a problem with those, so I recommend you go no further after replacing the filament supply wires.

[Yovan]

Ok I've done work on the amp, after powering up the filaments came on but only very slightly and although there is sound to the amp it's faint and sounds like it's going through a ring modulater.

[Leo_Gnardo]

Ok I've done work on the amp, after powering up the filaments came on but only very slightly and although there is sound to the amp it's faint and sounds like it's going through a ring modulater.

Measure the AC voltage at your output tubes' filament. Maybe you ran one jumper to ground, the other to 6.3 VAC. That would result in 3.15 VAC, dim filaments, very low power.

The ring mod effect, I dunno. OK I s'pose, if you want to get those early jazz-rock-confusion tones from 45-50 years ago, Larry Coryell, John McLaughlin, Jeff Beck. Nah... I didn't think so.

[Yovan]

Actually I think I might have knackered the amp - one of the tracks pulled off when I was soldering so I bypassed it with a wire and now when I power on the amp, smoke starts pouring out. Looks like a fire hazard now and I've probably reached my limitations. Hope the amp is still repairable but not sure if it's within my capabilities to be honest

.....


Kept tinkering for luck - think I bridged the wrong connection and was causing a short so I snipped one of the wires and now the amp powers up on one side, more power than before but still weaker than it was when it worked.

[Yovan]

Kept tinkering for luck - think I bridged the wrong connection and was causing a short so I snipped one of the wires and now the amp powers up on one side, more power than before but still weaker than it was when it worked.

[Yovan]

Tinkering continues....


kept digging to figure out what I did wrong - it seems that the green filament wires have a third earth wire which is melted (completely melted to one of the filament wires) somewhere down the line also one of the filament wires that was in the multipin had blackened so I assumed it was black (first mistake) and connected it to the black wire in the board. I'm not sure if my actions caused the meltage however I've disconnected the wires and the power transformer and removed it so I can sort out the problem.

I'm fairly sure there's some damage to the circuit but it's hard to say what the extent is, the filament pin which was burned was completely pulverised when I disconnected the wire, there was a track running under it but because I can't see under the board I'm not exactly sure where it goes or what it was connected to.

Perseverance