When to start repair.

[j.d.roost]

Just out of curiosity... Have you pugged in a cable from your effects send to your effects return and fr your pre out to main in if you have them to see if there is a difference. Also try using the pre out or effects send to drive another power amp or drive the amp from another preamp source into the effects return or main in to isolate if the problem is on your pre amp or output section.

I have tried it with two other known good preamps..and a pedal into the a channel input. Same results.

[j.d.roost]

Thanks so much for the pdf!

[guitician]

Yep....She no good..

Open? Or Shorted? doesn't matter, just curious.

[olddawg]

I have tried it with two other known good preamps..and a pedal into the a channel input. Same results.

That is not the answers to the questions I asked. The objective is to isolate the problem to either the power amp section or the preamp section. Running an external source into effects return or main amp in will isolate the problem. If you have no problem when doing that test the problem would be isolated to the preamp. Same thing for running the pre out or effects send into another amp. The first thing I would do is run an instrument cable between the effects send and return jacks and the pre out main in jacks. Those switching jacks are common failures in many amps. There is a pinned post about it.

[j.d.roost]

That is not the answers to the questions I asked. The objective is to isolate the problem to either the power amp section or the preamp section. Running an external source into effects return or main amp in will isolate the problem. If you have no problem when doing that test the problem would be isolated to the preamp. Same thing for running the pre out or effects send into another amp. The first thing I would do is run an instrument cable between the effects send and return jacks and the pre out main in jacks. Those switching jacks are common failures in many amps. There is a pinned post about it.

The mesa 2:90 is a Tube power amp.No sends/returns/no loops. Just input(s) (from an outboard preamp) and output ( 4 or 8 ohm to cab).

[olddawg]

The mesa 2:90 is a Tube power amp.No sends/returns/no loops. Just input(s) (from an outboard preamp) and output ( 4 or 8 ohm to cab).

Sorry... Not that familiar with various Mesa models. In that case I would inject a 1khz tone just before clipping with the amp on a dummy load. Then follow the signal with special attention to symmetry. Do you have a scope?

[J M Fahey]

FWIW I think "open" rather than "short" whan I see a "0" capacitance display.

Untill I found the cap. That connects the volume pot to the grid of v3a. (C16) on schematic. I "think" this a coupling cap.

I think you mean V3b .
Yes, it's a coupling capacitor and it open will open the signal flow.

Anyway, there's always pesky parasitic capacitance in any real world PCB and *some* weak and tinny guitar sound should still be heard through the speakers.

The fact that you don't report that makes me think that there's still some other problem.

[g1]

Open? Or Shorted? doesn't matter, just curious.

FWIW I think "open" rather than "short" whan I see a "0" capacitance dis

Agree with JM, with most meters on capacitance range, you will get "0" reading with leads disconnected (open cap), and "OL" reading with leads shorted together (shorted cap).

[j.d.roost]

FWIW I think "open" rather than "short" whan I see a "0" capacitance display.

I think you mean V3b .
Yes, it's a coupling capacitor and it open will open the signal flow.

Anyway, there's always pesky parasitic capacitance in any real world PCB and *some* weak and tinny guitar sound should still be heard through the speakers.

The fact that you don't report that makes me think that there's still some other problem.

Yes..open. My meter will display a short.

Yes. Very weak and zero bass and yes v3B (had a hard time reading the other schems.).
Output on that channel was about half to 1/4 of "normal" volume.
One of my concerns is it would work properly for a bet then cut out. Usually after I would
futz with the volume or standby switch. The cap is clearly bad so I don't know if I caught it
after it totally died or...it was able to get charged up somehow by messing with things.
I could not find any similar .1uf 400v orange drops...so I mistakenly ordered 600v..wich are too large to fit on most spots on the board.
I was able to get the 600v ones in c21 c22 and there was already a 600v in c23 (power section) so I replaced it anyway.C16 is a 200 volt cap that they don't make anymore but I found a 715p200v that looks to be small enough to fit in(its tight in there) http://www.ebay.com/itm/171323772028...%3AMEBIDX%3AIT .But I am not sure what to do about replacing the 400v spe715p400v caps.

I found some 716p caps that look like they will fit..but they look like they are built differently (sort of flat looking)

Sprague 716P Orange Drop capacitor, 0.1 uF @ 400 Volts [C-SPR-716P-104-400V] - $2.90 : NewOldSounds.com

Can I use them?
Also should I replace ALL caps (because I found a bad orange drop)?
Should I mess with the good B side of the amp..or leave it be?

[Tom Phillips]

...Also should I replace ALL caps (because I found a bad orange drop)?...

My input on that is a definite no.

...Should I mess with the good B side of the amp..or leave it be?

Leave it be.


Troubleshoot to isolate the problem to a circuit section and then to a specific part. Replace the bad part. Then test a repeat only as required. This is especially true for your Boogie CCA. Any extra unnecessary work increases the chances of damage to the board itself.

That's my 2¢.
Tom

[g1]

Agree with Tom, replace only the bad part. It is not a common fault and there is no reason to believe any other cap in the amp is likely to have the same problem.

The caps that look different but are the same value and voltage rating will be fine to use.

Yes..open. My meter will display a short.

Sorry to nitpick, but this is wrong. Your meter is displaying "zero". It is important to describe it as exactly that, and mention the range you are on.
You are interpreting it as "short", but short is a resistance reading. You are in capacitance mode. In that mode, zero means open. It would probably be best if we never used the word "short" at all, as it often just ends up causing confusion.

[j.d.roost]

Agree with Tom, replace only the bad part. It is not a common fault and there is no reason to believe any other cap in the amp is likely to have the same problem.

The caps that look different but are the same value and voltage rating will be fine to use.


Sorry to nitpick, but this is wrong. Your meter is displaying "zero". It is important to describe it as exactly that, and mention the range you are on.
You are interpreting it as "short", but short is a resistance reading. You are in capacitance mode. In that mode, zero means open. It would probably be best if we never used the word "short" at all, as it often just ends up causing confusion.

Understood. If I touch the leads together on capacitance mode it will climb quickly then blink zero.
If I test this cap (bad one) it will do nothing...like I have not put the leads on anything.

I have no formal electronics training..and can only "copy" pedal builds at this point in my life. Building and troubleshooting the pedals gave me enough confidence to at least try and fix the amp...I was pleasenty surprised that I was able to track down a bad part going on what I thought it could be. Hopefully I can get her fully up and running again.

I can't thank you all enough for helping me through this.

[j.d.roost]

My input on that is a definite no.


Leave it be.


Troubleshoot to isolate the problem to a circuit section and then to a specific part. Replace the bad part. Then test a repeat only as required. This is especially true for your Boogie CCA. Any extra unnecessary work increases the chances of damage to the board itself.

That's my 2¢.
Tom

Whoo Hoo! It seems to be fixed!
I can't crank it (baby sleeping) but will do a full burn in tomorrow night.
I ended up replacing all orange drop caps on the A side and all the electrolytic caps.
I cant thank you guys enough for the help.

[Henz]

Ok need a little more help.
Checked voltages Tuesday and from what I can tell everything checks out.
BUT.. a funny thing happened. I decided to toss in another 12ax7 tube (old penta tube) in place of the new mesa that was in the A channel inverter socket. Fired up the amp and played it for two hours with zero problems.
Chalked it up to two?? bad mesa tubes I tried in that slot.. Anyway long story short I was playing it again last night for about two hours and decided to try out the "modern" mode on the 2:90 (don't like it btw). When I switched back to normal mode I started getting a very strange "chorus" like sound from the amp (sounded like I had a pedal hooked up) and then 10 min.later the A channel crapped out again. Hit standby got the loud pop sound that I usually get when the a channel acts up and then the channel came back on for another hour with zero problems.
I don't think it's the switching section of this power amp that is causing the problem (tried all that stuff before when it would act up every 20 min or so) but I think it has to either be a cold joint or maybe a flakey coupling cap (because I also loose all bass responce when it craps out).
Maybe the slightly different internal ratings of the penta tube lets the resistors/caps operate for a longer time without issues? Fixing/testing/pulling the any caps ect. requires me pulling out the pcb.

Only thing I did not do was check the voltages when it had the problem.
I think this is key to fixing things. Problem is I can't get it to act up again.
Should I stick the mesa tube back in ?

Any thoughts on what else I can try?

Thanks for the help.

Greetings, Where are the 100uf caps with the LDR's? Just curious because i just re capped my 2:90.

[Henz]

Hello.
I have a mesa 2:90 that has a channel randomly dropping out (a-side). The volume will get halved + and the sound will go full treble. I have replaced all tubes (power and inverter) and cleaned all pots/sockets.
I have some slight knowledge of electronic circuits and have built a few stomp boxes. I also have been reading MANY posts/forums on tube amp repair (and the dangers involved with high voltage caps). Knowing that there are lethal voltages stored in those filters ect. I really want to give it a shot repairing this thing. I am auto tech (for over 20 years) by trade so it tears me apart having to send it out for someone to repair.
My question(s) would be..

Is there anyone here that started repairing amps without any formal training?

If so, aside from making sure the caps are drained (think I have read just about everything there is to read on amp repair safety) what other pitfalls do I need to lookout for?

I am not looking to blindly go sticking my meter into/onto in inner workings of my amp. I just want to find out what's going on with this thing and see if I can repair it.
I am going to need to check voltages on this amp (so now my chances of shock are much higher ).Any good safety tips that will help here?

I am only armed with "internet smarts " and a general good sense of mechanical repair knowledge. A also don't have time in my life to take an electronics repair class (don't think there are any in my area focused on tube amp repair/service anyway).
If there is a good web sight dedicated towards people wanting to get into repair/building (aside from this great forum) I am open to links too.
Also I don't have a scope so it's going to be things I can only check with my Fluke.

I know I am going to get a lot of "DON"T DO IT" posts here but that's not what I am looking for.. I am trying to do this as safely as I can.
Thanks for the help.
Jd

Mee!