low...low...volume. Mesa .50

[Garydean]

A 1990 Mesa Boogie .50 Caliber + is on my bench for repair. It came in due to a broken OT mounting bracket, scratchy controls, plus the tubes that were in it were about 5 years old. I removed the chassis and discovered an 1/8 inch of dust on the board. I swabbed the deck and cleaned all the controls and jacks with Deoxit D5. I visually checked for funky caps and checked the diodes with a DMM. I fixed the dangling OT by making a strap/bracket and secured it to the original holes on the chassis. I threw in 4 new 12ax7's, 1 new 12AT7 and 2 new 6L6's. Turned it on, tubes lit nicely but, I get a very, very faint distorted signal with the controls up to 10. I scoped it from the input to the output jack and the signal was also weak and distorted. Speaker obviously is fine. I'm thinking now to remove the chassis and take some voltage readings and trace the signal back until it gets strong, unless there's something I should check that's quicker to the point. This is my first Boogie and the thought of removing the circuit board to swap a component is making me a bit squirrelly. By the way the OT was still attached by one screw, but tilted. I did not test the amp when it came in due to it's condition. The owner of the amp did say it started making very strange noises.

Thanks in advance,
Gary

[drewl]

Well if you have a scope, start tracing the signal through the amp.
Even easier, see what you have at the fx send and return jacks.
You should have a decent sized signal at the fx send, and you can inject a signal at the return jacks to see if the power amp is working.

[Old Tele man]

...sound output that appears to be about one-fourth of what it should be is a classic indication of loss of '...one side...' of the push-pull path, either PI, output tube or OT, ie: 1/2 of 1/2 = 1/4th

[52 Bill]

If you are getting faint sound through the amp, then I'd look for a break in the signal path somewhere. Like Drewl sez check the FX loop jacks, check the tube sockets, and also look for broken wires at the pots and jacks. I thought that most of the parts were top mounted on the PC board in those amps.

Hope this helps.

[Garydean]

I'm actually only getting almost no sound at all. You can barely hear it.

thanks,
Gary

[Garydean]

Thanks for the tips. I tested the pots and found the dual gain pot peaks at 260K at 7.5 on the knob. From there to 10, it drops drastically and resolves at 56 ohms. doubt this is the the problem but I figured I would throw that in the ring. Once I close up my store I can head back and scope it. I'll let y'all know the results.

[Old Tele man]

...time to "inject" a signal and "chase" if from input through to output.

[Ampclutz]

If one or both of the fx loop jack aren't closing when unplugged you can jumper them with a guitar cable. If the sound comes back you know where to look... If the sound doesn't come back the jacks are probably OK. Quick, easy, no cost, reversible... Note the use of the word "probably".

Guess how I found out about this one. LOL.

AC

[Garydean]

the amp has a jumper from send to return right now. Silly question, but why is that?

[Ampclutz]

I don't know why there is a jumper from send to return on your amp. I did it on mine as a temporary fix for goofy jacks.

What happens when you remove the jumper? I'm guessing you talking about a short guitar type cable and not internal jumpering.

AC

[Chill]

If it's like the schemo at free info society, internal jumpering converts the effects loop from series to parallel.

[Enzo]

If the amp has a jumper from send to return, it is most likely because the return jack has a bad cutout contact in it.

[Garydean]

Good morning. Here's what I found while tracing the signal from the input. I made it through the preamp section and through the output tubes right up to the 8ohm jack lug which had no signal. I left the scope probe on the 8ohm lug and disconnected the load plug and the signal appeared at the 8 ohm jack lug as well as the tip tab on the jack.

My question is obvious.... the answer is not obvious to me.

This network of plastic jacks are all tied together just inside the chassis.

8 ohm..... no signal externally
2- 4 ohm jacks..... no signal externally
direct out jack....... no signal externally
send jack, Jumped externally with a 3" mono patch cable to the return jack

Gary

[Garydean]

Well after a few sessions with the scope I was able to get a perfectly beautiful 1,000HZ sine wave at the test point on my homemade lead but only after I unplugged it from the dummy load, apparently my dummy load was shorting the signal only at the output jack lug. I decided to now plug in the amp but got nothing for a few minutes then everything started working. There was static, like the filter caps were going south and a bit of 60 cycle hum but the full volume from the guitar was there. The dual, gain/pull for lead, pot controlled the volume of the static as well as the master volume and lead volume pots. So I'm thinking possibly a bad preamp tube. OR the effects send and return jacks or external jumper cable is bad. As soon as I fiddled with the external patch cord the static got worse....cool, I'll just swap the patch cord and see what happens. I turned off the amp, ran to my stash of cords and swapped the old for the new. Turned on the amp and back to square one with very low volume audible only with the controls at 8 and above. I put the original cable back and still nothing. I let the amp cool for an hour and still nothing. It's strange that when I traced the signal yesterday all seemed well. Loosing the signal when I plug into the load still puzzles me. Does that mean I have a short someplace? The caps are the originals from 1990 they could be leaking? What are your thoughts?

Gary

[Steve Conner]

I'd guess one of the secondary wires got yanked loose from the OT while it was dangling.