Mesa Boogie F-30 Dead Channel Issue?

[SYSDOA]

Hi All, Thanks in advance for any advice.

I've had three F-30's come in since July. When it rains it pours, I guess. This one in particular I've been working over for the last week and am stumped. The amp was delivered abused. The owner confessed to not paying attention to the way the amp was transported and noted problems with the amp months ago, but just changing settings to compensate for tone issues. One of the EL-84's was on 'fire' when the amp was turned on. There was power loss in the first several minutes of operation, and when the amp was put on channel one, there was a fantastic oscillating drone instead of signal. The pilot light was dead and the footswitch wasn't working. I opened the chassis and saw bulging filter caps, a burned up diode cluster as well as a burned out 1k 1/2W resistor going to the cluster. The plate resistors on all valves were fairly outside of their tolerances. I checked all the J175 fets with a tester to confirm they were still operational and switched out the two 5532 opamps just to be safe (If anyone has a quick and easy way to test opamps, I'm all ears. I just switch them when I'm having low, poor or no signal problems). I tested the chassis and now channel one master volume is the only one that works for both channels. The Channel two master volume almost acts as another tone pot, but the eq settings work correctly when they are switched respectively. The gain pots also work independently. I don't think I've got channel one back. I think it's just channel two still. I read something on the web about carbon traces building up and requiring drilling to stop power from jumping to the wrong leads. I don't want to get to much further into a hunch though without some input, if possible. Very frustrating. I've worked on some pretty ugly stuff and this one is getting to the top of my list! Thanks in advance, again.

[lowell]

Ok did you fix all the above issues?: plate resitors, power tubes etc? A schematic would help here as there are most likely relays and relay supply circuitry involved w this amp but I'm not familiar w this amp off the batt.

[SYSDOA]

Yes. I think you're right. Here's the schematic: http://music-electronics-forum.com/a...82165-f_30.pdf. I switched the plate resistors, power tubes, filter caps, bypass cap, 4007 cluster of diodes that were visibly burned and so fourth. Any suggestions are much appreciated!

[lowell]

Ok. Are the +16/-16v relay power supply rails good? Lets start there.

[lowell]

If the rails are good i might suspect ry2. It sounds to me like both channels are always on, therefore when you turn the rhythm volume down you stop hearing the rythm gain stage/tone stack etc - which is probably the tonal change you're hearing.

[SYSDOA]

Bummer. No luck. All relays have been swaped out. and the issue remains.

[lowell]

Are the rails for the relays good? You should also double check the reverse biased diodes across the relay coils if they are there. Are the relays working? Can you hear them "click" when you change channels? Did you ever check tube voltages to be sure they are good? I doubt that tube voltages are the issue but lets be thorough here. Finally, can you restate what's wrong and refer the channels in your description as LEAD and RHYTHM.

[SYSDOA]

Rails for the relays read correct at +15/-15V and there is an audiable clicking sound when switching from Rythm to lead and also lead + Contour. The problem is that the Rhythm channel works wonderfully. Then when switching to the lead channel, the controls for the lead channel are inoperative. The master volume for the rhythm channel is the only one that works even when in the lead channel. When in the lead channel, the lead channel master volume almosts acts as another tone pot. The tube voltages were correct when I initially tested the amp, but I'll check them again before I do anything else.

[lowell]

You said you replaced the relays... did you replace ALL of them? If RY1 is not switching you won't get the LEAD GAIN section of the circuit, and as you say the Rhythm master is always on, my only guess could be that RY3 is not switching... as far as I can see that's got to be the issue. Are you sure you're hearing EACH separate relay switch on/off? Make sure that the LEAD master pot is indeed grounded AND that there is continuity from the top of it to the 470k coming from V3A's plate.

[SYSDOA]

Thanks for stickin with me on this one lowell! I did actually switch all of the relays, but will double check the pot. it'd be pathetic if it wasn't grounded. Thanks for all of the good advice! In regard to the switching. I hear a pop through the speaker when I switch between channels. I assumed that was what you were asking. Is there a better way to confirm?

[lowell]

Ok, no the pop is not a sufficient means of determining that the relays are switching, I meant do you physically hear the relay click (not through the speaker). It may mean you have a bad relay, or the pop may be from 1 relay switching while the others are NOT. To be sure, I'd check for continuity on the relays in rhythm mode and in lead mode to be sure that they are doing their job. Did you ever check the diodes across the relay coils? If you replaced the relays and had a bad diode there that could damage the coils in the new relays.

[SYSDOA]

Hi lowell,

So I replaced the entire diode cluster that was going across the relay coils when I first got the amp. They were burned up and I even put little conduits on the leads, through the pcb from fear of carbon traces carrying power where it wasn't supposed to be. I was going to confirm continuity between the diodes as you had mentioned prior, but the customer came and asked to take the amp back, offered me to buy it from him and then left with it. I'm so sad.

Ohh, and I forgot to put the footswitch back in the enclosure before he left and now I had to mail it to him!

[lowell]

That's always tough when the customer gets impatient. Too bad about the footswitch too... sh*t happens. A learning experience nonetheless. I've dealt the seemingly impossible-to-fix repair and gotten very frustrated when I couldn't fix it, however I always look back on those and realize that I learned a lot and sharpened my troubleshooting skills, even though it didn't seem to end well.