Quad Preamp Issues

[[Beast]]

Oh, BTW, just some info on my rig:

PSSE custom made 4x12 box w/ V30s
Peavey Classic 50/50 power amp
Mesa/Boogie Quad preamp
Rocktron Xpression
Rocktron Mid Mate
Fender Strat w/ Floyd rose (neck: fender tex mex HB, bridge: EMG HZ 4)
Ibanez RG560 (SCs Shadow, bridge EMG 81)

I have to buy Rocktron Patchmate Loop 8, so I can start using only one floorboard once again and use Quad's reverb and two channels at the same time (eg Lead I & Lead II for solos).

First I though the EL84s in the power amp could be totally worn out (and they probably are), but is still can not be the reason for the crappy thin sound I'm getting out of the rig. With EMG81 I should get the Metallica sound with gain set to 6-7, not 10 I guess... I tried the rig with an Ibanez JEM 7DBK (?) and the result was just BAD. No bas, sound even thiner, punchier...

Any help will be great!

 

[[Beast]]

After retubing and changing the LDRs on my Quad most problems are solved, but one remains and it drives me crazy. It's the volume drops on the channel 1 of my Quad, mostly on lead 1.

It usually happens during the first hour of rehearsals, but in the last months it started to happen also after I've been playing for some hours. All of the sudden the volume on lead 1 goes down, almost totally out, and only this fizzy, thin, very low volume sound comes out of the speakers. It's like it was non amplified or something. It comes back after I increase the lead master volume. When I turn the lead master pot past 5 or 6, the signal becomes strong enough and it returns. There is complete connection between me turning the knob and the sound returning to normal. So far the hasn't failed (yet).

This problem annoys me very much, because it happened during live shows a few times and it breaks my concentration during playing. I've thought about replacing my Quad with a Triaxis or an Engl Preamp, but despite not having MIDI and being big, this preamp is THE preamp for me, so I want to get to the bottom of the problem. (As I mentioned, it has been retubed with 8 JJ ECC83s and all of the LDR were changed by a proffesional tech. That solved the problems with sound, but volume drops remained). The preamp was checked twice by a couple of very good techs, but they couldn't find the problem. I was able to "repeat" the volume drop to one of them, we let it play with the fizzy quiet sound and the tech couldn't measure anything different with a multi meter.

I've been told about two possible causes: electrolitical caps, which have probably dried out since the late 80s. Is it possible, that because they are probably dry(ish) somehow they have a problem with the signal strenght, meaning the signal must be increased (with the lead master pot) to pass through the caps? Or perhaps the lead master pot itself? Could this be the cause of the volume drops?

Please, if you have any ideas what could be the cause for this volume drops in my Quad, share them with me. There is no preamp like it and I'd hate to replace it with something without a spring reverb. Tnx.

 

[crane]

Replacing the big power caps is quite cheap and easy, I would do it. Same for the master pot.
The last thing I see is the anodes resistors. Cheap in parts, but you must remove the main board to do it (not enough space to do it from the top, unlike the big power caps).

 

[daveyboogie]

+1 anode resistors .... Caused me no end of s**t.

 

[GoneFission]

Bumping this thread because my '90 Quad has issues and this is the best tech thread I've found for these. PCB is revision 1E, this is an older dual-reverb pan model with the earlier switching scheme.

Only signal I can get it to pass at all right now is from the loop return input to the outputs. All the tubes are glowing, and I tried swaps into the good position, but that got inconclusive--even previously good tubes misbehaved sometimes on swaps.

So what are the culprits that I should be chasing? Bad Vactrols? Bad sockets or solder joints on them? Replace the current tubes as marginal? Are the Quads particularly sensitive to the wall voltage? Mine tends to be 125vac.

Anyone able to tell me how Boogie charges for service these days? I already had issues with the logic circuits before this, so I'm considering whether I should just send it in for a full service checkup and repairs.

 

[goldscott]

Hi @GoneFission,

I had the same exact problem. Measure the voltages circled here: https://photos.app.goo.gl/zMoiUYUqyCjsJCGN7

On the left side of the 220 ohm resistor it should be about 5.6-5.7V. On the left side of the 680 ohm resistor it should be about 9.5V.
If you are reading more like 0 - 1V, then you'll need to replace the 47uF 100V capacitor just left of that 680 ohm resistor (you can see my replacement cap floating on its leads as I wait for the real cap to come in the mail). I replaced the cap and it works like a charm again.