Quad Preamp Main ch2 issue

[JammyDodger]

Hey All,

I've been cleaning my Quad Preamp up for sale and I've noticed an issue. The Main Out Channel 2 output is scratchy / Intermittent.
I've never actually used this output (always used CH1) and just happen to find it by accident. As I turn up the CH2 volume it sounds scratchy up to around 2 and then disappears altogether. I immediately assumed that it was a bad pot and ordered another one from Mesa. I installed the new pot but the problem is still there. I've checked the schematic but there is not much difference in Ch1 and Ch2. BTW - the volume drop also effects the Recording output 2.

Any thoughts?

 

[rarebitusa]

not sure if I am looking at a proper schem but it looks like you can isolate V22b circuit by inputting into Fx Return B. Curious if the problem is there or before the fx return.

 

[JammyDodger]

Thanks, I'll give that a try.

Cheers, Mike

 

[rarebitusa]

Mike,

BTW, on the quad schematic I couldn't find the "recording out" points, but that's not surprising. If you poke around you may find some additional info on the forum that may shed light on other parts of the schematic that are missing. For example.

http://forum.grailtone.com/viewtopic.php?f=5&t=10231&start=45

 

[JammyDodger]

So I tried going directly into the Return B plug and got nothing from of the Main B out.
Zero, not even the scratchiness that I had before. Plugged into the Return A and the Main A output had good signal.

That would seem to narrow down the problem area in the B side of V22. But..... I checked voltages on V22 and got 448V on pin 1 and 38V on pin 6.
Something is really wrong here! Pin 1 and pin 6 should be 257V according to the schematic.

Cheers, Mike

 

[rarebitusa]

yea looks they are identical circuits.... and there is 450V on the other side of both 120k resistors?

 

[JammyDodger]

I measured wrong. There is ~260V on Pin 1 as there should be but only 34V on Pin 6.

I cannot seem to find where the 120k resistors are. My eyes are pretty bad even with the photo blown up.

 

[JammyDodger]

O.K. - I think I've found the issue. The 120k resistors are 'hiding' under the big orange 0.1uF Caps.
Sure enough, one of the resistors is open. I'm not sure if I can get an iron in there and I sure don't want to try and pull the whole board out.

I may try and piggyback the open resistor with a 120k.

 

[rarebitusa]

Mike... congrats nice find. Yea pulling the board can be a real pain. But you are wise to move forward carefully, yet it can be done.

 

[rarebitusa]

so it appears the solder joint in the thruhole is not flowed correctly, true? You may be able to carefully bend the caps over slightly, just enough to get a soldering iron tip in there? It's definitely a tight one there.

 

[JammyDodger]

All fixed up, working and ready for sale.

It's the end of an era for me I guess the Studio Preamp gone and now the Quad!

Cheers, Mike