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Studio Preamp issues (SOLVED)

Posted: Fri Jun 22, 2018 2:24 am
by pwu
I’ve got a Mesa Boogie Studio Preamp that is acting up. It’s worked perfectly fine in the past, but has been in storage for a few years and through a few relocations.

The function of the Bass, Mid, and both Output Level potentiometers are not working properly. Cleaning them didn’t help. They are working up to position “2” of 10, then the volume goes DOWN as I turn each one up. This exists in BOTH Rhythm and Lead Channels, ANY setting, and before and after a complete electrolytic recap, old/new tubes, cleaned pots. The Output Levels are making a scratchy noise around “2”, but only up to there. It’s like a transistor bias knob sound.

I’m having a tough time figuring this one out.
Does anyone have an idea of what might be causing this and what I should test next?

Re: Studio Preamp troubleshooting. Help please!

Posted: Mon Jun 25, 2018 3:50 am
by pwu
I just pulled and tested all of the Sprague Orange Drop .1μF caps, and they are ALL bad! The other values I’ve tested are ok. Just placed an order for new parts, and I’ll report back once I get everything installed.

Re: Studio Preamp troubleshooting. Help please!

Posted: Sun Jul 01, 2018 1:40 am
by pwu
Success! The new .1 caps fixed the issue! I thinking many other older Mesa Boogies have this issue with bad Orange Drop caps.

Now, I've got two new issues:
Output A isn't working. I'm guessing another cap crapped out, or a connection is broken/bridged.
EQ isn't turning off. I replaced the EQ LDR, but no change. Switch tests fine. Any ideas?

Re: Studio Preamp troubleshooting. Help please!

Posted: Mon Jul 02, 2018 1:47 am
by pwu
Output A is fixed. It was a broken trace between .1 cap and the 10k resistor.
Still haven’t solved the EQ not disengaging. If anyone has info or knows a thread that could be helpful, please post. Thanks in advance!

Re: Studio Preamp issues (SOLVED)

Posted: Fri Jul 06, 2018 12:49 am
by pwu
There was a trace that somehow got to ground from the EQ switch middle tab. I lifted the LDR lead from the board and wired the wire directly to it. Did the same to the EQ LED leg.
All is good now 8)

Re: Studio Preamp issues (SOLVED)

Posted: Wed Jul 25, 2018 2:57 am
by pwu
Image
For those curious, here is how she turned out. I know, some of the 10w resistors are overkill and too big, but their leads are insulated where needed and they work great. Yes, I fit a full size reverb tank in there: wired in parallel with the original smaller tank like the Quad Preamp. The reverb transformer and small tank are attached with 3M double sided tape. Position didn’t seem to matter regarding noise.

Re: Studio Preamp issues (SOLVED)

Posted: Sat Jul 28, 2018 12:10 pm
by TOOLGUY
Thanks for taking the time to post this. I actually had a bad orange drop in my 2:90 causing havoc.
Also I did a recap on my studio pre (Just the elec.) A few years back...but I noticed my eq and lead led`s are getting pretty dim. Was wondering if you noticed any change in brightness to them before/after the recap/repair? Have a feeling I will be hunting for Optocouplers soon...

Re: Studio Preamp issues (SOLVED)

Posted: Sun Jul 29, 2018 7:36 pm
by pwu
I saw no change in LED brightness. Mine are kinda dim. I think they’ve always been that way for the 10 years I’ve owned it.