Studio .22+ Problems

[Bren10]

We got an ols Studio .22+ for our next recording project and it's awesome. But, I'm having a few problems. I'm pretty confident I can fix these but I was wondering if anyone had seen these symtoms before and could possibly save me some time.

Problem #1: When playing in either mode, the amp intermittantly loses output. It gets super quiet and then loud again. It goes back and forth with no pattern. When it's quiet, It sounds fine but with no low end beef. Any Ideas?

Problem #2: Going into the effects return causes loud static at higher volumes.

By the way, does anyone know the values of the "Volume" and "Treble" pots?

Thanks

 

[Monsta-Tone]

I think that problem #1 is old Power Tubes. You should also replace the Phase Inverter tube (the 12AX7 next to the Power Tubes).

As for Problem #2, that could be the FX Loop tube or possibly the Send & Return Jacks need cleaning (with the amp OFF & UNPLUGGED, spray some contact cleaner into the Jacks and plug a cord in and out a few times). If these don't fix it, you might have to take it to a tech, or someone here might have another suggestion.

 

[Bren10]

Hey, thanks. I'm picking up new power tubes tomorrow morning. We'll see if that does the trick. I think I did replace the Phase Inverter allready. I think it's the second one from the left as seen from the back.

 

[MarkPurney]

Before you do anything else, there is a VERY simple way to test if the problem is related to the effects return jack. Just run a patch cable between your effects SEND and RETURN jacks. If the problem goes away, this means the normally-closed contact inside the effects return switch is making poor contact, probably due to surface corrosion.

Your entire signal passes through this contact whenever the effects loop is *not* being used. This little problem has caused a lot of people to needlessly buy new tubes or pay for servicing to fix what sounds like it could be a bad tube or output transformer.

 

[Bren10]

Man! MarkPurny, you're on it. The problem was indeed the effects return switch. Way to go! I replaced the power tubes anyway. I really needed to. The ones that came with the amp were unmatched and totally lame. I put a fresh set of Mesa EL84's in there and now we've got some boom boom workin'.

Now that that's fixed... I've got new problems. In both modes (Rythm & Lead), with the reverb up, down, or off, I'm getting this crackling sound at all volume settings. Especially when I play low notes. Anybody experienced this before? Like I said, the power tubes are new. I'm working on replacing the preamp tubes next week. The current preamp tubes are anchient but working. Could they be the problem?

Also, sometimes in "Lead Mode" I lose high gain and output. Could this be V1? Something else?

Otherwise, this bugger's ready to be mic'd up. Thanks for all the help.

 

[MarkPurney]

The first step is always to swap the tubes, because it's easy, and bad tubes cause a large percentage of amp problems. Since you don't have your new tubes yet, you can try rotating the existing tubes into different sockets to see if the problem changes at all.

If it's not a tube, I would suspect a damaged component, or a loose component that vibrates with the lower notes, making intermittent contact.

Resistor R131 (680 ohm) is the most common failure. This should be replaced with a 1k ohm, 2W resistor. This is not just a suggested mod, it's a very important change to the footswitch supply circuit. The original resistor will just cook itself to death if you still have a 680 in there.

Another part to check is capacitor C9 (470uF, 16V). It should be replaced with a 3300uF, 16V electrolytic. They say to only replace it the old one fails, but my attitude is to just change it, because it's an electrolytic, and they don't last forever.

You may find this information helpful. Schematics, and info that will help you find R131, C9, and some other recommended items to check for:
http://raktron.com/studio22/

Regarding filter caps, people have different opinions. My opinion is to always let the tubes warm up before turning on the standby switch, and then replace the filter caps every 15 years. I switched to 500V filter caps instead of 450V.

 

[Bren10]

Dude,
You are an amazing resource. We will list you in the "Thank You" section of our next record.

Thanks again,

 

[RussB]

A somewhat common issue with the 22's are cracked/bad solder joints around the power tube sockets. This is due to heat and the pressure that the tube retainer applies to the tubes.