Studio .22 with graphic - graphic stopped working

[skullsession]

I just purchased a used Studio.22 with graphic eq. Played and sounded fantastic before I brought it home.

Got it home...played a couple of hours, and realized that the graphic eq was no longer working. I switched out 3 of the 5 12ax7's...swapped out the old ones and moved them all around. (I only had 3 good tubes lying around)

Is that graphic eq tube driven? I'm kind of an idiot when it comes to this stuff, and I can't find a drawing anywhere that shows me which tubes do what.

Any thoughts or help is appreciated!

 

[Boogiebabies]

The EQ is not tube driven. It could be a dirty or bad EQ In/Auto switch or the LDR/Opto switch is bad.
On rare occasions one of the transistors on the EQ board go bad, but it's very rare.
Try switch the amp to EQ auto and see if it comes on in lead mode.

 

[skullsession]

I don't have the pedal to switch the channels. I tried to manually do it by shorting a cable as suggested in the instructions, but it wouldn't switch.

I wondered about the toggle switch. I suppose I should take it apart to see if it crapped out.

I spoke to the seller yesterday. He said the EQ has never been a problem before. (uh huh...) And he said he used to use a simple crate switch to change channels...so according to him, I should be able to switch channels as well.

 

[skullsession]

Alright. So I pulled it apart this evening and found a resistor that had come off the board. Found the spot R131 and soldered it back on. Funny thing though...the PCB under the R131 spot is cooked pretty brown like it's been overheated. Certainly been hot at least once directly below that resistor. Apparently hot enough to cook the solder so that the resistor would fall off.

I also removed the three-way switch and cleaned it up.

Fired it up and was able to use the Graphic EQ again. Played for a few minutes. Then I broke out a footswitch and was able to switch to the lead channel finally.

Then....like 4 minutes after I started using the lead channel, the EQ and channel switcher stopped working again.

I'll open her up again this week....but what's the deal if this thing is overheating that one spot on the board?

 

[Boogiebabies]

The supply for the accessories is usually off a low voltage tap on the PT, rectified and dropped with resistors for the proper voltage, ie EQ, LDR's and LED's.
R131 is a 1K 2 Watt dropper resistor right off the rectifier diode for the LV tap that should read on the output 8V switching off and 3V switching on.
The other side of the LV tap is the negative diode and resistor for the EQ, It's 1K and listed as ER5 and a 5K6 listed ER4. Across the 5K6/ER4 their are two 220uf/63V caps.
They could be bad as well. They could show a bulge in the top or leak yellow goo. I would look here first. If the LDR supply was surging I don't think the switching system would work even after cooling. LDR's do not like surges. The LED's in them die almost instantly over 40Ma and 3 Plus volts.

 

[skullsession]

This one is seriously beyond my skills.

Dropped her off in the shop today...because I sure did love the clean tones with the graphic in the loop.

Thanks for you help!