Crate vintage club 30 volume spikes?

[tacoman]

Hi all,
My VC30 has been acting up, after the amp has been on for a while the volume on the gain channel will spike and the sound gets real tinny, and on the clean channel it does the opposite.
I dont think this is a tube problem,but maybe it is? The channel footswitch and panel switch seem to be fine. I heard if the opto-isolators are bad it will cause these problems with switching. Anyone ever changed these out or had this problem? Or could it be another problem..

thanks
Greg

[yostamplifier]

i have a crate club 30 in my shop. i know the 5 optoisolators are usually the culrpit. but also there are two switching transistors that may be the real cause. Q1 and Q2 (located near the footswitch jack on the pcb). there are two test points for dc voltage when switched into each channel. the first test point is a jumper on the pcb near the q1 and q2 transistors: its pcb location is jw41. at this test point when switched to the clean channel you should be reading in dcv -10. when switched to channel 2 (distortion) you should be reading 0dcv. the second test point is located at the resistor r55 (where it meets the transistor q1 (the leg closest to the transistor is the test point). from here your meter should read when switched to clean channel 0 dcv. when switched to channel 2 (distortion) you should see -10 dcv. when transistors go bad they tend to get real hot, the more heat the more obvious the failure. let the amp warm up to get it to start to spike the volume and spray some freeze spray onto the transistor to cool it down real fast. you should notice the spike go away until the transistor can heat back up again from the freeze spray thawing. goto radio shack for the freeze spray. also you can hold a dust off upside down and spray to get the same freeze effect. this will help you track down the bad part. i would test the transistors prior to installing opto isolators, its a big difference in cost. opto isolators are about 30 bucks shipped for all 5. the transistors are 5 bucks shipped. i am also looking into the schematic for a couple mods to the reverb. i dont like the 10k audio taper reverb pots, it seems the value should be a lot higher. and i changed the reverb cap c20 to an orange drop .1uf 400v. more on that when i have more time.
yostamplifier.com

[yostamplifier]

I found the problem for this amp in my shop.. It turned out to be the effects loop input jack. Inside head on the pcb, looking at the jack, there are 3 contact points on the jack. The "tip contact" point, where the tip of the guitar cable (when plugged in) makes contact (to the upper tab) and 'lifts' the contact off of the lower/underside contact tab. This 'break" in the contact on the switch for the "tip" is what is causing the volume jump. What i did was take a dental pick and get under the bottom tab (this one is going to ground) bend it up so it makes real tight contact with the upper tab. Problem fixed..... i want to make an emergency male 1/4" plug for it that will put it back to short when the jack starts acting stupid again. just plug it in and forget about it. and believe me, its as simple as it sounds when it comes to the repair for this.