Previously, my amp was giving me the symptoms you were describing (with the solder joints breaking).  Sound cut out, hum appeared, pops, crackles etc.  Haven't gotten any of those since I started working on the amp again (from the start of this thread).

You say the resistors are supposed to be fairly hot.  I would assume they are too hot at the moment however.  I can hardly touch them for more than a second or two without my fingertip nearing the scorching point.

I'll post back if the problem arises again, but at the moment the amp seems to be working fine.

Thank you very much for your time.  It is highly appreciated.

And thanks for the link.  I'll definitely check it out.  I know the basics of the amp (preamp->poweramp, what those stages do, etc.).

The amp at the moment is essentially a learning project, as this amp has been dead for a while and I got new one in the meantime.

Fixing the solder joint got rid of the hum.  When it was humming, a guitar plugged into the input jack would not be heard, only the hum; changing anything (gain/clean channels) did nothing.  Fixing the solder joints did allow the amp to "work" as i was previously.

I work with electronics quite a bit.  I've got plenty of practical experience, but theory-wise I don't know so much.  I've got knowledge about how to handle electronics and such, so I should be fine actually doing the physical work.  However, I'm not very able alone to figure out the problem based on the measurements and other inspections I find.  The amp however is basically a learning project, so fixing isn't on the top of my priority list.

I can't test TP14 or TP15 at the moment.  I did have a function generator, but no dummy load to replace the speaker (I need a 4-ohm load, it is far too loud with the speaker connected).  I will probably end up building a switchable 16-8-4ohm load soon enough however.

I'll check the Zeners and their solder joints.

For the past few days, I've been playing guitar for an hour or so and the amp gave me not a single problem.  However, the voltage from +V is 17v still, and the 5w resistors are still extremely hot.

Sorry for the double post, but..

Turns out all my measurements in my past post were wrong.  Turns out the solder joints for C39 (very large capacitor) were cracked.  The broken solder joints were also what was causing the hum, reheated the joints and let the solder reflow and now the amp works like it used to when cold.  I measured the Zeners after playing for a bit.  One was a little over 16v (I believe 16.34vDC), the other was nearly 17v (16.97vDC).  My electronics teacher had told me that the Zeners should be extremely accurate to 16vDC, +- a few hundreths of a volt.

I haven't measured the rail voltages.  Where would I go about measuring them?  As you may have guessed, I am not the most electronically able person in the field of theory (such as, I don't know the basics about different kinds of components and how they should act, etc), but with some guidance I most of the time am able to troubleshoot problems if I know where to specifically look.

Edit:  Looking at the schematic, I see four things which I assume to be rails.  There is a +40 and a -40v rail, as well as a +V and -V rail.  I assume you mean to measure the V+ and V- rails, and they should be ~16v?

If the +V and -V are the correct places to be measuring, then I have measured them.  With the chassis as a reference point, +V (negative side of the Zener D25) is roughly 17V after a bit of playing, whereas the positive of D26 is around 16.3v.

Are D8 and D9 Zeners?  Their schematic symbol don't look like Zener diodes, but they are reverse-biased, no?

Starting the amp cold, I measured across the Zener diodes, one has 16.15vDC, the other is 16.65vDC.

The amp is giving me a constant hum, starting cold or leaving on for a while.  The hum doesn't go away.  It did once, after I shook the chassis (and in turn shook the PCB).  The hum I can record for you if you might want to hear it.

I rechecked the voltage across R72, and guess what, it dropped down to 5mV...  I readjusted it.  After letting the amp warm up a bit, still idle, I checked the Zener diodes again.  The first was 16.7vDC across and the other dropped to 15.3vDC across.  This is without any input, only the speaker connected.  Also, this is with the amp back in the chassis.

Greatest general paste in the world:

www.arcticsilver.com

I'm partial to the Ceramique.  It probably wouldn't last too long if you use it on many amplifiers, but I've attached and reattached at least 15-20 heatsinks to computer CPUs and GPUs and still have plenty left for several more.  It is quite cheap for what you get, and it performs above all others.  If you want thermal tape, Chromerics makes the best thermal tape around.

The only heatsink that amp has for the four transistors is the metal plate and the chassis.  Nothing else.

The resistors do not heat up C39.  C39 and C40 are both just lukewarm, nothing else.

Now that I look at those resistors, they do look a bit charred.

It does seem a large possibility that the preamp circuit is drawing too much current.  Any ideas how that could have happened?

Pics are here:

A pic of everything (chassis, PCB, speaker and everything else):
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0898.JPG

A shot of the PCB:
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0899.JPG

Shot of the fuse, power switch, transformer, and related areas (connections between transformer and PCB):
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0900.JPG

Shot of the DSP and footswitch jacks:
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0901.JPG

Shot of the resistors I suspect to have problems.  The three towards the left from front to back are R89, R88, R90.  The two to the left from front to back are R64, R61.
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0903.JPG

Shot of capacitors C39 and C40, and another angle of suspicious resistors:
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0905.JPG

Shot of R72 on the left and R74 on the right, AP1 to the far left:
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0906.JPG

Shot of the four transistors Q12, Q13, Q15, Q16.  The metal plate they are connected to had a thermal paste between it and the chassis when attached.
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0907.JPG

Shot of the Insert and External Speaker jack.  Note the Insert jack is missing (moved over to the external speaker jack which broke), but the only contact that is supposed to be shorted but wasn't is jumpered with a wire.  I haven't bought a switching jack to replace the lack of jacks.
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0908.JPG

Shot of the speaker:
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0909.JPG

Shot of the chassis and transformer.  Note the thermal paste in the BG, that is where the four transistors and their "heatsink" lay.
http://bostonrinkrats.com/solidxsnake/GFX120/IMG_0910.JPG


In the image of the suspicious resistors, the three on the left (R89, R88, R90) were getting extremely hot.  The two to the left (R64, R61) were still very hot, but not as hot as the three to the left.

If you need any more pics or sound clips (I couldn't recreate the problem while the amp wasn't in the chassis), shoot me a message.

The only part of the electronics that are cooled by the chassis are the four transistors (Q12, Q13, Q15, Q16).

I'm positive it's not a short in the chassis, there's not a single thing that could cause a short besides the two jumpers that contact the screws for the PCB.

I'll take some pics and host them.

Strangest thing.

Today, I was gonna work on my amp, measure test points and such.  I first wanted to create a soundclip of what was going on to show my electronics teacher (as well as people on this forum) to see if that would help narrow down what was going on.  I unattached everything from the chassis besides the transformer (all of the PCBs, the fuse holder and the power switch are not attached to anything), and then started playing to reproduce the problem.  After 10, 20, 30 minutes of playing, I didn't get one single problem.  I tried the caveman's testing for temperature by touching various components, and some resistors and transistors were BURNING.  Resistors in question are R61, R64, R88, R89, R90.  Transistors are Q12, Q13, Q15, Q16.  The transistors I mentioned are attached to a metal plate which is normally attached to the chassis with some screws (some TIM in between it) for the chassis to act as a heatsink.  However, even after the resistors and transistors were extremely hot, I had no problem.

This is very strange.  The only difference is that the board is not enclosed or screwed into the chassis at all, and I have no problems.  I can take a picture of my setup at the moment if you don't understand what I am saying.

Thanks for your help so far.

I brought the schematic to school and tried to explain to my electronics teacher what was going on, and she suggested it might be something from the power source.  She said to try checking the diodes, and the test points around the filter capacitors (TP14 and 15).

For the test points, an oscilloscope is helpful for the AC readings.  For the DC ones, a signal generator is required.  Thankfully, my electronics teacher lent me a signal generator.  The schematic says a 1KHz signal to the insert tip for the poweramp testpoints.

I'll post back with results as to what is happening if I can't figure anything out.

Once again, thank you for your time.

The glue was getting warm before adjustment (the voltage across R72 was 2mV then).  I haven't checked the resistors in suspicion since adjustment.

For voltages of the regulator resistors, I should not be measuring across the resistors, instead using the chassis or a ground as a common point, right?

The testpoints look as if they include the use of an oscilloscope, correct?

I can identify transistors and diodes, and know how to test diodes to see if they are still working.  However, I don't know exactly what you mean by basic measurement techniques for each.

R72 was not reading 7mV, but after adjusting AP1 a bit, it is now 7mV exact according to my cheap DMM.  This was without any input and with the speaker disconnected.  That was how I was supposed to measure it, correct?  With no signal/inputs and no load (speakers)?

I turned the amp on for all of a minute or two with the speaker connected but no input, and noticed the glue around the resistors towards the top-right of the PCB (R61, R88, R89, R90 and one other which I can't make out the name) was getting very warm.  Maybe there's a problem there?

For the record, all of the solder joints on the board are fine from eye inspection.  The input jack is missing.  My Ext. Speaker jack broke, thus I replaced the Ext. Speaker jack with the Input jack.  I made sure to solder the two points that weren't going to be connected with the jack gone (The jack was a switched jack, as in when a 1/4" cable was plugged in the switch was open, otherwise the switch is closed).  I still have yet to test to see where the problem is, the poweramp or the preamp.

I don't have the greatest electrical knowledge, I know the basics.  I don't have that many tools on hand either, I have a soldering iron and a cheap DMM...

The short was under the amp, it wasn't the amp's fault, human fault for the shorts (some metal rod got stuck under the amp between the chassis and the board).    The amp was doing the exact same thing before the short, so I'm just about positive the short had no effect on the amp besides blowing the fuse.

I do need the schematic.  Email me at MetallicAman91 [at] gmail [dot] com

The fuse that came with the amp stock was a slow-blow fuse.  My tube amp uses fast-blow fuses.

Is there any way to pinpoint what specific part is breaking down from heat?  I'd assume it's after the preamp stage as it occurs for the clean and the gain channels.  I can try rigging a fan above the amp to see if it stops/occurs later.

Several years back, I got a brand new Crate GFX120.  It's one of the older ones, not the ones with the red/black scheme, but blue/black color scheme.  It shot sparks from a short last year.  It is a 1x12 120w combo.  I recently decided to fix it up. Replaced the fuse (3A 250V Slow Blow), and the thing powers up and plays fine. However, after maybe 10 minutes of playing, the sound starts to become extremely inconsistent. For instance, I will play a chord on the guitar, and the sound fades to a very low and quiet level. It will keep fading in and out according to the output volume of my guitar (I assume). If I cut the volume on my guitar (turn down the knob all the way), then the hum from the distortion channel from the amp slowly comes back and stays consistent. This leads me to believe my guitar is "overloading" something in the amp. A visual inspection leads me to believe nothing is wrong with the amp (no blown caps or ICs etc.).  It is quite hard to explain.

Any clues? I can take pics of the amp, videos, sound clips etc.

I've confirmed it's not the speaker of the amp.  My gut tells me it's in the poweramp stage.