Fender Ultra-chorus (Non-DSP) right channel problem

[nagruv5150]

***Edit, this thread should be in troubleshooting, maintenance, and repair. I can't move it myself or I would, sorry***

I am trying to repair a right channel problem with this 2x12 solid state combo. If I turn it on and begin playing, it plays for about 30-45 seconds, then pops (sounds more like a crack) while still outputting music. After 1 or 2 pops, I get an extremely loud roar and the amp will not respond to musical input at this point. Turn off, let it cool, then repeat with exactly same results. The problem is only existent when I have both left and right speaker outputs connected, but does not matter if the drive channel or clean channel is selected.

Things I have ruled out:
1. Filter caps on incoming power supply have been replaced
2. no leaking DC on any IC's or directly from the speaker outputs
3. No difference in test point measurements between left and right channel outputs AC and DC
4. I disconnect the right speaker output and everything plays fine. No noise or anything. Left speaker output to right speaker plays fine also. Its definitely not a speaker problem.
5. Reverb tank disconnected makes no difference as well as chorus on/off

I do not have an o-scope, but a good multimeter. At this point the rails are present everywhere except on U10 (4560 IC). Pin 7 should be -15v, but I get +15v. From pin 7, I have continuity to R152 which also reads +15v. From the backside of R152 to C77, continuity, then from C77 to ground. I have googled as much as I can stomach and still can't find a similar problem on any SS Fenders. I have also attached the Schematic I am working with. This is the non-DSP unit. Any help is greatly appreciated.

Ultimate_Chorus_schematic.pdf

[teemuk]

As you said it yourself, it's something thermal-related. When the amp warms enough the problem appears. I think the cumulative loading effect of having both power amp channels in use is enough to heat up things but taking one channel out may not as easily exceed the critical point.

The fault can be a pain in the butt to track, it's likely just a microscopic solder joint failure in some critical point. You could try what you can achieve with a hair dryer blowing hot air into select places at the board while tracking when the amp begins to "pop" and finally "roar".

[MarkusBass]

Things I have ruled out:
1. Filter caps on incoming power supply have been replaced
2. no leaking DC on any IC's or directly from the speaker outputs
3. No difference in test point measurements between left and right channel outputs AC and DC
4. I disconnect the right speaker output and everything plays fine. No noise or anything. Left speaker output to right speaker plays fine also. Its definitely not a speaker problem.
5. Reverb tank disconnected makes no difference as well as chorus on/off

I do not have an o-scope, but a good multimeter. At this point the rails are present everywhere except on U10 (4560 IC). Pin 7 should be -15v, but I get +15v. From pin 7, I have continuity to R152 which also reads +15v. From the backside of R152 to C77, continuity, then from C77 to ground. I have googled as much as I can stomach and still can't find a similar problem on any SS Fenders. I have also attached the Schematic I am working with. This is the non-DSP unit. Any help is greatly appreciated.

U10 is in channel switching circuitry so the voltages on the output depend on whether you have footswitch plugged-in and which channel is selected.
Most of the other tests you conducted were not needed. E.g. the power supply is common to both channels so the problem cannot be caused by caps in the power supply. The same story is with the speaker. It cannot be a source of the sound (unless it gets such a signal from the power amp).
I would first check whether this "roar" is from the preamp or from the power amp. And you can do it easy by plugging a jack cable into Effects Return jack. If you find out that the sound comes from the power amp, I would check the diodes CR30-CR33. You can also check DC voltage between CR30 and CR33. Also check the power amp for cold solders.
Let us know whether this is preamp or the power amp.

Mark

[Steve Conner]

+1. Another tip: if the fault is in the power amp, and bashing the PCB doesn't provoke it, then it could be a transistor. I've seen hi-fi amps that ran TO92s so hot that they degraded and started making weird pops and cracks when hot. Teemu's hair dryer (plus some canned air for cooling) can help you localise this. If you don't have canned air, blowing on the suspect transistor through a straw has helped me in the past.

[nagruv5150]

okay, I went over all the solder joints again just to make sure none were cold, especially in both power amp sections. This includes the portion of the diodes that go through the heatsink. I also ran from the mono effects loop out to another amp and had no issue (speakers were disconnected due to lateness of this test). Across CR30 and CR33 (CR42 & CR45 as well for 2nd output channel) I see 1.6vDC after everything is warm. With both speakers connected and a jack plugged into the mono effects return, I notice an increase in 60hz line noise. At this point, I would definitely have to say power amp section, specifically the right output channel based on everything I have seen.

As for isolating the diodes, I'm having a little trouble on meter lead placements to see if one is opening up. When the amp is out of the chassis, it heats up quickly and thermal protection steps in.

Markus, The reason the filter caps were replaced on incoming AC was to eliminate another problem. Elimination of the speakers was my hoping that it would be an easy fix. The owner had been running it with the right channel speaker leads disconnected. Clean and overdrive channels work in this method, but overdrive is obviously not as strong without the 2nd output channel.

[MarkusBass]

With both speakers connected and a jack plugged into the mono effects return, I notice an increase in 60hz line noise. At this point, I would definitely have to say power amp section, specifically the right output channel based on everything I have seen.

This is the test that you should perform from the very beginning. But what is the outcome? Previously you mentioned "extremely loud roar" and now only "increase in 60hz line noise" . So the problem is still there or not at all? If you think that this is related to the power amp, just disconned the input (or even short it to the ground), connect a dummy resistors instead of speakers (40-50 Ohms) and start looking for differences between the two amps. If the are no DC differences, check them with oscilloscope.

Mark

[nagruv5150]

Okay, it seems the "roar" has gone. At least I can't reproduce it anymore with everything put back together. It must have been one of the diodes having a bad solder joint that I didn't redo earlier. No matter whether I plug directly in with both speakers connected, or with a jack plugged into mono efx loop return. I can even use another amp's efx send to this one's return and not get it. Even after about 10 minutes or so playing at various volume levels. This is for both clean and overdrive channels, and with chorus on/off.

The only problem I am left with now is noise in the overdrive preamp. After things cool off, I can plug in directly and play on the overdrive channel for 30 sec or so and all is well, then noise comes in. I can still play over it, so its no big deal. If I swap back to clean channel, its completely normal and no noise, so I at least know where the problem is in this case. I mostly use clean channel anyway with a pod xt or my bass directly.

Thanks for everybody's help and sorry for the lack of clarity in my descriptions.