Fender Ultimate Chorus power amp repair

[The Dude]

Did you buy the replacement parts from a reputable source? There are lots of Chinese fakes out there. Check all resistors in the path of the shorted parts. Often, if a SS device shorts, associated resistors burn open or at least towards open. Before operating the amp, check bias or voltage drop across emitter resistors and compare to the working side. Even though the amp doesn't have adjustable bias, measurements will often show if there is still a problem in the amp. Also verify there is no DC on the output before hooking a load/speaker to the amp.

[rybeka]

Yes. Got them from Digikey. Checked the resistors and all were in spec. I will check the voltage drop and DC on the output as you suggest. Might be several days until I get the replacement parts and get them installed. Thank you for your help.

[g1]

One thing I will mention, some of these era Fender amps had a heat sink bar that would only seat properly one way. If it was installed wrong, the screws would not sit in the indents, and the heatsink would not mate with the chassis correctly. Looking at it from the side should show whether it is fully seated or not.

[rybeka]

Thanks for the tip! I noticed the heatsink compound did not squeeze out when I reinstalled it and yep it was reversed. It fits like a glove now. Good call!

[rybeka]

OK, I have replaced the outputs (Q10 and Q11) along with CR46 (1N4448) and CR47 (1N5228B) which were shorted. Resistance checks against the good channel (left) are all within a few ohms of each other. As far as checking for DC across the CP3&4 and CP1&2 (speaker terminals), is it safe to power the amp without a load?

[The Dude]

Yes, it's safe (actually preferred) to power up without a load. Use a LBL and don't hook up a load until you verify there is no DC on the output.

[rybeka]

Thanks for the clarification. Leaving on vacation, won't get a chance to do this until next week. Have a great Thanksgiving!

[rybeka]

Tested both channels (CP 1&2 and CP3&4) there is 0 volts across there respective terminals. Reconnected the speakers and powered up. All seemed well with no signal so I inputted a 1000Hz on input 1 (normal channel) and it worked. As I shut down the amp after about 5 minutes there was this very loud screech (about 1/2 second). When I powered the amp back up, the same sound appeared and Q10 (TIP142) went up in smoke taking CR47 and CR48 with it. The left side was unaffected. Now I am back to searching for the root cause of this problem. After replacing the blown components, I tested the resistors, diodes and caps in the right side - no problems. Resistance checks to ground against the good channel were OK. I am really afraid to power it up again until I can identify the cause.

[rybeka]

Any ideas would be welcomed.

[g1]

Seems odd that Q10 would take out those zeners but the rest would be ok. I'd be more inclined to think a problem with those diode strings caused Q10 to blow.
Are the rest of those 8 diodes ok? (CR42-CR49) And the 4 resistors R127-R130 ?

[The Dude]

Did you check bias voltage before hooking up the load, as I described in post #16, and compare to the working channel? Most of the time, if there is a problem with the channel, bias will be off substantially.

[rybeka]

Here is the odd thing, bias voltages were the same and the components you mentioned were changed (again). I made a termination jack with 600 ohm resistors to ground and placed it in the stereo return jack.0 volts on both the speaker terminals. Reconnected the speakers. R&L sides of the amp worked flawlessly >20 mins when I injected a 1K signal at the stereo return jack I made. Traced the source of the squeal back to pin 6 of U6B. No squeal from the Mono side. The unit shuts down with a soft thump now that the input to the power amps is isolated. Maybe I am on the wrong track but it seems that the squeal is overdriving the right side and causing these problems?

[rybeka]

The problem appears to be with the chorus bias control. When I changed the setting, the squeal stopped but I now need to know how to properly set the bias to see if this was the real cause. The squeal was present even with U12 & 13 removed so I discounted the chorus being involved from the start.

[g1]

Do you have a scope?

[rybeka]

Yes, dual trace 100Mhz.