Troubleshooting Blown Fuse

[MattB930]

The amp is a crate gx-212+. Solid state amp. I got the amp from someone who got it from another guy who said it just kept blowing fuses. Sure enough it does. I traced a problem down to a bad tip142 as well as a shorted tip147. I ended up replacing 2 tip142's and 2 tip147's. I also noticed a bad rectifier bridge and replaced that. And while I was in there I replaced the filter caps. Measured ~50 volts from the PT as indicated on the schematic. Everything looked promising but as soon as I turned it on it had about a 1 second buzz then the fuse blew.

So now I am a bit lost what I should troubleshoot next. I have attached a schematic below. I appreciate the help.

Thanks

[Jazz P Bass]

1 second buzz.
You still have a short.
First, disconnect the speaker.
Retest the output stage, paying close attention to the ballast resistors (R72-75) & any resistors attached to the TIP output transistors.
Check Q10/11 & D10 for shorts.
No mention was made of the 16 volt supply.
Check the zener diodes +16 & -16.
Build a "Lamp Limiter"

[MattB930]

Thank you sir, I will check after work tonight. Appreciate the response.

[Amp Kat]

Q-10 &11 Drivers would be high suspect and not sure the 16 +/- rails pull enough to blow the fuse but guess it's possible. I would highly think the short is in the 40 volt supply so check the main rectifiers very good for diode drops and make sure they aren't shorted. I would even consider pulling the TIP147'S & 142''s and verifying it will come up without blowing a fuse and those +/- 40 voltages present on the collector pads.

[52 Bill]

And sadly, don't assume that the new parts that you installed are still good, because they may have been damaged on the power up.

[Jazz P Bass]

Another thought.
The TIP transistors.
Is (was) there an insulator between the heat sink & the transistor.
I do not reuse them ( don't ask )
I just install new ones.
With a small smear of silicone grease.

[km6xz]

Never test a solid state amp in questionable condition with a speaker load, or any load until you verify it is stable without a load, it will say a lot of blown up speakers and fuses.
Borrow a current metered variac if you can, it will save a lot of further damage by allowing testing at lower mains voltages and monitoring current draw.

[MattB930]

Jazz P Bass, there is an insulator (mica?) between the heat sink and transistor. I reused the old ones, I didn't think it mattered?

[MattB930]

Amp Kat, I checked Q10 last night(haven't checked Q11 yet) and it reads .579 one way on both the Base-emitter(one way) and the Base-collector(one way) and doesn't read from collector to emitter(either way). It checks out ok on these diode tests.
I will check Q11 later.

Jazz P Bass, to accurately check a zener voltage needs to be applied correct to check the reverse voltage on it?

52 bill, I'm hoping they didn't get damaged, I will check the TIPs later also.

Some surge or something definitely happened to damage the power transistors and blow the main rectifier bridge.