Sunn Concert Lead Burnt Resistor

[Jynk_O]

Hello!

After having luck repairing a few peavey VTMs with help from here, I decided to try to go back and fix a busted Sunn Concert Lead (Silverface). I got this knowing it was having issues as a learning project, but haven't had much luck. The issue occurring is R322 keeps burning instantly. I've tested the transformer for shorts between secondaries and the primary, as well as case without finding anything. The output transistors were replaced and all, including Q5, seem to be fine testing with a multimeter. I'm kind of stuck on where to go next or how to test further with limited equipment. If anyone has any advice on how to move forward or some suspicions, I would greatly appreciate it!

All carbon and wirewound resistors been replaced as well as electrolytic caps .

[g1]

Do you have a speaker or load connected? If so, remove it and check for DC on the output.

[The Dude]

^^^^^^ Yes, that. I'll add- leave the speaker disconnected until you very that there is no DC on the output and until the amp is repaired.

[Enzo]

And make yourself a light bulb limiter

[J M Fahey]

Q5 is shorted Base-Collector

Disconnect its base from R322 and retest.

Thereīs no other path for important current through R322

And yes, make and use a lamp bulb limiter, running through a minefield blindfolded is not the best way to find them.

[Jynk_O]

Sorry for a late response!

I replaced Q5, as well as Q2 and hooked up to a dimmer to the same results. I'm assuming the short would have to be mechanical then, but I'm not certain where that would be as I believe all the interconnections are correct. The only missing thing is that the mounting brackets only have one screw, but I assume that would suffice temporarily as both screws of the collector are connected.

Ive cleaned it up since taking the photo attached but connections are the same

[Helmholtz]

I replaced Q5, as well as Q2 and hooked up to a dimmer to the same results.

I hope that doesn't mean you used a phase control dimmer instead of the bulb limiter?

[Jynk_O]

No no sorry, I did use the light bulb limiter, I'm just mixing my words up!

[The Dude]

Are you maybe missing an insulator on one or more of the output transistors? Check transistor connections to ground and make sure nothing is grounded that shouldn't be.

[J M Fahey]

The only missing thing is that the mounting brackets only have one screw, but I assume that would suffice temporarily as both screws of the collector are connected.

BOTH screws are NEEDED, I couldnīt care less about the "electrical" connection, those screws hold transistors tight against the heatsink.

And follow/check WHAT is connected to the burning resistor, high current enough to burn it must reach it somehow.

As shown on schematic, ONLY path is through a shorted BC junction.

If not, then there is some other path, find it.

Current flows through metal paths (track/wire/shorted Silicon junction), or low resistance resistor (a few ohms).

Please measure Volts across R322 and post results.

For sake of completeness,showing the only possible paths for current into R322
and that .....



* Red through R301 must pass through 910 ohm in series, not enough to burn R322

* Blue through C301: ... itīs a capacitor! ... only possible if shorted.

* Green through shorted Q5 Base-Collector junction, which I suggested.

none other.

UNLESS you disconnected T1 driver transformer Blue-Vio windings and connected them to some crazy place,

AGAIN: measure voltage across R322 or report smoke/sparks/red glow.

[Jynk_O]

Sorry again for taking a while- band stuff kept me busy and the cold kept me away from the garage.

I went in today and took a good look to find some weird things going on inside this amp. Wires, from factory it seems, that do not exist on any schematics or in pictures of any other early-mid 70s concert leads. The most glaring is that an extra wire to the collector of Q1 that does not go to the transformer as per schematic. C301 removed but stability mod not done, so I did that. Set everything to what the schematic says (not sure where that green wire from q1 is from, because every schematic and all pictures show the brown connection to transformer. It seems like its got a mix of some of the differences between the red knob and silver face versions)

No burning on startup, R322 is now at 10.5v, no immediate burning but does start smelling (I didn't replace it from the last burn out, though, as I forgot to do so before checking voltage)

[loudthud]

See my comments in post 6 of this thread: https://music-electronics-forum.com/...-sunn-solos-ii

[Jynk_O]

Retested transformer out of circuit and it tests fine, in circuit there's DC offset of about 4v, which is absent when taking out the transistors! I also had a mix of old and new 2n3055s. With a lot of conflicting info online about what people recommend as a good replacement, does anyone have one they've had good experience with?

[The Dude]

Did you test them to confirm any defective? I would not buy them from Ebay and be sure to get them from a reliable source. FWIW: Digikey has options in stock.

https://www.digikey.com/en/products/...SlwB2AzAAxkU1A