Sunn 300 Coliseum Troubles

[gbono]

Sunn 300 Coliseum (circa mid 80's) - I replaced the blown 10A/32V fuse but after power up with the VARAC it would draw excessive current. I disconnected the secondary and connected a DVM. Using the VARAC - I can turn up the mains voltage (fan runs) no excessive mains current but the secondary only reads 30mV across the secondary leads.

DC resistance of secondary is .2 ohms. I'm assuming the secondary is damaged Also assuming this PT has a 100 V secondary (+ or - 50V rails). Anyone know where to find a replacement?sunn300PA.pdf

[Jazz P Bass]

Did it blow both 10Amp fuses?
Certainly begs the question, what in the world pulled that much current.
I would take a good look at the + & - voltage rails before putting in another tranny.
Check them both to ground, looking for a short.
I would make a guess at a 40-0-40 300VA replacement.

[gbono]

Double checked and the lead on the DVM is faulty. Must always remember that it is rare to damage a PT.

The schematic shows a 10 amp 3AG fuse though the fuse holder has a (6 Amp) marking?

Thanks for the sanity check - now I can start to see what's pulling down the supply. Here's what I know:

When I got this box there was one blown fuse (I think it was the positive rail).
No signs of damaged or overheated components.
Looks like all the original output devices
PS filter caps ESR measure in spec.

So I'll start down the posisitve rail (PNP output devices) and see what I can find. Next step test for over current with only rectifier bridge and filter caps comnnected.....

[Enzo]

Take an ohm meter to the bridge first. Just check the four sides of it for shorted.

And I wouldn;t bother being so specific as V+ side transistors. I'd just flip it over and go down the row. Check all the transistors.

[gbono]

Two shorted transistors in the output section - one of each flavor. Someone had previously "reapired" the +15V regulator transistor.

This amp was considered unrepairable by the last person who worked on it???

Clean it up and check the front panel features and rated output and it will be ready for battle

[52 Bill]

This amp was considered unrepairable by the last person who worked on it???

Early on you thought that the power transformer was bad. If you had contacted the owner and said that it would cost $200-$300 to fix he probably would have said that it was unrepairable, or at least not worth repairing. To some people having to pay for the two outputs would make the amp unrepairable.

[gbono]

Fixed the power amp but now the problem is the preamp. I can chase a signal to the input of the VCA (SSM2013) but nothing at the output of the VCA. I checked the bias on the VCA and the V+ rail is only 10V. I rebuilt the positive regulator on the PA board but I still can't get +15 rail back up to 15V. The negative rail is okay. I have to assume that some of the other dual OP-amps are damaged and pulling the supply rail down? So now I know why someone wanted to airlock one of these amps

BTW does anyone have a readable copy of the schematics for this amp?

preamp300.pdf

[Jazz P Bass]

The only easy thing that leaps out is C404. 10uf/ 25V
On the schematic it is at IC404B, pin #8.
Is there 0 volts on pin 5?

[gbono]

Thanks but there is 0 DC on pin 5. ESR on C404 looks normal. Even a cleaner copy of the schematic would help at this point it's either checking out all the 4558's or the airlock

[Euthymia]

That same thread where I said I was going to airlock the last one of these that was brought to me I also said that I uploaded the schematic Fender sent me to Ampix. It's on there.

This isn't Dion's poor old amp, is it?

The regulation for the 15V supply went on it and the resulting high voltage took out every IC on the preamp PCB. Repairable, yes, but desoldering and replacing all those opamps with no guarantee that the thing would not need even more surgery after that was not wise, in my recommendation to the client.

[gbono]

Euthymia - thanks for the schematics but I can barely read them. I assume the FMC files were pretty poor scans.

My son bought this off the SF Bay Area CL from someone in SF. The preamp board doesn't have any charred components. I can't get more than +10V at the VCA - rebuilt the transistor regulator for the +15V supply but still lossing volts somewhere?? I really don't want to trouble shoot 12x 4558 OP amps

[Euthymia]

Euthymia - thanks for the schematics but I can barely read them. I assume the FMC files were pretty poor scans.

My son bought this off the SF Bay Area CL from someone in SF. The preamp board doesn't have any charred components. I can't get more than +10V at the VCA - rebuilt the transistor regulator for the +15V supply but still lossing volts somewhere?? I really don't want to trouble shoot 12x 4558 OP amps

Okay, that IS Dion's old amp, and I was the one who deemed it not worth repairing, for just the reason you state. I'm pretty sure that dozen 4558's are fried due to the overvoltage. It would be a great project for someone who wants to learn to desolder IC's or something, but even after they got them all out of there and put in sockets, who knows what else they'd find that needed fixing?

For instance: good luck finding a replacement for the SSM2013.

She's singin' "Daisy," my man.

Dion loved that old thing and I patched it back together a couple of times before it just couldn't go any more.

[Jazz P Bass]

Look at the board & see if there are any common +15V traces that go to the opamps.
Maybe you could narrow it down by removing "banks" of ic's from the 15v rail.
Cutting traces is kind of crude, but hey. Ya do what ya gotta do.

[Jazz P Bass]

SSM2013
This guy got some
He is in Illinois.
Link: BRITISH AUDIO SERVICE

[gbono]

I don't believe the VCA is gone - I desoldered it and bread boarded it and it seems to work. It just needs more than 10V on +Vcc.

This amp had the -15V regulator rebuilt before. The +15 rail still had the TIP29 with an 80's date code - I changed out the zener and BJT but something down stream isn't right.

I'll get it back together - it's only a matter of time