Sunn 300 Coliseum PT

[gbono]

I have a 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?

[J M Fahey]

Hi GBono.
Your findings are contradictory, re-check to find which one is true:
1) if it draws excessive current, *connected to the  rest of the amp* but not when unconnected, your problem seems to lie somewhere on the amp and not the transformer.
2) to be able to draw excessive current, its secondary must put out a significant voltage into the load, so you can't have 30 mV.
Either your multimeter was still on DC setting or you made poor contact.
3) to prove it, plug the primary into a series lamp circuit: with the transformer "on" the bulb should barely show a weak orange glow; shorting any two secondary leads should make it glow full brightness
4) ¿10A 32V fuse? ... the schematic asks for a 6A 250V one !!!
The one you have there belongs in the motorcycle where it came from.
5) What kind of Coliseum 300 is it? Bass amp / power amp?
Can you post a couple pictures?

[gbono]

Thanks JM - 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
This is the 300 Bass amp circa 1985.

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.....

[J M Fahey]

With amp unplugged start by diode testing all output transistors; all 6 possibilitied between the 3 legs.
Any short makes that transistor suspect and candidate to be removed and tested outside, then continur measuring along the line.
You have 2 10A fuses protecting + and - rails, but the mains fuse is 6A as marked both in the schematic and, as you say , in the panel itself.

[gbono]

Two shorted transistors in the output section - it also looks like someone "repaired" the +15V regulator transistor. I'm waiting for replacement parts from On Semi.

Funny but this amp was considered unrepairable by the previous tech who worked on it???

Clean it up and check the features/output and it should be ready for battle 

[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?

[gbono]

Replaced all the preamp ICs and now they're in sockets. Can't get a signal into IC1 now - the signal "disappears" at R44 - unless the IC is removed from the socket

I have replaced the OP/checked all the components around the socket/VCC+- are good. What am I missing here? Something is loading the signal down at R44.

Preamp schematic is on the previous posting.