Sun Coliseum Problem

[J M Fahey]

Unfortunately that's an "in between" Sunn.
The early 70's were the classic ones, which everybody knows, so they are everywhere.
Then sales went down, maybe they even closed doors, and probably some new investor tried to start it again in the 80's.
Well made amps , but Musiciand are very fad conscious and probably these did not sell well enough.
Curtain down.
Then Fender bought the brand, mainly for the PA line .
Fender Box Powered Mixers were actually rebranded Sunn.
Now that I think of it, I'll check some 80's SRsomething Fender schematics, maybe one of them had the same Power Amp.
Bass and PA both need loud clean amps.
If I find a 300W into 2 ohms one, it may be the same.
If not, get some backup amp to practice, because repairing this one is possible and not expensive, but will take a lot of time and patience.
You will need to grab pencil, rubber and paper and start sketching the actual schematic.
Just draw two parallel lines, one on top, one on bottom, they are the + and - rails, go from right to left, starting with the power transistors which somehow go to either rail, either direct or through some resistor, and go backwards.
Maybe with just the right half is enough, because it's the part that suffers most abuse.
Look at the "old" schematic for inspiration and redo the drawing as many times as you wish to keep it clear and reasonable.
If you don't have some part value, label it with its PCB ID anyway (such as R41, etc.) , seeing where it goes I can estimate a reasonable value.
You can do it, but will take time.
No kids, pets, Wifey or TV in the room, so you concentrate.

[veganxxx]

would it be worth calling up here http://www.sunnamps.com/manuals.html and seeing if for $5 they might be able to dig up and mail me a schematic?

[J M Fahey]

**MAYBE** They have only those listed below  the page.
There's a "Coliseum 300" , "Coliseum 880" and "Coliseum Bass".
Yours might match one of them ... or none at all.
Problem is, those demands are processed by clerical employees, not technicians.
If you ask them for the one which has a 6 band equalizer or a compressor in the preamp, probably won't know what you are talking about.
Suggest you google the Sunn page in Ampage and aske them if somebody has it.
Or, of course, draw it by hand.

[gbono]

Try this

[veganxxx]

that looks like it!  I also did some snooping with the number written on the power board and got the info that the 20-0166 board was based on a Beta, this also looks very similar- http://sunn.ampage.org/site/svcmans/beta/ 

I will investigate closer when I get home.

Thanks for the info, very helpful to say the least.

[veganxxx]

FIXED!  Put all the right parts in all the right places.  Thanks to all who helped.  I guess the initial problem was a blown transistor and zener.  Hopefully those diagrams on the web will help others with the same board.

[J M Fahey]

Glad you solved it  :dbtu:
Thanks, gbono, your contribution saved us all a looong work. :dbtu: