Sunn Coliseum 300 problem

[Denis]

I've noticed recently that sometimes, not always, it will become quieter and quieter over the space of a few minutes, until it reaches the point that it's as loud at 6 on the volume knob as it was at 2 a few minutes before. Oddly, this usually happens when I have the Jap Epi plugged into it, although I think this is coincidence.
Anyone have any idea what's going on?
Thanks!

[Psycho Bass Guy]

Given the age of the amp, it sounds like the output transistors are dying and having brief moments of runaway.

[Denis]

Given the age of the amp, it sounds like the output transistors are dying and having brief moments of runaway.

Thanks! Is there any way I can test for this specifically and if so, is it something an amateur like myself can fix (or replace the transistors)?

[Psycho Bass Guy]

There's no way to test or repair this condition without test gear and a fair amount of tech know-how. The transistor substitution cross-reference manual alone is the size of a phone book.

[Denis]

There's no way to test or repair this condition without test gear and a fair amount of tech know-how. The transistor substitution cross-reference manual alone is the size of a phone book.

Ah, I see. In that case I'll pass and just take it over to MadScience and let them fix it.

[dadagoboi]

I had one of mine die in '77 in Los Angeles.  It was fixed by a guy in the San Fernando Valley which at the time had a bunch of aerospace surplus junkyards.  He replaced the power capacitors with 2 monsters that almost completely filled the chassis.  Afterwards I could get sound out of it for 30 seconds after shutting it off.

When you mentioned "Mad Science" it brought him to mind.  Love to hear how yours works out.

[Pilgrim]

Yeah, those caps.........

One of my daughter's ex-boyfriends had a Dodge stretch cab PU with a massive sub that filled most of the back seat.  He had a cap the size of a coffee can on the power lead to that thing. Scared me to look at it.

[Denis]

MadScience Works is one of the few remaining local amp repair shops.  He went into this amp right after I got it and it was producing monster volume! Hopefully I'll get to run it by there this week or next week and drop it off. His backlog is usually around 3-4 weeks though. Will post a report when I get it back.

[Psycho Bass Guy]

His backlog is usually around 3-4 weeks though.

...that just means he's good. Trying to explain my backlog was the worst part of my old repair business.  It takes time to diagnose a problem, order parts and perform a repair. Intermittent problems can sometimes take weeks to run down even under the best of circumstances.

[Pilgrim]

Denis, if you have a major emergency, let me know and I'll loan you my Coliseum-300.  It will cost you $$ for shipping, but it can be done.

[Lightyear]

Yeah, those caps.........

One of my daughter's ex-boyfriends had a Dodge stretch cab PU with a massive sub that filled most of the back seat.  He had a cap the size of a coffee can on the power lead to that thing. Scared me to look at it.

We used to charge up small electrolytic caps and toss them into the halls while in lab at college - about like joy buzzer when you touched the leads - with the leads bent so that this had to happen. The things you found funny at 19...  

[Denis]

Denis, if you have a major emergency, let me know and I'll loan you my Coliseum-300.  It will cost you $$ for shipping, but it can be done.

Man, thanks a LOT for the offer! I'm okay though, so no need. When I get mine back out of the shop, if you ever need to borrow it I'd be more than happy to return the offer!

[Pilgrim]

De nada.  Ever since I got my Genz-Benz Shuttle 6.0 with a pair of the 12" shuttle speakers, it has been sitting gathering dust as a backup.  Breaks my heart to have such a great amp sitting around, but I haven't bit the bullet and sold it yet.

Until I actually played through that G-B setup I would have sworn that I'd never spend so much money on an amp and speakers...but two hours convinced me.  Turned around and sold a heavy amp and two heavy speaker cabs to help fund it...never regretted it for a moment.

[Denis]

Breaks my heart to have such a great amp sitting around, but I haven't bit the bullet and sold it yet.
Until I actually played through that G-B setup I would have sworn that I'd never spend so much money on an amp and speakers...but two hours convinced me.  Turned around and sold a heavy amp and two heavy speaker cabs to help fund it...never regretted it for a moment.

I know the feeling. Lately I've been looking at unloading stuff I don't use, and I keep coming back to the USA Ampeg 4x10 and the 1x15. They are nice, heavy and sound great, but I like the sound the Sunns produce more. Decision, decisions.

How much do the Coliseums weigh, anyhow? I've never bothered to weigh mine.

[Pilgrim]

Right about 38-40 pounds.