Author Topic: new guy with a "new" 80's coliseum 300  (Read 4562 times)

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Offline foxfire

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new guy with a "new" 80's coliseum 300
« on: September 23, 2009, 08:17:11 pm »
i just picked it up yesterday, it's the one with the red sunn logo and 7 preamp eq's. anyway i don't think it's 100%. firstly the fan is dead and the chassis around the power amp gets pretty hot. way past "warm to the touch." i'll be replacing it tomorrow. secondly and more importantly the volume drops occasionally.

i'm testing it with my guitar. running it in to an older peavey 4x12 bass cab loaded with madison speakers. i'm not sure how efficient the cab/speakers are and i'm still getting used to the preamp(s) and all but there are times when it's plenty loud and then there are times where there is no way it's putting out anything close to 300 watts. not that i'm sure that i know what 300 watts sounds like. i know what my sound city 120 sounds like but that's tube...

so my questions are, given that the fan is dead should the chassis be getting that hot? and i know this is a big one, what should i look at/for when it comes to fixing the seemingly random/intermittent volume drop? there aren't any obviously burnt parts in the power amp and non of the caps "look" bad...

thanks in advance for any help you can offer, Rylan

Offline foxfire

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Re: new guy with a "new" 80's coliseum 300
« Reply #1 on: September 24, 2009, 06:57:29 pm »
so i installed a new fan which is keeping things cool, but it ain't fixing the volume loss problem. it seems that a swift tapping/pounding will bring it back up to volume so i'm thinking it must be a bad solder joint or wire? so let the hunt begin...

Offline loudthud

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Re: new guy with a "new" 80's coliseum 300
« Reply #2 on: September 24, 2009, 07:08:06 pm »
Check any of those ceramic box type resistors. They typically fail open when output transistors short.

Offline foxfire

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Re: new guy with a "new" 80's coliseum 300
« Reply #3 on: September 24, 2009, 07:34:03 pm »
don't suppose you'd feel like going into a bit more detail? this is the first solid state amp that i've had trouble with.

Offline foxfire

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Re: new guy with a "new" 80's coliseum 300
« Reply #4 on: September 24, 2009, 09:14:21 pm »
is the bi amp frequency control supposed to affect the sound if i'm not using the bi amp send/return function? i just read over the manual for this thing and i got the impression that it controls the mix going to the sends/returns not the signal going to the power amp?

Offline loudthud

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Re: new guy with a "new" 80's coliseum 300
« Reply #5 on: September 25, 2009, 11:11:19 am »

From the schematic, the signal path for the Coliseum 300 goes something like this. The input signal goes thru a gain control and is optionally routed thru the compressor then the Contour switch. Next in line is the multi band equalizer. Next comes the Effects Send/Return. Next is the Master Volume control. There is a Preamp Out jack that can get it's signal 'Pre master', 'Post master' or 'Post xover'. I'm not sure how the selection is accomplished, it looks like a jumper on the circuit board from the schematic.

From the Effects Return, the signal is routed thru the Master Volume and then to the Crossover circuit. The Frequency control affects both the high and low outputs simultaneously. The Hi and Lo outputs of the crossover each have Effects Loop Send/Return jacks. Next there is a circuit that combines the high and low signals from the Hi and Lo Return jacks. This signal is routed to the Power Amp Input jack where it dead ends if a plug is inserted into that jack.

The last little bit of circuitry is the CMOS signal shaper and then the signal goes to the power amp.

don't suppose you'd feel like going into a bit more detail? this is the first solid state amp that i've had trouble with.

You really shouldn't be messing around inside the amp if you don't have the training and experience necessary to safely do what repairs are necessary. Serious harm or death could result.

Offline foxfire

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Re: new guy with a "new" 80's coliseum 300
« Reply #6 on: September 25, 2009, 02:30:07 pm »
You really shouldn't be messing around inside the amp if you don't have the training and experience necessary to safely do what repairs are necessary. Serious harm or death could result.

how true, i had a theory that what was happening was that the low half of the signal was cutting in and out so while the amp was acting up i plugged a cord into the hi send jack and that cut all the signal. that seemed to confirm that there was something wrong with the low signal. so i pulled out the preamp board to look for cold joints/burnt parts and guess what? now i get no signal at all. the power amp is fine but i either made the problem worse or created a whole new problem. it goes in to the shop on monday...thanks for your help though. live and learn and all of that.

Offline loudthud

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Re: new guy with a "new" 80's coliseum 300
« Reply #7 on: September 25, 2009, 02:51:41 pm »
One thing that comes to mind is those Send and Return jacks have contacts inside them that can get intermittent. If you can spray any type contact cleaner (Deoxit is popular) in those, it may solve the problem.

Offline foxfire

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Re: new guy with a "new" 80's coliseum 300
« Reply #8 on: October 09, 2009, 09:52:48 pm »
it's finally fixed. long story short(ish) my amp guy charged me $112 which i thought was fair considering it wasn't passing any signal when i took it in. well within an hour or so of testing it it was clear that the original problem was still there. i promptly called my amp guy and set up to take it back to him and  since i never learn from the past pulled the chassis out and went to poking about. this time instead of making things worse i found the offending solder joint and now it's fixed.

anyway it sounds great. i wasn't planning on keeping it but it's really growing on me. even through my 8Ω cab it's loud as all get out. i may have to quick wire up a second 4x12 just for giggles and i'm looking forward to playing with effects in the bi-amp circuit. again thanks for any advise you gave me. even if it wasn't the problem this time there's always the next problem...