Sunn Coliseum 300 potential short?

[tonight, we ride]

Hey guys!

So a friend passed along an 80's solid state Sunn Coliseum 300 head to me that he claims has been blowing speaker cabs. Before it ended up in my hands he said that he took it too an amp tech that said it had a short but that he wouldn't work on it. I have some experience with doing moderate repairs on tube amplifiers but have yet to work on an amp that is entirely solid state and was hoping to get some input.

I built a light bulb limiter and when i turn the Sunn on the 100w light bulb i used momentarily is bright and then dims, but i have no reference as to what a functioning 300w solid state amp would look like in terms of the light bulb brightness. Would a 100w tube amp be comparable?

From looking around on the forum here it seems like a potential cause for blowing speaker cabs would be a shorted transistor in the last stages of the power supply that would cause DC voltage to be present across the rails of the speaker output. I checked most of the transistors (but honestly not all of them) for a short and could not find one. I also checked for DC voltage between ground and the speaker outputs and there was none.

Does anyone have any suggestions about what else I should check before passing this amp back to my friend? Are there any other problems that could cause a head to be consistently blowing speaker cabinets? Any input is appreciated! Thanks!

Highest quality schematic i could find is attached.

[gbono]

You might want to check the supply voltages - there are two transistor/zener based voltage regulators on the power amp board (+- 15V) for all the ICs. If you have a scope and a signal generator you can look at the output at different drive levels. The description of the problem does seem a bit vague and maybe the amp isn't the issue (bad speaker cable, etc).

[phatt]

I Agree with *gbono*,
300Watts at full tilt will blow the average guitar speaker Qbox setup.

A quick glance Looks like a DC servo setup to control FB (IC1 pwr amp schemo)

If that has issues then yes it could put DC on the speaker output and Yes with 300 Watts could easy blow even the toughest of speaker coils.

Wait till some of the clever chaps here read it they might be of more help.
Phil.

[Enzo]

It could be going to DC on the output intermittantly.  It also could be the owner simply overpowering speakers.

[tonight, we ride]

Thanks for the insight so far guys! Unfortunately I don't have access to an oscilloscope at the moment... really been needing to get my hands on one (especially since i already know the basics of how to use one). After my initial inspection of the thing I talked to my buddy that i'm fixing it for to find out if it's possible a speaker cable could have been what caused the damage to the speaker cabinet. Unfortunately he has no idea since he got the amp from his old bass player... both of us are hesitant to plug it into one of our own cabs until i'm absolutely certain that it's not going to do any further speaker damage.

I'll be sure to check out those zener diodes (won't be able to until the weekend since i currently work out of town during the week).

Is there a better way to go about checking for intermittant DC on the speaker out than just hooking up a DMM to the speaker output and ground and signal generator at the input?

Thanks for the help thus far, and if anyone has any further ideas/insight i'd really appreciate hearing it at this point!

[gbono]

Forgot about the DC servo amp on the PA board. Simple check of the rail voltages of IC1 on the output board would be a start - I think it might even be in a socket?

[tonight, we ride]

Now that's the kind of troubleshooting i hope for! I'll check the voltages on IC1 and those zener diodes when i get back into town tonight (I currently am working out of town during the week so haven't had access to the amp/my tools all week). Thanks again guys and i'll keep you posted on what I find!