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May 17, 2022, 04:32:19 PM

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Sunn Coliseum 300 Issue

Started by Drummer4gc, June 09, 2018, 02:48:52 AM

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Hey folks, I'm having an issue with a Sunn Coliseum 300. Here's the schematic:

With a guitar plugged in, it gives no sound unless the strings are plucked hard - then it farts and fuzzes a bit. It sounds great through the pre-amp out jack.

I don't yet have a dummy load to use to do all of the test point voltage testing, so I'm trying to see if I can figure it out before jumping to that. I hooked up a signal to C1, assuming that's the power amp in point, and got the same result. However, I'm confused by a connection issue - in the schematic, it looks to me as though R302 in the preamp should connect to C1 in the power amp through the molex connector, but that's not actually what's happening. Instead, R302 is on pin 1 of the molex and connects to the TP6 junction. C1 doesn't appear to be connected to anything, as that spot in the molex header (pin 2) does not have a wire connected to it. Is this correct? Does anyone have a picture of the internals of this amp to compare?

Thank you!


First thing clean both Send and Return jacks and try again.


IF FX loop is ok The maybe inject a signal right in front of C1 and see if the power amp comes to life. use pre output as your signal as you already know that works.


How about when you just put signal into Power Amp In jack?


Effects send and return jacks are good - they come before the pre-amp out in the circuit, which works great. I've injected signal at the power amp in with the same farty results. However, I realize this also includes the CMOS limiter. To try to isolate that, I've injected signal at C1 - same results.

However, as I mentioned in my first post, the input end of C1 doesn't seem to be connected to anything other than an empty molex pin. That's why I was hoping someone would have a picture, so I can determine if there is an error with the board connections.


Some more info/clues if they're helpful:

-in trying to set the bias according to the service manual, I get .01mv across r18.  Doesn't change at all with trimpot adjustment. Bias transistor q13 checks good, as do q4 and q5 and c13.

-I read in another thread to lift a leg of each cr2 and cr8 to effectively remove a limiter part of the power amp circuit to see if that was at fault. Did that, which promptly blew the 10A fuse F3 on the power amp board.

Thanks for any insight!


Well, that may not have been the best time to disable those limiter transistors.
Maybe when you have eliminated other problems and running on a bulb limiter.
Anyway, they were doing their job protecting the output transistors from over-current (due to some fault) and disabling them allowed excess current to blow the fuse.  The output transistors may have been damaged, so they need to be rechecked.
Were you running with no load?


What *G1* says plus,

Obviously you have received this Amp with history unknown so likely someone has messed with it and might have miss wired it.
So even if you get the molex wired up correctly damage may still be present. :-X

Q, This might seem silly but is it possible that the molex plug has been connected backwards? Most have a key so it only goes one way but not ALL do. :o

As to
Q6 & Q15,,,CR2,3,9,8,& other bits, As *G1* mentioned they are for short circuit protection of the output (Not part of the limiter circuit,, so Don't mess with that until you establish a few basic tests first,, post readings and wait for one of the tecks to reply.

1/ What is the DC voltage at the speaker output? (TP19 I think)
2/ Report DC voltage at TP7 and TP6.
If the amp is stable then those 3 TP's above should be close to Zero VDC.

This is a rather complicated circuit for the novice and the schematic is not so clear about connections from main to preamp.

Best I can make out regards the Pins at pwramp input;

Pin 2, is obviously input
Pin 6&3, are circuit Common
Pin 4, is +15V rail
Pin 7, is -15V rail

Now I'm guessing;
Pin 5, is some kind of DC control voltage which I assume goes back to the limiter circuit on the other board but where is unclear???

Pin 1, might be a feedback control point???

Maybe a close up picture of that connector might shed light on how everything is connected.

Oh yeah Lightbulb limiter build is here;
You will need that 8)


Thanks all - I've got a bulb limiter built and 3 new power transistors on the way (two were fried, one had an out of spec hfe). I also tested all the 5w resistors in circuit, most of which were out of spec, so new parts for that on the way as well.

Once I replace them, I'll post pics of the connectors, voltages at the test points requested, etc. Thanks again!

J M Fahey

Just one detail: low value (0.33 ohm and such) resistors do not "go out of spec", they are either fine or burnt open.
But regular meters have big trouble reading such low values.

So if a 0.33 ohm one shows , say, 0.6 or 0.8 ohm ... trust the printed value rather than the meter  :o

meaning: donĀ“t replace them unless actually open.

Worst case is that you may replace an "out of spec "  one ... and later find "the new one is also out of spec" ... which it is not.


I sort of wondered if there was a meter impedance issue...the .33s were reading somewhere around 1.5, and I've already got new ones on the way so I guess I'll know soon if they read the same.


J M Fahey

problem isn that meter does not read "just"  the resistor but also quite a few stacked resistors which are in series: 2 probes , 2 wires, 2 banana plugs, 2 or more rotary switch contacts inside the meter; all easily add up to 0.4 to 1.5 ohm, specially when corrosion starts to appear, simply because of damp air.

Normal is to short probes to each other, it will NOT show Zero, note that value and then substract it from resistor measurement.
So if you have , say, 0.6 ohm "parasitic" and resistor measures "1 ohm" , it actually measures 1-0.6=0.4 ohm .

Also notice that at best you get the first digit right, say 0.3 or 0.4 or whatever; FORGET distinguishing 0.33 from 0.39 for example. 

So, how do factories check production?

There are other ways, simply too expensive for the average Hobbyist or even regular Tech:
* impedance bridge, this is the one I use, can easily measure 0.005 ohms

or a "4 wire meter" which uses 2 wires to feed current to the resistor and 2 separate ones to measure.
Since there is no current passing through the "measuring only" ones, error is avoided.

But for simple "Go/No Go" everyday testing, normal meters and error substraction are enough.


I hate coming across repair threads with no closure or report back, so for anyone coming across this in the future:

Q15 was shorted, which looks like it was killing the bias. The "power amp in" wire in the molex connector was in the wrong spot, as suspected. Some of the outputs were blown, but I don't believe this was the original issue, I think I caused this when I removed those diodes to test.

It's up and running and sounds killer. Thanks for the help folks!

Jazz P Bass