Sun Coliseum Problem

[veganxxx]

Hi,
I recently had my coliseum short out so I did a bunch of tests which lead me to a short somewhere in the output section where there is DC voltage being output on the speaker cable.

I found a shorted transistor and a zener diode and replaced both.  I also replaced the IC just for good measure.  I still get a semi bright light with the light bulb limiter when everything is hooked up and a dim one when the output section is taken out.  I've checked all the caps, resistors, diodes, and transistors and can't find anything shorted.

When plugging it into the wall it smoked R47.  I haven't touched the bias pot.  Any ideas? 

[tonyharker]

A circuit schematic would help with your problem.  Have you got one you can show us?

[veganxxx]

http://www.richbriere.com/images/The_Su3.gif

this is one that has all the same components but everything is labeled different in numbers, which makes it difficult to see what the value of r47 or any other resistor should be besides the obvious .33ohm ones.  This is the sunn coliseum schematic that comes up in searches, not sure why it's incorrect.

[J M Fahey]

If it's the same schematic, part values match but part ID numbers do not, open it in some graphic editor, relabel them with what is silkscreened on the board, add a line saying "this is veganxxx version" (to avoid confusions) and repost it here.
It won't take long.
Recheck 3 or 4 times before reposting.
EVEN BETTER: relabel your board with a fine point sharpie.
Also label parts which are *outside* the board, such as power transistors, filter caps, wirewound resistors, etc.

[veganxxx]

will do when i get some time.  There is one component on the schematic not on the board- D.C. Offset.

Also, I replaced all of the large power transistors when I first started this project.  I just went over the board again and found a zener to be shorted that wasn't before.  I then checked the large transistors and sure enough one was shorted creating a short over the board.  Not sure if that was the cause of the problem or something else that caused the transistor to short.

[J M Fahey]

1) if yours does not have the dc offset adjustment pot (so no 470 k resistor either), don't worry, it's an "extra" added later, the amp works without it.
2) some new parts shorted again, because still there were unresolved problems.
Search here and build a lamp bulb limiter and use it until your amp is good.
Also use the amp *without* the speaker connected, until pronounced good.
3) that Sunn is well worth repairing, a beast of an amp.
300 *real* watts.

[veganxxx]

I have been using the lightbulb limiter as I said before the light is dim when the output transformers are not hooked up but brightens when they are.  I also have a dummy load to hook it up to as well do I don't blow any unnecessary speakers.

[J M Fahey]

1) which output transformers?
2) no load connecter. Neither speaker nor dummy.
It's not to protect it but to be able to do useful measuring.
3) label your parts as suggested.
Good luck.

[veganxxx]

I replaced all the MJ15022G's (the large ones).  One MJ15028C (smaller transistor) was burned out and I replaced that.  One 1N4744A zener was shorted and I replaced that.  Then one of the new MJ15022G's burned out so I replaced it again. 

The only weird readings I get are two of the MJ15022G's test bad in circuit with the diode tester, but are fine when removed.  The other are two resistors that jump all over the place when read, not sure if it's because they are in circuit or not.  This is with the board pulled out of the rest of the amp. 

Unfortunately after looking over the diagram I have, it's too different from my amp to use.  There are 7 .33ohm resistors in the diagram and only 4 in my amp.  I can't match the circuit to tell what's what. 

[J M Fahey]

How many output transistors do you have?
There must be one .33 ohm resistor for each one, connected to its emitter, plus an extra one, in this case, for the short protection circuit.
Post a couple pictures.
Measuring in board can cheat you.

[veganxxx]

I have 6 transistors.  4 are linked to .33ohm resistors and two are not.  The other three 5 watt resistors are 150ohm, 150ohm, and 2ohm and none of the last two emitters are hooked directly to them.  Here are pics marking my .33ohms in red, my replacement parts in green, and the resistor I'm not sure of the value because it was smoked in yellow.  I didn't want to blow up this board (message board i mean) so here are the links-

http://farm8.staticflickr.com/7085/6992562562_18b3429d6d_b.jpg
http://farm9.staticflickr.com/8011/7138643515_431cb5b961_b.jpg
http://farm8.staticflickr.com/7190/6992561308_b8017cab8d_b.jpg
http://farm8.staticflickr.com/7239/7138639987_b865f8f6bc_b.jpg
http://farm8.staticflickr.com/7264/7138639955_685afb8fec_b.jpg

Also, thank you so far for the advice, I appreciate it!

[tonyharker]

The pics you posted do not appear to be the same as the diagram.  Can you please state which model Coliseum you have as there seem to be a number of different types?  The details should be on a plate on the back of the amp.  Also is it a head or a combo?

[veganxxx]

These are the only reference numbers I could find.  It is a head.

http://farm8.staticflickr.com/7229/6994496260_368ac6d460_b.jpg
http://farm9.staticflickr.com/8010/7140584605_fbb9339131_b.jpg
http://farm8.staticflickr.com/7257/6994494404_8bd3749cc3_b.jpg

On the front is says Sunn Coliseum 300 Bass Amplifier.

[J M Fahey]

Sorry, it's not the "classic" Coliseum Bass 300 but a newer version, much less known.
Is it this one?:
http://www.talkbass.com/forum/f15/new-toy-sunn-coliseum-300-a-545484/

Yours has 4 of the 6 transistors as "main" ones, that's why you have only 4  .33 ohm 7W resistors; and the other two (Darlington connected) are used as current boosters for the drivers.

In fact, I even suspect that all 6 TO3 transistors might be complementary: 3 NPN and 3 PNP.
Do you still have the original transistors?
Do they all have the exact same codes?
The "classic" Sunn does not have an IC in the Power Amp.

[veganxxx]

Well now it seems like I'm the dummy.  There were a few different part numbers/brands so I figured someone replaced a couple parts before and because of the schematic and previous solid states I fixed, I figured all the main transistors would be the same.  Obviously not.  In the one pic you can see the original 4 that all have the same part # except the one on the left that's slightly different.  In the other pic you can see the other brand next to an example of the new 6 I put in.

Now the question is where do I get the right schematic to get all the right parts in the right places?!  I searched for a while but did not find my model style on the web.  I can probably assume that the 4 matching transistors go to the .33ohm resistors and the two matching go to the other two spots on the board.  I still don't know the value of the resistor that originally burnt (though it still reads 14ohms, who knows if that's the original value)

http://farm8.staticflickr.com/7129/7141712319_49f09ab9f6_b.jpg
http://farm9.staticflickr.com/8154/7141712727_054eda261c_b.jpg

Yes those pics do look exactly like my head, so I guess it's the 80's model. 

What a nightmare, wish I had the right schemo from the beginning.