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Sunn Concert amp; burned up diodes?

B

boxnife

Aug 5, 2008
44
0
#1
In the spirit of getting all my gear either moved out or back in use, I present my ole Sunn.

At a show I turned it on, left the stage for a while, then came back and played the show. There was no bass on stage other than the monitors and I thought the amp was malfunctioning.

Turned out the speaker cable was unplugged. I played another show with the amp and it limped along, but crashed eventually, emitting a bad smell. I didn't use the amp again.

When opened I found these 4 blue diodes (I think?) all with burn marks under them. Any ideas on what it'll take to get it going again and if I can do it myself with basic soldering/schematic reading skills?

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#3
Are those big grey Lytic caps early Christian or Romano Byzantine boss?
I should whip them out quick along with their brethren and flog them at Sotheby's. So what's' the betting one of them fakirs has not liked the volts it built up under no load much, has gone short taking the diodes with it or at least given them a right old kicking.
You don't tend to damage output transistors much with too many or infinity ohms.
What do you have as a mains fuse, a .22 shell case?.
Test the diodes having replaced all the lytic caps they must be about
1976 vintage.
:ninja:
 
#7
It looks like there's a part ID number under every component. That number should correspond to the ID #'s on the schematic. If you're lucky, the values will be listed there as well. (Those look to be 2 Watt resistors.) It's not that uncommon to see "fuse" resistors in high power circuits, sort of a dedicated "blow up first and save everything else" component.
 
#8
No I don't, if anybody knows where I could get the schematics for this one that might help. I actually was going to get it repaired and there are no shops in town that wanted to look at it. I'm not scared to try and fix it myself though.
 
#14
boxnife said:
what would that entail?
Click to expand...

...for a tech with experience, not much outside of tracing the signal path and replacing bad parts. The hardest part will probably finding a set of transistors, but none of it should be too expensive. It's a $100 repair at worst case. I strongly recommend that you do NOT try to fix it yourself as you'll just end up doing more damage and frying the new parts. Resistors don't burn up like that for no reason. They are a symptom, not the problem.
 
#15
Psycho Bass Guy said:
...for a tech with experience, not much outside of tracing the signal path and replacing bad parts. The hardest part will probably finding a set of transistors, but none of it should be too expensive. It's a $100 repair at worst case. I strongly recommend that you do NOT try to fix it yourself as you'll just end up doing more damage and frying the new parts. Resistors don't burn up like that for no reason. They are a symptom, not the problem.
Click to expand...

I agree and like Bassmec said you could see about upgrading with superior caps etc. and wind up with a mod that yields much better tone!
 
#16
You are asking all the right questions. But, this one is beyond simple.

If you want to really get into the repair side, you need a mentor. Someone who will show you the ropes. The Internet isn't so efficient at that. My suggestion is to seek out an older ham radio operator. That culture even has a name for them - an Elmer. Some of those guys are the best board level artists on the planet, and they can't wait to pass on what they know.

It is also a group dying to find the next generation to carry the torch. Do not underestimate their level of motivation. Great folks!

de KF9VV 73's
 
#17
PBG's right on.

Mine let out the magic smoke one night at a gig. Opened it up a few days later at a friend's house (good electronics fellow) and plugged it in. Whoo-wee! Sparks aplenty, and a little smoke too.

I have a light touch, so this amp never worked for me. Gutless unless everything was dimed. Nevertheless, I was a bone head and took it in to the local goofballs (honest description). It was out for three months while they tried to find output transistors (or so they said). This was about 5-6 years ago, and the price was 120. Biggest waste of money I ever made; I finally sold it for $50 bucks and got a decent amp.

If you want to use it again, Mr. PBG is right: leave the repair to a professional, it's likely your power transistors are in heaven right now.
 

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