Studio PA woes -

[Brian Comnes]

I have a 4 channel Studio PA, the 4 channel solid state one (there's a picture on this site at http://www.sunnamplifiers.com/studiopa/studiopas.jpg .  This thing is making intermittent crackling sounds (every three or four seconds) combined with variable whining sort of like an  old tube radio dialing in a station.

The problem is definiitely in the power amp section, since I plug an amp into the preamp out plug and there is no crackling from that point in the amp. The crackle is also independent of master volume level, its the same crackle on 0 or 10.  

 I swapped the 223055 power transistors and replaced a couple of 1000MF caps that were obvioulsy leaking, but still no luck.  

Does anyone have a schematic? I checked with Fender, no dice, the guy on eBAY selling Sunn schematics doesn't have it either.  As of now I am marching through the amp replacing transistors and caps, but a schematic would speed that up cause I could tell where in the circuit I was.

there are two socketed 1b407 transistors near the preamp in and a TIP30B that is soldered in. I swapped the positions of the 1B407 and that changes nothing.  I'm getting ready to desolder the tip30b and replace it. Does anyone know which of these comes into play earlier in the circuit?

Any help or ideas on where this problem might be would be apppreciated.  

So why do we have these  irrational attachments to old gear??  My two local amp techs said they wouldn;t touch it for less than $200.  Heck its only worth $75 but its a great tool for my daughter who is playing acoustic with a mic and learning to sing on PA gear.

Thanks in advance

[Bill Matthews]

The problem you are having is a cold solder joint.. I know what you are thinking, I looked all over, and everything looks fine.. Well go over it again.. Beleive it or not you're going to have to look over it REAL close to see it..  Get youself a jewelers loop, that will get you in close enough.  If you're still frustrated, get in touch me, I'll talk you thru it.. Good Luck

[Brian Comnes]

Bill Thanks for the update, I really do appreciate the offer

but  I got it fixed. here's how ......I swapped out the two BC107 transistors, they were socketed and were easy then I desoldered the TIP30B and installed a new one of those as well as a new 680MF cap. total cost about $8......(my local electronics store charges 2-3 times Mouser's price but there's no minimum and no shipping)  I also learned that solder wick is a great way to desolder a PC board, no comparison to that old solder sucker.

Plugged it in yesterday and so far after 4 hours of run time no crackles or extra noises  I don't know if there was a loose joint that got fixed in this part swap or not, my current guess is that the TIP30B was the culprit.

Anyway I called my amp techs and told them thanks for rrfusing to fix this thing cause I saved at least $75-$100 over what they would have charged and  also learned a thing or two about solid state......I'm a tube head for guitar but think that SS is OK for PA.  

Another Sunn units gets a new lease on life.  

My daughter is now happy too!  Happy New Year