Sunn Studio PA troubleshooting

[hijdra]

Hi, I recently acquired the solid state Sunn Studio PA (seen here http://sunn.ampage.org/site/museum/s.../studiopas.jpg) and am having some troubles with it. It powers up and gives off a very loud 60 hz cycle hum. None of the pots effect the volume of the hum and no signal goes through over the hum. Does anyone have a schematic for this amp (or one similar), or can anyone point me in the right direction of what to start checking?

Appreciate any help,
hijdra

[Enzo]

I have no schematic, but the first thing to do is see if the amp is putting DC voltage on the speaker. If ti is, disconnect the speaker immediately and don;t connect it until the amp no longer makes DC.

Looking at other Sunn products of the era, I am assuming the powr amp uses a split power supply, so there should be two large filter caps, one for either polarity. One terminal of each cap will be ground, and the other end will be hot. DO both have about the same DC voltage, ignoring polarity? Now flip your meter to AC volts and remeasure, this is ripple we measure. If either one has more than a fraction of a volt AC, then we may have a bad filter cap.

Is there a small transformer on the power amp board?

There will be some large power transistors. We would want to check them for shorted condition.

[loudthud]

The Studio PA (same as Studio Lead and Studio Bass) uses a single rail 47V power supply and capacitor coupled output. Drive from a PNP TIP30B mounted on the PCB is transformer coupled to a pair of 2N3055 output transistors. The main filter cap is a 3400uF at 75V that you arent likely to find a replacement that will fit. Use something between 3300uF and 4700uF at 60V or more and improvise a new clamp. The attached schematic is the Studio Bass version. The power supply and power amp are basically the same as the Studio PA.

[Enzo]

I stand corrected, thanks. I can't seem to open the schematic file. I'll log in after dinner and try again.

[hijdra]

Appreciate the help, yes, the main filter cap is definitely bad, I'll have to figure something out for that. I wasn't able to get the schematic open either, though hopefully after replacing the capacitor I won't need it. Thanks again, and I'll let you know how that works out.

[loudthud]

I was able to open the attachment yesterday. But today I just get a blacked out page, a message in the lower left corner of the browser that says /!\ Error on Page, and the only way out seems to be the back button. I'll email tboy.


Edit:
I was able to view the attachments in tboy's post but it was hanging on the one in my post. Now if I hold down the {Ctrl} key when I click on the thumbnail, I get the image in a new tab (I'm using IE-8) and it seems to work on all three. It's kind of a big image, 4328 x 2374.

[tboy]

I'm uploading another copy of the file as a test.

[tboy]

Hmm... same result. Here's another try:

[tboy]

...and the same result again. I don't know what the problem is, but I'll be looking into it. In the meantime, the attachment can be viewed if you right-click (command-click for Mac) on it and select "open link in new window."

[J M Fahey]

My vetust Firefox 2 running on W98 opened it automaically in a new window, with no problems at all.
Since it was either BIG *or* unreadable, I downloaded and opened it in IrfanView with no problems either, and scaling it at will.
By the way, thanks for posting.
PS: what did they claim the power output was?
Single 47V and driving 8 ohms looks too low for serious stage use.
I guess no more than 20/25W here.
Those 3055 are also biased quite "hot", in both meanings of the word.
A thermal-runaway waiting to happen.