RaeIstic STA-90 DC Offset

drdave3 · February 4

I just got a Realistic STA-90. I had one when I was younger, sold it and regretted it. Now that I have another one, I discovered that when I put it on A&B speakers, one of the woofers burned up. I checked the DC offset voltage. On A speakers, it is normal. When I go to A plus B or B or quad it can jump up to 19 volts! In other words, the offset is fine on A speakers, but when I switch from there it jumps way up. Any solutions?

msg · February 4

Nice. I have an STA-84 that I like a lot, though, unfortunately, nothing to offer on diagnosis or repair.

These guys might, though.

In alphabetical order...
@ALL212
@mhardy6647
@SCompRacer
@SeleniumFalcon

Gents? (applies to all but Ern; he's a scallywag)

mhardy6647 · February 4

That's puzzling.
A+B should do no more, and no less, than parallel the power amp's outputs. I cannot grok offhand how that could unbalance the power amplifier. Of course, if that receiver has a quad synthesizer ("Quatravox", in R/S-speak) setting, I guess something could be awry there. Those "ambience extractors", though, are passive -- they use a simple wiring trick to send out of phase signals to the rear speakers.

You might want to ask over @ audiokarma, @drdave3 -- of course, some of them will just say "recap it!".

Hint -- that ain't likely to help!

source: https://www.radioshackcatalogs.com/flipbook/1977_radioshack_catalog.html

Gardenstater · February 4

Main amplifier alignment instructions and schematics are in the Service Manual ("service info") you can download here:

https://www.hifiengine.com/manual_library/realistic/sta-90.shtml

SeleniumFalcon · February 4

The schematic for the output stage looks pretty normal, the output transistor pairs are fed plus and minus 39 volts, so that wouldn't account for the plus 19 volts at the output. @mhardy6647 is correct the speaker selectors seems to just connect the left and right outputs to two sets of speakers either separately or in parallel. Interestingly the headphone connection is fed directly from the output stage. What would needed to be done would be to trace backwards, finding the 19 volts and seeing where in the circuit it's being introduced. Either a transistor has shorted out, or a capacitor is leaking DC would be my guess.

drdave3 · February 4

Thank you very much. It just seems so odd that that voltage only appears when the B speakers are switched on.

Gardenstater · February 4

I'm thinking must be something wrong with conductivity of A position, ie. maybe switch needs cleaning.

RaeIstic STA-90 DC Offset

Answers