Pioneer SX-780 right channel hiss and strange meter output signal

DirtyDog

New Member
I just got a Pioneer SX-780. It sounds pretty good but there is an issue with the right channel.

As long as I have speakers hooked up to it and select the appropriate A or B selector the meters seem to function properly and both channels output audio properly. However, as soon as I release either the A or B switch so no speakers are enabled, the right channel immediately jumps up to about half way and stays there. After checking, I can hear a very high pitched hiss on the right channel both through the headphones and when listening to the right speaker. It is pretty low and hardly noticeable but the meter pegged halfway when no output device is selected is annoying, although when the speakers are connected the right channel meter drops back to normal and seems to function properly.

Any ideas what could be causing this?
 
Probably a flaky transistor or cap in the preamp, or phono sections. You don't say what function it's in when the 780 hisses. Just phono, Aux, Fm, Am or most or all?? If you have some shorting plugs (rca plug shorted together) install a pair in each section (one at a time) turn on and test. If it goes away in the phono function, start there. If it's FM, AM, AUX, TApe Monitor, it'll be after the phono preamp (which for the most part is a SIP IC). As the 780 is 41 to 42 years old, it's way past it's service life for the capacitors and there are some known problems with the POWER Regulator transistors, etc. I would suggest if you are going to keep it for a while, to overhaul it (recap, re-transistor the regulators, add heatsinks to them, and possibly do the POWER AMP Replacement MOD as outlined here...... http://audiokarma.org/forums/index.php?threads/stk-0050-replacement-for-sx-780-and-others.721181/

There are tons of 780 repair threads on here. take about 10 of them and study them. Parts lists are standardized with BOM for Mouser. all parts are equal or in 99% of the cases exceed specifications. Once done the 780 will surprise you. It'll run cooler, be reliable as a 57' Chevy, and you'll be able to push it hard. And it'll sound better than it does now.

Larry
 
As part of any service, cleaning the switches and controls would be a good start. In this instance, I think it would be imperative. Output wattage with no speaker load could indicate a driver or bias problem (not adjustable on the 780) and bears some investigation.
 
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