STA-77A distortion on right channel, stumped.

[G.Wess]

Have an STA-77A that I'm stumped on. Right channel distortion, also seems heavy on Bass. Present on Tuner and Aux inputs. Mitigated when switched to MONO, but then distortion is shared with Left channel. Pin checked everything on the amp and pre-amp boards to compare components between channels. All caps, resistors and transistors seem to measure equal between left and right sides, including output transistors. Eliminated everything before speaker switch by connecting directly to the output pins on the amp board. Hope someone can point me to some assistance. Thing looks way too cool to not be in service.

Peace, GW

 

[cademan]

How about trying cleaning the pots and switches especially the tape monitor switch.

 

[rkgren1]

Cademan gives sound advice. Clean the switch gear with contact cleaner.

If the cleaning does not clear things up, then your receiver may need a general refurbishment, electrolytic caps and transistors.
Though I have not had an STA-77A, I have had two other Realistic receivers and I enjoyed them both thoroughly.
I still have an STA-90.
Your -77A is surely worth fixing.

If you need to delve deeper, then task #1 is finding the service manual.
The AK 'Digital Docs' database has a schematic for the STA-77L.
I do not know how closely that applies to the -77A.

 

[G.Wess]

Thanks for the quick input gentlemen,
Went through the pots and switches a couple times with contact cleaner and compressed air. Sound in general improved on both channels.
But you specify the tape switch, any particular reason?

Service manual would be great, and the -77L seems to be a bit different from my examination.

Have checked parts by comparison the in the left and right channels (all resistors, caps and transistor on pre-amp and amp boards) , believing an offending part would
have obviously different ohm readings or ESR than it's counterpart in the left channel. But all were very similar.
.
Read on another thread were transistor testing can be questionable. The guy finally replaced the output transistors and fixed his unit even though the transistors tested to be OK. Recently I had to replace 2 output transistors on a QTA-720, which makes me consider taking a shot like the other guy had done.

 

[cademan]

But you specify the tape switch, any particular reason?

These are most problematic when they get dirty that contribute to channel drop outs and fuzz and distortion in both channels.

One of my integrated amps still tends to act up when I am listening to it. It will start to lower in volume and sometimes sounds real distorted in one channel until I flip the switch up and down a few hundred times. It will then work for a long time without any problems.

It may not be your problem, but it is a common problem.

 

[G.Wess]

Cool tip for now and future, I'll check it out. Thanks!

 

[G.Wess]

Worked the push buttons over once more really good, Hard to get juice into them for lack of the typical hole you
find on a pot. No joy though.
Just found from the schematic that the tone control board appears to be separated for left and right channels.
Thought just pre- and amp boards were divided. Gives me another chance to compare components between channels.
Crossing fingers and praying, really want to hear this thing come to life on both channels. It's a beautiful unit..

 

[Bob]

try inputting through tape monitor - this bypasses the input switching and the phono
stage.

then try using aux input, use left channel input - then move balance control to right. then
use right channel input and move balance to left. removes/isolates, etc

is there preamp output AND power input? if so, isolate preamp/line stage from
power stages.

these will isolate the distortion to a section, if not, then you'll need to
get a schematic and follow the sound.

there are several methods to try, tap signal on offending channel and
bridge to other channel as you step through the schematic. this allows
channel comparisons. use isolated tip probes, power off to place, power
on to test, DO NOT DO THIS LIVE unless you know exactly what you're
doing. one slip and the entire unit is toast.

use freeze spray on active components to note changes. again isolates.

if you're not familiar or not comfortable with schematic tracing then
think about sending the unit out.

 

[quaddriver]

shooting rubber bands in the dark, but for grins, if you are up to it, consider replacing the bias trans's only....I have found with some amps designs, if one goes bad or starts to, it acts as a current sink for the other due to a shared connection (again: some) and that causes one channel to have half of the sine wave muffed, even tho the drivers and outputs check out

 

[illinoisteve]

Is plugging in headphones the only way to turn off the speakers on that model? If so, the speaker on-off switch is a component of the headphone jack. When the contacts for that switch are dirty, corroded, or damaged, it can make one (or both) channels distorted or it can even take out a channel. I've seen this with two different make receivers and repaired it by thoroughly cleaning the headphone jack internals.

Obviously, if you have a separate speaker on-off switching, it isn't a headphone jack problem, though it could be a problem with that other speaker switch.

 

[G.Wess]

Transistors are cheap, and I'm nearly ready to reach for the rubber bands.
Speaker switch does have headphones only setting, but can also be run simultaneously. Distortion is less noticable with headphones only, which I had dismissed. I'll go over that headphone plug once really good, can't hurt.
Might be the weekend before I can take it further.

Thanks, everyone, for the suggestions. It's a cool puzzle with an even cooler result.
GW

 

[quaddriver]

headphones are much higher impedance, so at 'dont blow my brains out' levels very little current flows thru the outputs and if enuf of the sine wave is there, you hear it ok. but note you said 'less noticable', which means still present at mv levels.

a signal gen and a scope will help you out here...need to see what the signal looks like after the inputs and VAS. if thats ok, then its 'remove outputs' time and get it clean just off the drivers (with no outputs, the drivers are also low current devices thru 100-200ohm emitter resistors)

 

[G.Wess]

Downloaded a signal generator app for my Android. Also figured out how to test output by tap the boards with leads running through the microphone input on another amp.
Now just need to figure out how it all works together. So far only able to find sound at pre-amp output using amp's tuner while powered up, but not at tone board or amp board outputs. Hoping for a short learning curve that doesn't involved breaking more stuff along the way

Speaking of that--Did find a very questionable transistor on the tone board, have removed it an get a replacement this week. Accidentally touched the tuning needle cord with soldering iron removing transistor. Think I can splice/patch it with another bit of cord and some super glue?

 

[G.Wess]

Kept after it and did get it fixed, thought I'd share for someone's future reference.
Came down to right side output transistors, oddly same as a Quad Realistic receiver I was playing with recently..
They tested fine using the ohm meter test and measuring almost exactly to the good ones on left channel. IT ROCKS!
Thanks for all the input. Love putting these puzzles together. Jammin' and learnin' and jammin' some more-