Realistic SA-2000 with Bizarre Left Channel Distortion

W.T. Holt

Active Member
Hi folks, strange problem here.

I'm working on bringing this receiver back to life. When I got it there were a bunch of physically broken and burned resistors on the left side of the power amp. I replaced those and the output transistors and biased the amp while I was at it. When I powered it on for a test the left channel still had a nasty distortion in it. Since I already had it torn apart I figured I would go ahead and recap it too - so that has been don as well. Didn't change the problem though. Next I cleaned all the pots and switches with deoxit - still no change.

Now here's the weird part. I tested the unit through headphones, and when no speaker set is selected it sounds fine through cans. If a speaker is not connected to the left side and I listen through headphones it sounds fine as well, regardless of whether or not I have a speaker selection switch engaged. But as soon as signal is sent to a left side speaker the sound gets really grungy, both in the speaker and my headphones. It seems that having a load on the left channel is what is causing this distortion to occur.

I think I have also eliminated the pre-amp as a cause of the issues by sending the left channel from the pre-amp to the right side of the power amp. No distortion in the right side under those conditions.

This one is starting to test my diagnostic ability, so it would be a huge help if any of you all could give me a hand figuring out what to look at next. Happy to answer any additional questions that might shed more light on the issue.

Thanks!
 
Power amplifier front end / VAS component break down. Cold solder connection or solder bridges. Cracked PCB trace.
 
Thanks!

I feel I'm going crazy now though. Took a break from the Realistic and plugged in a Kenwood KR-3090... lo and behold it has the EXACT same issue. Tried both receivers running on power from a different outlet and then with a different set of speakers - still no change.

With my scope I can feed a 1 kHz signal through the amps and watch the distortion take place as I switch the speakers on and off. At this point I can only assume its an issue with either a transistor or a resistor on the power amp since I replaced all the caps on the power amp of the Realistic when I original encountered this problem. Please correct me if that is wrong thinking.
 
The same problem with 2 separate amps? Could it be that one of the speakers you are using is somehow shorted causing the damage to the amps? Or a shorted speaker cable?
 
Yep done that... 3 different pairs of speakers all doing the same thing.

I agree that it's really odd to see the exact same problem in two different amps, especially back to back like this. I've done everything to isolate the components and the fault really does seem to be in the amps themselves though.
 
With my scope I can feed a 1 kHz signal through the amps and watch the distortion take place as I switch the speakers on and off.

Try tracing signal with speakers engaged upstream in cicuit until you see no distortion.
 
Wonder is the distortion only on the injected signal, normal sources, or both? Or are the amps picking up a induced signal in the power amp side of the amp? One thing are the grounds/ ground connections internally good? Heard on somebody's amp had a interference pick up problem due to a bad ground connection from a board to the chassis here on AK.
 
Thanks for the help everyone.

I ended up finding the cause of the problem... I started switching transistors from the left side to the right side of the power amp until the problem switched sides as well. Turns out one of the small TO-92 transistors had gone bad.

I have since replaced it and the receiver is working great! now on to the Kenwood...
 
Hello,
Yes, I know this is an old thread however I have the same exact left channel distortion symptoms as W.T. Holt. I am an absolute no nothing rookie on all this so bear with me.

W.T. Holt stated he found it was the TO-92 transistor causing the issue. My question is as follows. I pulled up the service manual on HiFi Engine and zero transistors are labeled as "TO-92". They are all labeled TR ---.

Kindly direct me to the board location of these transistors and if you have it, a part number. I usually order thru Mouser.

Thanks,
Dennis
 
Hey Dennis,

TO-92 is a designation for the case style of the transistors, not a part number in a schematic or a model number of a particular transistor. KSC1845’s are a very common example of a TO-92 transistor.

I dont recall which specific transistor in this unit was causing my problem and I don’t have it any longer to go and check but if I were you I’d be looking at the small signal transistors in the power amp section.
 
Thanks W.T. though what you wrote went right over my old non electrical head.

I'll do research on what you relayed and see if I can sort it out.

Dennis
 
I just skimmed the service manual real quick. based on my experience with this amp TR602 through TR607 are the ones I'd be checking first.
 
Back
Top Bottom