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Realistic STA-2080 Distortion & Low Volume

Brivan

Brivan

Well-Known Member
I have a gorgeous Realistic STA-2080 that doesn't feel too good. Left channel has low volume level and the right channel is distorted. I injected a 100Hz tone into the aux input of the receiver and noted clipping/distortion of the negative peak of the sinewave when probed at the right speaker terminal (I tested both A and B to rule out a flaky relay). The sinewave was significantly smaller (but looked good) when measured on the left A/B terminals.

When the signal was measured at the preamp output, the distortion in the right channel was gone, but the left channel volume was still reduced. I measured DC voltage with volume turned down in both left and right channels at the speaker terminals - both sides were within +/-5mV of 0v.

So, it looks like I have left channel preamp issues (weak cap, I'm guessing) and right channel amp issues. Are there any known/common problem areas I should start looking in? I've re-seated the plug-in connectors on the amp boards. It would be helpful to start in a known problem area so I don't go blind wading through that schematic.

Thanks,

Brian.
 
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One of the output transistors in the right channel is open on one leg. From another Audiokarma post,
TR509B, which is a 2SD745, cross-references to a 2SC3858
TR510B, which is a 2SB705, cross-references to a 2SA1494
Digikey has the 2SA1494 in stock, but the 2SC3858 is obsolete and out of stock.

I ended up ordering two new, original 2SD745s and two, original 2SB705s from an Ebay seller.

The left channel now tests fine volume-wise. It's possible I didn't have the balance control centered.
 
Brivan said:
The left channel now tests fine volume-wise. It's possible I didn't have the balance control centered.
Click to expand...

Don't think so, more likely the new transistors are conducting and balancing within the range of gain. You have a scope so I'm sure you can run awhile and monitor.
 
Binkman said:
Don't think so, more likely the new transistors are conducting and balancing within the range of gain. You have a scope so I'm sure you can run awhile and monitor.
Click to expand...
I probably should have been a little more clear: I just ordered the new transistors today, so I was monitoring output with the original transistors still in place. I know I had less signal from the left channel at the preamp output last week but, since I got more or less the same from both channels today, I chalked it up to operator error. Unless there's something funky going on with the receiver. I'll get down to the bottom of it after the new transistors get installed. Will either be late this week or early next week. I'm guessing it'll be a good idea to set idle/offset/balance in the amps after the new outputs get installed.
 
Well? Go blind? I don't think so... ::D but refer tot he SM and check on basic voltages channel to channel.. Good news is you have 2 ch. audio so new transistors won't hurt but ebay sellers?? you better test them before you put them in but under power? hmm caps also needed perhaps .. not a cheap hobby... :D
 
Binkman said:
Well? Go blind? I don't think so... ::D but refer tot he SM and check on basic voltages channel to channel.. Good news is you have 2 ch. audio so new transistors won't hurt but ebay sellers?? you better test them before you put them in but under power? hmm caps also needed perhaps .. not a cheap hobby... :D
Click to expand...
Transistors were ordered from Semitron - they seem to have a pretty good reputation according to a few AK posts I've read. Good feedback rating on Ebay, as well. I was all set to order them cheaper through Digikey but, since they were out of one kind, I grudgingly paid more for a full set from Ebay. Yup, it can be an expensive hobby, but I remind myself of the money I saved by not having somebody else do it. Plus, my continuing receiver-repairing education is included in that cost. In another forum, there's a recent 14-page education of what to do when a Pioneer SX-980 protection relay starts cycling.
 
I installed the new output transistors about a week ago. Removing the old ones from the PC board isn't a job for the faint-of-heart. Having the traces on the same side of the PC board as the component makes it extremely difficult to remove the old transistor without lifting them (which I did on one). The original plan was to save the three good original transistors for spares, but when the trace started pulling off the board during removal of the first transistor, I abandoned that plan. I ended up cutting the legs off the transistors with a cut-off wheel mounted on a Dremel tool. This worked great. I was able to easily remove what was left of the legs and had no further issues with the traces.
 
You need(ed) a flush cutter. Same you would use for cutting off soldered caps leads. ie.exelite flat surface blade etc. and prices vary.
Without being obtuse.. ?? you spent too much time on solder iron and thus lifted the trace(s). Cutting off the leads is a better method and I can heat the joint immediately pull the scrap and save the trace side... You need to be like a duck and get out fast for soldering." :D
 
Binkman said:
You need(ed) a flush cutter. Same you would use for cutting off soldered caps leads. ie.exelite flat surface blade etc. and prices vary.
Without being obtuse.. ?? you spent too much time on solder iron and thus lifted the trace(s). Cutting off the leads is a better method and I can heat the joint immediately pull the scrap and save the trace side... You need to be like a duck and get out fast for soldering." :D
Click to expand...
I've got several pairs of flush-cut tools, but my concern was the legs of the transistor would tend to twist when I cut them, and I didn't want to damage the trace if/when that happened. Yes, I definitely spent too much time with the soldering iron on that one trace - I was having a helluva time getting all three legs freed up enough to let the transistor go. I usually don't have to use solder wick, but I tired it in this case, which put more heat on the trace. If I run into this situation again, I'll give the flush-cutters a try.

Unrelated to anything, but I just completed building my retro-style dim bulb tester. Made from an old decade box. Here's some pics:
Dim Bulb Tester (Small).jpg Dim Bulb Tester - Two Bulbs On (Small).jpg Dim Bulb Tester - Three Bulbs On (Small).jpg
 
Pretty!
Solder wick is passe but good to wipe a stubborn trace.. get a solder sucker plunger type sucker. Stubborn solder joint(s) need a dab with Q-tip w/ little flux for fast flow.
bink
 
Binkman said:
Pretty!
Solder wick is passe but good to wipe a stubborn trace.. get a solder sucker plunger type sucker. Stubborn solder joint(s) need a dab with Q-tip w/ little flux for fast flow.
bink
Click to expand...
I do have a solder sucker, which does a pretty good job (although the tip is about due for replacement). Thanks for the idea of adding flux - hadn't thought of that! In the past, I've added extra solder, which can help.
 
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