Realistic STA-860 - Another with Low Audio on Left Channel

Brivan

Well-Known Member
I could use some help on this one. I've seen other posts on here about low volume coming from the left channel of a Realistic STA-860, but no given solutions for the problem. Using an audio tracer and a voltmeter, I've narrowed the problem down to one specific spot, but every component I've checked / replaced has turned out to be fine. Voltages check okay. I'm comparing my readings to the schematic as well as to the right channel, which works okay. From the schematic drawing, I get weak sound from both sides of C507 in the left channel, but weak sound from one side of C508 and loud sound from the other side of C508 in the right channel. These caps are 5pf and, although I haven't replaced them, ohm out okay. There's other transistors, diodes and resistors involved in this circuit, but all have tested okay and I've replaced TR505 and TR507 in the left channel without any difference. I'm stumped on this one. I can post a close-up of the amplifier circuit so you can see what I'm working on, if desired. Any help would be appreciated!
 
In most cases the switches and selectors could use a good Deoxit treatment.
This is the first line of attack.
Yes - the issue can be more sinister but you have to eliminate the obvious.
The signal is only 250mV's and can easily jam up through one of the pesky function switches. And remember, the signal runs through every switch and selector.
I'll troubleshoot by eliminatiation. This set can't be separated easily but I'll desolder the signal input from the preamp to the main and jumper to the opposite amp. This will isolate the bad section to a pre or main amp.
I would not consider any transistors, diodes or even caps as problems. Resistors - yes, but you have to narrow it down to a circuit first.
Look for cold solder connections, best to reflow everything you can. It's easy and not as time consuming as guys think.
I'm freshening up an STA-2100 for a member. These sets are the bomb! Well built and the circuitry is topnotch.
 
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Zebulon1, thanks for the reply. I'm kind of past the point of the pots - everything in the preamp checks out okay - I have good sound all the way through. It's right around C507 where I start to get a dramatic difference in signal strength between the left and right channels. As stated, there are several other components involved at this point which could be suspect but, as of yet, I haven't identified anything. Yes, these are decent receivers and, at the time they were on sale, didn't get a big edge in the market. As has been stated, these are finally getting some admiration - myself included. A schematic is attached in case anyone wants to look at where I'm at.
 

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That's a nice looking receiver. Googled pic below.
upload_2017-10-31_21-6-43.jpeg

I have had 2 Realistic receivers and really liked both.
I still have one.

Definitely worth time and effort to fix.
I bet it will be a gem.

I like the reflow circuit elimination advice that Zebulon1 offered.
 
rkgren1, thanks! I didn't realize analog string-tuner receivers were being made this late in the game. Yes, I Zebulon1's advice will be helpful (thank you)! I did try cross-wiring the amp sections to no avail, but I did it without actually isolating them. I'll see which component is the easiest to pull to achieve isolation, and will cross them over - we'll see what happens.
 
Thanks for posting this issue. I’m really interested in how it turns out, since I have the very same problem with the STA-860, but I am a total novice when it comes to the circuitry work. I love the receiver and would hate to have to move on from it.
 
MarkTN - I did get the receiver repaired. Without having it pulled apart in front of me, I don't recall which capacitor was bad - either C507 or C509. That gave me good audio from all inputs except FM. The left channel again was weak when FM was selected. Another bad capacitor here - #227 this time. After getting it back together, I've noticed the tuner strength LEDs will partially illuminate, then gradually back off while the unit is on. That will be next (and hopefully last) on my fix-it list. In the mean time, I love this receiver! I had no idea this smaller-sized unit was such a great-sounding little powerhouse! I was originally driving it through an equalizer, but it turns out (for me, anyway) that an EQ isn't needed - this little butt-kicker sounds great without one! While I had it apart, I replaced the lamps with warm white LEDs - the receiver displays look great with those in place.

Let me know if you want to dive into repairing yours - they come apart easily enough and the components are accessible without having to remove anything internally. I have schematics if you want a copy of them.
 
What a great receiver the STA-860 is! Bought mine new in 85 when Radio Shack was having a sale on it. It retailed for $359.99 but the sale was $199.99..I remember it was kinda like a last of the Mahicans with the illuminated full dial as all the receivers then were all digital but I wanted the old school! Bought a set of Mach 2's as well to go with it!
 
atvmax99, I'm officially jealous of your setup! I'd eventually like to have an all-Realistic system, but my silly self bought another STA-860 off Ebay for the time-being. Interestingly, this one had a slight reduction in volume in the left channel, so I immediately replaced C507 and C509. Afterward, I had normal-volume sound from the left channel. I don't know what it is about those receivers with only the left side being problematic, but it's a simple-enough fix so I'm still happy with it. Next on my list is probably a LAB-series turntable - something with a wood finish in the LAB-250 - LAB-400 range.
 
atvmax99, I'm officially jealous of your setup! I'd eventually like to have an all-Realistic system, but my silly self bought another STA-860 off Ebay for the time-being. Interestingly, this one had a slight reduction in volume in the left channel, so I immediately replaced C507 and C509. Afterward, I had normal-volume sound from the left channel. I don't know what it is about those receivers with only the left side being problematic, but it's a simple-enough fix so I'm still happy with it. Next on my list is probably a LAB-series turntable - something with a wood finish in the LAB-250 - LAB-400 range.
Brivan..congrats on not only scoring a second 860 but getting it up and running as well! It's going to be nature of the beast with old electronics that the electrolytic caps are going to dry out and go bad. So far for the exception of my "Stereo" led not lighting when listening to the radio mine has been holding up but no doubt the led is going to be related to a bad cap I'm sure. If I get the gumption I'll end up signing out a Huntron from work and go over all the caps and replace a good bit I'm sure. Other than that the only other thing I had to do was replace the bridge rectifier in it in the early nineties, fortunately the power supply dept I worked in at the time had an almost exact replacement in stock with just slightly higher current capabilities. Enjoy ur 860's, at 65wpc 20 to 20khz @0.03thd it's quite a great receiver!
 
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