Realistic STA-860 Revival

June 21, 2024

The 1970s and 1980s were a magical time for HiFi stereo systems.  Living rooms were packed with racks of shiny aluminum faced gear pumping out room filling sound from radios, turntables and tape decks.  A friend had a Realistic STA-860 stereo receiver which he had owned for over thirty years and it was only outputting on one channel.  It was time to grab a service manual and crack it open and see if it could be brought back into order.

Once it was opened up I could see that it had stored every bit of dust it had seen in it's decades of use, so it would definitely need a cleanout to start with.  A visual look around the board didn't immediately show any component issues like bulging capacitors or things which were obviously burnt up.  I noticed that the controls potentiometers were inaccessible for cleaning without tearing down the whole front of the unit, which was too bad as I was hoping to be minimally invasive.

Before moving ahead any further it seemed like a good idea to test the unit to ensure I had a baseline of its current operation.  With it plugged into an isolation transformer and dim bulb tester for safety I brought it up slowly and it powered up without any smoke and the dim bulb stayed a low glow.  I wasn't able to push much power through the unit with the dim bulb tester in line because I only had a 100W bulb available.  So once the initial test for shorts was done I removed the dim bulb and just used the isolation transformer.  I was able to confirm that the right channel was absent.

I began using the Operational Check section of the service manual to confirm the transformer outputs which appeared to be within reasonable limits.  Next I injected a 1Khz sine wave using a signal generator to see if I could find where the signal for the right channel was dropping out.  It seemed like it was in the tone controls section the signal was getting attenuated, possibly the treble, but there was also unreliability through the op amps prior to the tone control.  In fact the more I tested around the more things seemed to be erratic and nothing was making any sense.

Since there was definitely something wrong in the tone controls section it seemed reasonable to work through the process of cleaning all of the controls and retesting to see if anything changed.  I'm a fan of DeOxit for cleaning controls, and I think you'll find many different viewpoints on the right product to use.  After tearing down the entire front of the unit and removing the dial string (what a pain) I finally had access to the controls and I was able to flush them out using the DeOxit I had on hand.  Retesting the unit showed some improvement, certainly the controls were no longer scratchy, but the channel was still out and the erratic behavior was still there.

Although the visual check didn't see any bulging capacitors it seemed like the next logical thing to check, as well as just taking a closer look around the board.  Testing capacitors in circuit is far from foolproof, but using an ESR meter I thought I would go through and check what I could, noting any strange readings that may require out of circuit testing.  In addition I checked diodes and resistors along the way.  The number of capacitors which seemed to have aged out and greatly increased in resistance was.... well... significant.  Honestly based upon some of the readings it was amazing that the thing was working at all.  Since this was a system my friend was hoping to use for quite a while longer I suggested that it might be worthwhile to recap the whole unit, he agreed and I ordered the parts along with some replacement dial lamps.

It turned out that the filter capacitors which I had ordered were not a perfect fit as the original 3-leg type were not available in the needed rating.  Also the holes in the board were spaced slightly further apart than the parts I had.  I was able to manipulate the legs a little, and then use the third hole in the board for each capacitor which was more closely spaced and originally not connected.  Using a jumper wire from the 'structural' hole I was able to get the replacements installed.

After replacing all of the capacitors and double checking polarity, I brought the stereo back up using the dim-bulb tester and confirmed no shorts.  Twisting the input switch to FM, I tuned in to a local station and it came in clear and distortion free.  But most importantly both channels were working!  So it's not always the capacitors.  But it's the capacitors.

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