Diagnosing a problem with my stereo equalizer unit: where to begin?
December 10, 2016 5:14 PM   Subscribe

I have a Technics Stereo Graphic Equalizer SH-8066 (circa 1985-1989), and the left channel is out. What is my first step in diagnosing the source of the issue?

The unit has been tested and the left channel is definitely out. We've pulled the circuit board. Where should we start looking, in our search to diagnose the source of the left channel not working?

I am seeking informed recommendations on how and where to begin the diagnostic process. This is a question about method. Just assume that I already know I could take it to a professional to have it diagnosed but that I'm too stubborn to do that. :-)

I have access to a multimeter, and my partner has knowledge of automotive electrical systems. We are vintage stereo enthusiasts but his knowledge of audio electrical systems is not as strong.

Photos of circuit board available upon request.
posted by nightrecordings to Technology (6 answers total)
The first step is to look at the electrolytic capacitors. Those are the stubby cylinders sticking up from the PCB. You will probably find one or more caps that are bulging slightly (usually at the top, but sometimes the sides), goo leaking out is also a possibility.

If you have bad caps, then look at their label to determine their capacity in units of Farads, and voltage capability. You can buy cheap replacements that will last a few thousand hours, or more expensive replacements that will last longer, your choice.

Before you desolder your capacitors, note the orientation of the polarity indicators. Putting one in backwards is the easiest way to destroy it. After that, desolder the old dead caps, and solder in the new ones.

Note that the capacitors may be in the power section, though it would be strange to have separate power supplies for the left and right side. If you need to work on the power supply, read up on safety, and lave the equalizer unplugged for several days to let any capacitors discharge. If in doubt, let a qualified technician do that. You can hurt yourself badly if you aren't careful.
posted by b1tr0t at 5:50 PM on December 10, 2016 [2 favorites]

[Just an update and then I'll quit threadsitting: The caps look good, but we'll take another look at them just in case.

We did find a temporary "workaround" - when we switched the output from the main output to the tape output, we were able to get both channels. However, we still want to find the root of the problem so it can be fixed.]
posted by nightrecordings at 5:58 PM on December 10, 2016

If the tape out works, then try to trace the connections back from the main outputs to where they split. There could be a bad contact, a cold/cracked solder joint, or bad op-amps in the way.
posted by b1tr0t at 6:18 PM on December 10, 2016 [3 favorites]

Or something in the switch itself.
posted by rhizome at 6:27 PM on December 10, 2016 [2 favorites]

Try wiggling the tape / normal out switch and see if you get output on the normal output dead channel. Switch failure with partial connections is common. If it's a rotary switch rotate it before and after the normal output setting. If it's a slider, try both the normal direction of movement and also gently wiggling perpendicular (if it's a left right switch try a gentle up and down).
posted by zippy at 7:18 PM on December 10, 2016 [1 favorite]

Bad caps in gear that age are certainly possible, and I've had them, but I wouldn't say they are probable.

Cleaning is an excellent start, use contact cleaner with a long hose thing (what do you call that stick a hole that goes in the nozzle?) and really wash out everything electrical that moves; switches, faders and pots.

Second point is to check the solder joints carefully with a good light. Dry joints are very possible. You can try flexing the PCB slightly with the unit in operation to see if the fault is intermittent, although be careful here.

Only then move onto component level testing.
posted by deadwax at 3:01 AM on December 11, 2016 [2 favorites]

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