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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 28th Jul 2020, 2:08 pm   #1
G6Tanuki
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Default Another PRC320 fixed!

On-air, I worked someone who was also using a PRC320 and whose SSB was really wobbly - something I've had to fix on these radios on a few occasions.

I got to look at it in detail - and, as expected, the problem was a flaky 'wet' Tantalum capacitor - 'C8', a 1.7uF in the stabiliser circuit.

If this is suffering from partial breakdown, it feeds noise on to the wiper of the 'set 110V' potentiometer which then gets fed to the synthesizer via the 'stabilised' 110V line. Looking at it with a meter typically shows the 110V line wandering up and down by a few hundred millivolts.

So, out with the tiny Tantalum and in with a yellow polyester 1.5uF 250V replacement. Which is much larger than the tantalum, so I fitted flexible 'tails' and covered it with heatshrink before popping the other ends of the tails into the PCB holes, soldering, and reassembling. It's *just* possible to tuck the new capacitor into a space in the PSU and still get the lid on!

Result: 110V line is rock-solid and there's no more wandery-SSB-signals. Owner of the 320 in question showed his approval in the form of a bottle of whisky.

Looking at the little capacitor, you can actually see signs of the contents having 'oozed' around the epoxy bung that makes up the positive end - see below.
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Old 28th Jul 2020, 2:19 pm   #2
HamishBoxer
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Default Re: Another PRC320 fixed!

Good repair. Pretty nasty those tants at times, as I knew when I was full time servicing.
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Old 28th Jul 2020, 3:23 pm   #3
G6Tanuki
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Default Re: Another PRC320 fixed!

Yes, Tantalum caps seem to have a really poor service profile: these little metal cased ones and the blue or green resin-dipped 'teardrop' ones both fail with monotonous regularity once they are a couple of decades old. I remember lots of failures of them in 1980s X.25 networking equipment.

I understand their need for use a few decades ago - lots of capacitance and low ESR in a small space - but things have moved on and modern aluminium-electrolytics work just as well. In this case I used the polyester ones (RS 185-4218) because I had a tray of them.
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