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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 25th Jun 2020, 5:14 pm   #1
G6Tanuki
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Default "I never got it to work", the story of a 15 PRC320

A few years back, at the Cobham/FRARS rally, I spotted a battered and looked-like-it-had-been-stored-in-a-chicken-shed PRC320 in a bin under one of the fleamarket-stalls. Casually enquiring of the seller he told me "I never got it to work". Now that either means he

[1]lost patience and gave up after a couple of hours of not-understanding-the-instructions,

[2]spent an entire weekend of soldering in replacement parts at random and most of the PCBs are missing tracks, or

[3]robbed it of most of the interchangeable modules so what's on-sale is an empty shell.

We talked. I bought it for 15. it could always provide knobs/meters etc for my other PRC320. Then "events" happened and it sat on a shelf for five years.

("Lockdown" time, and I was bored. In normal-times I'd have access to the entire test/instrumentation lab of my ex-company, but that's not accessible now, so this would be a "kitchen-table" refurb).

Job One: Clean off the crud. Going-over it with a paintbrush-dipped-in-white-spirit, followed by a gentle jet-wash, cleared off the grosser crud. [Thankfully the PRC320 is Royal-Marines-grade waterproof]. Then, after leaving it out in the sun for a day, I hooked it up to power/audio-gear/an antenna.

It was deaf. Really deaf. Wiggling a screwdriver into the antenna socket produced only a hint of a crackle. And even with the gain-control fully-to-the-right there was much less than the usual 'sharsh' background noise. But thankfully it didn't produce the PRC320 synthesizer wail-of-unlock on any band. There was hope!

Sticking a dummy-load on the antenna terminals and activating the PTT - lots of relay-chatter and strange audio-squeals! Interestingly, sometimes after this the 'sharsh' level would increase to what I'd expect from a healthy PRC320. More hope!

So I split the case [all those Allen-head setscrews...] Inside I found all the modules present (phew!) but module-5, the switched-mode inverter, looked to have been meddled-with. So out it came. Several of the TO5-cased transistors had been replaced, but - paradoxically - the 130V-rated tantalums (the usual failure here - they are expected to run at 125V) were original.

Having nothing better to do, I went *really* closely over the entire pair of boards with my magnifier - and - ["Hunting High and Low" - A-ha!] one of the original TO5-metal-can ICs had been replaced by a 6-pin DIL chip on a 'piggy-back' board. This looked rather professional, unlikely to have been done by an amateur. But between two of the leadouts was a tiny solder-bridge. I poked it and it broke away.

Next - a 'Kitchen Table' test-rig using resistors to simulate the expected transmit/receive loads on the different power-rails. Everything looked good! But before reassembling the module I swapped-out those troublesome 130V Tanties with modern 105C 160V-rated electrolytics.

Refitting Module-5, decent audio-level of 'sharsh' and no more relay-chatter-and-squeals when I hit transmit. But still deaf-as-a-post 90% of the time and 'odd' RF-power-out indications.

Cut-to-the-chase: the PRC320 has a two-BNC-connectors-on-a-metal-tube link between its RF I/O and its built-in ATU, the idea being that for manpack/mobile operations you use this to link to the internal ATU but if you want to connect a proper dipole antenna or use the 320 as a driver for a linear-amp in a base-station you unplug the link and have direct access to the RF I/O.
In my case the sliding 'trombone' section of the link was full of gruesome corrosion: disassembling it (spring out the plastic circlip) and cleaning using a fibreglass-tipped PCB-brush with lots of aerosol car-brake-cleaner followed by a squirt of Servisol - refitting it to the 320 gave me good receive and no odd transmit-shutdowns.

So, reassemble everything into the case [I only fitted four of the Allen-screws ftont and rear] and then an on-air test.

Yes, it's good-to-go! On 28MHz I was happily working stations in Austria, Germany and Poland.

Not bad for 15!

[Photo shows the 320 in my kitchen - yes this was a real 'kitchen worktop' rebuild!"]
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Last edited by G6Tanuki; 25th Jun 2020 at 5:31 pm.
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Old 25th Jun 2020, 8:23 pm   #2
turretslug
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Default Re: "I never got it to work", the story of a 15 PRC320

Great bit of fault-chasing there and a rewarding outcome- it's funny how things can be an ill wind sometimes, a stretch of un-hasselled time can work wonders for insight and clear-mindedness. I quite like broken things with someone's "I haven't got the time...." narrative behind it- it often translates as "I've been distracted by something else and haven't bothered with it" or "TBH, I haven't a clue but pride won't let me admit it, so I've left it well alone"- either way, it often means un-molested and a straightforward fix.

Fingers crossed it keeps ackling!
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Old 27th Jun 2020, 7:26 pm   #3
G6Tanuki
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Default Re: "I never got it to work", the story of a 15 PRC320

It was kinda methodical - the sort of follow-the-trail-even-if-it-does-not-seem-to-lead-anywhere stuff I tought my techs..

"Why did they do this?" was always a fun game.

The PRC320 in question continues to work well, though I still don't understand its "AM" behaviour.
Officially it's supposed to run a sort-of 'controlled carrier' AM, where the basic carrier-level is varied with a tine-constant of several seconds to follow the audio.

Talk, and the carrier-level rises quickly; stop-talking and the carrier drops-back to save battery-power [plenty of 1950s/1960s ham-radio gear used the same approach]

What seems odd on this particular PRC320 is that yes, talking on 'High Power' AM the carrier slides to follow the audio - but then when I stop talking it ramps-up the carrier to something like 50 Watts, before after a few seconds throttling it back to a more-sensible 'idling 10 Watts DC input'.

I'm not complaining though: working on 80/160M AM I'm regularly complemented on my audio-quality.
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Old 27th Jun 2020, 8:43 pm   #4
Andrew2
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Default Re: "I never got it to work", the story of a 15 PRC320

A courageous man indeed.
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Old 27th Jun 2020, 9:36 pm   #5
G6Tanuki
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Default Re: "I never got it to work", the story of a 15 PRC320

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A courageous man indeed.
In the days when I needed-to-work, I was the guy to who the 1st/2md/4rd/4th-level techs handed the "this problem is truly insoluble!" jobs.

And was happy to fold the problem back on them, pointing-out the signs and symptoms they'd missed, and - sometimes - after a walkthrough - offering them the opportunity to put their name rather than mine on the problem-resolution ticket.

"You've not failed to diagnose the problem, you've just not been provided with sufficient information or understanding".

I was always happy to see my techies go on to bigger and better things. And the juniors understood that if they approached me with a _real_ problem (rather than wanting me to fix things for them so they could slope-off early on a friday) I'd help them to understand their problem and guide them towards a solution in a way that made it a teaching-experience.

[only one exception to this: when I fired an entire software-development team because they refused to provide me with their code so I could do a line-by-line walkthrough. Part of mme thinks that by doing this I may have prevented a significant aviation-disaster]
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Old 1st Jul 2020, 12:30 pm   #6
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Default Re: "I never got it to work", the story of a 15 PRC320

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[only one exception to this: when I fired an entire software-development team because they refused to provide me with their code so I could do a line-by-line walkthrough. Part of mme thinks that by doing this I may have prevented a significant aviation-disaster]
Must have been military. In the civil aviation world they'd have been shredded by DO-178 MOPS for software certification, and its equivalents.

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Old 1st Jul 2020, 7:40 pm   #7
G6Tanuki
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Default Re: "I never got it to work", the story of a 15 PRC320

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Must have been military. In the civil aviation world they'd have been shredded by DO-178 MOPS for software certification, and its equivalents.
Not military or civil - but scientific-research. A bunch of postgrads are *really not qualified* to uniquely decide the power-levels/wavelengths of coherent light they can freely squirt-about inside/outside the airframe, whether it's sitting-on-the-slab or in-flight.

[NOAA didn't like me for what I did]
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