 The BEAST |
 Working on the Switch mode regulator |
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The BEAST |
Working on the Switch mode regulator |
 The faulty relay contacts |
 The loose bolt that caused a lot of problems |
| This is my Clansman PRC-320, in a moment of complete
madness I bought this transceiver as "Untested but complete" experience over
the years has taught me that "untested" means "Not Working" but the
guy selling it seemed honest and he did say "for display purposes" so I took an expensive gamble, and sure enough
it was definitely NOT working and in a bad way, "no fool like an old fool" came to mind, In reality
its was a collection of dud boards with some parts missing, But after the initial
disappointment I started doing a few tests to see how bad it really was, and
made a few enquiries about obtaining circuit info, I soon found info on this
set is hard to find, mainly I suppose as its still in use by HM forces, but
like all things Radio Amateur I soon found a guardian Angel who not only
knew the set but had modified one AND could supply some circuitry!!! Armed with some circuits a few measurements and golden advise it soon become clear that something very strange was happening, the set seemed to be half in transmit mode and half in receive mode, after cutting tracks and isolating boards I finally isolated the problem to one of the TR relay contacts being stuck on transmit, Without doubt this set is one of the most difficult and over complex sets of this vintage to work on, getting to the relay was incredibly difficult and then I found that a replacement relay was no longer available so in the time honoured Amateur way I decided to try a temporary repair by grinding off one side of the relay casing to expose the contacts, sure enough one of the contacts was welded across BOTH poles, I separated the contacts using a pin and cleaned them as best I could refitted it and hopefully switched on, NOW I had some noise in the speaker, but no signals, further tests showed I had an IF of 1.75mhz that I could receive and it seemed to be filtered ok, it was becoming obvious that there was no locked local oscillator, more test showed the 3v line leading to the Synth was down to 2v this effectively stopped the synth and the synth out of lock tone, another big problem, the voltage regulators are switch mode in these sets and not only hard to get at but difficult to repair, tests showed that the main series switched regulator collector was open circuit and the 3v was being fed via the driver load, changing this transistor corrected the problem and the synth locked up the VCO and AT LAST I had a receiver!!! After playing with and evaluating the receiver I pressed the transmit button Nothing :( but the supply current increased, after a few checks I found the set was actually transmitting a carrier on CW of a few milliwatts And here is a big problem, it is impossible to work on the main transmitter section with the two parts of the transceiver pushed together for it to work, you are left with two alternatives, you either make up a set of D plug jumper leads which is ok if you are repairing loads of sets, or you have to completely dismantle the transmitter section, this involves removing the 7 pin accessory section (taking care with the flexi leads) the antenna sockets and the transmitter tuning block and the transmitter PCB, my PCB took a bit of pulling to get it out as the PA transistors are stuck in the back which is the heat sink, though there is a cord there to help, finally I was able to plug it all in all hanging loose, first visual inspection and I noticed a bolt must have come loose in the sets history and fell into the PA shorting the main 24v supply to the heat sink, welding itself into place, at the same time blowing the printed circuit and presumably the relay contacts! whoops of joy! its very nice to find a rock solid fault after a lot of work :() with trembling hands I removed the bolt and repaired the blown track, keyed the TX on CW and got a good 20watt carrier out hurrah!!!! but no SSB :(( Boo, I quickly hooked some audio into the mic socket and on SSB got a good carrier (taking care to only transmit for a few seconds as the PA had no heat sink) so must be the mic in the handset, sure enough corroded contacts on the clip on sockets, taking my time I reassembled all the PA section again and finally decided to test it on air with just the two halves pushed together, two CW CQs on 7.030 mhz on low power about 3watts and DJ5GN came straight back giving a 589 report, so I had made it, a no hope job into a working set, there are still problems, sometimes the tx develops a nasty chirp when first keyed and their is still a problem with mechanical linking from the main turret to the rear tx tuner, these are odd faults that need to be sorted but only to be expected after the treatment this set must have had in its lifetime, So in the end these sets can be repaired, I still think they are horribly over engineered and with all the plugs and sockets not that reliable, I suppose the trade off with plugs and sockets is quick field repairs, ok if you have the spares bad news if not.. Anyway time for a cup of Tea :() Postscript 9/1/08 I had lots of contacts with this little transceiver on 40mtrs CW all over Europe, To be honest I'm only really interested in QRP CW operation usually on 40mtrs, I found the PRC-320 easy to use if a little limited, the battery fully charged lasts quite a while on LP CW so not really worth making a PSU for it, but it didn't have much nostalgia value like working old valve / tube rigs, it was a fun rig though, I can't say it was really for me, a little to modern for my tastes, I don't really like tuning with switches, but another interesting radio project completed :()
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