'Test Bed' PRC-320

Well, after some previous disappointments, I finally managed to get a second Clansman PRC-320 radio, for use as the development 'Test Bed' for the external VFO controller. Ended up paying a bit more than I really wanted though, but heyho,

In many ways im quite pleased with this one, all its front fins are good! and it has its original tuning chart. The battery clamps are a bit loose, as is the antenna socket, but other than that its only defects are the rear fin protecting the coax link broken off and a fair bit of scuffed paint...

... or so I thought!

Sold as 'tested and working' a few minutes of testing soon showed that this may be the case NOW, but it wont be for much longer, unless I step in very quickly with some repair work. It soon became apparent that the synthesiser was not locking correctly, and after about ten minutes would lose lock completely! Of course, I know what this unready tone means - the high voltage capacitors!

These capacitors provide the 110v line for the VFOs, and are metal cased tantalum electrolytics, well known for their poor end of life performance. Two of them are in the Synthesiser module 9, which is the oblong can on the right of the picture below (the one with all the circles on it), and two are in the PSU module 5, the big grey can front center.

Module 5 was found also to have a dent in the can lid! Luckily this just popped out with a bit of applied pressure. I have modern 160v radial electrolytics on order to replace these tantalum units, which should be with me by the end of the week.

At the top left of the radio and along the top of the cans, can be seen the pink wiring of the frequency decade switches. I will need to electrically disconnect these from all the synthesiser input pins, and the 3v supply to the switches, to prevent data contention. This is not going to be an easy task! And somewhere I also have to fit the microcontroller! The only space available is pretty much down the center line, between the IF filters and module 5, and the tuner turret!

So, Im now a bit miffed as to how to provide feedback to the seller.  On the one hand, although working, as described, the unit has a clear fault condition starting to cause an issue, something that the seller should have noticed if the unit was indeed tested shortly prior to sale, which by the last years date on the tested sticker seems unlikely. But then, on the other hand, its a fault I can nip in the bud now, and the rest of the radio, and indeed the case, is better than I expected. I dont think it really deserves positive feedback, but then I dont think it really deserves neutral either!

Another aspect of the controller development for this radio will be obtaining serial comms to the external unit without compromising the seal integrity of the radio. For this, it is proposed to sacrifice one of the two audio ports. To see just what a squeeze that will be, take a look below

Thats the audio accessory board, in its nice tight little space in the rear panel beside the PA. Theres an aluminium panel goes over the top of this as well.

The final picture is just a view of the underside of the radio. There really is very little space inside these for any additional circuitry, so fitting in a microcontroller capable of reliably taking over the synthesiser control, is almost certain to require some clever SMD work!