For some time ive been meaning to fault find an issue with my Clansman PRC320 HF manpack radio. This set has a slow return back to receive after transmitting, a delay that seems to increase the longer the pressel is held (i.e. longer in Tx mode), up to a maximum of about 2 seconds. This doesnt seem much, and indeed under proper R/T procedure in a military net would not be noticeable, as proper R/T procedure includes time between key up and speech to allow for stabilization of the equipment. But for amateur use, this delay is a nightmare! It means I miss the start of callsigns, and in contests or fast paced contacts such as SOTA - its possible to miss the entire transmission!
So, how to tackle fault finding such a problem? What can cause a transceiver to be slow returning to receive?
Several possibilities come to mind -
A. Sticking relays
B. Failing capacitors
C. Failing power supply
D. AGC faults
E. Semiconductor breakdown
Anyone who has ever been inside a PRC320 will know just what a maze they are! (if youve never seen inside one - piccies to follow!). They are a modular system, which on the face of it sounds easy to work with, but the modules are soldered in and bolted down - they are not plug-in units like on other Clansman equipments. Essentially, the radio breaks down into six sections, each consisting essentially of sealed or otherwise very awkward modules.
Part of unit 2, the rear section, is the reflectometer module 2b. As well as sensing the RF levels at the antenna, this also provides a master relay, which switches the antenna between Tx and Rx paths, and also 24V out to a pair of slave relays on other modules.
The whole rear section can be removed, this comprises the audio inputs, bandpass filtering, PA, and reflectometer sections. Because of this, it was the best place to start, as I could simply swap unit 2 with the same from my 'test-bed' radio, thus proving or eliminating the master relay...
So, I swapped the rear sections between the radios - and the fault stayed put. So not the master relay then!
The next, and in fact last, part that could easily be checked was module 5, the PSU. In the test-bed radio, this is an original unit that has had its horrid 1970s tantalum capacitors replaced with modern electrolytic units. In the faulty radio, it is a complete modern rebuild using DC-DC converter modules. But, this module at least connects with a plug and socket!
So, I swapped module 5... and the fault stayed put.
This leaves me with two relays, the slaves, that might be at faults. One of these is in a module hidden away within the tuning turret, so I decided to start with the other, which is located on unit 6, the motherboard - the only directly accessible circuit board in the whole radio!
Unit 6 consists of a PCB with various discrete components, upon which is mounted and soldered about five screened modules - each soldered in and bolted on. It can be raised into a 'service' position, to access the modules, but in this position the radio cannot be operated as unit 2 cannot be connected! It is also awkward to do this as the LSB mod gets in the way!
So I decided to meter out the relay and see if the change in voltages from the contacts matches the delay, indicating a sticking relay. I had already found just from the sound of the relays clicking that this relay seemed ok, but on metering the Rx 6v contact, discovered that the response times of my meters isnt fast enough to outrun the delay, and so the test proved inconclusive.
Im at a bit of a loss now. Unless I can locate a fault with the relay on unit 6, then im into the realm of module swapping! A rather daunting task! A last ditch method I can try is to tack-solder an LED and series resistor to the 6v Rx line from the relay. The LEDs response time should be near instantaneous, and so if the relay is at fault it should show.
If that doesnt work, then since I will have to start wielding the soldering iron, I may as well just swap the relay, and its associated components, out anyway, in exchange with those of the test-bed radio.
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