Friday, 8 April 2016

Fiddly Capacitors

After working my way through the circuit diagram of the PRC-320's PSU module, and reading an article from VMARS about how the PSU works, I started to wonder if the excessive start-up time might be down to one of the electrolytics in the 6v regulator, which also provides the square wave pulses feeding the switch-mode inverter for the 110v rail. There are four tantalum electrolytic capacitors in this section. Two seem to be simple smoothing caps, but two 4.7uF caps are connected directly with the mysterious ML1 I.C., and one of these also connects to a resistor and diode connected to the pulse transformer. Perhaps one of these sets the soft start delay, and is leaky?

Removing the module from the radio, and opening it up on the bench, required the careful desoldering of two of the interconnecting wires. But with it open on the bench, I could see the locations of the suspect capacitors.

One of the big 330uF tantalums in the 3v regulator looked worrying! A careful probe revealed this amber sludge to be most likely a blob of solder rosin, rather than electrolyte. Actually, im sure tantalums have a solid electrolyte anyway?

I found some 4u7 63v devices out of my stock, one of which was an axial type. Ooh how lucky I thought! C3 had to have a radial device which meant adding a bit of insulating tubing.

This was all well and good until I tried to put the module back together, and found that the axial device was too thick, and pressed against one of the transformers, preventing the two halve mating properly! Ive had to replace it with another radial unit, and a bit of inventive routing of the negative lead!

Tomorrow, I will test the module whilst it is open on the bench to make sure the levels are still good and see if the 110v rail rises at a sensible speed now.

Im also hoping to test the GSA and the PRC-352 tomorrow. Thanks to Mark, I now have the proper coax cable to complete the set-up.

The Baofeng BF-888S radios arrived today. All I need now is the programming cable.

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