Sunday, 29 May 2016

A good days radio

After a short walk and buying a few books, then picking mother up this morning, and doing a bit of gardening, I found I had time to get a bit of radio work done. Also, the replacement CIARL knob and decals for the RT-351 arrived in the post,

So, I cracked on with replacing the decals. The old original ones peeled off with the aid of a pair of pliers, but the glue underneath took rather a lot of scraping to remove. The spaces left then also had to be painted over. I had decals for the RT-351, all but the rear audio connector plate, which was in good enough condition anyway, the 20W amp, and a few spare MOD Record stickers.

One of the MOD Record ones went onto the LiPo battery pack. This got its first full charge today, in balance mode, and at relatively low current. Either way, I was taking no chances with it

But the 7A 12v PSU and the Turnigy Accucel-8 charger coped just fine. Whilst this was charging, I had the first go at modding the RT-351 for equal mic gain in both Whisper and Loud modes.

After studying the service manual, I came to the conclusion that the easiest way to do this would be to link the 0vW line to ground at the mode switch. That mod can be seen below, the purple wire.

Now, this did indeed work, and as a proof of concept was fine. The trouble was that by grounding this line, it put the extra 20dB mic amp on permanently. This is fine in Whisper mode, but in Loud mode there is awful feedback on the handset due to the mic picking up the sidetone! Of course I could have disabled the sidetone, but I kind of like it, it keeps the set authentic.

But it did prove that the mic gain and hence the Tx deviation level could be made the same between modes. So it was obvious that to do this without invoking the 20dB amp, would simply need the contact on the switch disconnecting.

As can be seen in the above photo, ive done this by desoldering the pad, insulating the pin, and then paring back the track a little. This was actually rather awkward, and required removing the switch from the case AND from the felxi-PCB! I would suggest to anyone else doing this mod to just use a scalpel and cut the track! After all, it can easily be reconnected with a linking wire if needed.

But, I now have an RT-351 where the mic gain and Tx deviation are the same and Whisper and Loud modes are now just volume control! One difficulty though is that this radio does not go into limit on over deviation like many PMR sets, so you can still over or under drive it, depending on how loud you talk, and so setting the deviation isnt easy. Ive done it on the Marconi 2955 with the old 'ooowww-lahhh' trick, to about 3-4kHz, but on air reports will be needed to see if further adjustment is required.

During all this, I also removed the SO-239 antenna connector from the roof of my car, which has yet again gone intermittent and to be honest is knackered, and replaced it with a 3/8" thread bulkhead connector. This means no need anymore for failure prone adapters either. The installation isnt yet usable, as I still need to put a right angle PL-259 plug on the coax and connect the antenna mount to the radio. Im hoping to get that done tomorrow.

And so it was back to the RT-351 and 20W amp, and the stickers. The paint now being dry. Im not sure if these were meant at some time to have been self adhesive, if so they dried out long ago (these spares are probably almost as old as the radios, and some of those are knocking on 35 years!), but some UHU impact adhesive soon sorted that out. Front and back views of the newly labeled radio are below, with the 4W SURF in place and the LiPo battery.

And below, the completely refurbished PRC-352 system, comprising Handset, 1.2m whip antenna, 4W SURF (Selective Unit Radio Frequency), RT-351 4W VHF FM transceiver, 20W amplifier, 24v 4Ahr NiCd battery, Adapter plate and GS carrier frame. The 10m select toggle switch can be seen on the left where the dessicator plug used to be.

I also found time to convert a DCCU charging cable for the 24v NiCds into an Accucel-8 charging cable for the 24v NiCds! This makes charging them much more convenient, and as the Accucel-8 can do discharge/charge cycling for battery conditioning, should help keep them in good order.

And now, late at night, im just waiting for the last battery to finish charging before I turn in, and have got some way to converting a cheap D-type RS232 to TTL converter into a programming lead for the Yaesu VX-2000.

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