An on-going tale of two radios, Part 2

By Gordon Bubb G7KNS

The first job is to find where the HV belt is coming from. It happens as the audio gear is plugged in so I opened up the case to look for trapped wires. None were seen, so out with the voltmeter and… look! 175 volts on the headphones! This is not right! In this set there are two audio output transformers, one for the radio and one for the intercom*. Both are floating with respect to ground but do have a common connection. I soon discovered that the intercom output transformer was leaking the 175v HT on the primary side through to the secondary. So, disconnect both secondary wires and all is now good. Very fortunate that it was the intercom, which I can live without, and not the radio side which was at fault.

Looking at the ex-Kuwait set’s power supply I noticed all the spare fuse caps were missing, as was one of the in-use fuse caps. The other two in-use fuse caps were the wrong type. Hmm. Oh look, what should be 1 ¼” glass fuses are in fact mains plug fuses with bits of wire stuffed under them to make them fit. Our friend Mr. Bodger has been here. The in-use fuse with the missing cap should be a tiny wee thing – see the picture with a standard 20mm fuse for comparison. These wee fuses crop up in a lot of Clansman equipment and I have never found a source of them. Chris G6HTH tells me the Post Office used to use them as well.

So, replace the mains fuses with correct ones and power up. I knew the receive power would not work because of the missing fuse but the transmit supply vibrator hums away nicely. So far so good. Fit a polyfuse in place of the missing mini-fuse and power up again. Not so good, the polyfuse trips straight away. Maybe it is the receive vibrator stuck, not an unknown thing to happen with elderly vibrators. The transmit and receive vibrators are the same so swap them over and now there is receive power but not transmit. Open up the vibrator and yes, two stuck contacts. Clean them up with wet and dry (it is nice and thin for slipping between narrow contact gaps) and all is now well, two working vibrators.

This picture shows the inside of the vibrator showing some of the contacts. Note that the case is brass, not the more usual aluminium.

* Being designed primarily for armoured vehicles, there is an intercom system built into the radio so that the crew members can communicate with each other.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s