Tuesday, 24 December 2013

A (Christmas) Magic Smoke Event . . .

I have recently been giving one of my "old ladies" in the guise of my 1982 vintage FT-101ZD Mk3 a "run out", promoting it to "main station rig" status for a few days despite the fact that I am aware that it is still in far from perfect working order.

It's basically a nice set, so that was no major sacrifice.

However, after the radio had been on for a few hours I suddenly started to get a distinct smell of "something" from it. I've smelt very many electronic faults in my time, but this one seemed rather to smell of a pan on the stove burning dry, but I quickly realised that there was no pan on the stove in the kitchen, and that the smell was indeed coming from my '101.

I removed the power and found a flashlight so I could investigate the cause, and spotted a little plume of smoke emanating from the left hand rear of the rig, and I also noticed that this corner of the rig seemed to be rather warmer than normal.

Straight away I disconnected all the external cabling, and, having allowed a reasonable time for the internal high tension power supply to discharge, I removed the top cover thinking that the sooner I could get at the innards, the more likely I was to identify the source of the problem.

Thus I homed in on the "Rectifier B" board and quickly spotted a rather distressed looking electrolytic capacitor which also seemed to be unusually warm to the touch.

Rectifier B Board in situ - PB-1968B

Now it may be recalled that this radio, bought originally as a “spares or repair” item from EBay, has had a chequered history having done questionable service as a high powered and illegal CB rig. It had no doubt been abused, quite possibly by someone less educated in the finer arts of the tuning and conservation of valve/hybrid rigs, this abuse being most keenly felt by the power supply stages. I had already rebuilt the “Rectifier A” board which had expired completely, and it now appears that I should have paid rather more attention to the “Rectifier B” board, which supplies screen and grid bias supplies to the output stages.

My own fault and hopefully any collateral damage will be minimal.   Looking on the bright side, as one of my “Twitterati” so nicely put it, at least it was “Christmas Magic Smoke”!

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