A Salutary Warning and Disaster Escape

[boater sam]

I have no love of AC/DC radio sets, particularly later ones. I find them to be cheaply made and poorly constructed.

But I have a few in the pile of "to be fixed one day" so this week I have been restoring a GEC BC402 to working order in between fixing boat things and travelling around.
It is not a thing of beauty, the chassis is soldered together tin plate, the cabinet dull and blistered bakelite. but I thought I would give it a chance.


Fired it up on the lamp, rectifier is way below par, no output on any waveband.
Reformed the triple electrolytic, fresh UY85 and we have a working output stage. Swapping the UABC80 gets MW and LW working reasonably.
Replaced the 3 moldseal caps and a waxy, the FM discriminator and output cathode electrolytics and it now worked on all 3 bands.

Check the valve voltages, output UL84 is drawing too much current, the cathode resistor had gone low, replace that and box it all up, fit the back and put it on soak test listening to the test match ( not very encouraging ) whilst I tidied up.

Got called away to outside for a natter. Should never leave a set on its own. 

10 minutes later see smoke coming from the workshop door. Enter to find the set gaily burning, still playing, on the bench. There is little immediately combustible around it and the CO2 extinguisher near the door quickly kills the flames and the sounds.
Total loss of the set only fortunately.

Probable cause? The failed again electrolytics overloading the rectifier, melting the plastic tuning slide till it dropped onto the rectifier and output valves.

Lesson learnt.

[ppppenguin]

The late and much missed Gerry Wells said that early GEC colour TVs were the only sets he'd seen that would catch fire AFTER you'd turned them off. Not sure about his explanation, Gerry was always one for a good story.

[BusyBee]

I am not sure about that, but I do remember someone runnig a (I think) GEC colour set (early model) with the back off. We were in an adjoining room and on hearing a bang went through to see it was snowing lol. If I remember the capacitor was in the line output stage somewhere, but, as it was a while ago, I don't remember much.

I am glad you caught it before it caused a real problem.

Tracy

[Old Sparky]

I once had a similar experience with an early 50s HMV radio. Fortunately it didn't catch fire, and it proved I should have checked before leaving it unattended. Someone had fitted the wrong output valve which was drawing too much current. That was may fault for not checking, but generall I do not try to reform 50/60 year old electrolytics, simply replace them!

[Kalee20]

Yes - though I generally reckon that AC/DC sets are slightly less likely to catch fire than AC only types.

I have had a couple of mains transformers fail and start overheating (red-hot windings and wax impregnation is a good fire starter). But AC/DC sets, with no mains transformer, don't have this failure mechanism, and are hopefully designed that the hot dropper is adequately cooled.

[boater sam]

A little update on the conflagration,
All the tools, pliers,  cutters, files, strippers and drivers on the magnetic wall tool rack behind the bench have now gone rusty on exposed surfaces.
Its not moisture, I used no water just CO2, but the smoke off the burning Bakelite. And it still stinks.

[boater sam]