Smoking mains droppers.

[john.north]

Hello all,
I have two radios with the same fault. They both work well but the droppers start smoking after a few minutes. One is a DAC10 the other is a Radio rentals 208 (Baird M218). I have not seen this before, thought it was probably dust or greasy fingers but the DAC10 is doing it a week later! I switched the Radio rentals on this morning and sure enough after a few minutes a thin wisp of smoke escaped from the vent holes. Is there anything I have missed, all hunts waxies and electrolytics have been changed.

John.

[Herald1360]

Measure the voltages across sections of the dropper and see if they make sense bearing in mind the expected currents. If they're OK, it should stop smoking eventually!

If they're high, you've missed a problem somewhere...... maybe a leaky valveholder or dodgy output valve?

[john.north]

Hello Herald,
Thanks for your reply, the rectifier and output valve have been swapped with known good ones from a EKCO U159. The valve holders looks like new but I will check when I get back to it. I will also check the volt drop as well.
Thank you.
John.

[paulsherwin]

They smoke for a while if they're contaminated with anything, particularly WD40.

[JacKam_]

John,
I understand, that you have two radios with the same fault after you serviced them. Correct?

Please let me know which resistor gets the load.

Jacek

[n_r_muir]

Hi

Have you checked the coupling capacitor between the anode of V3 and the grid of the output valve V4? Leakage there will cause the output valve to draw excess current, contributing to the overheating of the dropper resistors.

[paulsherwin]

No, the dropper isn't in the HT circuit. It just feeds the heater string. It's unlikely to be overloaded unless there's a short circuit somewhere, which is very unlikely in two different radios.

My guess would be that the OP is doing something as part of his servicing procedure which is contaminating the droppers. They run very hot and will burn off any contaminants as smoke.

[Nickthedentist]

They also tend to smoke if the set hasn't seen any action for a good few years.

[G6Tanuki]

Is it smoke, or steam?

I've had direct experience of droppers in radios that have been stored in damp conditions where the dropper former and sometimes the encaplulating glaze have absorbed atmospheric moisture - which 'steams-off' when the radio's next powered up.

In one case I switched the radio off in panic, realised the problem was just steam, switched back on - and the RCD tripped. The steam had condensed on the [cool] metal rod down the centre of the dropper, causing excess current leakage from the high-voltage end of the dropper to earth. I destroyed an innocent mains-suppression capacitor before I realised fully what was going on.

[Nickthedentist]

Yes, I suppose I meant steam rather than smoke, but the visual effect is the same and it's usually accompanied by odd smells.

[Herald1360]

Toasty dropper is a unique sort of fragrance

Almost as evocative as new mown hay, in its own way.

[john.north]

Hello all,
Yes as Paul says it has to be contaminant of some sort on the dropper which only feeds the heater chain, also the AF decoupler has already been replaced. I will switch on at the weekend in natural light and see if it still smokes, it is only slight and is hard to see under lighting.

[paulsherwin]

Run the radio with the back off and watch what's happening. It should be pretty obvious if anything is amiss, and where the smoke's coming from. After a few minutes, whatever is causing it should start to burn off.

[mark pirate]

I had this problem with a long dormant set, it took about 20 minutes to burn off the accumulated crud!

Mark

[john.north]

Thanks All.

I will run the set when I get back at the weekend with the back removed but when it was first powered up after many years of inactivity the smoke was definitely from the heater dropper. I will keep you all posted.

I have heard from the owner of the DAC10 and he reports that all is well.

[john.north]

Moderators please close thread.
Thank you.
John.

[Herald1360]

You never did say what the dropper section voltages were- since they're straight AC that would have been an instant indication that what now sounds like nothing, was amiss.

[sexton_mallard]

My DAC90a does this when not run for a while. It seems the dropper resistor is slightly hygroscopic and must vapourise off any acquired moisture as it heats up.

[Station X]

Originally Posted by john.north

Moderators please close thread.
Thank you.
John.

So the problem with the Radio Rentals 208 is fixed then?

[john.north]

Hello Station X,
Yes the smoke eventually disappeared and is working normally.
John.