The AM Forum

For the past two weeks, I have been noticing a burning smell coming from my rig http://schmidling.com/radio.htm when I buzzard.

I can not tell exactly which deck it is coming from but the power supply has no hint of the smell.  This leaves the RF deck and the mod deck.  I would have bet on the RF deck as it seems to come through the "window" (hole) in the front panel of that deck but a careful inspection shows not a hint of anything burning or even getting warm other than the tube.  There isn't much there to get warm other than the fil trans and the little power trans for the bias supply and they are both cool to the touch.

This brings me to the mod deck and the only thing hot there is the fil trans.  I have left the filaments burning for several hours and the smell never happens and the trans only gets moderately warm when not transmitting.

The mod deck is a pair of 813's, zero bias as triodes.  The centertap of the fil trans is grounded through the meter.

The question is, what effect does the mod current have on the heat carrying capacity of the filament transformer?  Are all 10v 10A trans equal?  Does talking create heat in it and could this be the source of the smell?

Lacking a better choice, I bought it from Antique Ele for $54 incl shipping.  Seems like a junky little thing compared to the one in the RF deck.  It's a Hammond T167S10.

Any thoughts?

js

Maybe it's the fires in Florida drifting up your way.  They say that the smell is so bad as far north as Atlanta that residents are advised to stay indoors.

Your plate bypass cap looks a bit small.  Is it heating up?

ALso see if there are areas heating up on the plate choke.  Check to see where it's self-resonant with a GDO and adjust turns if necessary.

I don't see a parasitic suppressor in the plate lead of your final tube, and don't know if it needs one, but it's one of those "can't hurt" things.  I had a parasitic suppressor go up in smoke on my HT-20 so if you've added one that might be another thing to check.

Could also be your RF plate choke heating up. Maybe there is some oscillation going on and the RF choke has a hot spot. I wouldn't worry about the heater transformer. The lack of a parisitic suppressor could be causing th echoke to deal with the heat.

Thanks for all the ideas.  Before pulling it out again and making changes, I checked  the neutralization and found that it was not right.  After futzing for about 10 minutes to get it right before the Noon Time Forum today, I got it about as close as possible.

Bottom line is, no smoke after many buzzard transmissions over a one hour period.

Thanks,

js NTF 3875 Noon M-F