[R-390] Smoke got out

[Roy Morgan]

Dog,

I would not expect a shorted tube, but rather a shorted filament bypass cap.  I have not seen the R-390A schematics for quite some time now, but I do seem to remember (vaguely) that there are some filament bypass caps.  This would seem more likely in the RF section than in other places.  Locating a short may be a challenge because cold resistance to ground of the filament line to a module may be very low, UNTIL you remove all the tubes.

Seems like the crystal oscillator is a target to investigate.  Not all that complicated compared to other modules.

I’d expect other folks could comment based on more concrete information/schematics.

You may decided to *replace* the burned wire(s), or you might only run a new wire on the outside of the harnesses involved.  Note that teflon insulation is possibly not as physically strong as other types, so overly vigorous re-lacing of harnesses may not be in order.

We have heard that the EAC-made radios have teflon wire in the harnesses.  I have one EAC here at hand and the wire does not seem to be teflon insulated.  Maybe I’m wrong on that.  When I get it out for overhaul I’ll look carefully.

Roy


Roy Morgan
k1lky68 at gmail.com <mailto:k1lky68 at gmail.com>



> On Aug 1, 2018, at 9:16 AM, dog <agfa at hughes.net <mailto:agfa at hughes.net>> wrote:
> ...all of a sudden I see a wire in the harness to P108 melting in real time and smoke starts coming out of the RF section over by the crystal osc.

> ...it appears as if the wire to P108-B has burned up. That's the 6.3V for the filaments in the RF, Crystal osc, the break in relay,...