[R-390] Sensitivity below 8 MHz II

[Perry Sandeen]

GM List,

Wrote: Altho' the xfmr tuned to a peak, further examination concluded the only explanation -apart from tube- or bad ground, would be the feedback capacitors in the oscillator circuit or the one in parallel with the xfmr.

Removing the RF module a pain and a shotgun approach not the best practice but removal and replacement of all 3 produced success. Full gain and voltage at test point per manual.

Thanks for the info Pete.  Perhaps the fact that the 17 MHz oven runs at 85C might have contributed to the caps failure.

Wrote:  Check at RF - (easy way is with a good ESR tester).

Ok.  No Argument.  It would work but how practical would that be?  First is the cost.  Second you’d still have open up the RF deck.  Then can it test in-circuit?    I don’t know the cost of such and instrument but I suspect one could buy all the resistors and capacitors for several R390A’s for the cost of such an instrument.

A shotgun approach is not a substitute for reasonable trouble-shooting. (If one needs help see Rogers’s excellent advice the Y2KR3 manual.) However, us non-professionals face test equipment limitations.  So sometimes an educated “best guess” is not only the most expedient but cost effective method.

An additional note.  Continued occasional postings to the list of mica capacitor failures, particularly the “postage stamp” variety, suggest that it may be time to replace all if one is doing a major overhaul or if one is having problems with a particular module.

A good example IMNSHO, is the I.F. deck where the paper caps seem to be notorious for devilish-to-find AGC problems.  Agreed it is a PITA but to me I’d rather replace all the resistors and capacitors once and be done with it.

FYI Ron Baldwin who is on this list has a hobby-business of selling mica caps.  Probably much cheaper than Mouser, etc.

Regards,

Perrier