[R-390] R390 troubleshooting - I'm stuck, 3/8A fuse pops

[frank hughes]

Hi,
I bought an R-390 from a "yard sale" parking lot table at the Orlando
Hamfest.
It looked very complete and possibly not previously mangled excessively.

The seller said it was "Only used on Sundays by a little old lady...."

Anyway, when I got it home and was able to remove some of the decks for
inspection, discovered the IF assy. had been tinkered with, and was
non-functional.

I put in another spare R390 IF assy., brought the unit up on w/ Variac, and
detected the delicate odor of carbon comps smoking.
Some of the resistors in the power supply were cracked.
After prolonged inhalation of these fumes, I found myself longing to vote
for Democrats..very strange.

Subsequently replaced all of the 47 ohm cathode balancing resistors in the
audio module (the B+ regulator section) and the power supply (rectifier
cathodes), with wirewounds.

Replaced the copper oxide relay selenium rectifier CR801 with diodes.
Ran all the tubes through a TV-10, replaced the defective ones.

The unit still pops the 3/8 A fuse.

B+ is going way over 180V.

Went to TM11-5820-357-35 and started the step-by-step troubleshooting
procedure for 3/8A fuse opening.
- C607, C101 are not shorted.
- measured from the 3/8 fuse to ground, = 148k ohms through V605, V606.
- Chassis ground to B+ jack shows 19k ohms, function switch=OFF
- Chassis ground to B+ jack shows 9.4k ohms, function switch= AGC, MGC. CAL
- Chassis ground to B+ jack shows 8.5k ohms, function switch= SQUELCH
So all this checks out.

The next section has me perplexed.
46. Checking Filament and B+ Circuits for Shorts;
d. Locating a Short Circuit. If the tests out-lined in "c" above indicate
that there is a short circuit in the receiver, determine in which
subchassis it is located as follows:has me perplexed.

All the tests in "c" passed, so if there is no short circuit, why is the
3/8 fuse opening?

I am not too proficient at test & troubleshooting, but really love these
radios, and want to learn how to do these things, so any advice for other
things I should try would be appreciated!

I have another R390 (pristine), obtained from KW4DH, used for SSB.
The audio quality is amazing. (diode load to a KRK ROKIT 5" Studio Monitor)

My goal is to use this second R390 w/ the 32V-2 for AM nets, if it can be
saved!

Thanks and 73
Frank
KJ4OLL