[R-390] R-390A blown fuse, now distorted line and local audio

[Jacques Fortin]

Hi Tom,

My guess is that V601B filament gave up.
So you have the 1st audio stage working, most probably, but the cathode follower stage that follows is inactive.
Quick fix: change V601 and check if the audio returns to normal.

If not, more investigation in the AF chassis is due.
That could be also the V602 that developed a gas leak....
Let us know what you found, anyways.

73, Jacques, VE2JFE in Montreal

-----Message d'origine-----
De : r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De la part de Tom M. via R-390
Envoyé : 9 avril 2024 19:38
À : r-390 at mailman.qth.net
Objet : [R-390] R-390A blown fuse, now distorted line and local audio

Hello all, 

looking for advice to help fix this problem.
I was operating my radio today when it exhibited some funny business, flickering dial lamps and then sudden OFF.Found 3A fuse was blown (but B+ fuse OK).
Not knowing what else to do I replaced the fuse and supervised the power on.  It powered on normally with no other fuse blowing or flickering, but I have low and somewhat distorted audio on both the LINE and LOCAL audio circuits.
Initially thought the AGC was out so I switched IF decks; same issue.
I checked the audio at the DIODE LOAD through an amp and it is lovely/normal.
So Q: where should I start looking to repair this audio problem?I don't know if the blown fuse was a red herring or not but it sure seemed real like bad stuff was happening right before.
Twenty five years ago I changed C603A/B (30uF caps on 1st AF AMP) and the PS electrolytics, so even though the originals are gone the "new" ones are 25 years old.  BTW the PS has the orig rectifier tubes in it; no solid state.
So to sum up, DIODE LOAD sounds fine, LOCAL and LINE audio are low and distorted after blowing the fuse.
Advice?
Thanks in advance.Tom


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