[R-390] R-390A low audio level with limiter off

[Leo Reynolds]

Hi Jacques,

Thanks to you and Don Heywood. I replaced C532 with a 100pF COG 600V ceramic capacitor from my stock and removed C537 as you suggested.

Audio in limiting and non-limiting modes is normal now. Measured voltages make more sense now too. I had previously measured the resistors and all were within tolerance except R533 (50% high) which I replaced.

Leo, KN4VXC in Carrboro, NC

-----Original Message-----
From: Jacques Fortin <jacques.f at videotron.ca> 
Sent: Sunday, May 5, 2024 11:18 PM
To: 'Leo Reynolds' <leoreynolds at protonmail.com>
Cc: R-390 at mailman.qth.net
Subject: RE: [R-390] R-390A low audio level with limiter off
Importance: High

Hi Leo,

The V507 plate voltage is way too low to be normal.
I believe that C532 developed a high leakage due to silver plating migration.
That can highly attenuate the audio signal as you experience in your set.
Just change C532 for a new 100pF, 300V SM and retry.
As you are there, you can also remove C537.
I made a lot of measurements on this circuit and the signal integrity will be better with C537 removed.
Check also the R527, R526, R532, R533 and R535 resistor values just to be sure they are OK.

Let us know the results.

73, Jacques, VE2JFE in Montreal