[R-390] 390 non-a rectifier

[Roy Morgan]

At 02:51 PM 3/23/2005, Dan Merz wrote:
>Hi, I'm getting a 390 non-a back to life

Hooray!

>... I realized that maybe with the 390 just putting in solid state 
>rectifiers may be the best thing ... the 8082 tubes/VR tube may take care 
>of the extra voltage and immediate voltage application... Am I right in 
>thinking this way

Dan,

I would say so. Here are some points about the matter:

1) The 5651 tubes are voltage *reference* tubes and not voltage regulator 
tubes. They establish a stable reference voltage for the DC 
amplifier/regulator circuits to operate from.  All the B+ current for the 
radio goes through the four sections of the two 6082 tubes.

2) You really should replace the eight 47 ohm 2 watt carbon resistor with 
higher power wire wound ones. (unless you do solid state and remove the 
ones in the power supply.)  Four in the audio module under the 6082's and 
four in the power supply module.

3) I strongly recommend a fan to keep the 6082's cool.  If you use solid 
state rectifiers they will dissipate even more heat than they would 
normally.  And under normal conditions, they need a fan!

4) The main B+ filter cap is an oil/paper unit mounted near the front 
panel. It rarely fails and is likely to last forever.  Right now I can't 
locate the voltage rating on it but I think it's above what you would get 
with solly state rectifiers by a safe margin.

5) I suggest you replace C608 in the regulator section. If it leaks, the 
regulator will not work right. It may well be a paper cap and prone to leakage.

>  or is there still a requirement to put a 200 to 250 ohm resistor in
>to mediate the voltage with the 390?

If you use marginal diodes, the inrush current may harm them.  The resistor 
would reduce the peak current and make them fail less, if they might fail 
at all.

Roy

- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
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