[R-390] 390 non-a rectifier

[Dan Merz]

Roy,  thanks for the perspective and tips. I see the cap at the front, C103,
10 ufd and there's C101, 10 ufd, near the power supply chassis itself,  a
formidable rectangular can.  I did some more snooping and saw that the 47
ohm resistors in the power supply chassis are not there - don't know the
story on  that.  The supply had bent over diodes stuffed in pin sockets when
I got it but I don't know the history of this radio beyond that.  I pulled
the audio chassis and id'd the 47 ohm resistors there, they look good and
measure 47,47,47 and 51 ohms.  I suppose the 5 watt 5% variety of wirewounds
would fit there and in the power supply chassis.  I noticed a couple of
Japanese cherry caps in the audio chassis on the vertical board so I'm not
the first in there doing something. 

I see what you mean about the 5651's being voltage reference tubes,  they
can only accommodate  3.5 ma current variation compared to the ones that I'm
more familiar with that take on all the current variation by themselves to
regulate voltage. The manual has a pretty good explanation of this
circuitry. I suppose considering the price of 6082's,  I should be more
concerned with treating them with respect than finding a sub for the 26z5's.
Best regards,  Dan  

-----Original Message-----
From: Roy Morgan [mailto:roy.morgan at nist.gov] 
Sent: Wednesday, March 23, 2005 12:18 PM
To: Dan Merz; r-390 at mailman.qth.net
Subject: Re: [R-390] 390 non-a rectifier

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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