[R-390] R390 Restoration

[Roy Morgan]

At 05:42 PM 7/8/02 -0400, Chuck Rippel wrote:
>I restored my Collins R390 over the long weekend.

Chuck,

Hooray. Glad to hear one more of them is back up running again.
The below comments are based on modest experience with the "Non-A", and an 
R-389.

>   It would develop a 60 cycle hum   ... a 100pf mica cap from one of the 
> 6082's to ground = 900K resistor.  ...theory:  the power supply subjected 
> to  further load, would add enough additional stress to overload the 
> supply just enough to make it not regulate and hum.

A scope on the 180 volt test jack would reveal such garbage on the B+. Your 
theory sounds reasonable. That 100 pF cap keeps noise and garbage off the 
grids of the 6082 series regulator tubes. If it leaks it will upset the DC 
conditions in the regulator amplifier and the regulator might lose control 
of the output voltage and hum.  Another cap, (C605, I think, 0.22uF) 
provides ac feedback to reduce B+ hum and if it leaks the system will not 
work right. I suggest replacing it if it is suspect.  It's also possible 
that the 5651 voltage reference tubes are tired and not working well.  The 
values of resistors in the voltage reference and dc feedback amplifier 
circuits should be checked.  Replace any that have drifted.  Work on these 
components is difficult due to cramped space.

>  I will have to come up with some solution to the heat generated by that 
> module though.  A CPU fan over one of the holes in the side might go far 
> to help.

Exactly. I strongly recommend a fan be run at all times on that 
audio/regulator deck.  I have made up a plate that holds a muffin fan 
against the outside of the radio blowing in.  The screws from the unused 
bottom plate and chassis frame hold it in place.  Angle slots at each 
mounting hole allow the fan to be slid in place after the radio is put into 
a rack or cabinet.

A better arrangement would be to mount a fan or fans inside the radio, 
along with a line bucking transformer to lower modern high line 
voltages.  Such an added system could include turn-on surge suppression, a 
power relay to save the microswitch, line voltage bucking, and fans.  The 
Non-A has a cast frame that offers places to mount such parts with a 
no-holes approach.

>Found the 1st RF tube replaced with a 6AK5 v/s a 6AJ5 as are all the
>oscillator tubes.  Also, there is a 6BH6 as the last IF amp.

Possibly another case of mis-guided tube substitution.

>...  the filter caps seem to be either papers a-la the 32V-3 HV cap or
>are oil filled's.

The first and only filter cap in the B+ system is a round oil-paper unit 
mounted near the front panel on the audio deck side (C10, I think). It 
almost never fails. It is at the output of the rectifier stage prior to the 
regulator stage.  The 180 volt output from the 6082 regulator tubes feeds 
the whole B+ of the radio.  Other than bypass caps such as 0.1 and 0.01 uF, 
I don't see any other filter caps in the schematic.

NOTE:
The DC amplifier tube filament (V607 - 6BH6) is part of a series 
string.  If that tube is not operating the B+ regulator output will rise to 
the full unregulated value. This is bad.

Be careful to NOT pull any of these tubes during operation:
V603   V604   V607   V509
6AK6   6AK6   6BH6   6BJ6
(Thanks to Norman Ryan for his list of the Non-A tubes and filament strings.)

I seem to remember that in the R-389 VLF receiver this dc amplifier tube is 
fed by the Ballast tube, but I don't have the details at hand, and could be 
mistaken.

A further note, on the Non-A IF alignment:
I remember doing a staggered alignment on the main IF strip in one of these 
radios. I cannot now find the reference to that procedure.  It might have 
come from a preliminary R-389 manual, or from the manual for a radio used 
in direction finding applications.  If anyone knows where that is located, 
I'd appreciate hearing about it. The goal is to do an alignment for best IF 
passband flatness to enhance AM listening fidelity.

Happy listening with the Non-A.

Roy


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
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