[MRCA] HV DC to DC converter

[David Olean]

I read some comments from AB5S on a DC to DC HV converter here back in 
early March. I was very interested as this looked like a good way to 
power up old boatanchors without chopping up the set with a transformer 
operated HV supply inside.

https://www.ebay.com/itm/183717171988

I purchased one to try in my Bendix RA-10FA receiver. It has a 220 VDC 
dynamotor that works quite well, but I hate to run that continuously and 
it does make a sort of racket in a quiet room.  So I tried the DC to DC 
converter route and  it works great. I knew it would generate noise so I 
built it in an RF shielded formed aluminum enclosure much like a large 
IF can. It had an RF gasket on the base. I fitted two RFI feedthru 
filters on the bottom for 28 volts in and 210 volts out. Even with the 
filtering I still had an awful level of RFI especially on the lowest 
band 150-400 kHz. I bolted it to the chassis near the dynamotor. The 
noise level was still awful and rendered the set unuseable.


I added a 25 MFD electrolytic across the 28 VDC input filter and a 47 
MFD 450 volt electrolytic across the output filter followed by a 400 uHy 
toroid choke in series on the HV lead, and a 0.5 MFD 400 Volt bypass cap 
to ground after it. The ferrite on the output lead with a cap after it, 
really helped. You can see the ferrite toroid in the picture underneath 
the IF can. I used black RTV to fasten it in position. The black RTV is 
VERY visible! Now I can barely hear a slight burble spaced every few kHz 
near the bottom of the 150 kHz band. When an antenna is connected the 
background noise rises way up and no RFI can be heard at all. 
Interestingly,  an initial check with a good scope showed that most of 
the RF noise came out of the 28 VDC input leads. You could see the 
switching transients and on each blip there rode a huge parasitic 
oscillation.  The HV output looked like a saw  tooth with not much RF 
seen at all.  The 47 MFD electrolytic helped knock down the sawtooth, 
but did not do much for the RF noise. The HV lead was the culprit 
causing most noise after the 28 VDC filter was bypassed.  The 0.5 MFD 
capacitor and the toroid were definitely needed there on the HV output.
        If you look at the original DC converter you can see a finned 
heatsink. I replaced that with a similar unfinned heatsink and tapped it 
to attach to the inside  of the IF can. The IF can is now part of the 
heatsink. The RA-10 draws 210 volts at about 60 ma and the DC to DC 
converter runs very cool. Any heat rise is most likely due to close 
proximity to the 6V6 audio output tube close by.
       The DC to DC converter has an output pot to adjust output 
voltage. It comes set at about 50 or 75 volts. I adjusted it for 210 
volts and it happily supplies regulated 210 volts whether it has zero 
load or drawing 60 or 70 ma of plate current.  The spec sheet says it 
will handle 40 watts, but I think that it also depends on the output 
voltage setting for the current margin. I did not delve into testing the 
limits!!  I am very happy with the outcome in my RA-10. I supply only 28 
VDC and it sits there quietly pumping out great audio on SW stations. LF 
broadcast stations in Europe,  and navigation beacons, all while running 
cool as a cucumber.
73
Dave K1WHS

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