Ray

Thanks for the reply.

I read the final report on the R-390/URR program. Collins mentioned designing the power supply to support a current load of some 120 mA but as the design was fleshed out, the current demand went to 200 mA. My thought was that this would be a worst case current with all power hungry functions selected and at a 125 volt AC line voltage. I was just curious to see if anyone had measured the typical B+ current at the mean line voltage at the time of the design, 115 VAC.

I have always wondered about the performance of the original design so I have collected a few carcasses so that I can cobble one or two working examples. Of course most of the carcasses are missing the same modules and those that are there are defective.

In the great scheme of things, spec is spec. I think that some of the complexity with respect to the regulated power supply and ballast tube were Signal Corps requirements. It would be interesting to get a copy of the original RFQ to see what the SC had in mind. In the cost reduction program, Collins was able to successfully argue that a regulated power supply was not needed. Not so with the ballast current regulator. Using the 12 volt versions of the oscillator tubes will eliminate the ballast and free up a tube socket for a possible product detector mod.

I am sure that the R-390/URR was not the only radio that was over specified. <grin>

Jim

Logic: Method used to arrive at the wrong conclusion, with confidence. Murphy

On Monday, June 23, 2025 at 09:59:12 AM CDT, Ray Fantini <[email protected]> wrote:

Cost reduction, wow that’s harsh! Don’t know any hard numbers in terms of current draw but know that I worked on many R-390A receivers and a couple old version straight R-390 receivers and would take a 390A any day of the week over an original R-390

The A has separate decks for band change oscillator and slug racks, way easy to align and maintain then the monster deck in the non A, mechanical alignment is simple on the A and a nightmare on the non A, the mechanical filters preform better then the crystal filters on the non A although a lot of AM operators will disagree.

As far as the regulated power supply thing goes always thought it was somewhat pointless and had the side effect of producing tons of heat inside the radio. Afterall the PTO and BFO has its own separate feed with regulator so what’s the point of having a regulated B+ for the audio and IF stages? And that’s not mentioning the weird version 6080 or whatever they are tubes they used for the regulators. A lot of this is like the 3TF7 thingy that I have often replaced with just a resistor. Never noticed any stability issues between correct ballast tubes and just a plane old resistor, course it’s not like I am using the receiver in the back of a Angry twenty six with a generator. Always leave that up to the person who owns the receiver if they want the 3TF7 or a resistor but myself don’t see any difference, maybe the regulated power supply is the same deal?

Well at least that’s how I see it, imagine a lot of others may see things differently but somehow just don’t see the R-390A being a low cost R-390 but a improved R-390

Figure this is something I can speculate on, way safer then trying to say what’s the proper method or technique for soldering!

Ray F/KA3EKH

From: [email protected] <[email protected]> On Behalf Of Jim Whartenby via Milsurplus
Sent: Friday, June 20, 2025 2:54 PM
To: reflector R-390 <[email protected]>; Milsurplus Reflector <[email protected]>
Subject: [Milsurplus] R-390A L-601 swing choke question

Has anyone measured B+ current and voltage in the R-390A/URR as the function switch is set to various positions? I'm curious as to what trade offs were made in the power supply during the cost reduction program. The differences between the two power supplies are substantial but the specifications with respect to line voltage specifications are quite similar, if not exactly the same.

The R-390/URR regulated power supply holds B+ variation to less then one volt for a line voltage variation of some 15%. The R-390A/URR seems to have a much greater variation in B+ for the same line voltage variation but it still meets the same spec. The papers on the reflector that look into the power supply seem to ignore the ESR in the inductors used in the R-390A/URR.

Does anyone have any data to share?

Regards,

Jim