[R-390] Solid Stating & an R-392 story

[David Olean]

All these comments about solid state conversions has prompted me to 
report on my latest "project".  I am just finishing up an overhaul of a 
Stewart Warner R-392/URR receiver. I really like these sets, mainly 
because they were in use while I was in the Signal Corps back in the 
late 60's.  At one point I ran a commo shop and saw all sorts of classic 
gear come thru the doors. We had T-368s, R-390As, and of course the 
R-392 and the T-195 transmitter. I don't think I ever saw an R-392 that 
needed repair. The (T-195 was another story. ) We were swimming in RT-524s.

Anyway, I picked up this R-392 radio and it had a severe oscillation 
problem on a few bands. Broadband noise would wipe out reception. Also, 
the dial accuracy was way off by about 10 kHz. I jumped right in and 
discovered that the oscillations were a function of a dirty bandswitch. 
They would come and go depending on movement of the MHz dial. I removed 
the PTO and rigged it up in my test fixture  that has a nice vernier 
dial on it. Indeed, the PTO has a 10 KHz error, and I was able to 
compensate by just tweaking the small series trimmer coil inside the 
unit. I got the error down to about 350 Hz anywhere in the range. I was 
happy with that, and put the PTO back in the receiver.

I knew that this unit needed an alignment as switching to the 2 kHz 
bandwidth position dropped the signal level drastically. I went through 
the entire alignment process and saw the radio getting hotter and 
hotter. The sensitivity was coming way up. I was impressed. I had to 
replace a broken ferrite slug in the RF section.  When I was finished, 
this thing was hotter than a 2 dollar pistol.  The first evening, I 
listened to the AM broadcast band and was horrified to find that all the 
signals would overload the radio. Even the lower HF bands were being 
overdriven and suffering from overload. The radio had way too much gain! 
As an example, a 1 microvolt signal would drive the tuning meter up 
about one quarter of the needle range!  It was then that I got looking 
closely at the set and noticed that there were no tubes in the two 26C6 
mixers. The tube shields were in place but if you looked closely, you 
saw a black plastic disk with a small screw head in the middle inside 
the tube shield. I took one of them out and saw an inverted 7 pin tube 
socket, with three pins stuck in the socket. On the back side there was 
a 2N3819 FET mounted and connected to those three pins. I figured that 
this may have been the reason for the oscillations and also the 
excessive amount of gain available.  I purchased a pair of 26C6s and 
plugged them in. What a difference! Now the radio acts normally and does 
not overload.

I gave the radio a new coat of glossy olive drab spray paint and it 
looks like it is almost brand new!

It looks like solid stating a radio is a time consuming process, as you 
must match the gain function in each stage or troubles will appear fast! 
I am sure the 2N3819s are great, but just dropping them in is not the 
way to go.

73

Dave K1WHS

On 10/30/2021 6:55 AM, Mack McCormick wrote:
> Perry,
>
> Charles provides an excellent template to SS your R-390a. He is a
> wonderful gentleman and we collaborated often. I am the only other known
> person to complete the project. Charles and I went through four design
> changes as I SS's a R-390a. It is a rock solid design now.
>
> Without knowing your technical/construction skills let me caution this is
> not a trivial project and requires at least intermediate level skills.
>
> I posted about my SS conversion on this list within the past three months.
> You can search for the thread if interested.
>
> Here are pictures of some of my work. Please note: The PC boards I made for
> the SS tube replacements are no longer the current design. I will provide
> you with the version 1 files but you will need to update them to the latest
> design using KiCad (free on DigiKey).
>
> Good luck and I'll be interested in your results if you undertake this
> project.
>
> https://photos.app.goo.gl/11AjyNCi2zV3T8LaA
>
> 73,
>
> Mack
> W4AX
> https://www.qrz.com/db/W4AX
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