The AM Forum

OK, I got new power supply capacitors for the R390A, and it is no longer destroying 26Z5W tubes.

One last issue is still bugging me though. I have no AGC action in the SLOW position. Fast AGC works 100%. MEDIUM AGC is squirrelly - when you first switch to medium, there is a several second blanking of the receiver followed by what seems to be normal AGC function. Switching to the SLOW position causes the AGC to become nonfunctional.

In the FAST or MEDIUM position, I can measure a fluctuating negative voltage on the AGC terminals on the back panel of -4 to -7 VDC or so. There is no negative voltage on the AGC terminals in the SLOW position.

V506 is good. I therefore assume the problem is R548, R549, C548, or C551. If I need to replace C551 and can't find an exact 2uf replacement, how should I do it? Is it OK to use an electrolytic at C551?

Thanks, Ron W8ACR

Ron,

NTE has it...…….….MLR205K630...……….2uf @ 500WVDC. Never found a good C551 yet, replace it. In the Slow AGC position C551 is connected to the AGC line. It is a mylar cap and will fit inside the original can or under the IF chassis.

Regards,
Craig

better to use a film cap... you can parallel two if you need to, or series two for higher breakdown.
Also, the value is not hypercritical at all. I would not use an electrolytic, if possible.

The AGC on the 390A typically suffers from a brief period where it goes wonky when switched.

You might want to look at the "Langford" mods from the old Hollowstate News - there are several
ideas for mods, including a "speed up" for attack which makes SSB reception better with a stock rig.
Other sources have other mods for the AGC issues.

Anyone with a R390/A should have these reference sources very handy.

#1: The Y2K manual, now at revision 3. A quick Google will get you there.

#2: https://www.r-390a.net/             This source has it all!

#3: Wei-Li's "Pearls of Wisdom"        https://www.r-390a.net/Pearls/index.htm

#4: The Gurus                       http://mailman.qth.net/pipermail/r-390/

Craig,

Thank you Craig and Bear. I have a new 2uf capacitor on the way.

Since this is an AGC circuit, I thought that the value of the capacitor would be fairly critical. I guess what you are saying is that if you change the 2uf to, say, 1uf, it would still be "slow" but just not as slow. Is that right?

Now I just hope that the RF subchassis doesn't develop any problems. I'm afraid that would be beyond my meager skill set.

thanks again,
Ron W8ACR

Yes.

I was thinking more about 2.2ufd or 1.8ufd, but you'd not notice much if you
went to 1ufd or 3ufd...

The 390A uses a "screwball" AGC time constant circuit.
The fast and medium caps are placed "normally" but the slow, iirc, uses
the tube's Miller capacitance... at least that's what my synapses say.

Thus there are several mods to the AGC scheme, some of which are in fact
an improvement.

Those are probably documented in one or more of the linked references, which
are copious and extensive.

The so-called Lankford AGC mods, intended for SSB improvement mainly, use a pair of
diodes to create a very fast attack. The BFO injection cap gets bumped up to ~50pf.
I tried that, and think it represents a degree of improvement, but the SLOW position
with those mods is problematic. Otoh, I basically use the MED all the time for AM anyhow.

There may be better circuits for improving SSB reception

Ron,

The R390/A receiver is not a half way fix-it receiver. They are old, get that fact between your ears. ;D Looking at some of my notes:
#1: Replace all know caps that have failed. Most of these are BBOD's
       a: two in the panel & frame section
       b: four in the RF deck
       c: nineteen in the IF deck
       d: nine in the AF deck

#2: Refer to the Y2K manual, chapter 6. Follow step by step alignment procedure. Mechanical is first! If the mechanical ain't right, ain't nuthin right! Then electrical alignment.

#3: Have fun finding the rest of the bugs.

The RF deck removal has never been joy for me, hate that part. The Y2K shows how, get help if needed. Has to be done! Believe me when you have completely gone through a R390/A, replaced the BBOD's, fixed the bugs; they are a joy to use.

They are not a band cruiser! Designed to be turned on, left on 24/7 for six months at a time, mostly on one frequency. On AM they are among the best of the vacuum tube battle mode receivers. The mechanical filters eliminate most of the frequency hogs in the AM getto. Not bad for a receiver first released around 1954. ;D

Properly restored you will hear other stations in the QSO that other can not!

Regards,
Craig