[R-390] Call for measurements - 100dB carrier reading

[David Wise]

Thanks for your debug tips, Graham.  I will check
tonight, time permitting.  Right now all I can do
is comment.

1. Any trouble in Z503 or the AGC amp in general
(besides runaway oscillation which would be obvious)
would lessen my AGC, not increase it.  I'll measure
it anyway, since I've seen a lot of posts about it.

2. Yes, if R201 (270K, top leg of V201 AGC divider)
were to increase, or if R234 (1.5M, bottom leg) were
to decrease (but that's rare), V201 would get less
than usual AGC, forcing the control loop to throttle
down the other tubes more.

A similar effect would occur if V201 were absorbing
electrons at pin 1, but I tried four different ones
that also test good, so I can rule it out.  On the
other hand, V203 (2nd mixer), and V204 (3rd mixer)
have high-impedance grid returns and could do the
same thing.  (So does V202, 1st mixer, but my
peculiarity happens above 8MHz as well as below.)
These are easy to check: drive the AGC bus negative,
kill the oscillator, and measure between the grid
test point and the AGC bus.  Should be zero.

3. AGC leakage is unlikely to be towards
anything but ground, which would lessen the voltage
not increase it, and I have replaced C551, the usual
culprit, but I'll check anyway to get early warning
on filter trouble.

4. Thanks, I'll check over my own radio first.

5. The design is poor.  The R-390 had a 15-ohm wirewound
pot, and its cost and complaints about its coarse
adjustment granularity led to use of a carbon pot
in the R-390A.  The smallest practical value was
100 ohms, and some unimaginative junior engineer
tacked a 22-ohm resistor across it to make it look
like 15 ohms.  But the whole R-390 adjustment range
is squeezed into the bottom end of the carbon pot,
and at the nominal setpoint, change is magnified tenfold,
so the slightest scratchiness (like mine) is unbearable.
I have a keen interest in design and low tolerance for
kluges, especially unnecessary ones.  This one really
irritates me.

6. I'll compare my diode load voltage to yours
although any discrepancy would be caused by the
AGC amp, 4th IF amp, IF cathode follower, AGC
rectifier, or detector, none of which can
exaggerate the AGC voltage.

I'm betting on R201.

Regards,
Dave

> -----Original Message-----
> From: r-390-bounces at mailman.qth.net
> [mailto:r-390-bounces at mailman.qth.net]On Behalf Of Graham Baxter
> Sent: Wednesday, August 22, 2007 11:39 AM
> To: r-390 at mailman.qth.net
> Subject: [R-390] Call for measurements - 100dB carrier reading
> 
> 
> Hi
> 
> I would urge you to test three things. One is the dc resistance of the
> AGC amplifier anode coil, Z503, should be no more than 17 ohms and
> peaked for max AGC voltage.
> 
> Second, with the multipin connector off the RF deck, check the DC
> resistance to ground from pin E, positive probe to ground. This should
> be about 1.8 Meg at any voltage up to 250.
> 
> The third thing is to test the dc leakage resistance of the AGC line
> in the IF module. Unplug the IF multipin connector. From pin 6 of the
> connector on the IF module to ground I see about 50 Megohms once all
> the capacitors have charged. This is with 250v test potential, again
> with the positive potential to ground.
> 
> Any significant leakage here will mess up the AGC, and sadly a
> potential culprit is one or more mechanical filters. Lets hope not.
> 
> I have a second EAC here. It is slightly less convenient to check the
> levels on that one because it is not near the test equipment. If it
> becomes important, let me know and I will check it. However, on
> broadcast signals it gives very similar S readings to the one I
> measured.
> 
> I have to say, I have no complaints about the standard R-390A metering
> and AGC circuit. The zero pot is a bit twitchy but once set mine tend
> to stay put. Is it possible you have a fault?
> 
> One item of interest might be the diode load voltage.
> With the measured 100mV unmodulated at 1.45MHz its about 14v
> 
> Graham
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