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

Wed Aug 22 17:56:09 EDT 2007

Hi Dave

I take your point about some of my suggestions appearing to have the
opposite effect to the one you are experiencing. Perhaps I should ask
you to just humour me and check them anyway! When I had filter leaks,
paradoxically my S meter read very high. But my diode load voltage was
excessive too.

I agree R201 would fit the bill nicely. I am also quite interested in R544.

Years ago I had a Collins R-390A with a Motorola IF. The meter zero on
that was all over the place. I replaced the pot and it was lots
better. A couple of days ago I found that old pot, and just out of
interest I measured it. It had gone up to 330 ohms. I was quite
surprised at that.

Let me know what you find out; I am hooked now!

Graham

On 22/08/07, David Wise <David_Wise at phoenix.com> wrote:
> 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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