[R-390] R-390 (non-A) - Low RF Gain or bad Antenna relay?

[k1jos at att.net]

Hi Barry, Jacques

Thank you.  I triple checked and V506 cathode reads 99 ohms to ground.  In addition I found another very odd resistance in the V507B (Neg Peak Limiter) where pin 8 cathode should read 500k (Limiter knob at full CW) and 450k with Limiter switch OFF - I read an open circuit or resistance > 10Meg.  I rechecked by removing the tube extender and reading directly at pin and same > 10Meg with my DMM.  The cathode of V507B is tied to the cathode of V510  (pin 3) and I confirmed that this reads open on my meter as well.  I tried to trace where this cathode goes via the bus diagram but my eyes gave up and will resume tomorrow. 

Nevertheless, it sounds like I need to pull the IF module and look under the hood.

Jerry  

-----Original Message-----
From: Barry Scott <72volkswagon at gmail.com> 
Sent: Wednesday, November 6, 2024 10:52 PM
To: k1jos at att.net
Cc: r-390 at mailman.qth.net
Subject: Re: [R-390] R-390 (non-A) - Low RF Gain or bad Antenna relay?

I just checked resistance from V506 Pin 7 to ground in my R-390/URR and it checks a few ohms over 820 ohms.

Thanks,
Barry - N4BUQ

On Wed, Nov 6, 2024 at 8:38 PM Barry Scott <72volkswagon at gmail.com> wrote:
>
> That resistance measurement seems quite odd.  R536 is supposed to be
> 820 ohms.  Unless C523 is bad, I don't see how there should be 
> anything much less than 820 ohms at pin 7 of V506 yet the resistance 
> diagram shows 700 ohms.  99 ohms seems very wrong.
>
> Thanks,
> Barry - N4BUQ
>
> On Wed, Nov 6, 2024 at 8:12 PM <k1jos at att.net> wrote:
> >
> > Update....
> >
> > I checked voltages and resistances per TM  11-5820-357-35 (09 March 1962) for the IF amps V501 - V506.  I don’t know if my R-390 had any mods, its SN 1256, but all the results were very close to the listed values except for V506, 6th IF amp.  The cathode should read  700 ohms to ground and 8.2vdc and I find 99 ohms and 2.3vdc.   The schematic shows the cathode bias resistor of 800 ohms and a 15ohm pot for carrier meter adjustment.  There is a decoupling cap 0.1uF off the cathode that if leaky could bring the resistance down to my reading.   But would this be enough to drop my gain so dramatically?
> >
> > Jerry
> >
> >
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