[R-390] Yet another question... sigh...

[n4buq at knology.net]

On Mon, 18 Sep 2006 10:38:08 -0500, "Cecil Acuff" <chacuff at cableone.net> 
wrote :

> Barry and group,
> 
> 
> Which tube pin on V507 2 or 7 is resulting in normal results....? 
(hopefully 
> pin 7)
> 

Yes, pin 7.  Good results here.

> How much signal on pins 6 and 7 of V506B gives you some output? (crank it 
up 
> and see) The IF may be driving much more than the .1V that you are 
injecting 
> resulting in the radio working somewhat.

The attenuated output will only do 0.2V (100mV with 200% override on the 
attenuator).  There is a 2V output that I use to drive the frequency 
counter.  Maybe I'll try that.

> Should be able to put a scope on 
> it and see what the level is there with a strong signal on the antenna 
port 
> and the radio tuned to it.  Are you using something like a .01uf DC 
blocking 
> cap in series with your generator probe? (should be)
> 
> Try injecting your 455 at pin 1 of T503 then at the diode load 
point....any 
> change?
> 
> I see opportunities at C531 the .1 coupling cap, R 527, some of the 
limiter 
> circuit components, L502, C562 in T503.  Possibly C530 leaky....
> 
> Looks like shielded cables out to the diode load terminals on the rear 
panel 
> and back.  Pinched or damaged cable?
> 
> If you can inject and get normal results at the grid (pin 7 of 507B) I 
would 
> back up and open the diode load jumper on the rear panel and see if I 
could 
> do the same from terminal 15.  That would eliminate the cable from the 
back 
> panel, the influence of the limiter circuitry, R527, C531 etc.....
> 
> Unless you have been injecting on pin 2 of V507A which then adds several 
> other possibilities.
> 
> If you can inject your signal at the diode load with just a reasonable 
> decrease in signal level due to R527, R527 and C531 then I would look at 
> what's going on between pins 6 & 7 of V506B and the diode load terminal 
14.
> 
> Have you tried using the NOS tube somewhere else in the radio to be sure 
> it's good.  NOS tubes are sometimes bad.....
> 
> Are you sure your signal generator is outputting .1uv and that it's on 
> frequency?  An HP 3400A RMS voltmeter pays for itself here...(for the 
level 
> part anyway)
> 
> Put a scope on the same point you put the RF generator and see what you 
> measure while injection your 455 Kc RF signal....
> 
> Just some thoughts after looking at the schematic....and considering 
other 
> opportunities you might be facing....:-)
> 
> The modulation frequency of your RF generator won't matter at this 
point...
> 
> Cecil....


Lots of good things to try.  The radio "works" with the NOS tube in place 
so I assume it's at least functioning to some degree.  I replaced a few of 
the tubes in the IF with NOS and got the AGC a lot closer to the MGC 
reading on the Carrier Level meter (less than 10dB now).  I'm thinking bad 
tubes was partly responsible for the 20dB difference between AGC and MGC.  
The radio "works" with all these new tubes in place but still trying to get 
it working "right" (lower distortion, better gain through the IF deck, more 
response with BFO off, etc.).

Thanks again,

Barry - N4BUQ