[R-390] R390A Hissing RF Deck

[Larry H]

Hi Tom,  Good work.  It sure looks like the 3rd mixer is the culprit.  Try
a different 6C4 and if that's not it, try wiggling the MB connector from
the vfo - some times the connection is poor and the mixer cathode has too
much resistance.  Try measuring the voltages on the plate, screen, and
cathode using a tube extender.  You need to be able to inject 455 kh into
the control grid and have it get through.  Have fun.

Regards, Larry

On Tue, Sep 1, 2020 at 5:20 AM Thomas Hoyer via R-390 <r-390 at mailman.qth.net>
wrote:

> Working on a 390A. Have not had this issue in the past with my other
> 390A's, it's an odd one (at least for me)
> This 390A is very clean and shows no evidence of past work, even the green
> screws have no chipped paint making me think that the modules may not have
> been removed very much if ever.
> As typical, I go through each module and check for / replace any out of
> spec parts including the caps around the filters. I also check the tubes on
> my Hickok. I rebuild the electrolytics on the AF module too.
> This one powered up ok but no received signal just a lot of hissing.
> Working backwards from the AF through the IF all seemed good. Once I got to
> the RF I was not able to get a signal through T208 at either 455 or 3 mhz.
> Resistance checks of T208 show the secondary at 15 ohms - not good. Removed
> T208 and bench verified that it wasn't passing much of a signal. Found a
> donor T208, bench tested much better, installed it and now the hissing
> sound is louder! Checked the parts (R's and C's) around T208 and the 2.2k
> resistor was measuring 3.3k so I replaced that.
> Removing the PTO connection from the RF deck lowers the hiss but it is
> still there. At this point I decided to shut down last night.
> Looking for suggestions. Thinking of removing tubes one at a time to try
> to isolate a section.
> Thoughts from those more experienced then I?
> TomW3TA
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