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

[k1jos at att.net]

I have checked all tubes and found two weak 6JB6 in the AF module which I replaced.  The primary issue seems to be inadequate RF gain.

 

I did a side-by-side comparison of strong and weak stations above and below 8MHz with my HF-380 and R-390.  I found the R-390 was deaf to signals with less than an S-6 reading on the HF-380.  I also compared signal strength to a Flexradio 6700 and the Flex and HF-380 gave similar readings.   

 

Would adjusting the overall Gain (TM-11-856, para. 71 a and b) be a reasonable next step?  If my R-390’s Gain is indeed low, the adjustment requires a signal generator with a Network, Impedance Matching device (CU-406/URM-25F, part of the AN/ URM-25”.  I have a Rigol signal generator where the load impedance can be set to provide correct voltage readout.   Is there any recommended method when not using the AN/URM-25 and impedance matching device?

 

Jerry 

 

 

From: k1jos at att.net <k1jos at att.net> 
Sent: Monday, November 4, 2024 8:14 PM
To: 'r-390 at mailman.qth.net' <r-390 at mailman.qth.net>
Subject: R-390 (non-A) - Low RF Gain or bad Antenna relay?

 

Several years ago, Don W5OR (SK) asked me to share administrator responsibilities with him to manage the r-390 mail list server.  While I have several R-390/R-390A, I never had the need to repair any.  So, this is my first post for help…

 

My R-390 has gone partially deaf.  It sits in a fan-ventilated rack which rarely gets rolled around.  Its last maintenance was in 2002 when Don Heywood did an incredibly thorough job.  It worked great until a few weeks ago when it became completely deaf.  My initial inspection found something bizarre, the RF cable had popped loose (see attached picture).  After reconnecting, it was only partially deaf.  I could tune in moderate and strong signals across all bands but I need to set Local Gain to 7 or higher just about where hum comes into the speakers.  The best description is what you might expect to hear if RF Gain is not at maximum. Also, another ‘clue’ is I noticed that switching to CAL does not remove or attenuate a CW or phone signal.. sounds same as if in AGC.  I checked that the antenna relay is working: 

 

*	On powering to STBY – the relay clicks and pulls in visually and checking both twin-ax connector pins, I confirmed both 2 pins are grounded.  
*	On switching to AGC or MGC – the relay clicks off and neither of the twin-ax pins are grounded.
*	On switching to CAL – the relay clicks on again and the twinax pins are grounded. The manual says in CAL, unlike in STBY, +180 volts de is applied to the rf. if., af, and calibration oscillator stages and in both STDBY and CAL, terminal 2 of CR801 is grounded to apply 6 volts dc to relay K101.  

 

I also checked Break-In and that works in AGC giving complete silence, but the manual says that’s achieved by grounding the AF output in addition to grounding the antenna inputs.  How does that prevent overloading the RF section with a local transmitter?

 

My 1st question is, why is there no signal attenuation when switching to CAL?  Is there anything in the electromechanics that could explain this?  Is this coincidental to the R-390 partial deafness or part of the same problem?

 

Pending some replies, my next step is to check the tubes in the RF and audio stages but I’m hoping that my description may sound familiar to the many experienced members of the list.

 

TIA Jerry