[R-390] R-390A intermittent loss of Diode Load voltage

[Jordan Arndt]

Hi Jim...

The RF and Xtal modules have been gone over pretty thoroughly after not 
having been used for a few decades. In fact, I bought it over 20 years ago 
as a parts chassis and it was already pretty rough looking even then, but 
after a lot of work, it's working really really well. I never thought to 
restore it before because it appeared to be too far gone.

Checking the tube seating was my first thought too, but no cigar with that. 
I haven't found a way to provoke the "symptoms", but I can sometimes correct 
the symptoms by going from AGC to MGC and back, or by going up a few 1mc 
bands and back, but that doesn't help too much because to me, the problem is 
in the detector/audio chain and the related wiring and interconnections, and 
parts external to the modules.

I'm assuming that a problem with an oscillator or in the RF circuits would 
manifest as a drop in AGC voltage and the Carrier Level meter, but that's 
not happening.

Of course, I could always be wrong...!!

Sorry for the long reply...

73...Jordan VE6ZT







----- Original Message ----- 
From: "Jim Whartenby via R-390" <r-390 at mailman.qth.net>
To: <r-390 at mailman.qth.net>
Sent: Thursday, February 17, 2022 10:53 AM
Subject: Re: [R-390] R-390A intermittent loss of Diode Load voltage


> JordanHave you pulled and reseated all tubes in the RF, Crystal Oscillator 
> and PTO? Might be a bit of corrosion on the tube or socket 
> pins.Regards,Jim
>
> Logic: Method used to arrive at the wrong conclusion, with confidence. 
> Murphy
>
> -----Original Message-----
> From: Jordan Arndt <Outposter30 at shaw.ca>
> To: Collins R-390 HF Receiver List <r-390 at mailman.qth.net>
> Sent: Thu, Feb 17, 2022 11:31 am
> Subject: [R-390] R-390A intermittent loss of Diode Load voltage
>
> Hi again group...
>
> This one has me searching for clues and I hope someone might have
> encountered this problem, or help me narrow down what to look at next.
>
> While this 1960 S-W 390A has had a lot of intermittent issues, I've been
> able to resolve all of them so far through testing and analysis.
>
> This latest one however has me stumped so far.
>
> While checking daily for intermittents by monitoring a very strong local 
> BCB
> station, I'm seeing an occasional random drop in the Diode Load voltage 
> and
> a drop in the Line Level Meter reading and the Local audio output, while 
> the
> Carrier Level reading and AGC voltage level, do not change. The voltage 
> drop
> can be from 2-3 volts to nearly 10 volts measured at the Diode Load jumper
> on the rear panel.
>
> So far, I've replaced the IF deck, the audio deck and the power supply 
> with
> known properly working modules, to no avail. I even replaced C101 with a 
> new
> .22uF film cap which did not solve the problem.
>
> What's left is the RF and crystal osc deck, and the interconnecting cables
> and thier connectors, which have all been cleaned and re-seated numerous
> times. I've also cleaned the fuses and the contact connections within the
> fuse holders because I had found contact issues therein.
>
> What's puzzling to me is that if the problem was in the RF/xtal modules, 
> I'd
> expect to notice a corresponding loss of gain which should appear on the
> Carrier Level meter and/or the AGC voltage, but I don't see that at all.
>
> Any thoughts or insights as to what to check next would be greatly
> appreciated...
>
> 73...Jordan VE6ZT
>
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:R-390 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:R-390 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html