[Milsurplus] [MMRCG] RadioMarine Corporation AR-8506-B Receiver Resurrected.

Tue May 3 09:48:13 EDT 2022

Speculation, C-141 is there to bypass short duration pulse noise from the possibility of damaging the primary of the output transformer. They return it to G2 being that's elevated to B+ so you in theory would not need as high a voltage isolation capacitor if one end were grounded or perhaps they felt that by introducing an out of phase signal on the screen from the plate that would make the action of the limiter more effective? Cant see the entire schematic but would assume that if that radio is employing a common B+  bus and that the decoupling capacitor is keeping all signals off that bus. If its just a high value 4/8 uf electrolytic maybe it has a high ESR at the frequencies where the tube would tend to go into oscillation. Going to assume that there is also a couple small 0.1 or something like that scattered around the radio on the B+ bus for just that function. Also if C-140 were open that can cause weird things to happen. But I think the only way that stage can go into osculation is for there to be feedback from the B+ bus.

Did they isolate B- from ground? Also going to assume that's a radio that was powered by 110 volts DC? Somehow they had to develop a negative grid bias for the audio amplifier and would be curious how they do that? Don't know much about Liberty Ships but going to assume that one side of the electrical system was +110 /120 Volts DC and the other ground? So how would you go about isolating B-?

Ray F/KA3EKH

From: MMRCG at groups.io <MMRCG at groups.io> On Behalf Of David Stinson via groups.io
Sent: Monday, May 2, 2022 11:36 PM
To: milsurplus at mailman <milsurplus at mailman.qth.net>; Boatanchors at mailman <boatanchors at mailman.qth.net>
Subject: [MMRCG] RadioMarine Corporation AR-8506-B Receiver Resurrected.

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WWII "Liberty Ship" RmCA AR-8506-B receiver
gets back to work.  This one was challenging.
(Got it going, Doug! TNX OM)
Still need to finish alignment.

Video:

https://photos.app.goo.gl/KqWcYNnnFaKxkj5n8<https://nam10.safelinks.protection.outlook.com/?url=https%3A%2F%2Fphotos.app.goo.gl%2FKqWcYNnnFaKxkj5n8&data=05%7C01%7CRAFANTINI%40salisbury.edu%7Ce04266c6578249145a7f08da2cb6144c%7C2472f1faf24f421badd7b01c4b49be07%7C0%7C0%7C637871457793775174%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=TJXTKHo0A2fgX9D4Zxzdwl91N4nlb1I9%2FzeLmesLNLM%3D&reserved=0>

Question:
I probably should know this, but I don't want
to assume.  Can someone please tell me
the purpose of the capacitor across the
Audio Output transformer primary?
C-141 from plate to screen of the 25L6.

https://photos.app.goo.gl/BFyqarNgQ9BuQfbF7<https://nam10.safelinks.protection.outlook.com/?url=https%3A%2F%2Fphotos.app.goo.gl%2FBFyqarNgQ9BuQfbF7&data=05%7C01%7CRAFANTINI%40salisbury.edu%7Ce04266c6578249145a7f08da2cb6144c%7C2472f1faf24f421badd7b01c4b49be07%7C0%7C0%7C637871457793775174%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=h3Tzrrkk2mAdMpCJNmwJzxweNVhcdhuxA4EuzjN7gKw%3D&reserved=0>

If it's included, or even reduced from .005
to .001, the Audio Output breaks into
"motor boat" oscillation. Lower value gives
slower reflex oscillation, but remove it
completely and it goes away.
The coupling and bypass caps
have been replaced, as have the power supply
filter caps of course.  Addition of more
bypassing on the B+ buss has no effect.
The bad resistors also replaced,
so the grid isn't floating.

Hints and Kinks about this rig
if anyone is interested.

GL OM ES 73 DE Dave AB5S

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