[Milsurplus] [MMRCG] Capacitor across AOT primary, was: RadioMarine Corporation AR-8506-B Receiver Resurrected.

[Ray Fantini]

Think we have diverged from the original question, why is the radio motorboating? And what's the function of C-141
Mr. Stinson has already informed us that by changing the value of C-141 that he can change the frequency of the oscillations, that's a direct indication of C-141 and T-101 forming a resonate circuit.
Indeed C-141 may have been intended as a audio shaping device but unfortunately I will always see it as protecting T-101 primary from short duration high voltage spikes that would potently damage the primary of T-101 but that's just my opinion, and as my wife would be the first to tell you I am often wrong.
But the question still remains, what causes the motorboating? And I am still sticking with the idea that there must be a feed back path on the B+  that allows C-141/T-101 to act as a parallel resonate circuit.
Maybe the answer is for Mr. Stinson to perform an experiment, Disconnect C-141 from pin 4 (G2) and temporality  connect it to pin 8 (K) and see what happens? Or perhaps if not the cathode of the amplifier maybe directly to the B- bus. Being that the chassis is not B- don't think you want to connect directly to ground but that may also be interesting to see what the result is.
Only been using the partial schematic so don't know what happens to the cathode of the amplifier tube and see that C-140 the cathode bypass capacitor disappears to some other point then B-, may have to go and see if I can find a complete schematic.

Ray F/KA3EKH



From: MMRCG at groups.io <MMRCG at groups.io> On Behalf Of Jim Whartenby via groups.io
Sent: Tuesday, May 3, 2022 7:20 PM
To: MMRCG at groups.io; milsurplus at mailman.qth.net
Subject: Re: [Milsurplus] [MMRCG] Capacitor across AOT primary, was: RadioMarine Corporation AR-8506-B Receiver Resurrected.


Ray
Yes, you can view it as a parallel "tuned" circuit in some sense of the word but it isn't in a positive feedback loop so it can't be the reason for any oscillations that you see.  Then there is the resistance of the primary which drastically lowers the circuit Q so again no oscillations.  The capacitor in parallel with the transformer will act as a low pass filter as per Fritz Langford-Smith.

Not a problem if you don't accept the explanation but it is never the less the reason for the capacitor across the AOT.
Jim
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