[Milsurplus] Advice Please: Frequency stabilizing a 80M ARC-5 transmitter

[howard holden]

Hi Mack, Don’t see what Mark and Robert suggested.  Have you been through the normal stability issues like loose screws, bathtub caps needing replaced, etc? You piqued my curiosity though, so I checked my CBY52208/ATA which I just happened to have set up on 80M. I usually keep the PA tube cover off. I keyed, and with 40W out, my signal started out at 3542.66, went to 3542.68, then slowly dropped to 3542.52 over a period of about 4 minutes. Put the PA tube cover back on (a lot of heat goes on there with it on) and checked it again. Started out at 3542.48 and dropped to 3542.40 over four minutes. Nothing near the drift you report. If you’ve done all the normal screw tightening etc., have you checked and made sure your SWR was reasonably low between the set and the BC-375? If not, perhaps you’re wasting a fair amount of power to drive the -375, and causing the whole set to heat up some. Possible also you’re running the set on too high DC voltage?

Tell the SDR boys they need to increase their bandwidth, which will give them better “fidelity”, or better yet, tell them to get some real receivers.

BTW there is no “ARC-5” transmitter. There’s the general command series, which are the AN/ARC-5, SCR-274N, ATA, etc. The oscillator sections in the HF versions are pretty much the same setup. A common misuse of the term “ARC-5”.

73, Howie WB2AWQ


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From: Mack McCormick<mailto:w4ax.mack at gmail.com>
Sent: Tuesday, November 8, 2022 7:41 AM
To: milsurplus at mailman.qth.net<mailto:milsurplus at mailman.qth.net>
Subject: [Milsurplus] Advice Please: Frequency stabilizing a 80M ARC-5 transmitter

I have an ARC-5 transmitter that I am using as a VFO for the BC-375 to prevent FMing from the BC-375 MOPA oscillator/PA.

The ARC-5 exciter is working great except for frequency stability. The peak carrier frequency starts about 1 kc low and after about 1 minute is about 1 kc high. Not really a big deal except it drives the SDR folks crazy when I'm on an AM net.

The 1626 oscillator is exceptionally well regulated using a FET in the external power supply. It is rock solid.

Do you have other ideas on how I might further improve frequency stability? I built a crystal oscillator but it does not have enough drive using a 12A6 vacuum tube to drive the 1625's above about 5 watts. I'm still troubleshooting this circuit.

I need about 20 watts out of the 1625s.

73,

Mack
W4AX
https://www.qrz.com/db/w4ax<https://nam12.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.qrz.com%2Fdb%2Fw4ax&data=05%7C01%7C%7C04e9d7477f17434cb8e108dac19fbea1%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C638035189091261394%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=h0rkyP8wsQcCcds1bw3DM79WFMZnrwvKB3%2FNTJg%2FzuQ%3D&reserved=0>

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