RCA BW-66F

[n5ama]

I'm trying to put together a 50s vintage broadcast station and starting with a BTA 250M transmitter coupled with a BW-66F modulation monitor and a RCA station receiver (MI 12116) and amplifier (MI 10235).

The modulation monitor frequency range is 500-2500 khz and needs to be moved to the 75m band. The current impedance match is with a chassis mounted small cylindrical transformer. Input level is low at less than a half watt with a maximum of 6 watts. The matching circuit is driving a 6AL5 which then goes through a low pass filter before going to the distortion circuit and meter.

I realize that this function is better observed with a scope and I plan to do that but I think there would be a "cool" factor using support equipment from the same age of the transmitter.

Anyone have a feel for how this could happen?

Tom N5AMA

[K8DI]

There's a manual here
https://www.steampoweredradio.com/pdf/rca/manuals/RCA%20Type%20BW-66F%20AM%20Modulation%20Monitor%20Manual.pdf

The schematic is included.   There's no tuned circuit, just a matching transformer into a pair of diodes.  Seems to me all you need to do is replace that transformer with one that's appropriate for 75m. Physical examination should give you some ideas. You may even be able to unwind say half the turns...

The 6AL5 dual diode should be more than capable of operating linearly at 3.8 or even 38MHz..all you have to do is get RF to it. 

Ed

[n5ama]

Hi Ed. Thanks for the reply! The manual is way better than the fuzzy one I have, so it will be much easier to follow the chassis parts location. I haven?t pulled T-1 off and pulled it apart to see what is inside. I suspect it may be potted.

I was fairly certain the primary wasn?t tuned because this monitor is for the entire broadcast band but I?m not sure what impedance it needed and where I would have to go with it to get the match I needed for 75m.

I?m tempted to put a watt of 75m rf to the existing transformer and see what happens. If I get some response that looks close to what I?m seeing on a scope, just call it good. If it?s way off, I?ll probably play with the turns to see if I can get something reasonable.

Thanks again,
Tom N5AMA

[wa1mtz]

Hi, I have one of those monitors, do not try to put 3.8 mc input! you will fry the input pot, the input xfmr was made to go up to 1600, all you have to do is disconnect it and replace with rf choke and a coupling cap (500p or so) then, from your ant connection from your xmtr to gnd, use an untuned air dux coil to gnd. something larger than your op freq, like a coil for 160, so it won't affect your 75m signal, then you want to tap up from ground end a few turns until you get a centre reading on the left meter on the mod monitor. will take at least a 50w or higher carrier to register the meter, you must be in the center of the scale or the mod% will be way off. also, check all the caps & resistors, many will be bad, as the units never got turned off. 73, mike

[K8DI]

Mike is right in that if you connect your transmitter output/antenna to the monitor, it'll blow stuff up.  But the BTA-250M and most other broadcast transmitters have a specific mod monitor output off a tapped coil pretty much as Mike suggests you build. I have a BTA-1R1 with the same mod monitor output setup. If I had that monitor I'd probably just hook it up and see how not sensitive it is at 3.885.  The only place its bandwidth limited is the input transformer.  

As an aside, the transmitter mod monitor coil serves as the safety choke as well, so removing it is unwise...

I've attached a snippet of the output of the '250M for clarity.

Ed

[W3SLK]

W1MTZ said:

Hi, I have one of those monitors, do not try to put 3.8 mc input! you will fry the input pot, the input xfmr was made to go up to 1600, all you have to do is disconnect it and replace with rf choke and a coupling cap (500p or so) then, from your ant connection from your xmtr to gnd, use an untuned air dux coil to gnd. something larger than your op freq, like a coil for 160, so it won't affect your 75m signal, then you want to tap up from ground end a few turns until you get a centre reading on the left meter on the mod monitor. will take at least a 50w or higher carrier to register the meter, you must be in the center of the scale or the mod% will be way off. also, check all the caps & resistors, many will be bad, as the units never got turned off.

If you had a GR-1931A you could use it from 0.5 to 60Mhz depending on what coil set you have installed.