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Old 6th July 2004, 04:50 PM   #1
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Default Field of Dreams (a pair of FRT502 modulators)

Check out this photo of four chassis, a few hundred pounds of vintage iron, four 3B28 rectifier tubes in each PSU and transformer driven 809 DHT P-P output pushed by a pair of P-P 6L6's in each. The easiest first mod is to drop in a pair of 811A's.

If anyone has the schematic for the FRT-502 AM VHF base station circa early 1950's I would very much appreciate receiving a copy.

Cheers,
Rob
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Old 6th July 2004, 09:40 PM   #2
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Wow! Awesome, where did you pick those up?

They would make an amazing and incredibly grunty project... What are the output transformers on them, will they drive speakers?

What kind of scary voltages do they run on?
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Old 6th July 2004, 11:31 PM   #3
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I got two of the complete transmitters in crates government surplus about 10 years ago. They were each in a very heavy steel rack cabinet and weighed about 800 lbs each. I stripped them and kept the pwr supply moules and AM modulators. One RF deck is still intact (used two 829B's as exciter and final). I actually put one of these on the air for a brief period with music when I got them just to play. Just the leakage into a dummy load could be picked up by my portable aircraft band VHF receiver for a 1/4 of a mile.

The output transformer would have to be changed as it has a hi-Z output winding meant to modulate the B+ line to the tube final.
The xfmers are all oil filled. Vitamin-Q oil and paper caps everywhere, etc. The best quality tube components everywhere.

If I were to make a plasma speaker which is on my long list of projects, then this output transformer would be useable as-is to modulate the RF power oscillator B+.

B+ looks like probably in the 900 volt range.
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Old 13th March 2007, 09:26 AM   #4
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Default FRT-502

Did you ever get the manuals on the frt unit?
I have one of these, actualy part of onel.

The power supply and the modulator.

I plan to use the P/S on one of my receivers. I really have only one question:

Does the P/S run on 220v or 110?

I suspect 220 with the 4 3b28's in the circut.

Thanks. Please e-mail me off list at usa_military_radio_doc##tor@adelphia.net

(Remove the ##'s)

Thanks !!!
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Old 13th March 2007, 01:40 PM   #5
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Hey Doc,

I have not yet been successful at finding any schematics or other documentation for this transmitter.

The primary on the PSU is 120 VAC, 60 Hz. This makes it perfect to set up for domestic use as a audio amp or whatever.

With all those xfmers it would make a good theft resistant electric space heater!
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Old 14th March 2007, 01:25 PM   #6
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Thanks, I'll just dig in..and hope the majic smoke stays in..
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Old 14th March 2007, 02:12 PM   #7
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Quote:
Originally posted by RadioDoctor
Thanks, I'll just dig in..and hope the majic smoke stays in..

FYI, there was a chassis rack panel that was a unit cooling blower with a squirrel cage and a 1/4 HP induction motor mounted immediately at the bottom of the rack beneath the PSU. I cannot remember offhand if the master Heinemann ckt breaker/power switch was on the blower chassis panel, or is on the PSU. I could find out as I still have the blower units.

If you end up circuit tracing I would really appreciate a copy of the schematic you come up with.

Thanx and good luck. I`d like to make a nice pair of push-pull triode monobloc audio amps with my pair someday if I don`t offer these for sale. They will support a P-P pair of 811A`s very nicely. Too bad of the limited frequency response of all the audio iron. I wonder if there might be practical work arounds. The circuitry around the ouitput stage looks good. P-P triode output driven by interstage xfmer that forms phase splitter also. P-P 6L6`s drive the IT. The linput stage is definitely lo-fi being set up to power a carbon microphone. That would have to go.

The PSU would have enough extra jam since the RF deck was not being used, to supply a healthy triode RF power oscillator being plate modulated (with the FRT`s modulation type Hi-Z secondary output xfmer) to drive an experimantal plasma loudspeaker cell. Dang, I`m getting excited about building this ....agan.
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Old 1st September 2009, 01:14 PM   #8
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Default RadioDoctor...any progress?

Quote:
Originally Posted by RadioDoctor View Post
Thanks, I'll just dig in..and hope the majic smoke stays in..
Hi Doc,

Did you get any farther with your FRT502 PSU unit? Traced a schematic perhaps????
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Old 3rd September 2009, 08:13 PM   #9
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Default Still looking for schematics

Bump. Hi, I'm still trying to locate schematic diaphragms for this equipment. Type AN-FRT-502. PSU chassis and Plate Modulator Audio Amp chassis.

Any assistance greatly appreciated!
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Old 3rd September 2009, 09:10 PM   #10
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You should ask there: http://amfone.net/Amforum/index.php

May be some of AMers would suggest you to swap output transformers: they can give you some with speaker outs, you give them yours, for a plate modulation.
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