working on a GPT-750

[Detroit47]

Hi I need a manual / schematic for this beast. It was restored by Howard Mills, and now he can't find the manual. The wattage fell off after it blew a fuse, and I have no idea what the operating parameters’ are.
73 N8QPC

[Edward Cain]

You can download it from BAMA. It's in the military section.

Ed

[ve6pg]

..you might try emailing al, ve3ajm..he was 2, i believe..
   ve3ajm@gmail.com


..sk..

[w3jn]

I have one, what do you need to know?

Load to 300 mA plate, drive at 60 mA *screen* current and yer good to go.  Best to tune for a peak in screen current rather than a dip in plate current.

Beware that the mil manual doesn't cover the modulator deck, if you are so lucky to have that version.

[flintstone mop]

IF memory is serving me. There was a GPT750 that Howard had to build an 810 modulator for the owner. And tuning wasn't run-of-the-mill either.

fred

[w3jn]

The very rare stock mod deck used a pair of 810s.

[Pete, WA2CWA]

The very rare stock mod deck used a pair of 810s.

I have listed in my inventory the GPT-750D2, AN/URT-17A. Big manual. Looks like about 1 inch thick with lots of schematic foldouts in the back. Do you know what the model number is for the mod deck? I took a quick look at the manual I have, and nothing jumped out for a mod deck mention.

[w3jn]

The URT-17 manual doesn't have the mod deck, as far as I know.  The URT-17 variants came with either the FSK exciter or the SSB (GPE) exciter.

The GPT-750 with the mod deck is the GPT-750(B)-2.  The mod deck itself is the RTM-2, and it has no URT() or mil nomenclature in the 15 October 1959 manual I have.

The URT-17A is the GPT-750(D)-2.

[W2NBC]

This link might help..

Hey JN!!

http://virhistory.com/tmc/tmc_pages/tmc_manual_page.htm

[w3jn]

Indeed, and there it is! 

Jeff u da man!  There's a lot of other cool TMC manuals there as well.  I had several of the components for the DDR-10 dual diversity receiver, but could never find a manual... gonna hafta download that just for curiosity's sake.

[Detroit47]

Hi Guys

Thanks for all the responses. Fred that is the one that he built the mod deck for. I guess it was a teletype in its previous life. It's not mine but a repair job for a friend. The thing is in nice shape. When I got over there to look at it I had no schematic nothing. I saw a smoked cap in the neutralization circuit. The owner said it was running fine tuned to spec's and it blew the breaker. After that the wattage fell off as well as the plate current. I put a new set of finals in with the same results, after I fixed the neutralization circuit. I figure that it lost screen or the plate voltage fell down. I’ll figure it out thanks for the help guys. I would like to thank you W2NBC for the link. The manual on BAMA will probably be all I need though.
73 N8QPC

[w3jn]

Well the thing is easy to check with the panel multimeter.

Locked 'n' loaded, you should get 20 mA grid, 60 mA screen, 300 mA plate, abt 2800V plate.

Beware that the plate voltage multiplier resistors were bad on mine so be suspicious of a low reading..

It'll rock 'n' roll on 160 if you slice the short across the last few turns of the final tank coil (you'll see it at the bottom of the coil).  The internal VFO won't go down that far, but using an external VFO it works FB.

[W2VW]

A Snap On toolbox cover fits right over these. The top is a little high for a workbench though.

[WA1GFZ]

Gee  the guys at TMC must have been hams. I had a GPT 10K tank coil with a mechanical short across the last two turns of 3/8 tubing.

[k4kyv]

The very rare stock mod deck used a pair of 810s.

And a VR-75 for the regulated bias supply.  Not sure about what they used to drive the 810's.

The KW-1 uses a pair of 6B4's as the audio  drivers, with the same modulator and PA tube lineup.  A pair of 6B4's or 2A3's is not sufficient to drive a pair of 810's to modulate 100% at that power level without distortion.  You need a quad in pushpull-parallel.  The Raytheon 250-watt broadcast  transmitter runs a pair of 810's at only 1600 volts on the plate, and they use a quad of 6B4's as the class-B audio driver.

I have found that a brute-force supply with a heavy bleeder resistor makes a better bias source for a class-B modulator, than an electronically regulated supply, since the bias voltage goes up or down in step with the plate voltage with variations in the a.c. line voltage.  This maintains better stability in the idling modulator plate current.

[w3jn]

12AT7 in push-pull  --> pair of 6L6GCs in push-pull --> pair of 810s in push-pull. All xformer coupled.

[flintstone mop]

I remember the fine line of TMC stuff during the military days. I liked the design of the cabinet. A lot prettier than the T368.
A pair of 4-400's in the final?
Hopefully one of the final tubes shorted out. I don't like 4-400's.
Fred

[W2JBL]

growing up in Mamaroneck I remember several local hams (now long SK) who worked for TMC and had GPT750's. one guy had the modulator deck and his had 4-250's, not 810's in it. but there were a lot of "specials" floating around town- for example a 100% factory built pristine GPR90 receiver I had for years with a 3.1KC Collins mechanical filter and product detector. I sold it at a hamfest years ago, and often wonder where it is today with it slipping dial drive and crappy bandswitch... it was a nice box though.

[flintstone mop]

The very rare stock mod deck used a pair of 810s.

And a VR-75 for the regulated bias supply.  Not sure about what they used to drive the 810's.

The KW-1 uses a pair of 6B4's as the audio  drivers, with the same modulator and PA tube lineup.  A pair of 6B4's or 2A3's is not sufficient to drive a pair of 810's to modulate 100% at that power level without distortion.  You need a quad in pushpull-parallel.  The Raytheon 250-watt broadcast  transmitter runs a pair of 810's at only 1600 volts on the plate, and they use a quad of 6B4's as the class-B audio driver.

I was wondering why that TX modulates so easy, even pushing the RF out a little beyond 300W. But I keep it around the spec.
Fred