it... is... ALIVE!!

[w3jn]

Had my first contact with the GPT-750 last night with Don k4KYV.  Got thru several issues with this ol' girl (including my endemic RF in the audio problems, a busted wire in the 25-conductor cable between the PS deck and the RF deck, modified/undocumented keying circuit, interlock problems, loose shaft coupler on the loading cap, lack of 220 power in the shack, VFO problems, etc).  First reports are that the mudulation is a bit thin and there's a lot of background noise (which I could see on the 'scope).  So I need to do some tweaking on the audio chain obviously.  Could be the mic (a Shure 55S that I've never used before), the cheap audio mixer (which previously injected Deutche Welle into the audio somehow  ), the multiple fans in the '750, etc.

I *really* like this xmitter a lot, and I have it paired with a GPR-90 for a complete TMC transceiver

Don made an excellent point about there being a flurry of activity on the lower end of 75/80 right after the band expansion, but recently AM stations have been few and far between.  Let's get down there and use that space now that we have it!

Here's a pic from this rig's previous owner

[K1MVP]

Hey, Johnny,
That is a NICE looking rig,--I think I like it better than the T3,
as far as looks.

TMC was a "quality" company,--I was down in W.Nyack N.Y. at
their assembly plant back in 66, and saw those rigs come off the line, and I remember most of their products were built for commercial
applications, as I recall.
                                 73, K1MVP

[Steve - WB3HUZ]

Congrats dude. Should be a very strapping TX and signal. Can't wait to hear it.

[Todd, KA1KAQ]

YES! Congrats from me as well, John. I agree completely with Don, let's make some noise down below 3800 before it all becomes SSB 'net city. Most nights there is no AM activity down there after 6PM.

Hope to hear you on the air this weekend. Can't wait to work you with your new-and-improved big strap. Don't worry about the lack of 230v power in the shack, just roll it out into the kitchen and unplug the range after dinner. Works like a charm. 

[w3jn]

THanks, guys!

Rene, I agree completely with you regarding TMC quality.  I've had lots of TMC gear Including over a dozen receivers, 6 or 7 SSB converters, etc) over the years and this is the first thing I've had that came from Mamaroneck that's needed repair.

It'll be fun getting the audio straight 

[k4kyv]

Wonder how the frequency stability of the GPT VFO compares to that of the PTO unit in the T-368?

I recall a few reports that the GPT has been known to develop a condition of intermittent frequency jump.

My VFO which is a highly modified T-368 master oscillator unit, is almost perfectly stable with regulated DC on the filament, and regulated plate/screen voltage, on the oscillator tube, and enough shielding and filtering of the power leads to allow the oscillator statge to run all the time without being audible in the receiver.

[WA3VJB]

looking forward to hearing those 4-400 soon, John.

Is activity down on 75M AM overall, or have people started slipping back into the rut.

I haven't had time to get on, myself, but curious to know whether the novelty of having more elbow room has worn off.

QTF, over

[w3jn]

Wonder how the frequency stability of the GPT VFO compares to that of the PTO unit in the T-368?

I recall a few reports that the GPT has been known to develop a condition of intermittent frequency jump.

My VFO which is a highly modified T-368 master oscillator unit, is almost perfectly stable with regulated DC on the filament, and regulated plate/screen voltage, on the oscillator tube, and enough shielding and filtering of the power leads to allow the oscillator statge to run all the time without being audible in the receiver.

See my post on this in the Technical Forum.

WIth 2 thermostatically controlled ovens and a fil regulator, it's pretty stable except for the frequency jumps (which are becoming less of a problem as I use it).  Absolutely NO signal audible in the RX (which is why I had trouble spotting you last nite, Don!)

[AJ1G]

When I was a 14 year old JN WN2ZPS in West Nayck (40 years ago now!), I knew who TMC was from thier ads in the 64 ARRL Handbook.  I knew of thier facility on Snake Hill Road (you could see the TMC logo on the building from the Thruway).  Never knew what went on there, but nice to know now that GPT-750s come from my home town!

Think they moved out of West Nyack sometime in the late 70s.

73, Chris, AJ1G

[Dave KA2J]

John,
Congratulations on getting the GPT-750!  I remember a while back you really wanting the one that was on ebay.  I'm so glad you finally got one and can match it up with a great receiver.  Enjoy!  Dave

[WA1GFZ]

Very cool John. Real quality! I wonder how you fixed the VFO?

[w3jn]

I didn't, Frank, it's still reading 60 KHz off and it jumps a bit from time to time.  A *LOT* of cranking back and forth on the Vee-FO knob improved it somewhat, but that cap wiper still needs cleaning once I figure out the way in.

[N3DRB The Derb]

I seem to recall the tim tron saying there was sum  real big time audio restrictions in that tx that had to be drilled and blasted out of there. True or False? I dont remember.

[WA1GFZ]

Very Good John,
I've learned from the best when it comes to getting inside a sealed can.
Solder or welds.  When you find the seam let me know what it looks like.
I take soldered crystal filters apart no problem. I've ground the lids off welded hybrids to get inside and repair them.

[w3jn]

I seem to recall the tim tron saying there was sum  real big time audio restrictions in that tx that had to be drilled and blasted out of there. True or False? I dont remember.

Dunno.  Ain't stock; 99% of these things were either CW only, had a FSK exciter, or one of those universal slobucket exciters.  Only a handful had the plate mudulator deck.  This xmitter was originally a CW only, so the middle deck was empty.  THere's a very nicely made hombrew mod deck in there with a pair a 813s and broadcash iron 

[N3DRB The Derb]

oh, well hell, you all set then.

[W2JBL]

John, you need to come up to Mamaroneck. i'll take you to the former TMC world headquarters and training center on Fenimore Road. there you can lay your hands on the biulding, and take a baptismal dive into the dumpsters i raided as a kid. after that your TMC troubles will be over...neighbor Gus W2JE had one of those, with factory modulator. he never let me run it on AM though. he was CW only and thought i was a crazy JN for wanting to run AM back in '70...

[Steve - WB3HUZ]

The GPT sounded great this afternoon on 3733 and 3712. You were very strapping, as strong as -37 dBm at times!!

Sounds like you have that thing dialed in.

[Todd, KA1KAQ]

Sorry I missed the fun, guys. Would love to have worked the beast up here. Unfortunately I spent the last 2 days disassembling and trying to repair my transmitter after losing audio. Looks like I may have a crapped out driver xfmr. Not a good thing... 

I'll go whine about it in the tech forum. But congrats again John, on your fine xmtr. If all else fails, I'll still be listening.