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Author Topic: HT-37 Mod's?  (Read 4089 times)
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WN3V
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« on: February 24, 2007, 06:35:12 AM »

Anyone have any mods for the HT-37 transmitter/exciter? I dug a little and didn't-really find much.
Thanks!

John,WN3V
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w3jn
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« Reply #1 on: February 24, 2007, 07:18:06 AM »

IIRC that's a phasing rig and should be very easy to mortify for HIFI.  Keys to success with SSB xmitter mods:

1)  Fatten up coupling caps (increase low freqs), decrease the value of any caps in the audio signal path to ground (increases highs).  You can get a rough idear of the rolloff by the old f=1/RC trick.   Calculate R as the parallel equivalent of the plate resistor and the grid bias resistor of the succeeding stage.  C is the coupling capacitance.  If it's in the 200hz-300 hz range C needs to be increased accordingly.

2)  Route the audio directly to the balanced mudulator, bypassing all the phase shift junk.  The Hallicrafters HT-30 does this for AM, not sure about the '37. 

3)  If you're using an external audio chain, don't bother with the above - just run yer audio (perhaps with a bit of RF filtering to keep RF in check) right to the balanced mod 'n' you're done, presuming you have enough munky swing on your incoming audio to properly mudulate the toob.

4)  Use of a scope-a-dope on the air is mandatory.  These things'll do 300% positive mudulation if you'll let 'em.  No point in that, you're just gonna get distorted audio reports (because there's not enough carrier for the receiver to recover).  Your carrier level adjust will be the carrier balance control, for the HT-37 I wouldn't run more than 20-25 watts of carrier.

Good luck and let us know how it turns out.
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« Reply #2 on: February 25, 2007, 10:18:47 PM »

see if you can get a hold of Pal W2BP. i was up to his place recently and after looking at his HT37 schematic i wrote out some changes for it. the big one involved getting rid of a dual pi section audio yellification filter in the output of the two audio output stages, which if let in place will screw you no matter what you do before it. also put a series string of zener diodes accross the output of the bias adjust pot, so when the PA draws grid current the bias volatge stays constant, like we would do in a class AB2 modulator. i did that to both of my HT32's (identical RF chassis) and it cleaned the SSB output up a ton- third orer IMD went way down after that change. i clamped it at 49 volts. the 37 has tremondous potential as a HIFI AM or SSB rig.
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