[Hallicrafters] HT-37 Bias Adjustments

[Al Parker]

Hi Bill,
    Well, that's the first measured indication that the later ones are
doing what they should.  Yes, change R-64 to a lower value to allow you to
adj the proper bias at abt half on the pot.  I have the same low voltage
probem in my HT-32, as I replaced the selenium and changed a dropping
res., but now also can't get much better than -44v.  I've gotta get into
it & fix that, and probably will make the mod. to get better stdby bias.
    I would have hoped that there would be more than 3v change.  If it was
easy to ck actual plate current it'd give us a better handle on plate
disipation.  Your actual temp readings will also tell the story.  We'll be
interested in hearing what you find.
73,
Al, W8UT
----- Original Message ----- 
From: "Bill" <k2wh at optonline.net>
To: "'halliqth'" <hallicrafters at mailman.qth.net>
Sent: Saturday, August 21, 2004 11:34 PM
Subject: [Hallicrafters] HT-37 Bias Adjustments


Hi guys,

Pulled the top cover of the HT-37 and here are the voltage measurements
for grid bias.
In standby mode, - 49 volts.  Measuring this same voltage in transmit, -
46 volts.

Following advice posted here, I put the rig in MOX and while
transmitting, I attempted
to adjusted the bias to - 49 volts.  Couldn't get there but I was able
to get to - 47.5 volts.
In standby, the bias voltage is now - 51 volts dc.  This minor voltage
changed has resulted
in a cooler running rig (maybe).  I don't have actual temperature
measurements, but it
does feel cooler.  I will run temp checks to confirm.

Possible reasons for this problem (?).  When restoring the rig, I found
SR1 bad.
SR1 was replaced with a standard silicon diode of appropriate ratings.
Both capacitors
C98 & C99 were also replaced.  When energized, the bias voltage soared
and I could not
get the - 49 volts dc.  At this point, I changed R64 (4.7k) to (15k).
This change
allowed me to obtain the requisite - 49 volts dc at TP2 in standby mode.
It is possible that
I may have to change the 15k to a lower value to be able to get back to
- 49 but I do not
think this will be necessary.

Those are the voltage measurements, I will get back later with
temperature changes (if any).

Bill (K2WH)