[Hallicrafters] HT-37 Bias Adjustments

[David C. Hallam]

Bill,

A couple of points before this discussion about bias gets too far out hand.

1. Bias should not be set in standby mode.  Operate switch should be in MOX
and Function switch can be in either upper or lower.  You should have zero
signal input, i.e., no audio.  Under these conditions, you should set the
bias to -49 volts.  If you can't get -49V, you probably will have to change
the value of the resistor you replaced.  As a side note, the manual for my
HT-32B states that -49V bias is for a line voltage of 117VAC and suggests
that if your line voltage is 120VAC, you set to -50V, or -51V for 112VAC.

2. The bias voltage should be somewhat higher than -49V in standby.  In
standby resistor R46 is lifted off of ground by the VOX relay through
contacts J - K thereby making the bias voltage somewhat more negative and
reducing the 6146 place current in standby.

3. Bottom line;  You should have a minimum of -49V bias in the MOX position
and zero signal.  If you can't get to that level, you have a problem that
needs correcting.


73

David
KC2JD


-----Original Message-----
From: hallicrafters-bounces at mailman.qth.net
[mailto:hallicrafters-bounces at mailman.qth.net]On Behalf Of Bill
Sent: Saturday, August 21, 2004 11:35 PM
To: 'halliqth'
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)



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