[Hallicrafters] HT-37/32 bias

[Bill]

Al,

You say the -49 volts must be measured in transmit mode.  I assume you
mean key down RF out.  The service manual on this adjustment does not
state this.  It states the bias should be set at -49 volts in MOX
position.  This could be an important clue to my hot tubes since it
seems this would allow them to cook at -49 volts in receive or standby
position.  In transmit, the bias would go even lower.

I did set my bias in the MOX position, but without any RF output simply
because I had yet to connect a keying circuit.  I will measure this
tomorrow and report back to all on my results.  Very interesting!  BTW,
I have just
started a restoration of an HT-32.

K2WH

-----Original Message-----
From: hallicrafters-bounces at mailman.qth.net
[mailto:hallicrafters-bounces at mailman.qth.net] On Behalf Of Al Parker
Sent: Sunday, August 22, 2004 7:20 AM
To: halliqth
Cc: k2wh at optonline.net; ab0zz at arrl.net; dhallam at rapidsys.com
Subject: [Hallicrafters] HT-37/32 bias

Hi folks,
    A few days ago Bill, K2WH, asked a question about bias in his HT-37.
This spawned a discussion and some research by several of us.  With the
help of Bill, K2WH, Bill, AB0ZZ, Mark, WB8JKR, and particularly David,
KC2JD, I developed a chart showing differences in the bias ckt of the
various HT-32's and the HT-37.  There have so far been no measurements
to
determine the differences in bias voltage from xmt to stdby in those
models that do change.  I did measure a small (1v) change in my HT-32
mkI,
but that one does not have circuitry to increase the voltage during
stdby,
so it was probably just due to change in loads.
    I think the major conclusion that can be drawn is that the stdby
bias,
and how it's developed, varies by model in these rigs that we more or
less
thought were identical in many respects, and at least the early HT-32's
didn't vary it.
    I've tried to make the chart so it will come thru OK in text form in
the body of this email, and had to resort to some notes to do so.
========================

   HT-32 Mk 1    late HT-32A      HT-32B         HT-37
1)  R61  10K         R61  5K        R167 10K      R61  10k
2)  R62  12K         R62  3.9K     R168  3.9K    R62  18k
3)  C98  10uF        C98  40uF     C194 40uF    C98  40uF
4)   C (for ref.)             K                     K                 J
5)see note   a.            b.                      .                c.

legend
1)  Bias adjust pot.
2)  Resistor, pot to gnd
3)  Bias filter cap
4)  Relay NO contact which is grounded on xmt
5) a.  HT-32 mkI doesn't appear to change bias during stdby.  Relay
contact K has no affect??
    b.  the late HT-32A (skem 089-701328G) and HT-32B pull bias down in
xmt thru R26 18k & R 25 220k to ground via contact K, effectively
allowing
slightly higher bias during stdby, the early HT-32A (skem 089-701328)
takes R26 directly to ground at all times, so no bias diference xmt to
stdby.
    c.  HT-37 parallels R62 with R46 46k in xmt, to effectively allow
higher bias during stdby

Note, the bias of -49v must be set when in xmt
====================

If anyone can make measurements of bias voltage in both transmit and
standby, this case maybe can be closed.

73,
Al, W8UT
New Bern, NC
BoatAnchors appreciated here
http://www.thecompendium.net/radio/
http://www.hammarlund.info



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