[Boatanchors] HT-44 mod de W7ZFB

[msix@nmia.com]

Several years ago I posted a mod (see below) to fix HT-44
double balanced modulator null drift and instability.
This fixed most, but some had to remove the 4 caps at the
12BH7 cathodes to solve it. Latest case was W8OTO. No null
until caps removed then OK.
These caps are not needed. It is safe to remove them.
It will increase long-term null stability.

Perhaps Duane will post this to the Halli list and other
places Halli fans can find it...

73 - John Conley W7ZFB
1400 Catron SE
Albq, NM 87123
PS: Due to a very effective but stupid spam filter pse put my
call on the subject line so I don't miss your msgs...
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* Homebrewer since 1947    CW     BoatAnchors    Norcal #930 *
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-----------------------old post-------------------------------
Date: Fri, 19 Jun 1998 19:32:58 -0600 (MDT)
From: msix@rt66.com
To: hallicrafters@qth.net
Subject: HT-44 12BH7 Mod

Got lots of requests for mod so will put on Halli list for all.
If questions, please ask but also please copy Halli list so all stay
in loop. Use above subject.

Look at 12BH7 cathode ckt. Note both cathode currents go thru null
pot, heating it, charring it and causing drift.
Don't know exact current for both sections of the 12BH7, but the
22K/2W plate resistors get plenty hot...so more than a few ma.
To make matters worse, one of these resistors is very close to one of
the pots, heating it even more.
Mine may not be typical but before this mod, I couldn't complete the
opposite SB suppression adjustments before the carrier had drifted
out of null.

Ideally we want no current thru pot, but no way to do this.
What we can do is put only one cathode current thru it, reducing
thermal stress on the pot by one-half, like we used to do back when.

Simple mod:
First, get rid of the two stock pots.

Put a 470ohm .5W resistor from one cathode to ground.
Put a 390ohm .5W resistor from the other cathode to the arm of the
new pot. Ground one end of the pot, leave the other end open.
End of mod.

I used 200ohm, 10-turn pots. The higher the resistance, the harder to
adjust. In round terms, all we need is the diff between 470 and 390,
or 80ohms so maybe even a 100ohm pot would work. Use a 10-turn if you
can but not essential. Use a good quality 4W carbon pot like Allen
Bradley or whatever. No cheapy jap pots. If scrounging/junk box fails,
try mouser as in www.mouser.com. They are always super nice and fast.

Now do the same thing to the other 12BH7.

Get the two 22K/2W resistors as far away from the pots as you can.
While at it, change them as probably heated out of tolerance.
If you go to mouser, might as well get 22K/3W while you are at it.

After I did this, I sniffed 1650 at the first mixer with the 2B.
Nulling reduced the sig from S9+20 to S6, or roughly 38db.
Not bad.
Better, the null didn't drift any more.

73 - John W7ZFB