[Hallicrafters] HT37 problem----solved?

[Roy Morgan]

At 06:10 PM 9/12/2005, Joseph Trombino Jr wrote:
>I have posted to this list my travails in getting my HT37 to arise from 
>the dead.

Joe,

And a noble trip it has been..keep at it!

>...I decided to swap out the balanced modulator tubes V4 and V5. (I had 
>installed NOS 12AT7's from the git-go with no joy) ...
>And why do only two out of four NOS 12AX7 tubes work???

Here are my thoughts:

Balanced modulators depend on balance (duhhh!) and that means 
non-leaking  and non-drifted capacitors, non-drifted resistors of the right 
value, and non-leaky tubes.

So I suggest:

1) You may have had a combination of things going on, which a somewhat 
leaky tube showed up.

2) Check all caps for leakage.  (In the Collins S-line the balanced 
modulator is fed it's audio via a cap that sometimes leaks.. this 
unbalances the thing and you can't null the carrier, or if you do, it 
drifts back in after a while.)

3) Check all resistors for correct value.  If the HT-37 is like the HT-32, 
it's not fun to get that module out for close examination and un-hooking of 
resistors that can't be measured easily in place.

4) Most tube testers don't do grid leakage tests very well, but you may 
learn something by measuring low voltages with a very high input impedance 
meter (VTVM) with the tube in and out, or with a tube with a dead filament 
in the socket.

By the way, a General Radio Type 1800A VTVM recently sold on the E-place 
for *very* little money... having two already, I restrained myself.  this 
is a grand old instrument of ancient age that can be used with it's grid 
open for nearly infinite input impedance on DC.  It is a restoration 
project in itself, but the mirrored, pilot-lamp-illuminated meter is a joy 
to read, and once resurrected the thing would last another 60+ years.

>Some might say to not trust a tube tester....if so, then why do we have 
>tube testers at all???

Dead horse topic number 97:  we have tube testers because we like them.. 
and they are very useful for some things.  Like telling very tired tubes 
from ok ones, and estimating the life left in a tube, and spending pleasant 
time in front of the fireplace on winter nights in the company of the 
family. A side benefit to this is the indoctrination of the 4-year old into 
what's an "octal" and what's a "compactron". heheh  At Dayton this year, I 
bought a double handful of metal octals for my RBB/RBC receivers at 50 
cents each. Even though the radios are not yet running (I need power supply 
to radio cables), my tube tester tells me I have a good supply of spares now.

>  only one of the balance pots appears to nulling the carrier....

Check for an open pot.  Add a bit of Caig MCL by all means (see www.caig.com).

>Has anybody had a similiar totally unexplainable, illogical experience???

Saying heard at MIT:  "When all the theories fail, we must rely on being 
very smart."

Roy
Whose HT-32 should be overhauled.. again.



- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Cell 301-928-7794
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