[Hallicrafters] HT37 problem----solved?

[pete wokoun, sr.]

Joe,
  Sounds like you got 1/2 of your balanced modulators working.  Both pots 
will definitely have an effect on the carrier nulling when working right.  I 
once had a balancing problem, turned out to be one of the balance pots 
themselves.  The carbon element just started disintegrating and flaking off 
in pieces.  You might try checking the value of all the resistors in that 
circuit, they should be pretty close to each other, the 390 and 15K ones.  I 
replaced those 390 ohm resistors with 2 watt ones all closely matched.  
Tubes should not be so critical when things are working right.
  If you have a  'scope you could check signal levels on each tube grid: are 
V4A and V4B the same (they should be) as well as V5A and V5B?  Then check 
the plate signal levels, ie V5A plate signal and V5B plate signal should be 
close to each other.  V4A plate level should be similar to V4B plate also.  
Rotating the balance control in each tubes circuit should alternately 
increase one plate level and decrease the other plate level.  Try pulling 
the tube opposite to the one you're checking the plates on to prevent the 
opposite pair from affecting your results.

Good Luck...
pete KH6GRT



>From: Bill Gerhold <k2wh at optonline.net>
>To: 'Joseph Trombino Jr' <w2kj at bellsouth.net>,'Hallicrafters' 
><Hallicrafters at mailman.qth.net>
>Subject: RE: [Hallicrafters] HT37 problem----solved?
>Date: Mon, 12 Sep 2005 18:41:40 -0400
>
>Hey, glad to hear you solved the problem, strange as it is.  Perhaps your
>tube tester is not giving you correct information.  That's the only thing I
>can think of.
>
>But, why worry.  It just goes to show you that your troubleshooting style 
>is
>still intact because you thought of swapping numerous tubes since nothing
>else could possibly be giving you the problem.  So in a nut shell, you did
>the proper thing and performed magnificent and unimpeachable
>troubleshooting.
>
>K2WH
>
>-----Original Message-----
>From: hallicrafters-bounces at mailman.qth.net
>[mailto:hallicrafters-bounces at mailman.qth.net] On Behalf Of Joseph Trombino
>Jr
>Sent: Monday, September 12, 2005 6:11 PM
>To: Hallicrafters
>Subject: [Hallicrafters] HT37 problem----solved?
>
>Howdy Gang:
>
>I have posted to this list my travails in getting my HT37 to arise from the
>dead.
>
>I had been finally able to get full output on DSB and CW but had been 
>unable
>
>to null the carrier in the SSB mode....I was getting full carrier output
>when I went to the MOX position or when I talked into the mike in the VOX
>position.
>
>OK...so I am playing around with it today and just for grins 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)
>
>First I tried a couple of 12AU7 tubes with no joy...then I tried two NOS
>12AX7's with no joy and then from among the NOS 12AX7's I tried another 
>pair
>
>and lo and behold I can null the carrier and can hear good SSB in a monitor
>receiver.
>
>I can't explain why this is happening....I originally checked the 12AT7
>tubes in my B&K 747B tester and they indicated GOOD....and I swapped out 
>the
>
>original tubes with NOS 12AT7's and still couldn't null out the carrier.
>
>Why is this happening????  The tube called for is a 12AT7 not a 12AX7...why
>didn't the several 12AT7 tubes work???
>
>And why do only two out of four NOS 12AX7 tubes work???
>
>I checked all of these tubes on my tube tester previously and they all
>indicated GOOD.
>
>Some might say to not trust a tube tester....if so, then why do we have 
>tube
>
>testers at all???
>
>OK...so now it appears that only one of the balance pots appears to nulling
>the carrier....the other pot does't seem to do anything yet the carrier is
>fully suppressed.......man, do I hate these old boatanchors (grin)
>
>It just isn't fair to play these games on a 60 year old ham who certainly
>isn't the sharpest tool in the shed......my hair is already gray so this
>experience won't hurt me (grin)
>
>I have been on this rig for several weeks off and on and thought it was a
>hopeless case.....just goes to show you about persistence, I guess.
>
>Has anybody had a similiar totally unexplainable, illogical experience???
>
>                                         73, Joe W2KJ
>
>
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