[Hallicrafters] HT-37 60 Hz Hum

[Mark Bell]

Hi Rick --

Wellcome to the list and the world of Halli-iron!!  It's addicting.

I doubt you caused the hum by converting to a 3 conductor
line cord.

Since the radio was working on OK day 1, and it's working
OK now except for the hum, I don't think the voltages will
tell you anything.   They can't hurt,  but I doubt that the problem.

I suspect that you need to replace the electrolytic
caps under the chassis.   Particularlly the two big orange ones.
You won't find any direct replacements as I believe they are
475V units;  I made an equivalent bridge for mine.   All the
others can be replaced with off the shelf capacitors.  Replace
the selenium rectifier while you're at it.

Inside the SSB subchassis there is at least one electrolytic
that needs replaced.   To remove the subchassis, you'll have to
remove the front panel.  Then unscrew the subchassis box, and
rotate the box upwards (with respect to the main chassis top).
Finally, unassemble the sub chassis itself.

You might try doing the cap replacements under the main chassis
first.  If that solves the problem, you don't have to do the sub chassis.
However, I'd do it anyhow.  There are two kinds of old electrolytics
as far as I'm concerned -- bad ones,  and ones that will go bad.

Doing the voltage checks isn't that big a deal.  First, attach the
negative lead to a good chassis ground.   Then, put one of  your
hands in your back pocket, and hold the probe with the other
hand.   This is no joke; you put your one hand in  your back pocket
so that you don't accidentally grab the chassis.  Been there done
that' -- ain't pleasant.    For negative voltages,  you'll do the same
except that the positive probe will go to the chassis.  If you're
using a DVM with low impedance,  the values may be off.  Best
to use a high impedance VTVM.

Keep us informed Rick, and again, welcome!!

73 Mark K3ZX




----- Original Message -----
From: "Rick Abbott" <wx9m at charter.net>
To: <hallicrafters at mailman.qth.net>
Sent: Monday, February 17, 2003 10:57 PM
Subject: [Hallicrafters] HT-37 60 Hz Hum


> Hi,
>
> New to list and new to Halli's! I just purchased a SX-110 and HT-37. Got
the
> sx-110 running and the HT-37 was running great at first fire up (1 day),
but now
> has developed very low audio and 60 cycle hum on carrier and audio. The
hum gets
> very pronounced once you adjust the carrier balance for minimum meter
> deflection. Could I have caused a problem by converting the line cord to 3
prong
> grounded chassis?
>
> I have checked all the tube resistances according to chart in manual
(except for
> the audio section which is enclosed and seems like a bear to remove
:-(...).
> Anyway all the other tubes are seemingly ok. I am a little leery of doing
the
> tube voltage checks. I guess a safety feature would be to add some
electrical
> tape to my dvm probe covering all but the very tip. Would the voltage
check
> identify problems that the resistance check did not? Anyway, are there any
tips
> to getting that audio box out?
>
> 73 de rick WX9M
>
>
>
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> dfischer at usol.com
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