[Hallicrafters] My First Boat Anchor Restoration: SX101 MkIII

[T David Cohen]

Jason,

Good luck with your "restoration" project.  The SX-101 is a great receiver
(as long as you don't have to move it very far).

If you're going to do any significant troubleshooting, you'll need a
suitable meter.  For the most part a VOM is OK when you're measuring B+,
line voltage or filament voltage.  Where a VOM becomes a problem is when
you're trying to measure voltage in a circuit where the input resistance of
the meter itself imposes a significant "load" on the circuit you're trying
to measure and that load has an impact on the operation of the circuit.  An
example might be measuring the grid bias on a tube circuit. Sometimes the
voltage source you're measuring is current limited (maybe passes through a
high value resistor) and the load imposed by the VOM will give inaccurate
readings.

The solution to the problem is the VTVM.   The vacuum tube in the VTVM
imposes a VERY SMALL load on the circuit it's measuring and consequently, it
has almost no effect on the circuit being tested.  Fortunately, you can find
very capable VTVM's on eBay for practically nothing.  The Heath V-7A is a
great meter and I think the equivalent Eico instrument is the 232.  If you
watch the auctions, you can probably get one for $20 including shipping.
The HPs are very nice in an old sort of way but are expensive unless you
just want one.

If you've got some money to spend, a new digital meter is a good bet.  Make
sure the input resistance is > 10 megohms.  The down side of one of these
new meters is that they are inconvenient when used as output indicators
during an adjustment process (an i.f. alignment for example).  There, the
trend you can easily visualize on an analog meter is missing.  As the
sampling rate goes down, the problem becomes worse.

You'll probably want a signal generator.  You can find cheap ones on eBay.
The problem you face with Hallicrafters receivers (and some others too) is
the 50 kHz second i.f. frequency.  Most common signal generators don't go
that far down so your choices become limited (actually Hallicrafters made a
signal generator just for that very purpose).  The military surplus URM-25s
work fine but a rebuilt one (and almost all of them need a thorough going
through) is going to be fairly expensive.   Another one that you can find
from time to time that works fine for old tube gear is the Aul Instruments
SG-1144u (they have their problems too -- particularly the power supplies).
The SG-1144's when they are working are very stable solid state generators
and are a real asset if you're working on old receivers. 

Finally, a counter might be helpful.  They do a great job is setting up
signal generators AND I use 'em all the time to check the output frequency
of local oscillators in receivers.  They make alignment MUCH easier.  Here
I'm partial to used HPs.  They work great, are inexpensive and are fairly
easy to find.  The Chinese alternatives you can find on eBay work fine too.

Again, good luck with your project.  

Dave Cohen


-----Original Message-----
From: hallicrafters-bounces at mailman.qth.net
[mailto:hallicrafters-bounces at mailman.qth.net] On Behalf Of Jason Hissong
Sent: Tuesday, November 15, 2011 10:51 AM
To: rbethman
Cc: hallicrafters at mailman.qth.net
Subject: Re: [Hallicrafters] My First Boat Anchor Restoration: SX101 MkIII

Glad I mentioned the meter I had.  Looks like I need to find a VTVM like a
Heathkit or an HP previously mentioned.

So much to learn!  It is a different world.

Jason N8XE

On 11/15/11, rbethman <rbethman at comcast.net> wrote:
> Pay careful attention to the MANUAL!
>
> AC is read with a 1000 Ohm/Volt meter!  DC is read with a VTVM!
>
> There is a BIG difference.
>
> The B+ is 280VDC.  It will bite you worse than 120/240 VAC!  DC WILL 
> cause the muscle(s) to clamp down.  If you get it in the palm, and are 
> grounded, you will NOT be able to let go!
>
> This is different than AC.
>
>
>
>
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