[Hallicrafters] SX-25 recapping question

[Bill Gerhold]

Mike,

I had similar problems when attempting to recap an SX-17.  The more I got
into it, the more I saw I was getting in too deep.  The band switch has wax
caps in it that are virtually buried and inaccessible.  Yes, all the
resistors were reading different than what the schematic said and yes, some
of the components did not agree with the schematic at all.  Numerous
undocumented changes were made during manufacture; changes were made by
previous owners etc.  The final straw was when I then found out; there were
numerous versions that required multiple schematics.  I even found hybrid
schematics with some of the correct parts and some with the incorrect parts.
It got to be too much trouble.  I even had to have an old RCA tube manual on
hand to identify the tube elements and pin #'s since they were not printed
on the schematic.

I opted to replace what could be replaced without having to disassemble the
receiver completely.  My decision was based upon the fact that the receiver
performance was adequate for the time period.  Additional testing such as
receiver sensitivity and receiver alignment revealed adjustments were all
that was necessary to bring the receiver back to or close to original
performance.

In addition, it is important to remember, resistance measurements of these
receivers was performed with a 20,000 ohms per volt meter.  A Simpson 260 is
perfect for these measurements.  A new high impedance Fluke would likely
skew your resistance readings since it is not loading the circuit down at
all.

Bill G.
K2WH

-----Original Message-----
From: hallicrafters-bounces at mailman.qth.net
[mailto:hallicrafters-bounces at mailman.qth.net] On Behalf Of Mike Everette
Sent: Sunday, May 21, 2006 8:23 PM
To: hallicrafters at mailman.qth.net
Subject: [Hallicrafters] SX-25 recapping question

Can anyone recommend a procedure for getting to the
two wax capacitors at the 6K8 osc-mixer socket in the
SX-25?

These caps are buried deep.  I was going to try and
remove the coil-mounting baffles, but it's so tight in
there that I can't even see how to get to some of the
leads that would have to be unsoldered.  I thought the
SX-28 was tough but this one seems worse.

There are 2 caps, an 0.02 uf on the screen and an 0.05
on the cathode (or is it the other way around...?). 
Plus I need to get to the grid resistor on the
oscillator, as it is marked 50K but reads 78K to
ground.  That is a tad high.

The mixer cathode resistor in my receiver read 417
ohms to ground, and the schematic/parts list shows it
as 250 ohms; but when I looked at the resistor it is
actually marked 400 ohms.  I think my manual is dated
1945 but the receiver serial number seems to point to
mid 1941 production, judging by the info on the
Western Radio Museum web site; maybe the 250 ohm value
was a later revision.

If anyone has been into an SX-25 in this area
sucessfully, could you please share your experience?

By the way, darn near every 1/2 watt resistor in this
receiver read anywhere from 40% to 85% too high.  It
looks a lot different under there now, more like a
home brew project than a simple recap.

I can share something else, maybe a pitfall to avoid. 
When re-using the 1/4 onch hex head sheet metal
screws, such as those that hold in the baffles and
secure other parts in these receivers, BE CAREFUL. 
They are old and possibly suffering from corrosion. 
They are either cheap steel, or in some cases maybe
aluminum; I found one that a magnet did not attract. 
If they are overtightened even a little bit, even with
a regular nutdriver, the heads will snap off.  I had
two do this.  Fortunately I was able to back them out
using pliers, from the top of the chassis.  It would
be better to replace these old screws with more
durable ones, than attempt to reuse them.

Thanks and 73

Mike
WA4DLF

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