Crawls Backward (When Alarmed)

IconProjects, musings about guitar builds, guitar repairs, vintage tube amplifiers, old radios, travel, home renovation, and other stuff.

Hallicrafters S-40A Power Transformer Removal

We now take a break in the Gretsch ukulele action to do some radio destruction.

Long story short, there's a fellow on the ARF who is attempting to rebuild a Hallicrafters S-20.  His (well, the radio's...) power transformer is dead.  I happen to have the remains of a Hallicrafters S-40A chassis, and the transformers are pretty much interchangeable.  So I'm going to test mine to make sure it works, then take it out and ship it to him.

First picture is the business end of the transformer.  I previously scavenged some tube sockets - one of them was the rectifier tube.  So I already have some loose leads.

The AC cord is long gone from the chassis.  So I'm going to use my AC test cord - with bare wire ends to connect it up to the transformer.

I just use two test leads connected to the cord on one end...
...and connected to the AC primary side of the transformer.  This is safe as long as you don't touch the bare connections!

A nice piece of kit to have when repairing old electronic junk is a variac.  A variac is essentially a big wirewound potentiometer that lets you vary AC voltage.  This is handy for slowly bringing up voltage on old radios which may not have been used in decades.

Here's mine - it's a GE.  Dig the cool knob.

I have it wired up with a voltmeter and an ammeter so I can monitor voltage and current.  If there's a lot of current draw, there is a problem - usually a short.  Better to have some control over the voltage rather than zapping full wall voltage on a piece of old gear.

Before I bring the voltage up on the variac, I connect my DMM test leads to the secondary leads I'm testing.  In this case, it's the high voltage B+ output.

With 120 volts AC on the input, I see about 700 volts on the secondary.  About what I'd expect.  This is without a load on the transformer - the voltage in a circuit will be a bit lower.

This tells us that that winding on the transformer is good.

Then I test the other secondary outputs - the 5 volt rectifier and 6.3 volt filament windings.  These too are good.

Here I've labelled the various wiring so we can tell what is what. 

Now I can physically remove the transformer from the chassis.  There are 4 11/32 inch bolts that hold it on.

The transformer drops out easily.

This puppy is heavy - about 7 pounds (3.4 kg).   Part of why these old communications receivers are called "boatanchors."  That iron is heavy!

BTW, see the Red Pepper waiting to be finished (blue arrow).

Top of the chassis.  It was pretty bare when I got it - and I really just wanted the tube sockets.  I'm going to take them off soon and throw the rest out.  There's nothing else of value there.

Other that the tube sockets, the other parts I'll save are the coils - the assemblies on the right side.  I think I can remove them as whole pieces.  Somebody might be able to use them.


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  • otd strans said...
    December 10, 2013 at 1:25 AM
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