Hallicrafters SR-150 Operation And Maintenance Manual Download Page 31

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By Walter Cates WD0GOF (UPDATE 10/30/2013)

SUBSYSTEM TECHNICAL INFORMATION CONTINUED

3-6. PA LOADING COIL L32

I became a victim of a long time, incorrect assumption. When I first started working on the SR series I was told

that the plate coil of the SR-150 was a weak element that it could be damaged by too much power or mismatch
on 15 and 10 meters. I thought "oh well" and accepted it and even helped the myth along. I have found many of
these coils deformed and melted and either repaired or replaced the coil and went about my business. While
working on a 150 with exceptional power out on 15 and 10 meters I thought it was time to look seriously into
the problem. Not having had any real problems in the output ckt of the SR-150 in the past I have not paid it any
close attention. When I looked at the circuit and the components I was a little surprised. The end of the coil that
melts is actually shorted out on 40, 20, 15 and 10 meters and is only used on 80.

So I got out my IR thermometer and did some testing. There are only two ways I can get that end of the

coil to heat up. First, heating occurred if I induced a higher than 1.8:1 vswr on 80 meters. There was no
abnormal heating on the other bands until I reached 2.5:1 vswr. Second, heating occurred if I detuned the
neutralization and tuned for peak power and not the plate current dip on 80 meters. The coil heats a little on all
the bands with miss-match or improper plate dip. However, on 80 the tightly wound end gets hot and right now.
On the high frequency bands that end of the coil is the coolest end and it seems to be a heat sink, perhaps even
dumping some of the heat from the other end.

Just to be thorough I tested the shorted portions of the coil on each band for any stray resonance. I performed a

series of tests using the grid dip meter, sweep generator and spectrum analyzer on the coil cold and under power.
I looked at the spectrum for spurs under various conditions of mistuning and found nothing unusual.

So I think this myth is busted. But what has become very clear is, you must keep vswr low and tune the

plate to the current dip. This is critically important on the 80 meter band.

The critical nature of the VSWR and plate current dip applies to the entire SR series HF transceivers.

The coils of the rest of the series are much more robust, with the possible exception of the SR-2000 (but then
the 2000 is in a class of its own). Damage to coils for the rest of the SR series under normal operating
conditions is rare. HOWEVER, VSWR and plate current dip are extremely important to the

life of the P.A.

tubes

. In addition an improperly tuned P.A. will generate

splatter, harmonics

and an overall mushy sounding

transmitter.

The coil can sometimes be repaired using a heat gun. Slowly heat up the plastic and with a knife spread the

coils. NOTE: These assumptions are based on data derived from a series of tests performed on only one SR150.