"Dave Goncalves" wrote in message
...
I have a 2A that I was unable to get the passband tuner setup so
that the pitch of the noise was equal on both sides of the center.
I setup a sweep generator, 0.1v pk-pk, sweeping from 40 - 60 KHz,

[Technical details omitted for brevity]

BUT! I remember basic troubleshooting. Failures have a cause. WHY
did the passband tuner go out of tune? Caps in the circuit failed?
A short somewhere? I'll find out, but I am wondering if my method
here makes sense.

Dave,

You have my respect! Far too many hams do their troubleshooting of
vintage gear by first using a tube tester and then using a VTVM to
compare measured voltages to a chart. I have always recommended
that they first study the schematic until they understand the design
fully. Then they can use this knowledge to analyze the symptoms and
make an intelligent choice as to which circuits are causing those
symptoms. For example. if signals are weak on one band only, there
is little to be gained from working on the audio stages. Yes, your
methodology makes perfect sense.

While my first receiver was a Drake 2-B, this was nearly 45 years
ago, and I have no direct experience with the 2-A. But I think I
can offer a few suggestions.

I doubt if any of the capacitors failed in the sense that they did
not short or open. Moisture ingress over the years may have shifted
their values a little to the high side however. The coils in the 2-
B were wound on phenolic forms. Moisture ingress in these is very
likely, particularly if they are the traditional linen cambric
reinforced phenolic. Moisture will increase the shunt capacitance
seen by the coils and will lower their resonance frequency. Baking
the passband tuner assembly overnight at 140 degrees will remove
most of this moisture. Allow the assembly to cool for several hours
before testing it to allow it to return to room temperature.

I believe the coil slugs are powdered iron rather than ferrite.
These are more prone to wear and having small pieces break off the
ends since they are softer. Check carefully to see that the slugs
are intact. A little silicone grease can be applied to lubricate
them.

Before unsoldering the slugs, try adding 5 to 10 pF capacitors
across one coil at a time and note if the shape of the passband gets
worse or better. I would not bother with adding or removing
capacitance from the coupling capacitors yet. Getting a nice
symmetrical shape to the passband should come first. Then you can
adjust the center frequency like Drake suggests.

One final thought - using a sweep generator and an oscilloscope can
sometimes be misleading. The sweep rate should be low enough that
you do not see ringing. Also remember that the oscilloscope shows
voltage on a linear scale. If you have access to a spectrum
analyzer with a log scale, you can use a broadband noise generator
as the input signal. If you do this, you may have to average the
signal over many sweeps.

If you need to trim the coupling capacitors slightly, consider the
old approach of "gimmick" capacitors made from two pieces of
insulated wire twisted together. I would use PTFE (Teflon) or PVDF
(Kynar) insulated solid wire for longevity. PVDF is commonly used
for "wire-wrap" wire.

I think your approach is the best one and I am sure you will soon
have found the problem(s) and have that venerable old receiver in
use again.

--
73, Dr. Barry L. Ornitz WA4VZQ

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