[Motorola] 5654 Tubes in 41V

[Geoff Fors]

Jeff-

The tube I mentioned using in the front end of VHF receivers was the 5654, a
direct replacement for the 6AK5.  I believe you can get them from Antique
Electronic Supply, which has a web page of something like
www.tubesandmore.com . They are not expensive at all. However, don't expect
it to necessarily be the answer to a low sensitivity situation, or to see
much more than maybe a 1 or 2 db improvement at best, unless your present
6AK5 is flat.

In the 41V's I used to service, I often found that a weak sensitivity
problem, AFTER I first tried new tubes in the front end, was often caused by
defective coupling capacitors between various stages in the front end.
These capacitors are those little tubular phenolic jobs with capacitances in
the area of 1.5, 2.0, and similar pf sizes.  Evidently they go open with age
and take away much of the stage coupling.  You can sort-of tell this is
happening if the alignment is completely peaked-out but the sensitivity
improves when you put a metal blade of an alignment tool in contact with the
body of the suspect capacitor.

Another unrelated weak spot in old boatanchor FM stuff is the selenium
rectifiers in the power supply.  The series resistance increases with age
and you will find the B+ voltage is way lower than it should be.  It's bad
engineering practice, but I usually just parallel silicon diodes across the
seleniums and haven't had any trouble.  In a utility base with seleniums, if
you do that, you'll blow the series lamp bulb in the B+ line because of the
initial turn-on surge, unless you add a series resistor to bring the
rectifier loss back to what the original seleniums caused.  The exact value
is a cut-and-try situation.

I should point out that most 152-162 MHz tube gear, and practically all
162-174 MHz gear,  will NOT successfully tune down to the 2 meter band
without some conversion effort.  Specifically, various tuned circuits have
to be "padded" down into the lower range.  It depends upon the strip being
converted as to how much you have to do.  For example, a standard VHF
G-strip receiver out of a Twin-V or T-Power originally on 165 MHz channels
is going to need about 8 capacitors changed or added to in order to
correctly align at 146.52.   A 152-162 range set may only need 4 to 6
changes; it depends upon production tolerances.  Otherwise, you will find
that some coils "seem" to peak when they are really just reaching their
maximum inductance level.  Many poor sensitivity complaints can be traced to
trying to use a "wrong range" receiver strip on the ham band.  I have a
specific set of conversion instructions for the G-strip gear and can post it
on my antique Motorola web pages if anyone needs the data.  It takes about
one evening to fully and properly convert a g-strip radio to the 2 meter
band.  I don't recall what conversion to 2 meters a 41V needs, but probably
there is some necessary.  The ones I have are still on local police
frequencies.

Gross sensitivity problems, along with a test set showing zero Meter # 2
activity on noise, is usually an indication of a defective Permakay filter,
a lifetime guaranteed product which isn't anymore...

Oh, and one final comment.  Don't forget the antenna relay.  Some of them
get pretty corroded and can really cause loss on the receive side.  Try
injecting a signal directly through the phono jack on the receiver, it
should result in the same sensitivity as through the front panel antenna
jack.

My sincerest apologies to all Saber-Spectra-Astro-MT1000 generation members
of the Motorola list-server, who by now must be completely confused and
angered by the sudden increase in bandwidth regarding equipment older than
they are !

Geoff  WB6NVH