hw-101 cw (part 2)

[Steve Harrison]

At 09:22 AM 1999-12-20 -0500, wa1vwl wrote:
>My guess is the PEC is nothing more than R's and C's which makes for some
>kind of phase shift oscillator. The only problem (it's a big one) will be
>guessing what values R's and C's were used.

OR... one could re-design the phase shift network. This isn't as
"Rocket-Scientist" as you might think, because this network is usually
nothing more than an arrangement (kind of a ladder-network) of series
capacitors and shunt resistors from each cap junction to ground. All
resistor values are the same, all cap values are the same. Each series cap
and shunt resistor has a time constant that resonates at the frequency of
oscillation. Each resistor-capacitor combination provides 90 degrees phase
shift at the desired frequency of oscillation. The total combination will
provide positive feedback at that frequency from the plate to the grid of
the sidetone oscillator.

You may be able to measure the value of a shunt resistor at the input or
output of the PEC with your ohmmeter which would at least give one
component value. If you have a capacitor tester, you may be able to measure
the total series capacitance of the PEC from input (plate end) to output
(grid end).

What could fail inside the PEC? Hard to say; but I'd imagine the caps might
have become leaky. Possibly, if the device was installed backwards (and
still worked, which it probably would not!), the end connected to the plate
may have been a shunt resistor and if there was not a DC blocking cap
between the plate and PEC, that resistor's dissipation may have been
exceeded by the plate voltage and may have burned up with time and changed
value drastically. Just guesses. If you see a burned or heated spot on the
PEC, that's probably what happened.

On KB9JJA's Heathkit pages, down near the bottom of the page titled
"Modifications", there is this in the HW101 modification paper by K4JPN:

"CW sidetone oscillator problems, first check V15, then PEC #84-22. This
problem can also show up as sidetone when the rig is in CW mode but not in
transmit; when this occurs, suspect V15 has short. Bleed through of the CW
sidetone when in CW receive can be cured by simply putting a tube shield on
V15."

I could not find a schematic of the HW10x or SB10x so can't
reverse-engineer the PEC immediately. I suspect the sidetone oscillator
circuit of the SB40x would also be similar if not identical. Perhaps the
schematic of one of the other radios has discrete components instead of the
PEC.

Why did they use the PEC? Probably for temperature stabilization of the
sidetone oscillator: to ensure that all the resistors and capacitors
maintained the same value as the radio goes through temp changes.

Another hint that K4JPN gives is to replace all El Menko-brand tubes.

73, Steve K0XP

Sponsored by the City of Tempe 

Listserver Submissions:  heath at listserv.tempe.gov
Listserver Subscription: listserv at listserv.tempe.gov - "subscribe heath 'name' 'call'"
Listserver Unsubscribe: listserv at listserv.tempe.gov - -"signoff heath"