Old Part Needed
 

Hello all,

I'm restoring a 50s vintage EICO 720 transmitter. I'm in need of a 50 k
potentiometer rated at 4 watts (or above) that is NOT wirewound.

The old part looks like a thin ribbon-like piece of resisitive material bent
into a circle and placed inside the pot case. The spring loaded center
contact appears to be in good shape, but the ribbon is fractured
(microscopically) in two locations along its length.

I have been unable to find a replacement for this part at any of the large
parts supply houses.

Does anyone know of a source?

Regards,
Peter Barbella
KB1LZH

On Tue, 06 Sep 2005 02:04:27 GMT "Peter Barbella"
wrote:

I'm restoring a 50s vintage EICO 720 transmitter. I'm in need of a 50 k
potentiometer rated at 4 watts (or above) that is NOT wirewound.

The old part looks like a thin ribbon-like piece of resisitive material bent
into a circle and placed inside the pot case. The spring loaded center
contact appears to be in good shape, but the ribbon is fractured
(microscopically) in two locations along its length.

I have been unable to find a replacement for this part at any of the large
parts supply houses.

Is there any hint of who the original maker might have been?

What's the overall diameter of the pot?

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

I would try "Electronic Surplus" in Cleveland -- their number is (216)
441-8500

fwiw -- the manual is available on the BAMA mirror
site --ftp://bama.edebris.com/bama/eico/720/720.djvu
you need DJVU to view the manual -- but it is easy to find on the web.



"Peter Barbella" wrote in message
news:LO6Te.14512$aG.10577@trndny01...
Hello all,

I'm restoring a 50s vintage EICO 720 transmitter. I'm in need of a 50 k
potentiometer rated at 4 watts (or above) that is NOT wirewound.

The old part looks like a thin ribbon-like piece of resisitive material
bent
into a circle and placed inside the pot case. The spring loaded center
contact appears to be in good shape, but the ribbon is fractured
(microscopically) in two locations along its length.

I have been unable to find a replacement for this part at any of the large
parts supply houses.

Does anyone know of a source?

Regards,
Peter Barbella
KB1LZH

http://www.surplussales.com/

I'm restoring a 50s vintage EICO 720 transmitter.
I'm in need of a 50 k potentiometer rated at 4
watts (or above) that is NOT wirewound.

I have an Eico 720 and I'm 99% sure that the drive level pot is
wirewound in mine.

It's just part of a DC divider that sets the screen voltage on the 6AQ5
driver/multiplier. There's no AC so wirewound should be fine.

Lotsa radios in the 50's/60's and QST construction articles show
drivers with wirewound pots on the DC screen divider.

It has to dissipate a good chunk of heat. The way that Eico built this
radio there are all sorts of resistors to drop from the HV to B+ for
the oscillator/driver, it gets real toasty due to all those dropping
resistors.

Last time I looked 50K 5 or 10W pots were available in wirewound, I
think Allied or Mouser had the Ohmite units. I don't think you'll find
this sort of dissipation available in a ceramic pot. I'd recommend
that you go for something more than 4W if you replace it because it
does get warm.

Tim.

I know that mine, like Tim's was wirewound, and I can remember hassling
with a replacement. The wire was very fine and susceptible to
breaking.

These days, if I wasn't too concerned with keeping it exactly as EICO
designed it, I'd probably use a power mosfet with a small heatsink, as
a source-follower driven by a lower-power higher-resistance pot. I'd
include a zener from gate to source, and maybe a 10 meg resistor from
gate to source (in case the pot wiper opened) and maybe a 1 meg
resistor from pot wiper to gate (to limit max pot wiper current). That
would provide a fairly "hard" voltage output at the FET source, which
could be "softened" by adding a resistor in series. It may be useful
to drop something like 220k ohms/1W to ground from the output, if there
isn't already a resistive load to ground in the circuit. The parts
would likely be easier to find than an appropriate direct-replacement
pot, though maybe not.

Cheers,
Tom

Last time I looked 50K 5 or 10W pots were available in wirewound, I
think Allied or Mouser had the Ohmite units. I don't think you'll find
this sort of dissipation available in a ceramic pot. I'd recommend
that you go for something more than 4W if you replace it because it
does get warm.
=================================
Unless you want to restore this valve equipment as much as possible to its
original state ,you could of course adjust the screen voltage with a
carbon track potmeter , using a high voltage transistor ,a high voltage
zener diode and a few more passive components.

Frank GM0CSZ / KN6WH

Unless you want to restore this valve equipment as
much as possible to its original state ,you could of
course adjust the screen voltage with a
carbon track potmeter , using a high voltage transistor
,a high voltage zener diode and a few more
passive components.

True. This transmitter only has a single HV winding, so all the B+'s
for the non-final stage go through big power dropping resistors, and
then there's all the screen bias dropping resistors too.

I haven't added it all up, but this transmitter probably dissipates
close to 70W when not key-down in dropping resistors just to avoid a
second winding on the power transformer. If I were to re-do anything
I'd start by adding another power transformer for screen bias and
non-final B+'s.

Tim.