[R-390] COSMOS PTO, starting over again.
Jim Miller
[email protected]
Sat, 12 Jul 2003 19:22:23 -0400
Check to be sure the linearity disk isn't slipping on the shaft. That was
the cause of the problem with the one in my Cosmos article on the web. The
disk would slip and cause all kinds of problems with repeatability. It
appears to be welded to the shaft with some kind of tiny electronic weld
that can break loose. It can be repaired if this has happened. Also you
would be wise to build a little test jig for the PTO. It will save you a
lot of time in the long run. I built one with some cheap lumber, some
metal brackets with holes threaded to match the PTO mounting screws, a few
plastic couplings, a foot of brass stock shaft, and John Harvie's PTO
calibration sale attached to a plastic disk. I will take a picture and post
it on the web for any who want to do the same. I used a counter attached to
the PTO, and a lapttop with an Excel spreadsheet that plots the tuning curve
(I type in the counter measurements at each quarter turn).
The little test jig eliminates the need to swap the PTO in and out of the
receiver, and provides relatively easy access to the tuning screws.. You
will soon go bonkers if you keep doing that. I'll see if I can get that on
the web also. It took me a few hours to build it, but saved a lot of hours
in return.
It's nice with the spreadsheet to be able to "see" the linearity error on
the screen and know what areas need tweaking. Before you decide that the
Collins PTO is better, keep in mind that it's stack of linearity "discs"
inside the PTO are horrible to work with, and you have to keep taking the
shield can off and back on again. I can linearize a Cosmos in an afternoon
if it is working. Took me a week to do a Collins (a few hours a day). Jim
N4BE
----- Original Message -----
From: "Phil Atchley" <[email protected]>
To: <[email protected]>
Sent: Saturday, July 12, 2003 2:32 PM
Subject: [R-390] COSMOS PTO, starting over again.
Hi.
Y'all are gonna to get tired of all these Cosmos PTO's. But! Let me tell
you, you're not as tired of them as I am of working on the blasted thing.
In trying to adjust the linearity of it after having changed out the bad
6BA6 tube I noticed that I wasn't getting "repeatability" on the linearity
screws. I could tweak one, tune away and come back and it'd be off. In the
most severe cases by a couple KHz, most of the time a couple hundred or so.
What was maddening is that it demonstrated no discernable pattern as to when
or where. . .
So, I've stripped the PTO down completely AGAIN. All the way down to the
inner sanctum where the linearity "guts" lie. Upon pulling the "spring ring
thingy" and checking the plunger of the linearity coil I noticed just the
tiniest bit of roughness or grittiness in its action. Very minute, but
possibly enough to prevent repeatability of the core position.
So, I unsoldered the wires of the coil and removed it thinking that would
allow me to remove the Plunger and clean/lube it. WRONG!! It is a sealed
assembly with the coil.
So I did the next best thing. I used some semi-synthetic grease and applied
it to the part of the plunger that was visible (after wiping that part
clean). I did this a couple times, each time working the plunger in as far
as it'd go to spread it around. NOTE: DON'T use oil as it may run down the
plunger and contaminate the coil core. The plunger now is smooth and silky.
Don't know how many years it'll last before the grease hardens some but I've
had good luck with this Valvoline DuraBlend on many items. It "seems" to
stay "greasy" and not harden any.
Next, I am backing all the linearity screws out again and pre-setting them
one turn clockwise to give me a good starting point and the BIG job of
tuning will start again. Luckily I have that down rather pat. As I
mentioned before, I tune it IN the receiver. I leave the rear mount
attached to the PTO (but not screwed to the receiver frame), leave the front
screws unscrewed and that allows me to hold it in place against the Oldham
couple for tuning and EASILY lift it out for tweaking.
NOTE to Barry: This doesn't put any undue shock on the PTO as I lift it out
very easily and tilt it back for adjustment. I HAVE to be gentle so as to
not knock the tuning shaft position out of place.
HEATER NOTE: This time upon disassembly I completely removed the thermostat
and the heaters from inside the PTO. I NEVER use them anyway and they just
complicate the teardown-rebuild of the unit. Yes, I'll keep the insulation
intact as I'm sure it adds to the thermal stability.
One FINAL note. IF this doesn't get it this time I'll be giving up on this
PTO and will be looking for a Collins or other unit ;-)
73 de Phil, KO6BB
****************************************************
* DXing THE WORLD WITH THE FOLLOWING EQUIPMENT. *
* RECEIVERS: *
* '67 EAC R-390/URR Surplus Receiver. *
* YAESU VR-5000 Wideband, REALISTIC DX-392. *
* Home built Longwave "Hybrid" CW receiver. *
* Others which pass thru or are waiting repair. *
* ANTENNAS: *
* Coil Loaded 18.5 Meter Long Sloper. *
* Roof mounted Hustler 6BTV 6 band Vertical. *
* Scantenna Scanner antenna at 7 Meters high. *
* LOCATION: Merced, Central California. *
****************************************************
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