Palomar 300A missing power transformer.

[nomadradio]

Talk about a never-ending story. The eternal search for a transformer to run the Palomar 300A you just bought from the neighbor's grandpa's basement. Of course the folks cleaning out the house looked at the 300A RF deck and said "somebody will pay for that". Then they looked at the heavy black lump with a 3-foot cable, 8-pin plug and no name. Couldn't tell what it was so it got sent to the landfill .

This iron in the fire is finally getting warm. Bought an industrial steel enclosure that cost almost as much as the big transformer.

The published dimensions tell you the inside clearance. Big enough for the Antek AS-3T275 transformer. Just one problem the lip around the opening is almost big enough to get the toroid inside the box.

Oops.

Angle grinder time. Gotta find my hearing protector first, though.

The small transformer stacked on it supplies 12 Volts for the heaters in six tubes. The big transformer has two 6-Volt secondaries, but they won't supply the full 6 Amps (or so) needed.

Gonna pick up some rotor cable to feed the 8-pin plug.

It's coming together.

Gonna find out if anyone will pay what it costs to whip this trick together. I'll make it open source, so anyone who wants to can duplicate it.

Back to work.

73

 

[nomadradio]

Finally managed to put this together. Got the opening of the box gouged out holes drilled and a cable made.

Found some rotor cable, Wireman (Certified Quality brand) 8-wire with two larger-gauge wires. Those are to power the brake solenoid in a CDE-type rotator.

Pretty sure his catalog number is "302". Just one problem, it's stiff as a board. The outer jacket is incredibly rigid and thick. Just the thing to make it last a long time whipping around in the breeze under your beam. Not so helpful for this. Gonna have to find something more flexible to make more of these.

Figured I'd use the two fat wires for the 12-Volt tube heaters, since they draw about 6 Amps.

But it has eight wires. Ended up doubling up two pairs of wire for the 12-Volt circuit. The fat two wires are number 16, and the other six or number 18. If I double up two #18s they have more metal in them than a single #16. So I used the fat wires for the 120-Volt primary, two pair of #18 for the 12-Volt secondary and the remaining two #18 for the high voltage.

The P308 Jones plug is a PITA.

Only had to gouge one edge of the opening to get the big transformer inside. The small one went right in.

The big toggle switch is the high/low selector for the high-voltage secondaries. The 300A with a full-wave bridge rectifier gets selected with the switch one way. The full-wave doubler version of the RF deck gets half that voltage. The switch simply takes two 275-Volt secondaries and puts them in parallel or series.

Figured I would label the switch "Operate" on the low-voltage side and "Explode" on the high-voltage side. Even if I hide the switch inside, you gotta know someone will flip that switch just to see if he gets Mo Watts.

And if he calls to complain that his RF deck exploded inside, I'll just point out the truth in labeling part of what he ignored.

Haven't tried it. Got it to this stage when it was time to go home for dinner. Better to try it out fresh and wide awake in the morning.

Besides, there is a feature I need to add. When Palomar first sold the 300A they had a warranty problem with transformers failing. Turned out that a short on the 12-Volt circuit in the RF deck was the culprit. The low-voltage secondary of the transformer would not pull enough current when it was shorted to trip the main fuse. As a result, the 12-Volt secondary would just cook until the whole transformer was roached. Palomar responded to this by installing a 15-Amp fuse and fuseholder in the 12-Volt side of the power transformer. Back then we insisted on adding this "feature" any time an older 300A came in for repair. Decided what I should do about that is put a 1-Amp breaker in the 120-Volt primary of the small transformer that supplies 12 Volts AC. Got some of those, just gotta drill the hole and move wires.

73

 

[nomadradio]

The Palomar 300A power-transformer replacement project got stalled by an odd issue. The cord that goes from the transformer to a 8-pin Jones plug was too stiff. A generous customer had offered me a leftover length of "Wireman" or "CQ" brand rotator cable for this. Seemed sensible. Has one pair of fatter wires meant for the rotor's brake. Figured to use that for the 120-Volt primary circuit. Just one problem. This stuff is as stiff as PVC icemaker tubing. Maybe stiffer. The jacket is incredibly thick. Now, hanging from your beam down to the tower flopping around in the breeze alongside your rotator is hard service. This stuff should last a long time in that service. But if I moved the transformer box, the wire would pull the plug on the other end enough to skew the plug in its socket.

Not good.

Found some more-flexible rotor cable, Jetstream JT1816. This stuff is about perfect. Time to pick up where I left off.

Hooked it up, found only about 10.5 Volts on the meter light. Handy spot to see what the heater voltage is. Not gonna cut it. The 50 Volt-Amp transformer I had chosen to power the heaters is not big enough. Well, live and learn. The reason it's there is that the two six-Volt secondaries on the big transformer are only good for about 4 Amps. Figured a little over 6 Amps with six tubes. So I cobbled the unused 6-Volt sides of the big transformer in parallel with the small transformer's output. I had cut the 6-Volt wires on the big one short, but not too short to hook up if needed.

Turns out they're needed. So it's back to the drawing board on this one. I would rather have one small transformer big enough to run the 12-Volt side of the amp. This one ain't it. Brings up the reading at the meter light to 12.5 Volts. Clamp-on meter says it draws seven Amps. Close enough for now. Not pretty, but that's not necessary for a prototype.

I did make good on my threat to label the high/low HV winding selector switch. This amplifier has a full-wave doubler HV circuit, so it needs the lower 270-Volt setting.

Big attraction being able to power this amplifier up, I finally get to try my substitute relay circuit board.

And it works, thank goodness. I have two more of this prototype board. It needs a couple of refinements before it becomes a product.

And the transformer project needs more than a beefier heater transformer. Needs an enclosure that doesn't cost me north of fifty bucks. This galvanized box was the first one I found that looked like a good fit. Didn't do a lot of shopping around.

Time to do some of that now.

73

 

[nomadradio]

Now that this wacky idea works, reality sets in.

The cost of materials for this thing, as always, will be lower if you shop around. Just the same, leaving room for the labor to build the thing is gonna make it unreasonably expensive.

Here's what goes into it.


Bill of materials for the Palomar 300A universal replacement power transformer.

1) Antek AS3T275 6 lbs $62+SHPG
1) Antek AS1212 4 lbs $26+ "
1) DPDT on-on toggle 15+Amps $6-$20, depending on source
1) bracket to mount the thing
1) 1 Amp circuit breaker $6-$30, "
4 feet) Jet Stream JT-1816 8-wire rotor cable $8
1) P308CCT 8-pin male Jones plug $12-$30
2) 3-terminal tie strips for primary wires $3-$8
1) clamp/gland nut for the cable $2-$3
1) enclosure, at least 6" by 6" by 6" $30-$60

Worst-case from above range of prices:
Material cost alone $247

Best-case $155

I'd have to be building a lot of them to get the labor much below 100 bucks. Finally found some enclosures with a more-reasonable price than the electrical box we used first time.

This could turn out to be another of those "Just because you can doesn't mean you should" ideas.

73

 

[Naysayer]

just because you can, doesn't mean you should. I get that lol.