Will a 21FAP4 work for a Predicta?

[madlabs]

Hi All,

I have loacted a 21FAP4. Will this work for a Predicta Holiday?

Thanks!

Jonathan

[tom.j.fla]

A 21AFP4 is not a direct swap for the 21EAP4 pix tube. However a 21FDP4 is a direct swap, just make the wiring change for the 6.3 volt heater of the 'FDP4 tube. All the best, Tom

[madlabs]

Tom, I'm wondering if the 21AFP4 will work even if not a direct swap. It's OK if the neck sticks out. I'm at work for a 48 hour shift with only a phone, so it is hard to see and compare the data sheets. The only thing I see different so far is the neck is 3/16" longer. I haven't been having any luck finding direct replacements for the Predicta. This tube is available and reasonably priced.

Thanks!

Jonathan

[N2IXK]

Here are the datasheets.

The FA has an additional focus grid, and a different pinout. Circuit mods would be needed to make the swap.

[tom.j.fla]

Well you can use the 'AFP4 but there are wiring changes that half to be made to the tube socket for this tube to work(basing is different) and the heater voltage is 6.3 v so the change has to be made for that as well. When you can download the data sheets for all 3 CRT's on a pc and study them and then decide. All the best, Tom

[madlabs]

Thanks so much for the help!

Changing the socket wiring and the heater voltage doesn't seem too bad. However, circuit mods sounds a little more complex. Considering the difficulty (and I'm guessing cost) in finding a NOS original tube it seems worthwhile. What kind of circuit mods are you talking about, N2IXK?

I do see some posts by banderson on this forum about using the 21AFP4. Sorry to be pushing, but if this tube will work I'd like to jump on it before it goes away.

Jonathan

[N2IXK]

I'm confused. Are you asking about using a 21AFP4 or a 21FAP4?

[madlabs]

Man, dyslexic much?! Sorry about the confusion. I have located a 21FAP4.

Just saw your location. We must be neighbors!

Thanks and sorry again.

[bandersen]

I'm pretty sure all you need to do is swap pins 2 and 3. No mod necessary for the 6.3 volt filament. The set uses a series filament string designed for 117 volts and it's very likely you have 121 or more at your outlets making up for the 4 volt difference.

[madlabs]

Bob, thanks for the info. I'm going to go ahead and get the tube. It seems the only way to get the proper tube is to get another set and take your chances.

I live off the grid and could actually set my line voltage. But I'm guessing other things wouldn't like it.

JOnathan

[bandersen]

A few volts up or down isn't going to make much difference. Get the set running then you can measure the actually current draw in the tube filament circuit. Better to go a little lean than run them too hot.

As for the original CRT - I've yet to run across one that couldn't produce an image. Dim and washed out yes, but certainly good enough while restoring the set. So if yours has a good filament I expect it will produce an image.

[madlabs]

Bob, good to know. I'm curious, would you check the set with a slow variac ramp up before recapping or just dive into the job?

Jonathan

[bandersen]

Depends on the condition. If all the wiring looks sounds and the electrolytics aren't oozing, I'd try a power up. Do you have an isolation transformer ? This is a hot chassis meaning that the metal chassis is wired directly to one side of the AC line which poses a shock hazard.

[madlabs]

The condition is quite good, no oozing caps, wiring looks sound. The only replacement parts I see are three caps, all replaced from the top. From the caps looks like late 60's vintage. The flyback has some cracks but looks and smells good.

I do have an isolation transformer and a variac.

Thanks!

Jonathan