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Moonferret
May 31st, 2017, 07:48 AM
Hi Folks!

I've a Kaypro II on the repair bench (Kitchen table) that has a short on the CRT. Is it possible to just drop in another tube from say an old Mac SE/Classic? Has anybody done this? Sadly, I've not been able to find any schematics for the CRT section of the Kaypro machines.

I have done similar swaps on various 12" CRT's but I'm not sure if there is anything that might cause an issue with these tubes.

Cheers,
Dave

KC9UDX
May 31st, 2017, 01:26 PM
Are you able to get specs on the two CRTs?

Where is the short? You may be able to burn it out, or, shake it free, depending on the short.

Chuck(G)
May 31st, 2017, 02:24 PM
Yup--back in the day, a good tech would have a hefty electrolytic and a power supply and just zap the short. Worst case, you're no better off than you were before.

Druid6900
June 1st, 2017, 07:21 AM
I guess we weren't that good then. We just used a B&K CRT rejuvenator...

Chuck(G)
June 1st, 2017, 07:34 AM
Same idea--I know that several companies made them, including Heath. Sencore is the one I recall most used. At any rate, here's a good description of the issue and its causes (http://www.thegleam.com/ke5fx/crt/sencrt.pdf).

The "rejuvinator" blew away shorts in the same way that I described.

NeXT
June 1st, 2017, 06:51 PM
Same idea--I know that several companies made them, including Heath. Sencore is the one I recall most used. At any rate, here's a good description of the issue and its causes (http://www.thegleam.com/ke5fx/crt/sencrt.pdf).

The "rejuvinator" blew away shorts in the same way that I described.

Well, so long as you didn't overdrive the filament too much. Otherwise you also blew that out and left you with a fancy piece of glass.

KC9UDX
June 1st, 2017, 07:14 PM
I've really abused some CRT filaments over the years. Running for a decade at 12V instead of 6V to overcome a weak cathode, for example. I've run them as high as three times the rated voltage on the bench and never wrecked one that I can recall.

I've done the same with receiving tubes.

I should add that in the past, with CRTs, by the time you did something like that, there was nothing to lose; you could always send it out and have a new gun assembly put in. Hopefully (though it doesn't look promising at this point) we will get back to that point in the future.

gekaufman
June 2nd, 2017, 07:21 AM
Unfortunately rebuilding CRT's is an almost lost art, a good read is at the Early TV museum's site:
http://www.earlytelevision.org/crt_project.html

pcdata76
June 3rd, 2017, 09:03 AM
Monochrome tubes have usually the same tube socket and pinout between the same/near sized tubes so replacement is way easier than a color tube. You also don't need to fight with alignment (purity, convergence) as in the case with color tubes. Simply swap the yoke from old tube to the new, put CRT socket and go.

To be on the safe side, you can check and verify the pinout (at least heater pins are easy to determine using a multimeter) before swap.

PS: Do you know where is the short? Between filament-cathode or cathode-grids? If the short is between filament and cathode, you can continue to use the tube with the short by separating the power supply of the filaments totally from the monitor itself by building a small power supply consisted of a small transformer and a simple regulating circuit. In this configuration, the newly built power supply should be totally separeted (no common grounds for example) from the monitors circuit.

As an alternative, if possible due to space constraints around the core, you can wrap 2-3 turns of insulated copper wire to the core of the flyback transformer and feed the heater pins directly from the ends of that cable after separating the heater pins of the tube socket from the neck board. Before making the connection, measure voltage and verify that it's around 6-7 volts. http://www.electronicrepairguide.com/images/flyback%20core.jpg Note that this method works only for fixed frequency monitors. Do not apply on any multisync monitor.

Moonferret
June 9th, 2017, 04:11 PM
Hi Folks!

Thanks for all the replies & sorry for not replying sooner. I had no idea you could fix a short in a CRT like this. Good to know :)

In the end, I decided to swap the CRT out. The original had a few marks on the screen and also suffered from screen burn. I'm guessing the previous owner used Wordstar a lot!

The tube from a Mac SE worked great and only needed some minor adjustments. Just need to fix the foam pads in the keyboard now.

Cheers,
Dave