141T variable persistence failure fixed!

Chris van Lint

Hi all,

The problem has been solved. After re-checking and re-checking voltages, waveforms and magnitudes, as well as checking the integrety of crimped cables, fitting a new CRT socket, there was still no joy, with my suspicion focussing more and more on the "junction box". Well I was close........ When I could not fix the problem, I put in yet another CRT, this one also almost new. It came out of a mainframe with powersupply issues in which I had just replaced the tube. Again no joy.
Back to the junction box. I decided that maybe I should find some way to check the 6.6KV post accelerator voltage from the tripler. This is not easy, as the tripler is a sealed unit and the output HT wire is moulded into the junction box, from where it is routed to the CRT. I opened the junction box and poked the HV voltage probe at the HV input and behold only about 1.1KV. I replaced the tripler, which is a real pain and time consuming, as it requires almost removing the HV power supply. I switched the gear back on and you guessed it - the variable persistence worked.

When I first posted my question, I mentioned that I had not checked the 6.6KV post accelerator voltage, because that should be OK, since the unit worked fine in conventional mode. Nobody came back to me to tell me that I was wrong, wrong, wrong. This remark is not meant to be a smart a*** snipe, because I am grateful for all the suggestions and ideas. The point I am trying to make is that to engineers familar with storage CRTs, I imagine it would have been so obvious that the post accelerator HV is part of the variable persistence system, that my remark was overlooked. Not being an electronics engineer but a mere amateur (dabbler if you like), I was firmly convinced that the post accelerator HV was only involved in the conventional operating mode. I barely understand how conventional CRTs work and the variable persistence version is a complete mistery to me.

Anoher lesson learned, another problem solved. Once again my thanks to all who bothered to respond.



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