A curious thing in this Predicta

[AlanInSitges]

Hi all,

While waiting for parts to arrive for my Predicta, impatience got the better of me and I decided to see what she would do. I brought it up over the course of about 20 minutes on a variac, and when I hit about 60V got some hum from the speaker and a dim, distorted raster in the center third of the screen. So far so good, as I was preparing to shut off the power I heard the dreaded POP, loss of picture and sound, and a little whisp of smoke from where I could not tell.

I set it aside but today decided to figure out what had failed. I suspected the failure to be part of the B+ supply, maybe one of those doubler capacitors shorted. I removed the chassis from the cabinet, and connected it up leaving the CRT socket unplugged, thus opening the filament string and leaving just the B+.

But no. Fusible resistor is AOK, no sign of ever having been hot. I put an ammeter in place of the fusistor and gave it a little bit of juice, around 20V. It was drawing less than 1mA.

But! Something smelled hot. After touching around I found the thermistor to be fairly hot. It measured 200Ω, about halfway between cold and hot spec. I started measuring resistances to ground from the thermistor through the filament string because it seemed strange to me that it would be getting hot at all given the CRT is unplugged and the filament string is an open circuit in principle:



From the filament side of the dropper, I got 94Ω to ground. Pulling the tubes one at a time starting with the 12DQ6, the resistance measured infinite when each tube was pulled, until I got to the 6AW8 (V4 above). Pulling the 6AW8 the resistance to ground didn't change, it seems that whatever is going on has got to be between V5 and V4. But I can't find anything - none of those little ceramics are shorted, V4's pins 4 and 5 measure infinite to ground. How is this possible? There is also no resistance chart like most Sams have.

Anyone want to give me a sanity check? Thanks.