Predicta static crashes
 

Hi All,

I was watching my Predicta Holiday the other night and after an hour and a half or so I started getting some static crashes or pops. They were not affected by the volume control, with the volume all the way down they were just as loud. A little percussive testing didn't cause them to happen, just whenever the set felt like it. Any ideas where to start to look?

If the crash repeats when you wiggle a tube, that tube’s socket might be funky, a weak point in Predictas. Sockets might have internal breaks or intermittent connections to the PC board.

Phil Nelson
Phil’s Old Radios

Yes that was my first choice for troubleshooting. I've done 2 Seventeeners, that have an almost identical chassis to the Predicta, and I actually had to replace several sockets in both. I think it's so interesting the Predicta is such a hot vintage find because of design and it's absolutely bare minimum expense and component quality inside...really a "maximize profit margin" TV.

Thanks for the replies!

I haven't done a tube wiggle yet. I'll have to get the chassis out. However, all of the sockets on the PCB were replaced when I got the set running. So a socket is possible but at least I am not using the crappy stock sockets.

It is odd, it sounds like silver mica migration type of crashes but as far as I know there are no such caps in a Predicta.

Thanks for the help! I really don't know where to start looking for this one.

It has to be 'downstream' of the volume control in the audio chain, since it's unaffected by the vol.control setting.

So obviously true. Duh. I'm a little embarrassed. The symptoms do indeed tell me where to look.

So, finally looking at the schematic, there are two ceramic caps, one coupling and one de-coupling. Those aren't known for this kind of problem as far as I know. Prime suspect is probably the 12CA5 socket, it did happen after a period of use and being fully warm. I am not where the set is for a few days but I'll start with a wiggle of the 12CA5.

Any other suspects?

Thanks!

Resistors can be noise sources.

jr

In this case all the resistors were replaced a few years ago with modern replacement. I haven't experienced resistor noise, do they cause pops and crashes?

Yes, tired, old carbon resistors that perhaps have been over heated can produce snap, crackle and pop noises as current is passed through them.

jr

Good to know, thanks!

They really aren't. They used standard resistors and capacitors like everyone else. Yes, the sockets weren't the best. GE used the same sockets in their Coaxial sets on the same time period. GE and RCA used the same type of wire wrapping. Comparing it to an Admiral, Emerson or GE from the same year and you'll see much worse. Then there's Muntz....

At least in those other brands they tended to leave the underside of the board easily accessible so people could better service their junk.