Zenith yoke woes... - Videokarma.org TV - Video

nasadowsk · #1 10-12-2019, 08:49 PM

Ok, I'm diving into my 24H20 chassis set, and, I can't get any good sweep/HV out of it. With the yoke connected, I get zero HV and some decent loading of the flyback. If I disconnect the yoke, I get some HV (7kv or so), and the fly seems happier. I'm not sure what else it could be but a bad yoke at this point. Any ideas, or do I need to start hunting one down? See attached schematic snippet, below.

thanks guys!

old_coot88 · #2 10-12-2019, 09:09 PM

How much voltage do you have on the screen grids (pin 4) of the 6BQ6's? It calls for 170V.

nasadowsk · #3 10-12-2019, 09:20 PM

Yeah I was getting about that :/

nasadowsk · #4 10-13-2019, 03:56 PM

Well, I tried swapping the yoke out for a Merit MD70F I have (listed as a replacement in the Sams), and that didn't help anything. So, now I'm back to it not being the yoke :/

Kevin Kuehn · #5 10-14-2019, 01:30 AM

Have you already replaced all the old paper and electrolytic caps? You didn't mention it so I had to ask.

nasadowsk · #6 10-14-2019, 01:35 PM

I'll admit to being a bad boy and not having gotten to all the lytics in the power supply - I have new ones jumpered in though, so I think the PS should be ok enough, I'm reading about 10-15 volts low on B+, and haven't checked the AC ripple yet. I know, I know, I'm gonna get a listing tonight and order the rest of 'em and solder 'em in....

old_coot88 · #7 10-14-2019, 02:11 PM

Whut about the screen grid (pin 4) voltage on the 6BQ6s?

If this voltage were very low or missing, the set might make a little HV with the yoke out of circuit, but lose HV completely with yoke connected.

nasadowsk · #8 10-14-2019, 04:26 PM

Ok, I'm getting 150 volts on the screens of both 6DQ6s. I did pull them and take a look, they both look...tired...

Probably at this point, I think i'm going to rebuild the PS completly and get that ok before going forward. I'm getting a faint raster (I haven't mucked with the trap, but you can see something there), and about 7kv.

Electronic M · #9 10-14-2019, 04:43 PM

If there is B+ boost in your set and it has any paper caps filtering it those could be loading the HV down to the point it won't work properly too....best course of action on any set older than 1960 is change all the caps in the set first, troubleshoot second.