Zenith 2250R - Videokarma.org TV - Video

Kevin Kuehn · #1 09-12-2018, 09:03 PM

Riders also calls for 15k screen grid. I suppose you've already changed the HV rectifier and checked it's 1.2 ohm filament resistor?

Kevin Kuehn · #2 09-12-2018, 09:09 PM

Interesting that Sams calls for a 3.3 ohm HV rectifier filament resistor, but Riders shows 1.2 ohm. And I don't see any notes in the Sams parts list specific to model type variations.

*Edit* Oops, I do see a note in Sams that says some sets use a 1.2 ohm. The Riders schematic is specific to your model/chassis.

Zsuttle · #3 09-12-2018, 09:16 PM

Yeah, changed the HV rect and checked the resistor, mine is the 3.3ohm variant

Kevin Kuehn · #4 09-12-2018, 09:33 PM

I'm not sure if 3.3 is correct for your set, it's not according to my Riders schematic that lists your specific model. But I also don't know if that alone could account for your 35% lack in high voltage, I kind of doubt it.

I guess it's time for someone smarter than I to step up and make a suggestion.

Zsuttle · #5 09-12-2018, 10:55 PM

Someone just check this and my logic. Assuming that the screen on the 6BQ6 doesn't provide any voltage, the resistor (R15) faces the whole 340v of the B++. Given this, we can determine the "should be" resistance of R15 based off of the given voltage drop from the Sams voltage chart. If I did my math correctly, the resistance should be about 8K ohms. Why isn't it 8K ohms then?

old_tv_nut · #6 09-12-2018, 11:58 PM

Quote:

Originally Posted by Zsuttle

Someone just check this and my logic. Assuming that the screen on the 6BQ6 doesn't provide any voltage, the resistor (R15) faces the whole 340v of the B++. Given this, we can determine the "should be" resistance of R15 based off of the given voltage drop from the Sams voltage chart. If I did my math correctly, the resistance should be about 8K ohms. Why isn't it 8K ohms then?

I don't see how you can calculate a "should be" resistance value without knowing the nominal average screen grid current.
The average screen grid current can vary with the peak current and the length of time that it draws current, which I would think is a function of the drive control. So, I would look at the drive control value and also the adjustment procedure in SAMS. I would expect the DC screen voltage measurement varies with the drive adjustment (check if it's so?).

Zsuttle · #7 09-13-2018, 10:12 AM

Well, I gave it a shot. I can't find the alignment describing how to adjust the horizontal drive in my Sams. Either way, I've messed with it and it moves the HV up some, but only to about 7.5kv. I noted earlier that the horizontal drive potentiometer tested a little high (34k vs 25k ohms) Not sure if that could be affecting anything.