Low B+ Voltage

[vts1134]

I'm in the middle of a 630ts restoration and I've run into a problem that I need some help with. The schematic below shows the end of the power supply circuit. The screen grid voltage supply is +135vdc. My +135vdc supply is very low, in the neighborhood of +60vdc. Is it more likely that something is dragging down the supply downstream, or that I am not receiving enough voltage upstream? What steps can I take to narrow down the problem?



Thanks for any help/advice you can give.

[maxhifi]

1. Measure the +275V test point, and make sure it is close to 275V

2. Measure R185B and R186C out of circuit, and make sure they're within 20% of spec

3. Have C223A and C224B been replaced? I wouldn't really trust a 60+ year old electrolytic capacitor which hasn't been used in 40 years.

4. If all of above is good, it means there's excess load on the +135V supply.... figure out why.

[vts1134]

All of the above are good. I have replaced the capacitors and the resistors are within 5%. My +275 is dead on +275. I figured that if my problem was upstream my +275 would be low as well but I'm still learning so I thought I'd ask.
Any suggestions on how to narrow down the problem downstream?

[maxhifi]

What's the +135V supply feeding? The way that supply is designed, just using ohm's law there should be about 100mA through R185B. (275-135)/1360

If you're getting 60V, this means there's about 158mA - this is too much current, and you need to find out what's causing the extra loading.

You can use the voltage dropped across cathode resistors to figure out if a tube is drawing too much current. Keep in mind that if the TV is not adjusted properly as per the conditions the schematic's measurements were taken under, it may also affect the voltage readings.

[vts1134]

Fixed a bit of a mistake on my first post. The +135vdc supplies the screen grid voltage.

[maxhifi]

I just downloaded the schematic out of interest - very cool you have a 630, I am jealous

+135 supplies lots of stuff, all over the TV, not just screen grids, but also plate supply to IF stages and a bunch of other stuff. You're going to have to break it down into blocks according to the schematic, and figure out where the problem is the painful way.

[vts1134]

I'll try to remove +135vdc section by section and see where I'm loading down.