LowB+ in a CBS Ch 817-6 - Videokarma.org TV - Video

Doug66 · #1 11-25-2016, 09:02 AM

I've been working on and off with the set since April, giving up and coming back to it.

I started with no vertical, but a new transformer fixed that. Low B+ was next, about 100V too low.

The only thing that would bring the 340V back up was to disconnect it from pin 8 of the flyback, but that fried C3. I found a NOS new and used flyback as well as a yoke and bought all 3. I put in the yoke which made no difference. Trying the used flyback gave me no light on the screen, but the voltages were still too low.

I put the original flyback back in and this time I did con nect the 340 Volt and 330 volt yoke leads. That kept the voltage right. I connected the 340V lead and all was good. Connecting the other yoke lead to 330V caused it to drop.

Realizing the problem is in the 330V line, I disconnected everything going to it and isolated the problem. 330V goes the R99 then R98 onto pin 4 of the output tube 12BQ6. Disconnecting R98 from pin 4 brings the voltage back up. R98, R99 and C86 have been replaced, and I have tried another good 12BQ6.

I'm out of ideas.
Thanks

kvflyer · #2 11-25-2016, 10:43 AM

Did you replace the filters in the B+ line?

Electronic M · #3 11-25-2016, 10:44 AM

Don't you mean 6BQ6? Have you checked to see if the chassis mounted tube socket is shorted?

old_coot88 · #4 11-25-2016, 11:21 AM

Have you checked R106 for correct value? That's the one from the power xfmr centertap to ground.

dieseljeep · #5 11-26-2016, 09:42 AM

Quote:

Originally Posted by old_coot88

Have you checked R106 for correct value? That's the one from the power xfmr centertap to ground.

I always wondered why they did that! It's not used as a bias source or anything. Maybe they included it to compensate for low line voltage, either reduce the value or jumper it out completely.

old_coot88 · #6 11-26-2016, 10:24 AM

It musta just been a dropper as you say since it's not used as a negative voltage source.

Kevin Kuehn · #7 11-26-2016, 10:56 AM

A resistor in that position can act as a slow blow fuse if a filter cap shorts hard to ground. Before blowing it protects the power transformer from over current. Basically it acts as a shock absorber.

Username1 · #8 11-26-2016, 09:58 PM

C87, C88 replace.
6W4GT Check for H-K shorts, replace.
6BQ6 are the voltages on K and G1 correct?
Does the H osc work do you get HV and deflection H ?
R96 R97 & C2 Good ?

If C3 is a low voltage rated cap, I can see it frying if the voltage
widens between the 340V source and the 330V. Red Herring.

Skip This --> Also look at the Z Z heater source, look for caps to ground
possible shorts.

.

EdKozk2 · #9 12-04-2016, 11:56 PM

Doug 66,
Is that a speaker leaning against the picture tube? Make sure your HV cage is installed before tracing yoke/damper ringing problems.

Doug66 · #10 12-05-2016, 03:46 AM

Ed. Now we're getting somewhere on the vertical. I did get a Stancor A-8141 from Playthings of the Past, but the unit in the box was a 4 wire, so I hooked it up like the original.

Yes, the speaker is resting on the chassis and touching the CRT strap. Currently, I do not have the HV cage cover on the HV cage, but I will put it back on.