LowB+ in a CBS Ch 817-6
 

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

Did you replace the filters in the B+ line?

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

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. :scratch2:

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

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.

Hey Guys,
Thanks for the replies.
R106 was replaced when I 1st recapped the set. The original sand coated resistor was still there, so it had to go. I just checked, and the replacement still reads 65 ohms.

As far as the filter caps, I have double and triple checked my work to make sure those are installed correctly. A one point I even took out the new replacements and installed another et of new replacements just to make sure one was not shorted.

I also checked the socket on the 6BQ6. Only pins chowing continuity to ground are pins 2 and 7. I also disconnected one end of C86 to make sure it was not shorted and pulling the voltage on pin 4 down.

What about resistor R100 (47 0hm)?

You mentioned that cap C3 which is in parallel with it, had 'fried' at one time.

R100 (47ohm) has been replaced. Yes, C3 did fry at one time when I disconnected the 340V line from pin 8 of the flyback. However, that did bring the voltage back to normal.

Explain this to me please. if I'm reading the schematic right, the 340V going thru R100 and C3 should be producing the 330V source. It's not. My 330V source is coming off pin 3 (blue wire) of the yoke. I disconnected both horiz wires from the yoke (pins 3 and 1 of yoke. Pin 1 goes to pin 7 of the flyback), and my 340 volts returned to normal. However I had no 330V. I measured the blue wire (pin 3) which was hanging in mid air, and it had 330V on it. Co

Connecting the blue wire back to where the 330V source should be put voltage on all circuits calling for 330V, but too low. That's when I disconnected everything connected to the 330V and found the problem seeming to be at pin 4 of the 6BQ6.

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.

.

It also makes life a little easier for the vacuum tube rectifier. Lower peak currents due to the filter cap recharging between full wave cycles at 120Hz. You can kill a rectifier tube if you have too much filter cap and too low source impedance on it. SS diodes are much more forgiving here.

Hi Doug,
I'm assuming you recapped the whole set. Looking at the entire schematic,did you check what circuit variation your set has. If you use the dotted line wiring changes, make sure your width coil T7 is grounded on one end. The other end goes to pin 5 of the flyback only. The blue wire/ pin 3 of the deflection coil only goes to your 330 volt source junction of R100,R105,C1 and C3. If you didn't notice, the plate circuit of the video output tube 12BH7 is also connected to the 330 volts. Are you getting a 525v, B+ BOOST voltage at pin 10 of the CRT ? Check C85 and C84 the horizontal feedback
caps if your set uses them. Make sure C5 and C6 connected to the red lead of the vertical output transformer didn't short when you cooked C3. A short there would load down the B++ and flyback.
Ed

Hey Guys,

Here is what I have come up with:
C87 & C88 were replaced when I recapped the set.
Also the 6W4 damper and 6BQ6 are new.
On the 6BQ6, K and G1 are fine on voltage. In fact G 2 which is pin 8, and the one in question actually has 130V only 20V low despite the big drop in the B+.

I do have hi voltage. Plate of 12BH7 SB 290V and I have 241.

R96 and 97 have also been replaced as well as C2.

EdKozK2, yes my set does use the dotted line circuit and my width coil does go between pin 6 of the flyback and ground (pin 6). B+ boost voltage on pin 10 of CRT only has 435V.

Here is where I get confused. The red wire of the yoke goes to the 330V source. On my set, that wire IS the 330 source. If I disconnect that wire and leave it hanging in mid air, it still has 330V on it, and there is nothing on the negative side of C3 and the 47 ohm.

Do you also have 330V on the yokes blue wire when you disconnect the red? Ed just mentioned that only the blue yoke wire should be connected to the 330V source. Of course if the blue is connected and you disconnect the red you will also measure 330v on the red, because it's a very low resistance path though that yoke winding to the blue wire. Or are you saying that the blue and red yoke wires are reversed on your set? Do you measure 340v at the top of C3 and R100?