ctc12 clone beam current

[timmy]

Originally Posted by DaveWM

the HV adj should not "seem to work" it should work, in a very linear fashion.

Disconnect the pot from the circuit and confirm (preferably with an analog ohm meter) that it is working as it should (500k to near 0 resistance as you adjust).

Confirm the voltage readings at the shunt tube grid and cathode, make sure the grid varies with the pot.

Confirm the total bias of the shunt tube is within specs of the sams.

At some point I recall you saying the HV would go to 30kv with out the CRT anode attached.

If now you are saying the max is 19kv without the CRT attached then only 3 things can be wrong.


Remember you are working with high voltages all over the place (400v around the cathode and grid of the shunt, 30kv at the anode) so be careful.

1) bad shunt tube

2) bad shunt tube bias

3)no loner getting 30kv with shunt tube disconnected.

this set is nuts, lol, lol, the 30kv was with the shunt tube cap off. i have tried 4 shunt tubes all had the same result , no change. as for the voltages at the shunt tube they were where they should be. and 19kv was with the anode wire off the crt. as well as the currant too was ok. i had unsoldered the pot and checked the ohms and again was good.

[DaveWM]

lets break it down

all the following with CRT anode off to take the CRT out of it.

shunt cap off
30kv at anode lead

shunt cap on
19kv to 22kv

HV setting to max

so the shunt is drawing current with pot a the max HV setting

two things can cause it to draw excessive current

1)bad shunt tube (gassy/grid emissions), replace with a KNOWN working one.

2)incorrect bias (voltage divider resistors/pot)
leakying cathode/grid cap on shunt tube (upsetting the bias as well).

It has to be one of the above.

Did you alter any wiring around the shunt? does the cathode resistance to ground change the the HV pot?
do you have resistance checks for the tube pins on the shunt.
did you REPLACE the grid to cathode cap on the shunt? if so did you make sure the replacement cap was good?

[Electronic M]

I wonder if your HV reg tube socket is making proper contact with the tube pins? If not that would explain correct voltage at the socket, but incorrect behavior of the tube.

[timmy]

Originally Posted by Electronic M

I wonder if your HV reg tube socket is making proper contact with the tube pins? If not that would explain correct voltage at the socket, but incorrect behavior of the tube.

is it possible the wire burned inside the insulation ? if it did then i would pull the wire slightly and if it stretches then that could be it so ill look into that. the pins look very good and the tube fits tight in the socket, but i could check that again.

[miniman82]

19kv with the HV pot maxed and the cry disconnected means a shunt regulator problem, does it have one of those 100meg resistors in the circuit somewhere? Might have drifted. You need to take readings of the regulator tube cathode and grid, post them here. The bias on the regulator is certainly funny...it's dragging your HV down for some reason.

[timmy]

Originally Posted by miniman82

19kv with the HV pot maxed and the cry disconnected means a shunt regulator problem, does it have one of those 100meg resistors in the circuit somewhere? Might have drifted. You need to take readings of the regulator tube cathode and grid, post them here. The bias on the regulator is certainly funny...it's dragging your HV down for some reason.

it has 2- 1.5 meg resistors and i checked them and they drifted very little 1.6 and 1.59 not so bad within range. i already posted the voltage findings and the ma from all the few wires coming off the 6bk4 reg tube. please check further back on this thread for the voltages they are here. i think there were a brown wire, a green wire and i think a yellow coming off that tube.

[Username1]

I think you should try switching places of those 2 1.5meg ohm resistors......

Turn the horiz centering control.... Does it do anything....? What exactly does it do....?

Turn the focus control, do you have a range on it....? Does it do a lot, or not....?

.

[zeno]

A long shot but there is usually a low ohm resistor
in the HV rect filament inside the HV cup. Also if you unplug the CRT
socket does the HV come up ? May have been covered but the thread is
getting long !
And dont forget the H. blanking tube, causes all sorts of odd problems.

73 Zeno

[timmy]

Originally Posted by zeno

A long shot but there is usually a low ohm resistor
in the HV rect filament inside the HV cup. Also if you unplug the CRT
socket does the HV come up ? May have been covered but the thread is
getting long !
And dont forget the H. blanking tube, causes all sorts of odd problems.

73 Zeno

i think i tried another blanker tube with no change and there is no resistor on the hv rec socket. the crt plug off only comes up alittle bit.

[timmy]

Originally Posted by Username1

I think you should try switching places of those 2 1.5meg ohm resistors......

Turn the horiz centering control.... Does it do anything....? What exactly does it do....?

Turn the focus control, do you have a range on it....? Does it do a lot, or not....?

.

the horiz centering control is just about all the way to one side it will swing from one side to another but to center the screen the pot is all the way to one side. the focus does have range although it could be a bit clearer. and the 1.5 meg resistors i was going to try 2- 1 meg resistors but that i think will only kind of change the pot position from maybe maxed to the half mark.

[Username1]

Well changing the resistors from 1.5 to 1.0 would not do much.... I figured if they
were off, like the one you said was 1.5 and the other 1.6 and the pot .5 meg, then
it may have changed the range of HV if it shifted the bias on the grid of HV Reg.

So, with no plug on end of crt, hv changes by how much...?

And no HV lead attached to tube HV goes to how much...?

Can you make both of those readings again with no other work done in between...?

And one more.... HV connected, Plug on tube.... Brightness at cutoff - Dark Screen..... What id HV.....?

Make each of these tests with the HV adjusted to MAX.....

And write out each reading, and condition for the reading....
Acceptable answers are not "same as before" and or "they are good".
We need the condition, and the value - please.


And the cap between the grid and cathode of HV reg, you replaced that one....?

Anyone got an objection to running the set without that cap for a test...?
Just in case it's not working at that high voltage....

As a matter of fact, you should make these tests on both your sets with the same
chassis, or allow someone with the same set chassis to do them and have an
ongoing comparison..... And for each of these tests, please also record
the B+ Boost..... I think it's 390V.....


.

[timmy]

Originally Posted by Username1

Well changing the resistors from 1.5 to 1.0 would not do much.... I figured if they
were off, like the one you said was 1.5 and the other 1.6 and the pot .5 meg, then
it may have changed the range of HV if it shifted the bias on the grid of HV Reg.

So, with no plug on end of crt, hv changes by how much...?

And no HV lead attached to tube HV goes to how much...?

Can you make both of those readings again with no other work done in between...?

And one more.... HV connected, Plug on tube.... Brightness at cutoff - Dark Screen..... What id HV.....?

Make each of these tests with the HV adjusted to MAX.....

And write out each reading, and condition for the reading....
Acceptable answers are not "same as before" and or "they are good".
We need the condition, and the value - please.


And the cap between the grid and cathode of HV reg, you replaced that one....?

Anyone got an objection to running the set without that cap for a test...?
Just in case it's not working at that high voltage....

.

hv without the crt is 19kv adjustable by the pot but 19 at max. hv with crt 22kv adjustable by pot at max. the cap you are talking about is the 033 i think and was changed a few times with new ones. at the cap that is the place for the jumper for the hv current and with that jumper section open with no meter in between the hv will not adjust by the pot and it goes over 30kv. brightness down dark screen the hv is between 22-23kv hv pot maxed brightness up the hv drops more so depending on whats on the screen. the b+ is in place it is good that was the first thing i ever did check and i find myself still checking it, lol,lol...

[Username1]

And what are the voltages at pin 1 and pin 5 of the 6BK4 Shunt when the HV adjust
is at min, and max....?

.

[timmy]

Originally Posted by Username1

And what are the voltages at pin 1 and pin 5 of the 6BK4 Shunt when the HV adjust
is at min, and max....?

.

6bk4 reg tube pin 1 402 volts and pin 5 390 volts but i dont remember if the bright was down when i had checked it as i hope this helps.hv current dark screen hv pot low 1.64 pot high 0.93 bright up pot low 1.16 pot high 0.20

[miniman82]

If you mean .93ma with a dark screen and HV set to max, that's your problem. Most of the Sams I've read for roundies specify .8ma with a dark screen, HV set to whatever the schematic calls for. If you're pulling any more than that, chances are you're getting close to melting the 6BK4 as you're likely exceeding it's dissipation. You see HV schematic information has more to to with the regulator tube than it does the chassis.

For example, the 6BK4B (which is what most will have in their chassis) is rated at 40 maximum plate watts. Taking into account a 20% tolerance, this tube can be expected to dissipate somewhere around 32 watts continuously. Typically it comes to around 25 watts though, RCA designed the shunt circuit conservatively to maximize the shunt tube's life.

So for your example with a dark screen and the HV pot at 'low' and drawing 1.64ma (assuming a 23kv anode), that comes to 37.7 watts- easily above what RCA would have designed it to do, and dangerously close to the tube's maximum dissipation. I strongly suspect that if you look at the tube's structure under those conditions with a darkened room, you'll see the plate glowing which can be seen reflected off the bottom part of its structure (the part containing the grid and cathode, it's usually shiny so you can see stuff in it).

For the other example: .93ma@23kv=21 watts which is much better but again, outside what I would consider normal for a shunt tube which should be closer to .8ma. Obviously whenever you have something on screen, shunt current should drop as the CRT beam current picks up where the regulator left off. You should check to make sure the shunt tube's cathode current drops as brightness increases, if not the regulator circuit is suspect. Also take a look at your B+boost voltages, as that's where your regulator is taking its reference voltage from. If B+boost is jacked, so will regulator cathode current as the grid receives it's marching orders from whatever B+boost is doing.