ctc12 clone beam current

[DaveWM]

more from Timmy

with the voltage divider in place but not attached to the filament he get 180v
attaching the filament drops it to 90v, not right.

the filament string reads infinite with a ohm meter (disconnected from the voltage divider) yet when connected drops the voltage.

I advised Timmy to disconnect the .1 cap, as its the only thing that is connected to ground and the filament. The cap replaced but could be the issue if it leaks when voltage is applied. I assume Timmy used a 630v cap for the replacement. FYI I always check new caps going in for value AND leakage at rated voltage.

Timmy understands that the voltage at the 180k/150k should be 180vdc and SHOULD NOT DROP when the filament wiring is attached. All the test where conducted with the shunt tube out of the socket.

Timmy please confirm the above esp the test were conducted with the shunt tube out of the socket (and I hope the horz out tube out also, so you dont have to worry about HV being produced).

[DaveWM]

more updates

further looks at the schematic shows the same filament supply goes to other tubes with elevated cathodes, the CRT and the second IF tube.

thinking maybe a HK short, the CRT is not connected, so he pulled the 2nd IF still dropping the 180v to 90v.

.1 cap off shunt to ground, disconnected no help.

Timmy will trace all wires, disconnect one at a time until 180v is restored and the dropping point is isolated. I did see a couple dog bone caps on the IF strip, grounding that 2nd IF tube filament. So maybe disconnecting the IF H1/H2 (2nd IF heater)supply may be a easy place to start.


Realizing the CRT is at risk (of a HK short) , I think I will routinely check the DC potential of all my set, just in case of some drifting of that voltage divider.

[Username1]

C 309 and C 310 up there filaments to ground at the IF tube also.....

Yah, don't pass on the thought that it could be an internal tube short... I chased one
of those once, refused to believe a tube could do that... Also crud or a wire or something
actually on any of the tube pins, or socket, top or bottom.... Maybe that set is haunted... Cursed....


-WOW- This problem is a real Hum-Dinger !

.

[DaveWM]

Timmy
c309 and c310 are the sams c25 and c26 those are the two dog bones we discussed.

SB is using the RCA numbers from the rca schematic.

[DaveWM]

PROGRESS!!
c26 (C310) burned to a crisp, a likely source of leakage due to carbonization.

he is going to replace and hook all back up, retest pin 1 and 2 of shunt to see if down below max of 225 (was 310).

if that is the case then should be safe to try a good shunt without fear of HK breakdown.

[Username1]

Question.....
When Timmy Makes the reading off the resistors and gets the 180V, is the filament
winding from the transformer still connected....? He is just removing the tube
feeds right...? And it's good DC that you are reading....

How about the old "tin Wisker" trick on any of the tubes that are on PC boards...?
Burnt looking PC board material near those smaller tubes....

AH ! I see you slipped a post right under me ! !

Good deal ! ! ! Fire that baby up ! !

.

[Bill R]

Very interesting problem indeed. I will definately use this information when I do get around to restoring my 12. My parts chassis has no B+. I have never powered up the one in the set since I need a CRT for that one. I may just repair the parts chassis so that when I find a tube I can just swap chassis. I can connect it to my test jig for serviceing.

[DaveWM]

ok here is where we are, just got off phone with Timmy..

HV is holding at about 23-25kv with scene changes. I think it should be better but at least its not down to 19kv.

Max HV with brightness down is about 28kv min is about 22kv.

gray scale procedure does not seem to work, that is service switch to setup, bias at min and try to get all three to just make a line.

Gray scale was eventually set by eyeballing a raster for overall grey/white tint.
did not get too into setting low lights and high lights with screen pots and drive pots.

interesting thing was with just a raster the kv held right at 25kv while adjusting the brightness pot from full dark to full bright, so the regulator took care of that, but seemed to jump around with reg programming.

I am tired, don't know how Timmy can hang with it but I did make a couple things to check

swap the shunt tube from one set to the other (the shunt he was using was from yet another set). He has two silvertones so I suggested comparing the shunt performance side by side.

check the bias pot (it seemed hypercritical, just moving off min would bloom and kill all HV) wanted him to use an analog to judge how smooth it is, only has digital, is suggested at least to confirm the resistive element had not opened at the far side (and the wiper was making the element complete in one sweet spot only).

check the Sam L14 transformer for continuity 7 ohms one side 12 the other.

The setup (using the switch) acted very odd while he was doing it on the phone. Did not work right at all. This bothers me.

[Username1]

On those control pots, I don't have one of those sets, but he has had this problem for
years, and probably played with those controls much more than they were ever meant
to be used in their life... They could be really in need of cleaning, or if they are PC board
based the solder connections to the board could be faulty...

Also that cap you guys just replaced was not subject to that much of a high voltage
while it was working.... 200V ~ There may be other caps same manufacturer that can
be getting ready to go.... Blanking, or Video Amps.... Coupling caps....

Does the set tend to blank-dark, or go white and cut the HV that way....?
I guess it would be a good thing to also check to be sure all that other stuff
around the flyback is all connected, nothing left unsoldered...

If you leave it alone for 20 minutes with a show on, does it do it on it's own
and can you point to specific types of things that cause it...?

Did you replace that 6GM6 that also had the DC added to the filament ??
Might be a good thing to do, just temp swap it with his other set....

.

[DaveWM]

I did not see the CRT so cant say for sure if its blooming and killing it or biasing off killing it but since the HV is dropping I have to assume its blooming too much current. I just want to make sure its not opening on the hot side as soon as the wiper moves, going to ground potential, resulting in a full on bias of the CRT.

with the HV somewhat working I would recommend looking into getting the CRT bias checked. a couple meters on the G1 and K should show whats going on. again I prefer analog as its just easier to see. is the K all over the place with the adjustment of the bias pot? this what I think but it would be good to confirm the G1 is holding steady.

Since he had two of the same chassis (silver tone RCA clones) on thing he could try is to monitor the K/G1 voltages off both when switching from normal to service and then see what happens when adjusting the bias and drive pots.

Also see how responsive the G2s are compared to a working chassis. again I really think a good analog meter is much better for this.

[timmy]

ok L14 measures 11.6 and 6.6 ohms so thats ok. the crt bias measures 6.20 k from terminal end to end and the wiper, the ohms are smooth. what is an acceptable ohms for the bias pot at full counter. all resistors around the service switch test perfect. i even took the pot apart to see if any burn marks or anything that dont look right, it looked new. i am curious as to which wire and where it goes is the actual one that triggers or determines the hv regulation because if the regulator is good now then if there is anything else further up the line in the circuit somewhere that maybe off a bit because this is the biggest improvement in this set since i have it. so its just about there. 6gm6 i changed even though it tests good just the heater lit very fast as opposed to another new one i have so i changed it to avoid anything later with that tube if the heater was stressed in any way.

[Username1]

Well, if he's into it, you could pick a few on ebay and send him a link to a couple
of acceptable quality meters and he can just get one or two...... There were a few
smaller Tripletts that are on there all the time, and are good to have around when you
"need analog" like this case... And they won't break the bank... Under appreciated meters...

.

[DaveWM]

pot and L14 sound good.

I suggest since it seems hyper sensitive to blooming you try the following.

start with just the raster, setup for the good 25kv from low to full brightness.

connect a meter to the crt red cathode lead (yellow/red) note voltage and adj crt bias, see what happens It should smoothly decrease in voltage as you turn clockwise.

do the same for the other cathode leads yellow blue and then yellow green, but not only test the bias pot but the two drive pots as well on those colors (green and blue, there is no red drive pot).

see if all the controls result in smooth gradual voltage changes.

[timmy]

just checked the hv pot its .513 meg full counter and 110 ohms at max, does this sound ok ?

[DaveWM]

yep that is correct.