this is whats blowing me away because this is the arrangement it was befor

so which is it?

ok dave i have to run out yet again so i am going to confirm any of my mistakes and i will get back to you tomorrow. i see your quote from me so i am going to test the lugs again and get it right this time as we really got far ahead of things and i guess it messed me up,lol,lol. ill send you a PM with the number.

Timmy and I have been in contact over the phone.
I believe he has the flyback hooked up right.
A test was done with a known good 6BK4 as follows:

bad set 6BK4 installed in known working set, Known working set exhibits the same problems so the 6BK4 is causing the problem

Known good 6BK4 is installed in Bad set, 6BK4 quickly appears to short out around the filament leads. Tube is tested by retrying in known working set, confirmed prior good tube is toast.

So we concluded the regulator tube is the issue AND the bad set seems to be creating bad tubes from good tubes.

http://scottbecker.net/tube/sheets/137/6/6BK4.pdf

Timmy is going to check the HK voltage potential to see if it is exceeding 225v per the data sheet. I am thinking a problem with the voltage divider resistors (180k/150k) off the B+ that is used to float the shunt heater upwards to prevent insulation breakdown around the heater.

Timmy will 1st check the known working set pins 1 and 2 of the 6bk4 socket (HK) voltage and then compare to the bad set. I advised him to simply disable the HV for now by pulling the horz out while testing. remove the 6bk4 and read the voltages right a the socket.

I would also like Timmy to check voltage pin 1 to ground (shunt cathode) and pin 2 to ground (shunt filament). again the difference in the two voltages should be less than 225vdc




If there is a problem, I would guess the 180k side of the voltage divider (filament to B+) may have opened up or drifted up dropping the DC voltage on the filament.

Very good thinking..... But with 400V on the cathode I believe he said was there,
or it was on the grid, I bet it's the second resistor, R124 150K to ground... open
really pushing the potential up.... Yah I downloaded that tube spec sheet trying to see
what it's operating parameters are, and any notes on breakdown.... Didn't get lucky...
Good work, be happy to see this thread finished and locked forever....

Hope you get good news from him soon ! !

So how long does it take for that tv to kill one of those reg. tubes....?

.

the K is about +400v so if the heater drops to less than 175v he will have trouble. If
the bottom of the divider opens, the heater would go to +400v, 0 difference vs max of 225.

I guess it could go slightly positive (10-15v) with respect to the cathode ("not recommended to go positive" per data sheet) since the voltage divider begins right at the 405v supply (before the choke and vert out trans where the K voltage comes from, which puts it about 15v lower IIRC)

but anyway its something to check that has not been check yet, so fingers crossed!

Ahh. So you think the source resistor is opening, taking away that reduced potential
needed to prevent heater cathode breakdown.... And once that happens you
think that is not effecting the cathode voltage ??? in a downward direction from
whatever short develops Heater to Cathode.... Interesting....

Oh-Ya My fingers are crossed.... Poor guy a couple-a years chasing this problem....
I wonder how many times this cropped up in the field.... and how long it took
other people to find it....

.

it happened very quickly while he was try to adjust HV. Must have been just seconds into the process.

Timmy can you recall how long?

That's my thinking its such a simple circuit and reg stuff all checks out.
IF it turns out to be the problem you can bet I will be checking those two resistors out regularly, darn 6BK4's are not cheap.

My thinking is this

New tube goes in, gets smoked enough to drop the K down due to leakage just enough to screw up bias, Thinks tube must be good since it brand new, etc...


I had Timmy pull the back off a known good set to observe the look of a functioning tube (how the filament changes intensity as it regulates). I warned him of the X ray exposure so not to do this long just see how it should look when working. Needless the say the bad set tube did not behave the way the good tube set worked.

How would a leaky cap of pin 7 of the regulator tube affect it's operation? Just curious.

That should bring down the dc component as well.... But he said all those caps
were replaced....

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SB is right, that was discussed as well.

all the usual suspects were check or replaced some more than once.

the two 1.5 meg resistors check and replaced
the .0033 grid to K disc cap, replaced SEVERAL times
the HV pot checked for operation
The .1 Filament to chassis, replaced.

when he said he saw sparking around the heater that is what got me thinking to check heater potential. Voltage checks should give us the answer.

I sure hope this is it as I am out of ideas.

Sounds like you nailed it !
I don't remember anything about sparking....
Fingers still crossed.....

.

That came up in conversation, after I told him to try a working tube from another set. He has the cover off so it can be seen, which may have gone unnoticed before if it was just installed with the cover in place. This is why long distance diagnosis is so hard. Its nearly impossible to cover all the bases. But to his credit he did note it and report back, with out that little nugget I would not have thought to check it. The knowledge of how a working regulator tube looks is kinda hard to describe, which is why I wanted him to look at one in a good set. So while powering up the bad set he was looking for the correct operation.
I just really really hope this turns out to be the problem.

I knew it had been replaced, I was just thinking what it's function was. I assumed it was a ground path for the ac pulses on the tube.

:scratch2:ok the .12 cap that i replaced was in question as to what values i put together to make the .12 , so here it is .1- 630v and a .022- 630v in paralle. ok now on to the voltages, pin 1- 6bk4 to ground 407 v maybe the grid here was up a bit at the outlet, pin 2- to ground 94 v . pins 1&2 - 315 volts and pins 1&7 - 315 volts at first start up this 315 was 330 volts but then dropped. each filiment wire to ground was 94 volts so after checking these voltages i did not go into the other set to compare because if this voltage is to be under 225 volts then clearly those resistors must be a problem and i will have to check them. the 180k and 150k thats next....:smoke:

Yah, doing the math, should be around 180v give or take tolerances....

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I just pulled the back of mine, 177v pin 2 to ground about -210 pin 2 to pin 1.

I am posting up a very short video, should be there soon. I will leave the set out in case Timmy gets the HV working and has focus issues, I can look at how I have mine setup.

my video ran out before I could get the pin 2 to ground but I do show the 2 to 1 DC voltages.

https://www.youtube.com/watch?v=ALle...ature=youtu.be

here you go timmy

well i got a problem here the 150 k and the 180 k i had already put resistors in series awhile ago and the 150 tests at 156k and the 180k tests at 182k so im looking for maybe a mistake on the 400v source or the 390v source maybe i crossed one of those sources. i just cant believe this crazyness. i need a smoke first.

if the resistors check out you will need to go back to where they connect up.

the 180k goes to B+400v check there with meter, then check where the 150k goes to ground should be 0v there, then check the junction, should be +180v there. has to be.

if you replaced the resistors you need to make sure you connected up to the correct location, esp the B+ side. During our talk IIRC you said it was connected to the output of the 1s diode of the doubler, that is NOT where the 400v is but more like where the 200v is. the 180k goes to where the 400v is.

if you have it hooked up right (180k to 400v) and the junction is still around 90v, it would mean you have a leak to ground somewhere, disconnect the wire at that junction and check the voltage at the junction of the two resistors again, if it pops up to 180v from the 90v then..

perhaps that .1+.022 cap. if this is the case try disconnecting the .1+.022 cap (lift the ground side) and check the voltage again.

ok i changed the resistors here and one end 180k starts at the diode and confirmed 400v , then goes to a tie strap and then the 150k from the tie to ground its all in the right place.

what junction, the filiments to ground ? so at this point it could be any cap from a positve source to ground causing the filiment voltage to be hi or am i off here.

disconnect the wire that connects to the junction and so all you have is the two resistors in series, then read the voltage at the junction.

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).

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.

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 !

.

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.

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.

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 ! !

.

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.

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.

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....

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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.

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.

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...

.

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.

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

yep that is correct.