WG 19K4901 is VERY blue

[kid sinister]

My friend has a Joust machine with a Wells-Gardner 19K4901 monitor that has a problem with the video.  After the game has been on for a few minutes, the screen will turn a very bright blue, with lighter, stationary diagonal lines going from the upper right to the lower left.  I can hear the gameplay continuing and can barely discern the text on the screen, so I know that the game hasn't frozen up.

Now this is the first arcade machine that I've ever worked on, and I've been super careful to discharge the tube and everything.  I have been into hobbyist electronics for a few years now, so I'm pretty good with a soldering iron.  The first thing I tried on it was a cap kit.  While the colors have definitely improved, after a few minutes the blue problem comes back.

So I pulled monitor out again and checked all of the soldering joints.  I noticed that the casing was flaking off of the thermistor on the main board.  It's my understanding that the thermistor is part of the degaussing circuit, which couldn't cause the problem I'm having.  Anyway, I redid all of the questionable joints and put everything back together.  I turned it back on, same problem.

Not sure what to try next, I tried hitting things.  Not hard mind you, I just lightly tapped various components with the handle on my screwdriver to see if something was loose.  It was then that I noticed something: if I tapped the edge of the neck board with my screwdriver handle, the screen would briefly flicker back to the normal colors.  If I tapped it enough, the colors would return to normal... for a few minutes.  I think that I have a loose connection in the CRT socket.  My question is how do I go about fixing it?  I've already contacted Wells-Gardner, but they don't carry a CRT socket that old anymore.

Second, should I be worried about that thermistor?

[qrz]

symptom is " blue w/retrace "
with power OFF, lift the collector lead of the blue driver xistor ( neck board).
with power ON , measure the voltage from the collector pad to ground .
 there should be around 145 volts dc present . if this voltage is low, there may be a heater cathode short in the crt
or a short in the spark gap ( part of the crt socket )
if voltage IS present , the xistor may be leaky/bad... replace .
there is a way around the hk short with out replacing the tube . 
pse advise what u find ......

qrz

[Luigi]

We had the same problem (Joust) with two differences:

1) it was red
2) it was right from starting the machine

We replaced both "red" BF871s on the neck board and it was ok. Don't know if this helps with your problem 

[kid sinister]

symptom is " blue w/retrace "
with power OFF, lift the collector lead of the blue driver xistor ( neck board).
with power ON , measure the voltage from the collector pad to ground .
 there should be around 145 volts dc present . if this voltage is low, there may be a heater cathode short in the crt
or a short in the spark gap ( part of the crt socket )
if voltage IS present , the xistor may be leaky/bad... replace .
there is a way around the hk short with out replacing the tube . 
pse advise what u find ......

qrz

I pulled that collector pin off of the pad and tested it with my multimeter.  When I first started it up and the colors were normal, my multimeter said that there's 173.8 V present.  But after the monitor turned blue again, the blue collector pad measured 30.1 V.  Which is my problem?

[qrz]

with the collector lead lifted , the pad should stay at the supply voltage ( 173.8 in ur case)
 the pad dropped to 30v when the cathode shorted to the heater filament. causing intense blue w/retrace
the proper fix is to replace crt   
but, in many instances , simply isolating the filament from ground will do the trick .....

to wind ur own filament coil , obtain a length of stranded insulated wire .
run the wire just over half way through the gap between the exposed core and the transformer
(this should be opposite the coil)
tightly loop the wire around the core for 3 parallel turns ( will produce approx 2 volts per turn)
once the coil is wound , gently twist the wires together to secure in place (a wire tie works as well )
this completes the coil.
next is to remove the filament connections from the original circuitry.
when in doubt, just pull the appropriate leads at the crt socket .
strip one coil lead and connect to fil lead . strip the other and connect to a 2 ohm 2W resistor .
connect the other side of the resistor to the remaining fil lead .  secure resistor with proper ventilation
if done properly , the filaments now have an ground isolated power supply which negates the cathode
grounding effect caused by the the crt in the original circuit.

qrz

[kid sinister]

I'm confused, where is this exposed core and transformer that you are talking about?

[qrz]

ok...,
the ihvt is the big black thing with the red wire going to the crt.
there is a square thingy that is going through the black part . that is the ferrite core .
also, should be a metal clip on it .
or search : "flyback, ihvt, or tv high voltage transformer " and see what u get


*all we are doing is making a new isolated filament winding to replace the original ground referenced winding.
in the "good old days" ,  pic tube brighteners did the same basic thing , but with a slightly higher output voltage.
haven't seen any on market in , well.... 30 years or so !
and yes, i am "old"  . thank you very much 

* is this something the forum would like to see demonstrated on video . such as youtube or similar ?

qrz

[SavannahLion]

* is this something the forum would like to see demonstrated on video . such as youtube or similar ?

I dislike YouTube but you get a yes vote from me.

[kid sinister]

ok...,
the ihvt is the big black thing with the red wire going to the crt.
there is a square thingy that is going through the black part . that is the ferrite core .
also, should be a metal clip on it .
or search : "flyback, ihvt, or tv high voltage transformer " and see what u get


*all we are doing is making a new isolated filament winding to replace the original ground referenced winding.
in the "good old days" ,  pic tube brighteners did the same basic thing , but with a slightly higher output voltage.
haven't seen any on market in , well.... 30 years or so !
and yes, i am "old"  . thank you very much 

* is this something the forum would like to see demonstrated on video . such as youtube or similar ?

qrz

Oh! I didn't realize that the flyback had other names too.  Learn something new every day.  I have a question though.  What exactly are these "filaments"?

I'm at home right now and the monitor is back at my friend's house, so I can't put together this new coil at the moment.  Oh well, I'll need to go shopping for some parts anyways.

Seconding the Youtube video request!

[qrz]

filament is simply another name for a vacuum tube heater element. 
IHVT is a modern term for "integrated high voltage transformer" 
meaning the rect diodes are inside the assembly.  no more 3a3's , triplers and so forth......

 filament is a bit "old school" .   when i 1st began playing with radios they were all tubes, and frequency was labeled as kilo and megacycles rather than in hertz.

back in the day, REAL radios glowed in the dark.....oh, that sweet smell of ozone and toasting carbon......

guess need to make a video , or just stills....?  hmmmm   .......

qrz




 

[kid sinister]

I found more elaborate instructions here that describe the same process.  It sounds like I need to wire up this coil, isolate the KB pin on the neck board from its pad, then connect one end of the coil wire to the KB pin and the other end of the coil wire to the pad for KB pin.  Does that sound right?

I imagine that I'd need to solder back that collector pin on the blue driver transistor eventually...

[qrz]

yup, thats it . though,  i like to add a 2ohm 2w resistor in series with the filament as a fuse/current limiter.

and reconnect the collector lead or NO blue ....

qrz

[kid sinister]

I assembled the isolated filament winding and connected it to the KB pin as directed, and I'm still having the blue problem.  What do I try next?

[qrz]

for the sake of argument , take a resistance reading from either heater pin to chassis ground.
if winding is isolated , this reading should be infinite.
there is also the possibility of corrosion in the crt socket causing problems in the spark gap.

pse advise readings

qrz

[kid sinister]

for the sake of argument , take a resistance reading from either heater pin to chassis ground.
if winding is isolated , this reading should be infinite.
there is also the possibility of corrosion in the crt socket causing problems in the spark gap.

pse advise readings

qrz

I assume that those would be the two pins marked as "H", pins 9 and 10, right?  Pin 9 tests near 0 ohms, Pin 10 tests around 1.5 ohms.

EDIT:
wait a tick, do I even have this isolation winding hooked up correctly?  Right now I have it connected in series with the KB pin (pin 11) and its pad.

[qrz]

pin 9 and 10 MUST be disconnected from the original circuit ( the ground loop is through the flyback )

the newly wound coil then connects to pin 9/10.  i still recommend a 2 ohm 2 watt resistor in series with the heater .
                                   9   /\    10   (or visa versa)         PSE pretend dashed lines are solid   
   __________________/   \_________________
   )                                                                           l
   )                                                                           l
   )  3 turn  coil wound on flyback                            l
   )                                                                           l
   )__________________/\/\/\/\/\_____________l

                                       2.2 ohm  2w
there should be no measured resistance to ground

qrz

[qrz]

well, it looked good prior to post . tried to edit - was still ok, but not correct when posted   

the dotted vert lines on screen right are to be considered solid.

the heater depiction was also correct and not so in post -      well, i tried .....

qrz

[kid sinister]

I finally hooked up the new isolated ground correctly.  So far it hasn't gone blue... but the original problem only showed up after it being powered on for a few minutes.  We'll see if it acts up again.

I have a new problem though.  The focus on screen is now way off, beyond what I can correct with the focus knob on the flyback.  The reds are the worst off focus.  It's possible that the reds are bleeding to the right (the reds were doing that originally, I thought the HK short was the bigger problem to fix first).  But yeah, red text is unreadable.  What should I do?  I've already installed a cap kit back when I tried to fix my blue problem.

[SavannahLion]

pin 9 and 10 MUST be disconnected from the original circuit ( the ground loop is through the flyback )

the newly wound coil then connects to pin 9/10.  i still recommend a 2 ohm 2 watt resistor in series with the heater .

[qrz]

add 1 more turn to the coil . sounds like the filament may not be quite up to optimum temperature.

TNX,  SavannahLion for posting what i'd intended ..

qrz

[kid sinister]

add 1 more turn to the coil . sounds like the filament may not be quite up to optimum temperature.

Thanks, that worked!  Everthing is much more clear now!

I still have one more question though.  The reds are still bleeding to the right.  What could be the cause of that?

[qrz]

likely blooming due to weak red gun.  could try to rejuve ...or add a 5th turn to the coil for a short time span .
might burn off some crud ....


qrz

[kid sinister]

How short of a time span are we talking here?  I'm wondering by what time should I turn it off before I do any damage to the CRT...

[qrz]

i much prefer the rejuve process as it is brief .  discharge crt , raise fil volts . apply high voltage to grid - poof , gunk
(hopefully) burnt out . ( via sencore CR70 )

 on an operating set , i assume raising the fil voltage for a time may provide similar results .
hotter cathodes can produce additional electrons . which can allow a higher beam current. though, the beam current may dislodge debris from the cathode surface . hopefully not causing an electrode short or lodging in the shadow mask or screen phosphor in the process.
 the risk is reduced by using the CR70 or similar device .  as hv is manually discharged 1st

hmmm , may have recently seen a post by Rick@niemendisplays.com on a "live"  rejuve......?

qrz