WG K7600 with Keystone Image

[Fiddler]

Problem Description


I have a WG K7600 Series Monitor that has developed a keystone raster.  This occurred after C204 on P456 (Yoke circuit board) shorted and blew the fuse within two to three seconds.  After Ireplacing C204, eveyrthing returned to normal, but I was left with a keystone raster.  I've probed all over the board and exchanged some components, but nothing has had any affect on the raster.  Can anyone offer some suggestions as to the cause or what I might have overlooked?

Components Replaced

• All major electrolytics
• HOT
• C36 (retrace cap)
• C38 (width)
• L2 (Linearity)
• R69 - 3.9K (value was off considerably)
• D15, D18
• R98, R99 (values out of spec)

Raster Details

• The top of the image covers about 3/4 of the screen width.
• The bottom of the image covers about 1/2 of the screen width.
• The sides of the raster are straight (not wavy).
• With the exception of the shape, all other aspects of the image are normal.

Voltage and Oscllator Measurements

• Horizontal and Vertical oscillator frequencies as measured on DMM seem normal
• B+ is steady at 123 VDC
• All voltages on pins of vertical and horizontal ICs are within tolerance
• Anode voltage measures 19 KVDC
• Collector voltage on HOT measures about 126 VDC  (this doesn't seem right)
• All of the "Do Not Measure" voltages do not exceed 130 VDC (doesn't seem right either)

Deflection Coils

• Vertical Resistance is 13.6 Ohms.  Each leg measures 7.8 ohms.
• Horizontal resistance is 4.6 ohms.
• I've reversed the leads to the horizonal coil, but the keystone raster did not invert (although the image reversed as it should).
• Adjusting the width coil will widen the image, but it retains its keystone shape.
• The deflection coils have not been moved or jarred.

Miscellaneous

• Solder connections have been reflowed and cleaned with flux solvent.
• The flyback has no cracks.  It does not arc, smell, etc.
• The circuitry around IC2 is slightly different than that shown on the Wells-Gardner schematic for the 7600 series. Pin 5 is grounded on the schematic, but Pin 5 has a bypass capacitor to ground on my PCB.

Observations
Since the keystone did not invert when the horoizonal leads to the deflection coil were reversed, that would seem to indicate the coil is bad.  However, I am doubtful that is the case.  I have also read that their are circuit problems that can still cause this keystoning.


I do not have an oscilloscope or ESR  meter so I'm unable to determine if there are any malformed waveforms or leaky capacitors.


Unless I start spending a lot of money, I've run out of ideas.  I hoping someone with a lot more experience at this than me, might be able to lead me out of my haze..


Thank you.

[grantspain]

are you sure its a k7600 as i thought they were for 39" tubes

[Fiddler]

I'm sure.  It's a 19K7689 built in August 1991.  It is an original factory unit and has not undergone any user modifications..

[grantspain]

is it one of the zenith built chassis as that year is a standard k7000 series
horizontal resistance sounds high for a 19" cga though,normally around 2.4 ohms

[Fiddler]

It is a Wells-Gardner chassis.  The WG name is present in at least three places including the serial number.

[Fiddler]

2.4 Ohms would almost double the current in the coil.  Could my higher resistance readings be the reason that I'm not getting a higher voltage on the HOT?

If the 2.4 reading is typical, that would not seem to indicate my winding has a short.  And since I get deflection, it is not open either.  So if it's not shorted and not open, what is it? 

[grantspain]

yeah zenith built chassis for wells gardner back in the day,even zenith chassis will have wg serial numbers
a photo of the chassis would prove it though
although helpful does not explain your trap issue,i have never seen that fault on a k7000 series and i have fixed a few hundred of them

i have a u2000 with a similar fault and i am waiting for a replacement deflection yoke as someone has bodged on a standard tv deflection yoke

[Fiddler]

How can I tell if it's a Zenith chassis?  Just like an Electrohome, I thought a Zenith chassis had a different schematic?  The chassis I have matches the WG schematic with the exception I noted.  Am I working under an invalid assumption?

[Fiddler]

This is my first arcade monitor repair, but I've repaired a reasonable number of television sets over the years and never encountered this symptom.  What I don't understand is how the shorted cap could have damaged something else.  There was a burning smell that would occur before the fuse would blow, but I never could pinpoint its source.  But with the cap shorted, B+ essentially went to zero, so I don't see how damage could have occurred in the HV section of the monitor.  If anything, I would have thought that the regulator might have failed under that condition, but its fine.

[grantspain]

the pinout around the flyback is different,like i said post a photo of the chassis
they work exactly the same way,same specs etc but info is king on tracing obscure faults and you have a fault that is very obscure

c204 you mentioned is on the neck card but tracing the schem it follows to h def circuit,maybe worth check the caps at c37 and c38

[Fiddler]

Here are the photos in attachments

C38 has been replaced.

I have not replaced C37, but I thought I removed it from the circuit at one point with no change.   C37 capacitance is good, but I don't know it it's leaky.

[Fiddler]

Since you alerted me to something I did not know, I did a little research on the differences between the Zenith and WG chassis.  Based on what I read about the Zenith flyback and tube, I have a WG chassis.  Do the pictures confirm that diagnosis?

[grantspain]

yeah its a later k7000 wg built chassis

[princess prin prin]

2.4 Ohms would almost double the current in the coil.  Could my higher resistance readings be the reason that I'm not getting a higher voltage on the HOT?

If the 2.4 reading is typical, that would not seem to indicate my winding has a short.  And since I get deflection, it is not open either.  So if it's not shorted and not open, what is it?

One of the two windings (wired in parallel) that make the horizontal coil is broken/open, that's why you have 4.6ohm instead of the regular 2.3ohm. Same thing happened here:

http://forum.arcadeotaku.com/viewtopic.php?f=4&t=29295

http://forum.arcadeotaku.com/viewtopic.php?f=4&t=29295&start=20#p462919

You can verify that easily by desoldering each horizontal winding.

[Fiddler]

I checked both windings and you are correct.  One of them is open.  Thanks for leading me to the source.

Do you have any idea how a shorted C204 could cause this to occur?  When I look at the circuit, I don't see a way for much current to flow through the horizontal coil under this condition.  Am I overlooking something?

[princess prin prin]

It was probably due to corrosion. Remove the yoke and take a look.

[Fiddler]

As suspected, there was slight corrosion on one of the windings that left about a 1/8" gap.  I was able to clean it off and repair the winding so that continuity was restored.  I placed the yoke back on the neck and I had a perfect picture!.  Thanks so much for your help.