monitor problem - Wells Gardner K7500

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dacflyer
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monitor problem - Wells Gardner K7500

Post by dacflyer » Mon Nov 30, 2009 4:49 pm

i have a strange problem with a K7500 monitor, the picture is weak, not very bright.
the brightness is turned up , very little change, and so i turned up the screen, little change, the B+ is good and the screen voltage is about 450 max.
when i turn the screen up, i get a white raster , not very bright either, and i never get any retrace lines in the screen when turned up all the way. the colors are good, i even tested the tube, its still strong.

any suggestions ? anyone have any video game exp with this monitor ?
heres a link to the schematic, if that helps any..

http://www.wellsgardner.com/pdf/Schemat ... 5_inch.pdf

thanks..

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Re: monitor problem - Wells Gardner K7500

Post by Janitor Tzap » Mon Nov 30, 2009 5:40 pm

Hmm.......

What sort of voltage do you have at the heater filaments?

I see that there is a 3.3 ohm 1 watt resistor that is on the filament voltage line.
If it has changed value, that would decrease the voltage & current.
Thus, the CRT wouldn't reach the 6.3V that is needed for it to operate correctly.
If the heater voltage is fine.

Trace the brightness control circuit.
Look for a resistor that has changed, or opened up.
Also,
Check the brightness control itself.
Check connectors, or plugs for bad solder.

Good luck.


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Re: monitor problem - Wells Gardner K7500

Post by dacflyer » Tue Dec 01, 2009 4:33 am

i have good filiment voltage , brightness does vary, i think that circuit is ok, i am thinking that the flyback voltage is low , but i have good B+ going into it, and a good picture otherwise, i just cannot get enough screen to cause retrace,,Hmmm
i might call WG today and ask them...

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Re: monitor problem - Wells Gardner K7500

Post by Janitor Tzap » Tue Dec 01, 2009 3:37 pm

Your Screen Voltage sounds about right.
But, whats the voltage at the High Voltage Anode?
{The schematic shows 26KV, -/+ 2KV.}

Did you check the resistor and capacitor, {R547 1K & C529 .001uF}
on the G2 Screen Control line?

It is possible that the flyback has gone bad.
On the Schematic, there are internal resistors, and a capacitor within the Focus/Screen control area of the Flyback.
I've had Flybacks fail, in which the resistive Pot for the Screen, or Focus had opened up.
Or the carbon substrate of the Pot had changed its value.


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Re: monitor problem - Wells Gardner K7500

Post by dacflyer » Fri Dec 04, 2009 3:28 pm

i have no way to measure the anode voltage, but i am able to draw a 2"+ arc, so i know i have decent HV.. i have checked the parts on the G2 circuit, i even replaced the flyback with another one, problem is still present. i have replaced a handful of HV caps 130v and up.
mostly found 2.2µf caps dry. and a few others..
might have to start checking the lower volatge caps as well..
i love my 88A cap checker. :D
BTE. i found i had some brand new 2.2µf thats were testing as bad also.. Hmm, go figgure.

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Re: monitor problem - Wells Gardner K7500

Post by Janitor Tzap » Fri Dec 04, 2009 5:19 pm

dacflyer wrote:i have no way to measure the anode voltage, but i am able to draw a 2"+ arc, so i know i have decent HV.. i have checked the parts on the G2 circuit, i even replaced the flyback with another one, problem is still present. i have replaced a handful of HV caps 130v and up.
mostly found 2.2µf caps dry. and a few others..
might have to start checking the lower volatge caps as well..
i love my 88A cap checker. :D
BTE. i found i had some brand new 2.2µf thats were testing as bad also.. Hmm, go figgure.
Yes,
The 88A is a nice unit to have.
As for the defective cap's....
Well I did warn in another thread about bad Electrolydic Capacitors that are still out there.

Those 2.2uF caps were on the CRT Board right?
Heat from the CRT tends to dry them out.

Hmm....
You said you tested the CRT, and the Gun emission for each gun (R\G\B) was good.
Check the signal inputs to the monitor.
R\G\B, composite, Y, H, etc.....
Make sure the signal isn't weak.
If that checks out.

Then it has to be some where in the brightness control circuit.
A resistor or Pot has changed its value.
I noticed that it also has a ABL (Automatic Brightness Level) circuit.
Poke around there as well.


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Re: monitor problem - Wells Gardner K7500

Post by dacflyer » Fri Dec 04, 2009 8:10 pm

the caps are on main board, theres just 1 cap on neckboard, and it just filters the B+ for the video drivers. it checks good also.
i am at wits end here.. i have never seen a reason why i could not get retrace lines when screen is turned up all the way..
could ABL be the problem ? i am not familliar with this circuit.
of all my years of exp,. this one has me stumped..

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Re: monitor problem - Wells Gardner K7500

Post by Bob Scott » Sat Dec 05, 2009 10:28 am

If I had this problem set, it would be a time that I regret taking notes on what the typical CRT pin voltages are supposed to be on a good working set. If the problem is not the CRT itself, one of the CRT pins has a voltage that is not right. If all colours are dim, then the problem lies with a circuit that effects all three guns, like the different circuitry for the cathodes, grids, or screens.

I would find a working set and measure the voltages. It doesn't have to be the same make or have the same CRT part number. Most of them have similar voltages.

Have you checked the factory list of probably solutions for K7500 problems?

http://www.wellsgardner.com/pdf/repair/K7500.pdf

DARK PICTURE R310, R823, R811, T700 ADJUST VR905, VR906, G2
-=VA7KOR=- My solar system includes Pluto.

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Re: monitor problem - Wells Gardner K7500

Post by Janitor Tzap » Sat Dec 05, 2009 11:06 am

Bob Scott wrote:If I had this problem set, it would be a time that I regret taking notes on what the typical CRT pin voltages are supposed to be on a good working set. If the problem is not the CRT itself, one of the CRT pins has a voltage that is not right. If all colours are dim, then the problem lies with a circuit that effects all three guns, like the different circuitry for the cathodes, grids, or screens.

I would find a working set and measure the voltages. It doesn't have to be the same make or have the same CRT part number. Most of them have similar voltages.

Have you checked the factory list of probably solutions for K7500 problems?

http://www.wellsgardner.com/pdf/repair/K7500.pdf

DARK PICTURE R310, R823, R811, T700 ADJUST VR905, VR906, G2
Good find there Bob! :grin:

Well we can rule out T700.
And the G2 Voltage.

So, it has to be one to the Pots, or resistors.


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Re: monitor problem - Wells Gardner K7500

Post by dacflyer » Sat Dec 05, 2009 12:48 pm

i have already checked all that is mentioned , in the WG site,

the resistors are just fuseable type for the sourceof the circuit.

the problem i cannot understand is,, why i cannot get any screen overdrive to cause retrace lines, all video voltages measure fine, i do not think its in the guns, but something that effects the G2... Hmmmm

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Re: monitor problem - Wells Gardner K7500

Post by Janitor Tzap » Sat Dec 05, 2009 2:03 pm

Hmm......

Well there is the G1 circuit as well.

On the Flyback; Pin {8?} has the R810 that connects to the ABL circuit.
There is a Cap C811 that is going to ground on that line.
Is it open, or changed value?
R310 shows two Caps near by C310 & C812.
Have you checked them?

Also don't be fooled by good in circuit readings on the Resistors or Caps.
The A88 ESR type meter will tell you if the Cap is bad or good.
Shorted, or Open.
It won't show how much the cap has changed in value.
And in some circuits this value can be critical to its operation.

The same is true for DMM reading resistors in circuit.
I've had countless problems with reading resistors in circuit.
Only after I pulled up one leg did I get a true reading.


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Re: monitor problem - Wells Gardner K7500

Post by dacflyer » Sat Dec 05, 2009 6:58 pm

thanks for the added info, i'll check that some more, i might have already, i marked the caps with black marker that i have checked already..most that i have checked are bad :P
these monitors either are used to death or they have some really cheap caps in them.

NOTE>> i just got another monitor exactly the same chassis, but different tube.. 25" is what i have been working on, and this other is a 27" but same chassis.
i will be doing some testing and swapping and let you know what i find out later.

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Re: monitor problem - Wells Gardner K7500

Post by oldtechie » Sun Dec 06, 2009 2:12 pm

I worked part time for an Owner/Operator of arcade games about 14 years ago, then again in 2008. He is now selling his games.

The monitors run a lot of hours and get pretty warm in those game cabinets. It is common for the electrolytic caps to dry out go leaky. A LOT OF THEM. They even sell what is known in the industry as "get well kits". These are simply capacitors that normally go bad, selected for whatever make & model monitor you have. They package them in little plastic bags and it is really handy for the technicians, but IMO the cost may be a little higher if you are doing this in volume. For someone who can solder and this is just a hobby, it is ideal!

I can honestly tell you that about 90% of the monitors I have repaired have needed a lot of caps replaced.

I have an ESR meter. But if the monitor is fairly old I change out the electrolytics. If by chance I still have a problem, then at least I know it is NOT just a bad cap. IMO, this saves me a lot of time.

BTW: I have seen bad caps cause a monitor to appear dead many times.
In the monitors that did NOT use switching power supply they would go into high voltage shutdown.

In monitors with SMPS the problem can be in the power supply or elsewhere. A 25 cent cap can keep the SMPS from starting up.

Having that extra monitor should help a lot. I know that's how I would do it if I knew the caps
were all good and still had your problem. I think Wells Gardner has testpoints listed for voltage
readings too. But nothing is better than having a "working unit" to use for comparison IMO.

Let us know what you find. -:)

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