Wells-Gardner U2000 problem

[segundosombra]

Hi all,

I have a problem with my wells gardner monitor. I replaced the horizontal transistor in order to get working the chassis. After a few days it got faulty. I have replaced that transistor several times and I got the same result. I don't know what to do.

If anybody know how to fix this problem, please help me.

Thanks in advance.

 

[channelmaniac]

Make sure you check out the high voltage caps and any diodes coming off the secondary circuits of the flyback transformer. Those tend to go leaky and cause the transistor to run hot & fail early.

RJ

 

[ttooddddyy]

Are you using the same type transistor or a substitute, if a sub are the specs adequate for the application. They should prefereably be higher.

If there is a mica washer between the transistor and the heatsink make sure there is no break down of insulation there.

Make sure to tighten the transistor well to the heatsink and use silicon heat transfer compound for good measure.

When the board is running tap around the horizontal output stage with an insulated screwdriver to check for dry joints, if you notice any instability(jumpy pic or intermittent noise or line collapse) you need to locate the poor solder points, this often leads to repeat failure.

Check the +B, if too high the horizontal output device may overheat and fail.

Any noises coming from the flyback transformer, like hissing or crackling ?breakdown here may also lead to hot failure.

It is possible that the hor oscillator going way off speed could cause this, check that the hold pot is not scratchy, even a cap kit around the horizontal osc/sync stage may help.

Also breakdown of the insulation on the deflection yoke windings can be a problem, may be worth removing for a look see.

 

[segundosombra]

Thanks guys for the reply.

@ttooddddyy: since I don't have too much knowledge about it... how many volts should I get from +B?

 

[ttooddddyy]

TIP- do not measure it on the collector of the horizontal output transistor- it will fry your meter ! Measure it at the cathode of the +B rectifier diode.

I do not have a manual for that model, but you should see it clearly marked on the schem. Its usually 120-130 volts. It is critical that it is set correctly.

 

[segundosombra]

OK, after several days testing my thing I can say it works fine. I just changed that transistor by another one with higher specs and I isolated it better.
Now I am trying to fix a screen problem... the left half is brighter than the right half... can you guys help me again?

Thanks

 

[ttooddddyy]

Good news.

Regarding the other problem, two things to check out.

180 volt supply to the video output stage, if low try the 10 uF smoothing electro.

Improve video grounding by running seperate connection from mobo ground to monitor ground.

 

[segundosombra]

Hi ttooddddyy!

I tried your tips but screen remains the same.

Here are pics of the screen showing that problem, maybe these are more elocuent than me:

Is there any other way to fix it?



Thanks

 

[ttooddddyy]

Has the monitor had a cap kit recently

 

[channelmaniac]

Also, what was the original part number transistor... and what is the part number you replaced it with?

RJ

 

[segundosombra]

Has the monitor had a cap kit recently

No.

what was the original part number transistor... and what is the part number you replaced it with?

Original transistor was C3686 and new is BU2520.

 

[ttooddddyy]

No.

Would be worth a try.

Original transistor was C3686 and new is BU2520.

Any letters after the BU2520 (if its an A it is OK, anything else may not be)

 

[segundosombra]

Yes, that is BU2520AX and I replaced all caps that channelmaniac mentioned... still not working fine.

Any other sugestion, please?

 

[ttooddddyy]

Yes, that is BU2520AX

Its touch and go on the wattage rating of that device, thats the price to be payed for the isolated case Anyway I wouldnt worry too much about it at this stage.

2SC 3686 = 1500/800v...7A...120W
BU2520AX = 1500/800v...10A...45W (lower wattage rating due to the isolated package)

Any other sugestion, please?

So you have done a full cap kit ?

 

[segundosombra]

I have done a horizontal output cap kit. Do you mean a full mobo cap kit?

 

[ttooddddyy]

What about the 10 uF which smooths the 180 volts supply to the rgb video outputs.

What voltage do you have there ?

This does look cap related or maybe a leaky signal diode in the video blanking.