Bestech 27" EGA/CGA chassis - width and pincushion help

[castlesteve]

Maybe someone out here can help.


First, this is easily my 4th Bestech monitor... and i've yet to see one properly labeled... like what model it is or anything.  It is very close to the 23d27d schematic that you can find online here... but it is different.   Some of the ICs are marked differently (just a couple)... and some of the traces that I have followed show a different path.

Anyway, my problem should be simple for anyone who has also seen this.  The monitor works perfectly EXCEPT cannot be adjusted for WIDTH or PINCUSHION.

Things I have already checked/replaced:

1)  POTS on control board.  (the control board is different than the one on the schematic slightly).
2)  All electrolytic caps in the WIDTH and PINCUSHION path with new parts
3)  ICs 402, 301, 302 with new parts
4)  Q303 with new
5)  C115,C116  (width caps I would assume)
6)  tested all diodes
7)  Board does not have Horizontal Position switch (like schematic) SW301 .  Not even marked on board anywhere. 


The only thing I havent tested is L301 or L302.  However, most all schematics never tell the values (in mH) of these parts.  The Wells Gardner schematics only call out part numbers which mean nothing to me.  I have plenty of those chassis I could pull parts from.

Also, none of the inductors have adjustments on them.  Has anyone ever seen an inductor go out?  This board looks brand new... I seriously doubt these parts fail.

Any suggestions or leads to a proper schematic?  I'll even throw in the towel and send this chassis out for repair if someone is willing.



Thanks,
Steve

[southmonitor]

I have found that problem many times with other brands of monitors. Bad pincushion and not adjusting H-Size. The problem could be in order of ocurrence the following:
1-Bipolar Electolithic Cap responsible for Pincishion (should be easy to identify with no polarity markings normally from 4.7 to 10UF/50V and very large for it's capacitance value). Replace only with a Bipolar Cap designed for H-Out circuits that can work at that frecuency. NTE is a good source for replacements.
2-Pincushion correction output Transistor. Usually midsize transistor TO-220. Take it out of the circuit to properly meassure. I have seen them meassuring close to good and they are bad. I you have one just replace it to be sure.
3-Pincushion correction Diodes (leaky). Normally fast diodes next to the Horizontal Output damper diode and to the H width coils. They meassure very close to good but they could be bad. Replace them if in doubt. They just affect pincushion but not H-size adjustment though (What I have seen)
4- Less frecuent is the IC Operational Amp responsable for pincushion. Frecuently LM358.
Hope this will help.

[castlesteve]

Thanks.  I'll try the cap and diodes.  I've replaced the ic and transistor
already.  I'll reply with my findings.

[castlesteve]

well it's funny.  This monitor doesnt have a BiPolar cap like the Wells Gardner... but does have a 47u160V off that transistor.  I've already replaced it and tested it.  Pulled it and replaced it again.  I cannot find any width or pincushion caps that are shorted.  Replaced that one suspect fast diode.  Still no go.

I've officially wasted a week on this.  I'm throwing my hands up at this point unless I can get an accurate schematic. 

Someone who already owns this board could get a bargain if they want to buy it from me. 

[grantspain]

this is a common problem on hantarex polo 2 chassis-its almost always down to the east/west correction diodes-i am betting your is the same

[castlesteve]

Are we talking about D303/D304   or  D307/D308?  I've included a schematic that im using that is close but not actually my board.

Oddly, my board doesnt even have a SW301 (H-POSI) switch or even markings on the PCB.  I imagine it would only have one of these diodes.


I've already replaced D304 and D303 tested ok.   I've also replaced Q303, c319, tested the resistors in the general area, IC302, C323, c319, R335.

Thanks for your help.  If you could help me point out which diodes you are referring to... ill give this another shot tonite.

Steve

[grantspain]

well you have it covered as i would have said d303 and d304
this monitor was working beforehand?

[castlesteve]

It was in a machine when I purchased it.  I didnt notice the pincushion issue with the current game as it didnt have any vertical lines to give it away.  The seller kind of camo'd it in my opinion.

The monitor works perfectly fine otherwise.  It's just like it is set to full width and no pincushion correction.  So i would imagine that a critical cap went short (or open) that would otherwise be used for adjustment (or timing).  Could the width or pincushion coil go bad?

-Steve

[grantspain]

i am wondering whether this chassis has been changed and does not match the yoke

[castlesteve]

it should still have SOME adjustment.  Anyway, its a samsung tube, an annodized black monitor frame and unique monitor chassis.  It all looks original and matching.  Also, this is a pretty recent looking monitor as far as clean lines, no dirt etc. 

I slapped on a WGu2000 27" chassis for fun as the yoke impedence was pretty close... and it works perfectly (and adjusts as expected).  So the yoke and tube seem fine.

I'll drink some beers tonite and try again.  well.. maybe that's my problem

[grantspain]

i have seen on occasion when people have swapped chassis and completely mismatched the yoke to chassis thus given a serious width problem-if its the original setup then ignore

[southmonitor]

Complicated problem hummm...
You are right I don't see the bipolar cap either in the schematic and belive me that was the more likely candidate. Well in this case. I would double check:
1-Q303 (Replace it even if it appears to read good)
2-D303, D304. (Pincushion Diodes I was telling you). Replace them even if they appear to read good.
3-L303
4-C115, C116, C326. Meassure them with your Fluke in Cap mode. More than 20% deviation and they affect pincushion and width; but eventhough you should still be able to adjust H-size.
5- I don't belive you have a CRT Chassis missmatch. Eventhough you could be able to adjust H-size, even if it is too wide.
6- Use a Scope and meassure the waveform at pin 7 IC 302. Take a pic of the waveform and post it here. I want to see if it looks good or not.
I had exactly the same problem with an NT-2702S and it was what's in this schematic Q303.
If you are willing to pay for shipping back and forth, I will take care of it for you for free just for the fun :-) www.adrianelectronics.com

[castlesteve]

I'll check the diodes again today. I've replaced q303 now twice.

If it still is messing with me, I'll take you up on the offer... But I'll throw some $$ in with it.

[grantspain]

thats a really nice offer from southmonitor,it would interesting to see what the problem is

[castlesteve]

I totally agree.  it is very generous.

[southmonitor]

You said you have used a Wells-Gardner Chassis with that Monitors Tube. I need to know the Model number of that Wells-Gardner in order to see which Tube I can use to check your Chassis since I don't have the original Bestec with me. Just in case you decide to send it to me. This is a stationary problem that is always there, that's a very tough one; but I'm more worried when the problems are intermitent or happen in a flash after warm up, etc. I have a Tatung VS14228 that shows lines and shakes in the top of the screen after 5  min of plug in. I'll take care of it this week end. I also have a WGM2574 for Mrs Packman that goes off after a couple of hours and when resetted it starts ok for 2-3 Hours again and so on. What is it? I don't know. I'll replace the SMPS IC this time and see if that's a fix. Like I said when the problems are steady they could sometimes be fix if you have the parts.

[castlesteve]

Yes the condition is regardless if just powered up or on for hours.

I'll get the tube/yoke info.  But, I believe for testing purposes any Wells Garder 25"/27" k7000, u2000, u5000 and likely k7400 will suffice for short term work.  Just not the k7500 with it's low impedence yoke... It'll smoke the board.

Also, I've fixed plenty of flakey WG boards like how you are mentioning.  First thing I usually do is replace the HOT.  I've been troubled many times with HOTs that fail temporarily when warmed up.  Then I'd check those 2 small transistors that usually fail on older boards... I'll have to get back with you on the part id's 

[castlesteve]

Oh yea, one more thing with k7400s, if you did a cap kit recently, double check the B+... you may need to bump it down just slightly.  It may be tripping your ABL circuit when warmed up.

[southmonitor]

Same thing was happening before I recapped the Monitor. Actually I recap it because of this problem. I replaced the LA7850 and now it takes longer for the prob to happend. I thought about the B+ but it is Ok. I'll try the SMPS chip and the HOT just in case. I hate this kind of problems and one can spend the whole life trying without success. The other problem is that when it fails everything is off and no meassurements can be taken. I'm pretty lost on this one... I'll start changing components ramdomly (with some logic of course) to see if I get lucky. The other prob is that I have to wait for hours to see if it goes bad again...

[castlesteve]

OK, so i took a couple days off from this thing.  Last nite, went back thru it... replaced another E/W diode, same thing.  

I'm out on this one.  

southmonitor, I'll take you up on that proposal if you still are interested.  I'll include $$ so you will at least be paid for your time and whatever part you may find is bad.

PM or email me.

Steve

[southmonitor]

OK. Steve, you are welcome to send it to me. Please let me know which wells-gardner tube did you used for testing. I have some 25" tubes for k7400, k7500 and U5000. If you can let me know all parts designators that you already chaged. I'll have some fun.. You can get my info in my website www.adrianelectronics.com.