Mtc900e bright white screen but image is there

[stevebm1]

Just turned on one of my cabs and got greeted with a grey white glow over the game image,the chassis is an mtc900E,I have checked the b+,it’s stable at 125v,tried turning the screen volts on the lopt,the image gets the usual lines if I turn it up to high,but never goes black when turned right down,the neck card is getting +12 and -5,I swapped out r131,but it made no difference,the issue is still there,any ideas what to check next?
Thanks in advance

 

[stevebm1]

 

[stevebm1]

so I think I may have found the issue,the 200v test point on the neckcard is only reading 50v,so what components do I need to check to hopefully fix this?

 

[DaveR]

Discussed here

Hantarex MTC900e White screen

I’ve an old Hantarex MTC900/e, I’ve never tested it until yesterday, plugged it in and it was dead, after finding some posts from grantspain I found he’d mentioned the most common issue with these chassis is corroded coil on the TR20 which indeed it was corroded and broken, I repaired this and...

www.ukvac.com

 

[stevebm1]

Discussed here

Hantarex MTC900e White screen

I’ve an old Hantarex MTC900/e, I’ve never tested it until yesterday, plugged it in and it was dead, after finding some posts from grantspain I found he’d mentioned the most common issue with these chassis is corroded coil on the TR20 which indeed it was corroded and broken, I repaired this and...

www.ukvac.com

Thanks for the link,but that thread just ends,it does not solve any issues?

 

[stevebm1]

So r131 is fine d52 is fine,are we thinking bad hot or bad lopt?

 

[big10p]

Out of interest, does the focus pot on the lopt have any effect? What circuit is the low voltage on the neck board in?

 

[stevebm1]

Yes,focus pot works,I found a dry joint this morning on one of the traces to the lopt,I reflowed it,and I now have 50v at the neckboard,where it should be 200v,not sure what components to check next?

 

[big10p]

I'm not familiar with mtc900, but I'd look at the schem where the low voltage is, and see what's in the circuit.

 

[stevebm1]

Replaced the 4.7 uf cap on the neckcard and I know have this image,the colours are bleeding,but the focus and screen volts on the lopt are working

 

[stevebm1]

If you turn the screen volts to minimum the lines go,but you can barely see the picture,it seems i’ve gone from a too bright screen to too dark,the brightness and contrast pots have little effect when changed,it’s still way to dark

 

[M K L]

Try replacing the other filter cap on the 200V line, C72 (47uF 250V) and maybe also the ceramic cap C74 in parallel with D25.

Check some other outputs of the flyback, e.g. 25V (cathode of D28) and 15V (cathode of D27).

 

[stevebm1]

All those voltages are good

 

[tb2000]

If you've not done them already, do the three small caps at the rear of the power supply section on the PSU/RGB interface board. Caps C25, C36 and C83.

 

[stevebm1]

Already changed those,it made no difference

 

[big10p]

I think at this point I'd be reflowing everything. 😀

 

[stevebm1]

I actually have a spare neckcard,which im toying with swapping out,as there is 200v,at r131,on the chassis,it just drops to 50v,when the current neckcard is attached,all 871’s are ok and resistors are all in spec on the current neckcard,so it’s a bit of a mystery

 

[M K L]

Lift one side of R145 on the neckboard and check the voltage on the loose leg. If it's not correct, check it going backwards on the line (other leg, B6, D25) until it reads good.

 

[stevebm1]

Swapped the neckboard,with the spare I had,made no difference,image still exactly the same

 

[stevebm1]

had another look at this this morning,as it's too windy to venture outside,I swapped the left hand board out from a fully working chassis,and the fault is still there,so this proves,the neck card is working ok,the power side is working ok,so the fault must be on the deflection board somewhere,would a bad lopt cause a dim and blurry image?