Problem Toei cm-r20 brightness Issue?

[gunblade]

you would need to monitor voltages like i previously mentioned, 6.3v of heater for example - but you need a bench meter for this
i think eht is stable otherwise picture size would fluctuate, screen volts is likely ok but you can read that with a good quality multimeter and will be around 400-600 volts
i imagine you checked bad connection by tapping the chassis, something this age will suffer from corrosion and oxidisation of connectors

what does VR13 do on this chassis?

 

[Ronnie Dent]

Thanks Grant, I think VR13 is black level I'm sure I read somewhere?
I have a Fluke 177 so a decent meter could this not read heater volts? Where would I read screen volts?

I did try tapping the board but it changes nothing!

You probably already have this but will attach the service manual
Edit file is too large!

Link to service manual https://files.stardustarcade.com/PD..._Service_Manual_ (Toei_CM-R20_and_CM-R14).pdf

Thanks Ronnie

 

[gunblade]

no way a fluke 177 will read heater, you need a 87v or better
screen read at neck board where screen wire goes in

i think its your black level pot or brightness pot causing your problem

 

[Ronnie Dent]

I will change both those pots to make sure, so screen volts thick wire on the neck socket to ground? 4 to 600 volts AC ?

Thanks Ronnie

 

[gunblade]

screen volts is the larger gauge orange wire on the neck card, your meter will need to be able to read upto 1000vdc
the white large gauge wire going into the crt socket is the focus, that will be around 3000 to 6000 volts - not required to read as your focus is fine and you would need a hv probe to read with a multimeter

i am convinced its the black level pot, just smacks of this sort of fault

 

[Ronnie Dent]

I’m a bit unsure on the black level pot! Top near fuse is vr11 b+ the one to the right is vr6 that’s marked B.BKG? The bottom one is vr13 I can’t find that on the schems all 3 are glue

Thanks Ronnie

 

[gunblade]

vr 13 ,its 10k and marked on the schems - near d16 zener diode
you told me earlier in the post vr13 was black level

 

[Ronnie Dent]

vr 13 ,its 10k and marked on the schems - near d16 zener diode
you told me earlier in the post vr13 was black level

Sorry Grant, I'm sure I read that on another forum but can't find it now! I see it now on the schem but it does not say what it does!

 

[gunblade]

adjust it then and see what happens

 

[Ronnie Dent]

adjust it then and see what happens

Got the chassis out just now, if I have a 10k pot I will change it first, I see some tracks around it too!

 

[Ronnie Dent]

Found a 10k pot but it's too big for the board! I removed VR13 sprayed it with contact cleaner and metered it, seems smooth up and down the ohms, put it back and retested, so turning it anti clockwise increases the brightness and vice versa, making it brighter means I can turn down VR12 (brightness) and get a decent luminace, but I'm still seeing the issue however turning VR13 back and fourth mimics what I'm seeing as the fault, so either the pot is flaky or something related to that pot!

Thanks Ronnie

 

[gunblade]

i don't muck about with old pots, i would have replaced both and then if still issue remains concentrate on that circuit - you could have a resistor or diode or transistor failing and most likely from corrosion

 

[M K L]

Q25 and VR13 are in the ABL circuit which uses the lower end of the flyback tripler to monitor the beam current (not sure it was a good idea to turn VR13 which was factory adjusted and sealed). One end of the VR13 pot is connected to the flyback pin through R147 and R148. The other end is tied to the 12V line through L9 and R141. The zener D16 regulates the voltage to 9.1V which goes to the other end of VR13 through R145. Check that you have about 9V on the cathode and if that checks out, test D17.

 

[Ronnie Dent]

Q25 and VR13 are in the ABL circuit which uses the lower end of the flyback tripler to monitor the beam current (not sure it was a good idea to turn VR13 which was factory adjusted and sealed). One end of the VR13 pot is connected to the flyback pin through R147 and R148. The other end is tied to the 12V line through L9 and R141. The zener D16 regulates the voltage to 9.1V which goes to the other end of VR13 through R145. Check that you have about 9V on the cathode and if that checks out, test D17.

Thank you, I will check that out, I just had it on for a few hours and it never done it that I saw, Would I need to be reading D19 while the screen was flickering?

Thanks Ronnie

 

[Ronnie Dent]

i don't muck about with old pots, i would have replaced both and then if still issue remains concentrate on that circuit - you could have a resistor or diode or transistor failing and most likely from corrosion

I know I just don't have that stuff to hand, I think I have 3 types of new pots but all the wrong values, I will order some and get them replaced

Thanks Ronnie