Problem Toei cm-r20 brightness Issue?

[Ronnie Dent]

I had a previous thread where I had a green screen issue which was resolved but I have another issue!

The brightness of the screen comes and goes, sometimes the image disappears completely. It seems ok at power on but after a few mins it starts going full etc! I took a short video to show it.
The chassis has been re capped the B+ is a steady 102v regardless of what the image is doing. Would like to try and fix this, thinking it might be a bad transistor as there are loads of them on this!

Thanks for any help or advice

Ronnie

Can’t post a video here I will load to YouTube and add a link

YouTube link

 

[gunblade]

can you post a photo of the chassis
i think q25 has something to do with brightness circuit if its the chassis i think it is
also does the screen pot function normally
i also remember having a resistor break down causing a similar fault whereby the background/cut off pots would not function, can't remember if it was r149 or r152 but it was 2.2 OHM metal oxide

 

[Ronnie Dent]

Thanks Grant yes the screen pot seems ok, but at times the image completely disappears ! Tapping the board does not seem to change anything,

Here is a couple of pics I can get more if needed?

 

[gunblade]

these chassis suffer track corrosion due to cap leakage and particular around the flyback area, you can see the copper tracks on the component side - perhaps one of these is almost rotted away
if you had the tools then monitoring the heater voltage and ht would be very helpful but you would need a ht probe and bench meter for these tests

check those components i mentioned in the previous reply

 

[Ronnie Dent]

Thanks Grant, whe I recapped the board I repaired some tracks component side and did check well all those tracks I even removed the heasink beside the flyback to better check those, but will have another look when I pull the chassis. Will have a look at those components you mentioned.

Thanks Ronnie

 

[Ronnie Dent]

Just had a chance to take this chassis out, so R149 is right by the lopt, and we talked about this in the other thread Grant, it's wire linked and looks factory. Q25 is in the bright area as marked on the schems as is Q24 I could pull these ant test them?



Thanks Ronnie

 

[Ronnie Dent]

Here are a few pics and one showing some track repair and R149 linked

 

[gunblade]

no its q25

 

[Ronnie Dent]

no its q25

Thanks Grant, I pulled Q25 it tested good but I guess it could be the issue as the picture is fine for first few mins. it's 2SA733 I think, is there a compatible I might have?

 

[gunblade]

2sa1015 should be ok

 

[Ronnie Dent]

Not got anything 2SA! I did compatible check 2N5401 came up? My stock is limited !

 

[gunblade]

its a bit overkill on spec but will probably work ok but you would need to twist the base and collector legs to match the 2sa733 pinout

 

[Ronnie Dent]

its a bit overkill on spec but will probably work ok but you would need to twist the base and collector legs to match the 2sa733 pinout

Thanks Grant, I might balls that up! I will just order some

 

[Ronnie Dent]

I managed to man up and figure out the correct way to fit 2N5401 so refitted the chassis but the results are the same, it's fine for around 3 mins then it starts it's fading etc!

Thanks Ronnie

 

[gunblade]

bummer
not something as daft as a bad brightness pot?

 

[Ronnie Dent]

The longer it's on the worse it gets, the pots there is the screen one beside the lopt and what I think is the brightness at the other side, will have a fiddle and put some contact cleaner on

Thanks Ronnie

 

[gunblade]

you should also check any glass diodes, they have a habit of corroding so you can push them with a screwdriver and they will just break in half

 

[Ronnie Dent]

Will do Grant cheers, the bright pot seems ok but really needs to be fully turned up to produce a semi bright image this is when it's first turned on and stable, when the image dissapears I can turn up the screen volts and see flyback lines but no image, I hope this is not terminal!

Thanks Ronnie

 

[Ronnie Dent]

Chassis out again, checked for fragile glass diodes but all seem firm, I see a few tantalums, is the only issue with these shorting and exploding?

Probably barking up the wrong tree! But from the manual? these 3 are 2SA733's also

Is it worth putting back the old Q25 ?

 

[gunblade]

yes to everything