1084S-D Repair

[Chris_hod]

Hi All! I was recently asked to try and repair a 1084 monitor. Power switch had gone so I've replaced that but the main fault is the screen dims after being on for approx 5mins. I can still just about make out the picture.

I've replaced all the caps on the heater board and reflowed all joints there as well as I figured that's where a lot of drying out would occur but alas the fault remains. I've had suggested that it may need the flyback replacing but from what I've seen when those go the screen tends to not come on at all and it makes a noticeable noise. Not to mention how hard these are to get hold of.

Anyone else had similar faults before and found the culprit? Any other caps I could turn attention to?

TIA
Chris

[lesta_smsc]

Not sure if you've replaced this particular capacitor but there is a filter capacitor near rectifier diode on CRTs. If they start under performing the voltage output drops and the picture is dim. It may not show as blown but checking voltage will let you know if it's a problem.

[cdh]

You might also try walking the board with a can of freeze spray to see if you can isolate the component(s).

[Chris_hod]

Originally Posted by lesta_smsc

Not sure if you've replaced this particular capacitor but there is a filter capacitor near rectifier diode on CRTs. If they start under performing the voltage output drops and the picture is dim. It may not show as blown but checking voltage will let you know if it's a problem.

Thanks I'll check this out!

[Chris_hod]

Originally Posted by cdh

You might also try walking the board with a can of freeze spray to see if you can isolate the component(s).

I've seen this done but I don't really understand how it works/what it does? If I spray a component that's defective the screen could brighten? I'm a novice when it comes to CRT's unfortunately...

[lesta_smsc]

Originally Posted by Chris_hod

Thanks I'll check this out!

Any luck?

[Toni Wilen]

Does heater also dim (yellowish light coming from neck of the tube) when display darkens? If it does, it probably is bad connection (or bad capacitor) between tube pins and power supply.
EDIT: Picture size stays the same when it darkens?

[cdh]

Originally Posted by Chris_hod

I've seen this done but I don't really understand how it works/what it does? If I spray a component that's defective the screen could brighten? I'm a novice when it comes to CRT's unfortunately...

Yes, you mentioned that the screen will dim after approximately 5 minutes. What you'd be hunting for with the freeze spray is if a component or solder joint is cooled back to a cold power-on state, does the display go back to being bright. So, you can walk the components on the board, one-by-one, until you find the area where cooling it makes a difference. From there, you can try tapping that component with something non-conductive to see if it's a solder joint issue or the component itself is bad.

If freeze spray isn't available, most cheap air duster cans can be flipped upside down and be used like freeze spray.

[Chris_hod]

Originally Posted by Toni Wilen

Does heater also dim (yellowish light coming from neck of the tube) when display darkens? If it does, it probably is bad connection (or bad capacitor) between tube pins and power supply.
EDIT: Picture size stays the same when it darkens?

Yeah the heater stays bright when the picture darkens and the picture still fills the screen.

[Chris_hod]

Originally Posted by cdh

Yes, you mentioned that the screen will dim after approximately 5 minutes. What you'd be hunting for with the freeze spray is if a component or solder joint is cooled back to a cold power-on state, does the display go back to being bright. So, you can walk the components on the board, one-by-one, until you find the area where cooling it makes a difference. From there, you can try tapping that component with something non-conductive to see if it's a solder joint issue or the component itself is bad.

If freeze spray isn't available, most cheap air duster cans can be flipped upside down and be used like freeze spray.

Cheers for the explanation!

Looks like this is my next step, I replaced all the caps that handle the mains power on the main board and it's still not helped.

[Chris_hod]

Update/further question. So I've tried going round the board with freezer spray and found nothing. Last desperate attempt to repair is replace the LOPT. My question is can I replace with an equivalent or does it have to be a like for like? There's a DCF1580 fitted but I've got a MSH1FCT31 which I know is an alternative for this monitor. Anyone know?

[alexh]

There is a very knowledgeable guy over on Amibay called Roy Bates, his knowledge is so good I started to think he might have been a TV engineer

https://www.amibay.com/member.php?4968-roy_bates

[roy bates]

i need to post a warning as the op sent me a mesage concerning this monitor,dont use a MSH1FCT31 itll blow your horizontal output transistor,no idea where that number came from but its wrong i suspect it came from gona,the correct part is MSH1FCT81,which i dont think anyone will get

what you could do is see if the main b+ drops off as a basic test,it should be 110 volts at the collector of the hot
i think this could be something other than the lopt,as its on and pictures goes after five minutes,is that correct?
normally a faulty lopt wouldnt make a picture at all,you can have strange effects like, the heater be on and no picture but not a picture that fades away after five minutes

[roy bates]

to be perfectly honest,not sure i could help anyway
the entire thread is vague,from a repair perspective

there is a service manual for for the 1084s-d1,you need to look at it and start probing around for faults and testing voltages as a start,i dont like to guess where the problem is,if there was any sort of hint of fault finding i could of maybe helped a bit,i dont mind,but if i have to help,then there would have to be more in depth conversation and more fault finding

[roy bates]

from what i can see in the thread the picture goes dim? correct?
but,its still there and you have full deflection? correct?
so ill have a vague guess,see if you have 12 volts where 12 volts should be,there is a linear regulator powered from one of the lopts pins,see if it puts out a constant 12 volts on its output,i think its also used on the brightness and other controls on the front,and on the tube neck board,for gun gain

try that and measure the main b+ either at the collector of the horizontal output transistor or...at the diode 102 near the supply which probably easier,but make sure all voltages are relative to ground