GT65-2 monitor / anode cup reattachment.

[arkleyjoe]

Hi

Hoping someone can help.

I have taken apart the GT65 monitor in order to diagnose a fault. It currently just shows a green screen no border at all on two known good 464s.

Aside from that fault, I safely discharged the anode so I can safely work on the CRT. However I can not workout how it goes back in to place. Can anyone tell me how the little clip in the anode cup locks back in to position on the CRT?


Kind Regards


Joe

[Bryce]

It should just push back on and the four metal tabs will hold it in the socket. Can you post a picture of the plug, maybe you've bent the pins?


Bryce.

[arkleyjoe]

Hi


Thanks for coming back to me @Bryce much appreciated.
I have attached some pics of what I have.


Joe

[Bryce]

The connector is completely broken. I doubt you will be able to repair that in any way that would safely work.The rounded section should be clipped around the cable, the two pins hook under the hole in the tube.

Bryce.

Edit: I found a picture of what it should look like. The pins (red arrow) hook inside that ring on the CRT, bend these outwards if they aren't rock solid inside the ring, otherwise you have an extremely dangerous monitor. The bend (orange arrow) presses against the outer glass of the CRT and the cable should be pinned in that double bend between the hairpin and the glass. Check that the cable hasn't frayed or broken where it's soldered to the pin. The rubber plunger should then be pushed over the double bend.

[arkleyjoe]

Ahh that is great thank you!!


Actually the clip I think is still is pretty good shape, it will just need the legs made a bit wider to grip correctly.  My photos are just from funny angles. However I think I will tidy up the solder joint as the wire is a little frayed from where i have twisted it.


Thanks for your help I will let you know how I get on.


Joe

[arkleyjoe]

Hi @Bryce

I have finally had time to take another look at this and I still can't quite work it out.

I have removed the wire and taken some photos of the clip sat in the hole. Is that how it is supposed to fit / sit? It feels very loose. It does not feel as if it is locked in?


Joe

[Bryce]

That's pretty much how it goes. With the cable and suction cup it should be a little bit more stable.

Bryce.

[arkleyjoe]

Cool thank you @Bryce I will get in put back together

[Bryce]

Try it with the case removed first and stay at a safe distance. Turn down the lights and see if there are any sparks escaping from under the suction cup. Any sparks or crackling noises mean it's a bad connection.

Bryce.

[arkleyjoe]

Hey @Bryce

Just thought I'd let you know that after a very tidy (if i say so myself) re solder it went back together really well. For the benefit of others I guess i under estimated the role of the suction cup (clue in the name  ) in holding it in place. 

I did as you suggested, powered up from the other side of a dark room and checked for crackle / sparks etc. All good.

Main problem is that my original problem still exists.

All I get is a plain green screen from two known good 464s that work great on a scart lead. I say plain, nothing on screen at all no boarder just green.
I assume that because it is showing green it shows the high voltage part of the circuit is working? Is that a valid assumption?

Does it seem likely this will be something simple like caps or a transister? 

Joe

[Bryce]

Well congratulations on getting it back together without killing yourself. That's an achievement in itself.

Yes, if the screen is glowing green and there's no crackling you can assume that the HV is good. As to the actual issue. What equipment do you have to take measurements? An chance you have a scope?


Bryce.

[arkleyjoe]

yeah I was pretty pleased to still be standing.


Yeah I have a scope handy.


Joe

[Bryce]

Good. Then the next step would be to trace the picture signals. Luckily, the GT65 (and most monochrome monitors) are extremely simple circuits. The picture is derived from just two signals. The Sync, shown in red, controls the scan path of the beam. This seperates into several paths to control the high voltage source (for blanking during beam return) and to control the actual left/right/up/down position of the beam. My guess is that this signal is good, otherwise you wouldn't have a pure green screen, you'd have tearing and flickering all over the screen.
The LUM signal, shown in green, controls the brightness of that beam. This is more likely the issue on your screen. As can been seen, it passes through very few components before it's fed directly to the back of the tube. The three transistors on the left do some very basic signal conditioning and biasing and the last transistor (which is on the PCB on the back of the tube) is the driver transistor. My guess is that something in this path has failed. By following the signal with your scope it should be very easy to establish where the signal ends and hence what has failed. My money would be on the driver transistor. Let us know how you get on.


Bryce.

[Gryzor]

Is he still alive though? Should we call someone?

[arkleyjoe]

Hey,


Ok so it has taken me a little while to get back on to this one, work went crazy etc etc. (no i did not kill myself digging around in there @Gryzor  )


@bryce as you predicted it seems the problem is somewhere with q605 in that there is no signal coming from that. What is curious is that the transistor tests fine out of circuit.


I also noticed that the d702 is open both ways when doing some general ticking around.


I need to spend a little more time with it, but will not get time to do that until next week.


Joe

[arkleyjoe]

oh gutted!!!!


I went back to have one more look before packing this away . . the ground clip on my scope probe pinged loose and made contact, big sparks. Now I have an even more dead monitor and my 464 is giving a pink screen with some garbled characters.

[arkleyjoe]

actually, while it has damaged the monitor further I am pleased to say actually my CPC464 is fine!