CTM644 problem - blown flyback?

[Bignumbas]

I have found the CTM640 and 2 x CTM644 monitors I bought had 2 bad LA7800 chips so I have been using the one good one I have until the boat finally gets in from China with the 10 replacements I ordered a month+ ago.
I put sockets in all the monitors for this chip to share it. 

The CTM640 just worked with the LA7800 only had to adjust the sub brightness and 5v to spec.
It did have a small problem with the brightness control causing white flashes but Audronic suggested I spray cleaner in the pot and that fixed it.
One CTM644 worked it had the good LA7800 but did not have 12v out, just needed a new cap C523.

The problem child is the other CTM644.
It had a blown fast fuse ICP501 and the LA7800
I soldered in a temp ceramic 2a/250v and now I get a red hot R436 (15ohm 7w) cement resistor which feeds the flyback transformer pin 2.
I have replaced Q405 and checked all the caps, diodes and resistors that come off the circuit.
The 5v an 12v out work fine
Suggestions anyone?

I found this  http://www.ebay.com/itm/TRASFORMATORE-EAT-HR7674-/330748824875?pt=Componenti_elettronici_passivi&hash=item4d022a8d2b
from here, you need google translate: http://www.howtofixit.gr/forum/archive/index.php/t-67118.html
Looks like I am not alone with problems fixing these screens.
Good price it the freight is OK.

[Bignumbas]

Now I have really blown it.
I took out Q404 to check it out of the board.
In doing so I accidentally dislodged T401 (coil 305Y001) and broke it.
Does anyone have  one of these in a dead monitor or know where I can get one.
I found it is used in a Toshiba TV/DVD  MV13K1R unit so they must be out there somewhere.
It is Toshiba part no. 03305Y0018 in their service manual.

[Bignumbas]

Audronic says he has the part and is posting to me.


Thanks Audronic


I found it online in America for $3.20 but they wanted $18 to post it.
It could be sent in a letter.

[Bignumbas]

The 5v and 12v out work fine.
I have 34v on Q404 collector which is correct and it is new.
I have 65v on Q405 collector which should be around 100v and it is new.
R436 is red hot on power on and Q405 heatsink is hot to.
IC401 is new, IC 502 is the correct voltages
IC402 is good and IC501 is too, I swapped them with a known working monitor which is still working.

I have checked all the caps that could cause a short to earth.
The only thing I can think of now that could be causing this is the windings in the flyback for pins 2,8 and 7 shorting thus pulling a lot of current.
Does anybody know where I can get a flyback to test this?

[Bryce]

Are you sure you've installed T401 in the correct orientation? Installing it 90° turned would cause what you are describing.

Bryce.

[Bignumbas]

The T401 can only be installed in one position because it has 2 pins on one side and 3 on the other so unless you want to drill holes it's not going to happen installing it incorrectly. This T401 came from a PCW to replace the one I broke.
Which brings me back again to the inaccurate schematics suppled by Amstrad because these components have 5 pins not 4 as described,  there seems to be a centre tap which is not documented.
I will look into this and try to document it for posterity.
Also  I have a sticker on the back of one of the 644 covers I have proclaiming it to be a 644-2, I have seen no documentation on this v2, unless its the amendment which misses the 5 pinned T401. 

[Bryce]

And the PCW part is definitely the same value as the CTM part?

Bryce.

[Bignumbas]

Yep, the same, the monitor had the same symptoms before I broke the original T401 and replaced it.
OK, I will swap the coil with the one in my working monitor to test it.
Stay tuned....

[Bignumbas]

Ok, I transplanted the T401's between the 640 and 644  and the 640 still works  with the replacement coil I got from Audronics PCW.
These coils are really fragile, the one in the 640 was a little loose in it's base and when I put it in the 644 it was open circuit on the inner winding, the one with really, really fine wire.
With this open circuit I had the correct voltage on the Q405 of 107v, but nothing on Q404 and R436 was cold.
So I did some micro surgery on the coil and miraculously managed to connect the coil tail back, put some hot glue on it to stop it moving.
After putting it back I have exactly the same symptoms again, correct voltage on the Q404 of 34v and low 67v on Q405 with the really hot R436.
Does the problem appear only when the flyback circuitry tries to oscillate then or is it something else?
This is beyond my depth of electronics knowledge.

[Bryce]

If things are getting hot, then too much current is flowing. It's time to find out where it's flowing to (to GND obviously, but through what path).

Are you getting 180V across C430?
Is D407 good?
Is R444 good?
Does the 67V go back up to 100V if you disconnect one end of R436?

Remember, that R436 is a 7W resistor! It will get warm or even hot during normal operation.

Bryce.

[Bignumbas]

R436 would give me a burn blister if I touched it hard.

[Bryce]

That's too much current

Bryce.

[Bignumbas]

I had thought that.
The brown burn mark on the bottom of the resistor is a dead give away too.
This is why I am thinking it's a a faulty flyback.
I have swapped out every other major component but I don't want to do the flyback.
Although its not so hard to to do...... only 10 pins...and it may fix it and I would know if that was the problem then.
But then I would have to find a replacement.
Worst case is that I will still have one dead monitor.
I will think about it.

[Audronic]

@BigNumbas


What is the number on the Flyback Xformer
Or a Photo.


I may have 1 on the board that the little transformer came from ?


Ray

[Audronic]

@Bignumbas


Photos if security will allow me to send them ??


Frustrated


Ray


I took an other photo and Bugg er me it worked
Ray

[Bignumbas]

This maybe of interest to those fiddling with CRT monitors who are curious and brave enough.


https://www.repairfaq.org/sam/flytest.htm

[Audronic]

Hi Mark


Attempt 3 to try and send more Photos


Ray

[Bignumbas]

As you can see from the photos from Audronic, he has a spare CTM644 board which he is sending to me.
It was just lying around in the corner of one of his 4 vaults of stuff.
Thanks Audronic, can I come down to your place with my metal detector for a treasure hunt?

[Audronic]

@bignumbas


Certainly come on down.
By PushBike it is only 1893 Km.
It might take a while.


Ray

[Bignumbas]

Fitted the replacement board supplied by Audronic after I put T401 back.
It fired up immediately .
I had to degauss the screen using some hard drive magnets. The colours are now good over the whole screen
The colours look ok when I try a game, some games have the blue border around the game window but when I just have the prompt there is no blue background just yellow writing on black. If I turn up the brightness it just goes to a bright greeny yellow tinged background.
I tried it with another cpc and the problem was still there.
I have adjusted the sub brightness to spec of 123-124v.
Is there a colour test pattern available for adjusting the colour?

[Audronic]

@Bignumbas


The De-Gaussing coils should clear the screen after about 3 Power ups (wait 5 Minutes between power ups for the Thermistor to settle )


Well Done


Ray

[Bignumbas]

Screen colour is good now.
I tweaked VR805 (blue intensity) and VR503 (Blue brightness)
Now I have a correct blue screen and when I turn up to max brightness its still blue but washed out because of the over brightness.
There does seem to be a bit of strobing?rippling? with the screen, maybe I need to test the caps on the horizontal drive circuits.
I am not under fluro lights, just natural daylight.