avatar_Sebastian Blanco

Got amstrad pcw 8256 but the fun don't lasted very long ...... :O

Started by Sebastian Blanco, 19:19, 01 July 16

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GeoffB17

Well, I'd be interested to know what sort of thing might cause the STK to fail.


Something to be warned about??


Obviously, if it's just plain old age, will happen whatever at some point, then there's nothing I can do but keep fingers crossed.   I don't want my PCW to die!


Geoff

Bryce

The STK runs hot its entire life, so it's a part that's prone to fail. All that dust around it won't have helped either. Luckily they are still relatively easy to source at a reasonable price. You just need to make sure that you add thermal paste and bolt it securely to the heatsink when you replace it, otherwise the new part won't last very long.

Bryce.

GeoffB17

Thanks.


So, my PCW has NEVER been THAT dusty inside, and has ALWAYS had reasonable ventilation.  So I - hopefully - should worry less?


Geoff

||C|-|E||

That layer of dust can definitely make a big difference, if yours was always clean and well ventilated I would say that you could worry less, yes  :)

Bryce

The capacitors around it are more likely to fail first.

Bryce.

Sebastian Blanco

Quote from: Audronic on 00:58, 11 July 16
@Executioner


If you want an STK7308 I have one here (Victoria) In a WORKING colour monitor board.
Cost " a Smile " .




Ray

Thank you very much for your offer but theres no need for you ti kill a color monitor for this part seem to be quiet cheap on ebay i will go on and purchase one.

Sebastian Blanco

Quote from: Bryce on 09:23, 11 July 16
Hmmm, the STK is most likely broken, but I'd also question your multimeters functionality. What model is it? Pins 14 and 15 are connected together both inside and outside the STK, so they can't possibly have different voltages.

My suggestion would be to swap the STK and all electrolytic capacitors around it. When you have the STK and caps off the board, check that the diodes are stilll doing their job.

Bryce.

True bryce i check and rechecked the voltage across the pins and get this values, my multimeter is a very cheap chinise unit and i  have been using it from like 15 years. I will get a new better one and check again but i verified other sources like the main unit input and got 218v so seem to work OK.

Already purchased a new stk gonna change all the caps and check again after removing the stk

Sebastian Blanco

OK have a news STK (not installed still) before i replace it i what to check so i don´t get it kill.
I have got also a brand new multimeter much better than the previous old one.
Lets see on pins 14 and 15 of the STK7308 got 305v DC on both pins. But 7.9v is what shows on the schematic i think must be something very wrong on another part of the circuit.
Also tested the big resistors the zener diodes and all the diodes in the input, and they seem to be fine removing one point and using the semiconductor test on the multimeter.
Got almost 0v on the q5001 transistor legs.
I´m not sure how the current from the input is drop from 308v(that is what i got DC at the end of the bridge diode) to 7.9 on the 14 and 15 pins.
Removed the dead STK7308 and tested again but got 305v on legs 14 and 15 still.



Audronic

HI Sebastian.

The power supply circuit (DC) is floating above GROUND ? (It is AC coupled via C5013 to ground only)
If you do your measurements with respect to PIN 1 (-ve Lead (Black)) then to pin X that may help.


Please remember that this area is LETHAL

Good luck
Procrastinators Unite,
If it Ain't Broke PLEASE Don't Fix it.
I keep telling you I am Not Pedantic.
As I Live " Down Under " I Take my Gravity Tablets and Wear my Magnetic Boots to Keep me from Falling off.

Sebastian Blanco

Thank you for your reply Audronic i have tried using the multimeter from pin 1 ground and pin 14 but i still got 306v like before.

Quote from: Audronic on 01:15, 01 August 16
HI Sebastian.

The power supply circuit (DC) is floating above GROUND ? (It is AC coupled via C5013 to ground only)
If you do your measurements with respect to PIN 1 (-ve Lead (Black)) then to pin X that may help.


Please remember that this area is LETHAL

Good luck

Audronic

Quote from: Sebastian Blanco on 01:25, 01 August 16
Thank you for your reply Audronic i have tried using the multimeter from pin 1 ground and pin 14 but i still got 306v like before.
Hi Sebastian
I am not sure that i explained it OK.
Please try this:-
The Black Lead (-ve) on the Meter connect to PIN 1
and connect the Red Lead (+ve) to PIN 14.
Test the Voltage on DC


We are not testing to Ground.

Good Luck       Ray



Procrastinators Unite,
If it Ain't Broke PLEASE Don't Fix it.
I keep telling you I am Not Pedantic.
As I Live " Down Under " I Take my Gravity Tablets and Wear my Magnetic Boots to Keep me from Falling off.

Sebastian Blanco

I think i understand you correctly Audronic i take a picture in case i am metering it wrong.

:o
Quote from: Audronic on 02:21, 01 August 16
Hi Sebastian
I am not sure that i explained it OK.
Please try this:-
The Black Lead (-ve) on the Meter connect to PIN 1
and connect the Red Lead (+ve) to PIN 14.
Test the Voltage on DC


We are not testing to Ground.

Good Luck       Ray

Audronic

Hi Sebastian


Yes the points that you are using for testing are OK.
I forgot that you did not have the iC Connected..
I think that you will have to solder in the Chip WITH HeatSync compound, and the do some testing.


A picture that i am using ( I don't have a real Machine ) Please download it an see if that is the correct circuit.


Ray
Procrastinators Unite,
If it Ain't Broke PLEASE Don't Fix it.
I keep telling you I am Not Pedantic.
As I Live " Down Under " I Take my Gravity Tablets and Wear my Magnetic Boots to Keep me from Falling off.

Audronic

Hi Sebastian


PS:-
If you follow the Positive line through the transformer to the Point 10 (Circled) and then to Pin 14 you will se that it is The output of the Diode Bridge 300 Plus volts
Ray
Procrastinators Unite,
If it Ain't Broke PLEASE Don't Fix it.
I keep telling you I am Not Pedantic.
As I Live " Down Under " I Take my Gravity Tablets and Wear my Magnetic Boots to Keep me from Falling off.

Sebastian Blanco

Thank you very much for your help, i see the same on the shematic i was just worried that i got 300v in place of the 8v that is show on the shematic i just don´t wanted to risk killing my new STK7308 gonna solder the new one and post the results  :o


Quote from: Audronic on 02:57, 01 August 16
Hi Sebastian


PS:-
If you follow the Positive line through the transformer to the Point 10 (Circled) and then to Pin 14 you will se that it is The output of the Diode Bridge 300 Plus volts
Ray

Sebastian Blanco

OK installed the new one and surprise ... 310v between pins 1 and 14 so another dead STK  :picard:
All the other pins got almost 0v

Audronic

Hi Sebastian


Sorry i cannot help further


All of the components around the I/C will have to be checked.\


Good Luck    Ray
Procrastinators Unite,
If it Ain't Broke PLEASE Don't Fix it.
I keep telling you I am Not Pedantic.
As I Live " Down Under " I Take my Gravity Tablets and Wear my Magnetic Boots to Keep me from Falling off.

Executioner

Quote from: Audronic on 00:58, 11 July 16
@Executioner

If you want an STK7308 I have one here (Victoria) In a WORKING colour monitor board.
Cost " a Smile " .

Ray

Hi Ray,

Sorry, I haven't been keeping up with this thread until now. I'm not sure that is the part I'm going to need as I haven't even looked at the board yet. Is that the most likely failed component?

Audronic

@Executioner


By the look of the faults reported on the Forum it looks like thats the hardest working part.


Ray
Procrastinators Unite,
If it Ain't Broke PLEASE Don't Fix it.
I keep telling you I am Not Pedantic.
As I Live " Down Under " I Take my Gravity Tablets and Wear my Magnetic Boots to Keep me from Falling off.

dlfrsilver

Quote from: Sebastian Blanco on 03:23, 02 July 16
Yes i have tested removing all hardware possible, even the digial board but the unit never turn on


Those machines are VERY old. Check the fuse and also change all the capacitors. Those are more than likely prone to die since their old age.


Most retro hardware must be revised, you can't buy them and say "hey !! Let's power that or this up! ".


The joy won't last long. All the replaceable parts must be changed.


Please post a photo of your PSU belly open, and focus on the capacitor.

JonB

Regarding testing the power board, what is the safest way to run it under test without connecting to the CRT? Remove the flyback transformer?

JonB

OK, I have got it upright and braced against the far end of the chassis with a wooden stick.


After replacing R001 (I found it was open circuit) I am seeing the same as Sebastian - 313v on pins 14/15 of the STK.


I found this interesting tidbit from the pages of an electronics trade magazine:


Amstrad PCW8256 We are beginning to get PCW8256 word processors back with power supply faults. The raster produced is small and the STK7308 chopper control chip IC5001 screams. All is returned to normal when the i.c. is replaced.

Suggests that it may be common, but we have a different problem. Already I tried to substitute the STK out of a working PCW into my broken one, but like Sebastian, this didn't fix it, and I have this nasty high voltage on pins 14/15. I am trying to find a datasheet for the STK that explains its inputs, but so far no joy (just short form sheets).


I also checked the voltages on Q001, they are all wrong (should be -2.0, -1.3, -2.0 and I have zero or mV type values).


The full measurements for the STK: I am getting 0v at pins 1-9, .46v at pin 10, 0v at pin 11 and 313v on pins 14/15, all WRT pin 1, which although connected to the heatsink, is not GND. WRT GND, I get -154v (1-11) and +154v (14/15).

JonB

I have checked all resistors in the high voltage circuit RS001 - RS011 and they all match what the schematic says. No easy fix there.


robcfg

Hi,


I just found that the NTE1870 may be a replacement for the STK, and its datasheet is much easier to find.

JonB


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