• Please review our updated Terms and Rules here

Amstrad PC 2286

If the original capacitors are polypropylene film capacitors, they should be replaced with the same if they don't measure right. I would not recommend using generic ceramic capacitors in high voltage scenarios, unless they're safety rated Y capacitors.

You'll need to make sure you know what the operating voltage of the original capacitors are, if you just assume some value that turns out to be wrong, you could do a lot of damage.
 
Seems there are four film capacitors on the pcb. Their markings are:
C614 and C615: 1H104J R -X
C631 and C634: 1H333J R -W

Tried to get datasheet for these. Looks like the first ones are 0,1uF and the latter are 35nF (or 0,033uF).
But, I cannot find what voltage level they should endure. Just a guess where they are located on the
pcb (probably not on the 230VAC side), 0,1uF/50V and 0,033uF / 50V might be allright. Can anyone confirm?

All four are confirmed as polyester capacitors. C614 & C615 are 0.1uF, C631 & C634 are 0.033uF. But, if I traced correctly, they go right to the AC in! I'm hoping to send you pics of the PCB by the end of the week. So, it might be good to hold off on ordering replacements for these until then. That way, you can follow the traces for yourself.

acgs, I've sent another inquiry to the museum, if they are able to help me with either scans or perhaps
a good quality mobile phone image of the schematics pages of that book. I tried to be very polite and
thanked them for saving rare book even if they are not able to help me.

Fingers crossed that they can help you out. It would be really great if we could track down the Amstrad copyright holder, whoever that is now.
 
Thanks for your comments again!

I have pulled all four film capacitors. My cheap tester gave values ranging 99 nF - 102nF for the two and 32nF - 34nF for the other two.
They appear to be close to their values, but I cannot really say what tolerance or range would be acceptable.

I have now substituted also the big diode and basically all IC's I ordered before and the majoc caps.
I've also made new orders for serveral other components that were not available domestically. I expect very long wait time on some of these.

But, I now also have voltage pinout for Amstrad 2286 motherboard connector:
Pin - voltage
1 - +5V
2 - -5V
3 - +5V
4 - -12V
5 - GND
6 - +12V
7 - GND
8 - GND

This is great; during long wait time for next components, I now can also try to convert at or atx psu and check if the Amstrad 2286
motherboard at least is in working condition, or not.
 
> All four are confirmed as polyester capacitors. C614 & C615 are 0.1uF, C631 & C634 are 0.033uF. But, if I traced correctly, they go right to the AC in! I'm hoping to send you pics of the
> PCB by the end of the week. So, it might be good to hold off on ordering replacements for these until then. That way, you can follow the traces for yourself.

Very, very interesting!

I'm not expert here, but it seems to me like four diodes (D601 - D604) act as bridge very close to power switch. AC is thus converted
soon to DC and the "HOT" side of the pcb is not AC, but DC.

C614 and C615 are in fact in series together and between GND and +5V. This +5V is
directly connected to over voltage protection circuitry, IC602 (MB3761) leg 8. That leg is the IC's VCC.

However, I measured voltages of IC602 a couple of weeks ago and leg 8 (VCC) was showing +4,9V, which is
close enough to be correct. This IC VCC ranges from 2,5V up to 41V. I think those film caps (C614, C615) are probably
fine, because IC602 is getting good voltage on its VCC. I will be putting them back on the board.

I had measured other voltages of IC602 back then, here is a summary:
Leg (what we have / what we should see)
1) 0,07V / 3V (In-B) - probably a problem
2) 4,8V / 5,1V (HYS-A)
3) 2,4V / 0V (In-A)
4) 0 V / 4,9 V (Out-A) most likely a big problem
5) GND / GND
6) 2,4 V / 4,2 V (Out-B)
7) 0V / 5,6 V (Hys-B) (not connected? value not affecting circuitry)
8) 4,9V / 5V (VCC)

Right. Leg 4 should have 4,9V. It goes to Base of Q601. It's expecting to have 4,9V and getting zero instead
and thus giving wrong signal / voltage at its collector. Q601 collector is then connected to the biggest IC601,
the switching regulator.

This leads me to speculate that the overvoltage circuitry is turning the psu into OFF.
Now it does not yet help to pinpoint where the fault is, but it is at least a clue.
IC602 could have failed - or it is just fed wrong voltages at its inputs.

Well, more head scratching to do!
 
Alright,
this illustrates a bit better what is going on with the psu.
As I pointed out earlier, IC602 is getting wrong input voltage at pin 1 and also outputting wrong 0 V at leg 4.
Thus Q601 is getting wrong zero volt at Base and passing through wrong 4,93V.

There are also wrong voltages coming out of Q603 pins 4 and 6.

I put the film capacitors back before I made measurements. Voltages are against GND pin 5 of IC601.

I measured voltages on the "cold" side of the transformer. -12V and -5V are generated from leg 8 of the transformer.
When power is switched on, this goes high briefly, I measured up to 55 Volts. Then very soon voltage goes to zero.
+12V and +5V are generated on the transformer cold side from legs 12 and 13/14. These go up only maybe fraction
of a volt when power is switched on. And soon voltages are just zero.

This makes troubleshooting really tricky. Because voltages go to zero on the cold side of the transformer
very quick after power on, it is not possible to measure voltages at specific points to figure out or get a hint of what component
may be faulty. What I have to assume is that problem is in the +12V or +5V rails causing the overvoltage protection to
shut down psu operation. Or the overvoltage circuitry itself is faulty.

At least this should narrow it down a bit. I do not think the problem is at -12V and -5V rails. These are not connected to
overvoltage circuitry at all.
 

Attachments

  • 1.gif
    1.gif
    84.7 KB · Views: 10
I have pulled all four film capacitors. My cheap tester gave values ranging 99 nF - 102nF for the two and 32nF - 34nF for the other two.
They appear to be close to their values, but I cannot really say what tolerance or range would be acceptable.

Varies. Modern film capacitors are usually between 3-5%, but can be as much as 10 or 20% Older film capacitors are more likely in the 10-20% range.

Anything outside 10% should be treated as suspect, and outside 20% is a reject and should be replaced.

But the values that you got would indicate that those capacitors are probably fine to keep using.

There is continuity in the coils which makes me believe that the transformer is ok.

Continuity on a transformer doesn't tell you anything except that there is a connection between the pins being tested.

Transformers have a variety of failure modes where continuity won't tell you anything. The insulation varnish on the wire can degrade and cause coil shorts to themselves or other circuits on the transformer and cause big problems. Ideally, you'd have a good transformer to get known good resistance readings from and compare them to the transformer you have. A ring tester would also be ideal, but those things are hideously expensive.
 
Yes, that's a good point. I can't take for granted the transformer is ok, especially knowing
that the unit has suffered prolonged times of extreme temperature changes and humidity.

Now I hope it is not it. I'll probably be out of luck finding suitable replacement. Well, there are
still several components that could be the cause. I've go my fingers crossed I can still find out what's
bugging the unit.

There is 2286 with keyboard on ebay at UK for auction at the moment without monitor.
I will not bid on it, but if I remember to follow up, I will see how high the price goes. It appears
to be working unit, but with dead cmos batteries.
 
While waiting for the ordered components I decided to try the Amstrad 2286 motherboard
with AT psu now that I have the power connector pinout.

Aaaand... the screen remains black after I turn on the power. I'm using modern vga display that I know is working.
The only sign of life is quick blink of LEDs of the keyboard and when power is switched off again there is small 'pop' on the
pc beeper speaker.

I also tried with vga card on the ISA slot and not only with vga port on the motherboard but that had no effect.
Checked voltages on the motherboard side of power connector(s) and they seem close enough. At least that
leads me to believe there are no shorts on voltage lines.

RAM chips seems to be tms44c256-12N chips, 256kx4bit chips. I don't think I have anything that I could test these out with.
They are all socketed. I tried to pull and push them back, did that also on ICs 149-152 that were socketed but that did not help.
I'm guessing these bigger ICs may be bios and keyboard ROMs.

There are no sim rams on the slots. I think they are not mandatory since the smaller sims make 1Mbit of RAM. I might find
30 pin sims from some other old pc I have lying around to test the board with, but I'd need to know jumper setting to make
the change. I'm guessing the smaller RAM chips need to be pulled out before 30pin sims could be installed.

I'm wondering if failed RAM chip will prevent the motherboard from POSTing? This new trouble was a shame and
a bit surprising too, since the motherboard does not look corroded.
 
Transformers have a variety of failure modes where continuity won't tell you anything.

That's what I was thinking when I questioned the transformer's condition. Also, I thought that voltages higher than expected coming from the transformer might point to it being faulty. Wouldn't this be a correct assumption?
 
I also tried with vga card on the ISA slot and not only with vga port on the motherboard but that had no effect.
Checked voltages on the motherboard side of power connector(s) and they seem close enough. At least that
leads me to believe there are no shorts on voltage lines.

Inserting a VGA card in a slot will not be the only thing you have to do to try getting video output external to the onboard Paradise adapter. You have to deactivate the "internal VGA" first. That is done with an option link (AKA "jumper" for the rest of us).

All the relavent info regarding the option links is on the hardware page I linked to back in post #14 of this thread under the heading "1/2/3xxx DIP switches and option links." Where the "1/2/3xxx" in the heading can be translated into "model #s 1xxx or 2xxx or 3xxx", or in our case "2286" specifically. So that you don't have to hunt down the link in post 14, the URL is https://web.archive.org/web/20101216023809/http://web.ukonline.co.uk/cliff.lawson/atpcs.htm.

RAM chips seems to be tms44c256-12N chips, 256kx4bit chips. I don't think I have anything that I could test these out with.
They are all socketed. I tried to pull and push them back, did that also on ICs 149-152 that were socketed but that did not help.
I'm guessing these bigger ICs may be bios and keyboard ROMs.
Yes, how many banks are there? You may try swapping them around. Also just reseating them can sometimes help.

I agree RAM swapping is a good idea to try to get the machine running. But, for a complete understanding of memory configuartions on this machine, read the section "Adding memory" on the above link. Adding memory is really one of the best things you can do with the 2286. I've been able to install a full 16MB on these machines maxing out the 80286 addressing capabilites. The motherboard has an onboard LIM EMS system also. This allows you to use the entire memory as extended, EMS or XMS memory and in combinations just like it was a 386. (The special MEMM.SYS driver available on the system disks is needed for using the RAM as EMS.)

There are no sim rams on the slots. I think they are not mandatory since the smaller sims make 1Mbit of RAM. I might find
30 pin sims from some other old pc I have lying around to test the board with, but I'd need to know jumper setting to make
the change. I'm guessing the smaller RAM chips need to be pulled out before 30pin sims could be installed.

Again, the info you need is in the "Adding memory" section. But, briefly, you guessed right: you would have to pull out the RAM chips first, the system wasn't designed to use both memory types at the same time. Also, not just any 30 pin SIMMS will work. The details are on the above link. (Hope no one got sick of me saying that over and over.)

I'm wondering if failed RAM chip will prevent the motherboard from POSTing? This new trouble was a shame and
a bit surprising too, since the motherboard does not look corroded.

Especially if the first addressable RAM chips are bad, the POST probably won't run. While I can't be sure on the 2286 because I never had any bad RAM chips installed on one, I know that some BIOSes do use the first 64K for data storage, etc during the POST.

One final note: While searching the internet for more data about the 2286 service manual, I came across this thread on another forum:
https://www.cpcwiki.eu/forum/other-retro/amstrad-pc-2286-help/

Coincidentally, there is another 2286 in the process of being restored. I thought you might want to join that forum and invite the other owner/restorer to join in on the process over here. Perhaps the 2 of you can benefit from the help from both forums.
 
Last edited:
That's what I was thinking when I questioned the transformer's condition. Also, I thought that voltages higher than expected coming from the transformer might point to it being faulty. Wouldn't this be a correct assumption?

That's what usually happens when windings start shorting together, the voltage rises. It's why old linear wall warts should always be checked regularly, because they can go up astronomically in voltage.
 
thank you for excellent technical link, just what I needed! Also interesting that discussion
on the second link about 30 pin sims - that 3 chip sims may not be compatible. Useful to know this too.

Reseating did not help, but I will try swapping the ram chips and see what happens. If that won't help, I'll
be looking for some 30 pin sims. I did also suspect that I have to disable on board paradise VGA but had no
idea how - until now. I will try that as well, using the trident vga card.

I took a picture that illustrates sim and ram banks. There are 8 RAM chips, but also three socketed (logic?) chips on
their left side. I wonder if they are related with parity checking... and if they must (or must not) be removed when
sims are in place.

Edit:
Ah, yes, I noticed that at least the red power-on led is working, when I power up that AT psu.
One small indication that something is right with the unit, keeping hopes up.
 

Attachments

  • 1.gif
    1.gif
    214.1 KB · Views: 13
Last edited:
Ahha, the other Amstrad project had this issue: "replaced the VGA connector as it wasn't outputting a signal".
Yeah, I need to make sure I don't have the same fault with my unit.
 
The poor old Amstrad has been on a break during summer.
A bunch of components did arrive from China during summer though and I replaced the into the psu. They
did not help, it's still dead. I guess suggestions that the transformer is bad may be correct.

I have changed my focus to the motherboard. I tried powering it up with an AT psu and today with ATX psu.
Obviously I needed to solder correct voltage cables to the original psu connector first.

I'm stuck with black screen of death still. I have tried 4x4mb simms, 4x256kb simms and the default ram chips
that are on the board. Tried out the daughter board paradise vga and alternatively using isa vga card on the expansion slot.
Tried both flat screen and CRT monitor. No dice.

Checked voltages on the motherboard near the connector, all are present and reasonably close to the specs. Also checked voltages
on the ISA slots, voltages are also there.

I'm starting to get a bit concerned that perhaps when that original psu went bad, it may have damaged some (or many) thing(s)
on the motherboard. If that happened it is quite a pity.

There are several small electrolytic caps that I have not changed on the motherboard. In fact, I have not changed any components
except the RAMs, but perhaps those electrolytic caps could be an issue causing black screen? Everything seems a bit far fectched
though.

Thought has occured to me; what if I cannot get life to the motherboard. I suppose plan C might be putting random 386 motherboard
and at(x) psu inside the unit and go with something like that. However, this would lower the fun factor quite a bit. Not because
I know what's inside the unit (it is close enough), but because I could not use Amstrad original keyboard and mouse. This really
lowers the authenticity a bit too much.

I'd still like to try something out with the original motherboard, but I'm slowly running out of ideas what to try next.

Edit:

> Inserting a VGA card in a slot will not be the only thing you have to do to try getting video output external to the onboard Paradise adapter. You have to deactivate the "internal VGA" first. That is done with an option link (AKA "jumper"
> for the rest of us).

Yes, by the way, I want to add that I have set jumpers accordingly with each setup that I've tried.
Thanks again for the link above, it has been valuable here !
 
If the PSU sent a spike to the logic board, anything could be fried. You'd have to start pulling ICs and testing them individually. A logic probe and oscilloscope would also be required.
 
You're probably right, there is no easy way with this one.
Pulling ICs I can do, mostly without causing more damage as well. Getting oscilloscope
and learning how to use it... this part may be an obstacle for now.

I could also make a wish to santaclause, please bring me (or find me obtainable) working motherboard.
But have I been nice enough? Maybe it takes a few years for this wish to come true.

More likely it is the logic probe for this christmas. And more pondering what else I can do with what I've
got at hand.

A few years ago I made small circuitry to write 8kb eeeproms and succeeded in that. I used those to make
replacement 1541 ROMs. I could find that stuff and see if it could be used to read ROM dumps of some
of the Amstrad motherboard ROMs, perhaps with some modifications. Then, I'd at least know if the ROM
chips are ok and not fried.

Unfortunately, I still have not been able to source schematics for the motherboard. Not a game stopper
though, if I go on to checking individual ICs.
 
I've been giving your problem a lot of thought and have a few suggestions. First, just to be sure the simple things have been resolved, have you have made sure that the speaker is functional? If so, make sure that the volume control is working and turned up all the way. If any beep codes are being produced, you have to be sure they're actually audible.

Next, the 2286 has an uncommon design where 2 "motherboards" are plugged into each other. The bottom board is the main board (the real motherboard) and the top board is the expansion board (like a riser card) for plugging in ISA cards. I remember, once long ago, having to disassemble a 2286 and remove the top board. After putting it back together, carefully plugging the top board into the main bottom board, I couldn't get the computer to boot. After several frustrating attempts to find the problem, I had to disassemble it again, and found that the top board wasn't really plugged in right even after all my checking.

I also found that you might not get the computer to boot if you don't put the 2 boards back into the case first. Trying to boot with the boards on the workbench is difficult. The top board isn't rigid enough to rest by itself and can flex if not supported properly. Coupling that with the connectors to the bottom board not being strong enough to stay properly plugged in when the top board isn't supported, and you can have no end of intermittent problems. So, make sure to install both boards in the case and plug them in carefully before you try booting it.

Hope that helps.
 
Back
Top