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avatar_Ynot.zer0

CPC 464 diagnostics

Started by Ynot.zer0, 17:50, 28 April 23

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Ynot.zer0

So...I just paid £50 for a CPC 464, new keyboard style with a small size board.  Received it, plugged it in.  nothing.
Took it apart, it was very grubby, cleaned it up - nothing looked obviously damaged / broken.
When I plug in the monitor cable and PSU, if I check on the board ground (all around the outside) for ground and touch the center of the PSU I get a nice 5v reading.
The on/off switch doesn't seem to be doing anything - maybe that it defective? I tapped/bridged it, still nothing.
I disconnected the tape drive completely, ie. removed the white cable connector on the motherboard.

On one of the original large motherboards I have, I found that if I touched pins (from the top moving down) 1 and 6 together, I would get the BASIC screen prompt output - basically bypassing the tape player and on/off switch.  However, doing this on this motherboard I get nothing.  I tested on a SCART cable TV/monitor as well as a CTM644 proper monitor - nothing outputs at all.

I've pressed the 2 socketed chips firmly in, so they are seated okay.  It seems like power is getting into the board and through it (I can check voltage on the pins and see 5v coming through), so is there a monitor / visual output chip that could be faulty?  The blue capacitor next to the PSU input looks okay, as do all the others (I can provide a photo if needed)

Does anyone provide a UK service (or even in Europe?) where I can send the board for diagnostics?  or even attempt to fix it myself if someone can advice me what to prod & poke.  Seems a shame to bin it, or stick it on eBay & make it someone else's problem (like the person I just bought it from), the board and keyboard / casing is in really good condition, albeit oddly a mix of spanish and english writing on it, maybe that was a thing?

any thoughts? pointers?

Rabs

What do you get, a black screen or grey? Is the power led on? Sounds like you have checked power, I would next check for clock and M1 on the Z80. Do you have an oscilloscope or any means of checking signals?

Ynot.zer0

I get no screen output at all. No LED comes on either. I don't have an oscilloscope, just a trusty old caltek AG1021 device that I used for checking car electics - it does show me DC V, ohm resistance and DC mA, if that's any help.

Rabs

Quote from: Ynot.zer0 on 18:27, 28 April 23I get no screen output at all. No LED comes on either. I don't have an oscilloscope, just a trusty old caltek AG1021 device that I used for checking car electics - it does show me DC V, ohm resistance and DC mA, if that's any help.
No LED? Are you sure you have power to the board? What voltage do you see on the Z80 pin 11, should be +5v.

eto

Quote from: Ynot.zer0 on 17:50, 28 April 23The on/off switch doesn't seem to be doing anything - maybe that it defective? I tapped/bridged it, still nothing.
The switch is often dirty / corroded - and this is probably the primary fault of any "non working CPC". Just open and clean it. 

You can also bridge pin 1 and 3 of the connector on the motherboard. See it in this video: 

Ynot.zer0

I did say I'd done all those things.

I even removed the board, and as you say pin 1&3 tapped together give no output, as shown in photo.

You cannot view this attachment.

eto

I might be mistaken, but pin 1 and pin 3 don't seem to be connected here.

But you said you have measured the 5V on the board? What's the exact voltage when you check Z80 between pin 11 and GND (as Rabs already suggested)? Does it show (close) to 5V?


Ynot.zer0

That's a great video btw, just watched it all and it helped understand a bit more if what's going on.

Apologies I assumed pin 1&3 were in reference to the red & white cable wires from the connector to the tape drive, I put my own 2 wires on and tapped together but nothing on screen.

If I put 1 voltage connector (+ve) on the input psu +ve and put the other connector (-ve) on pin 11 of z80 it reads a nice strong 5volts.  That is the right way to read it isn't it?


Rabs

Measure between pin 11 +5v and pin 29 gnd.

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Ynot.zer0

ah, when I do that... I get nothing  :(   hmmmm.....

eto

Quote from: Ynot.zer0 on 22:22, 28 April 23ah, when I do that... I get nothing  :(  hmmmm.....
That means that the 5V is not coming to the mainboard. This might be much easier to fix than a fried IC.

Do you get 5V directly at the power connector? then follow the traces and see if you still have 5V. Maybe there is a crack in the PCB.

Rabs

#11
You may want to check the solder joints on the power connector as they are prone to dry joints.

You cannot view this attachment.

You can check continuity between pin 3 on the tape connector and the 5v centre point on the power connector or pin 1 and pin 11 on the z80. You can also check pin 2 on the tape connector, which is GND.

You cannot view this attachment.

Ynot.zer0

Many thanks to Rabs & his forensic investigation skills, we discovered that there were a few breaks that were not visible to the eye, by bypassing them we now have a fully functioning MC0044B motherboard.

Huge thanks for the time, effort & skill shared by the community - this CPC 464 lives to play another day.
I now need to invest in an oscilloscope in order to debug hardware more effectively  :)

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