YABA - Yet Another Broken Amstrad

[ShadowTD]

Three years after I first registered on here, a house move later and I'm finally getting around to getting all my stuff out of storage.  Starting with one of the two CPCs I bought as 'untested' - it's sadly a cost reduced model but I've got a new soldering iron and a borrowed oscilloscope and no true sense of how out of my depth I am (GCSE electronics, don't fail me now).


It was obviously dead when first powered - no continuity from centre pin to +5v.  Fixed that with a bodge wire and now I've got either a stable white square or occasionally rolling distortion.


With the second one the tape drive moves when pressing play - the first one doesn't.  No beeps when pressing delete.  I've seemingly got a clock signal at pin 6 of the z80 and 1mhz a pin 21 (?) of the UM6845, which also seems to have H and V sync coming out of it (which makes sense given the usually stable image on screen).  I can also seemingly see signals going in and out of the RAM.  None of the chips appear to be running 'hot' - the z80 is warm after about 3-5 minutes on.


Any suggestions on how to proceed would be greatly appreciate!

[Bryce]

Most likely just a dead RAM IC. Have you any device that replaces the lower ROM so that you can test the RAM with Geralds test program?

Bryce.

[ShadowTD]

Thanks for replying!  No, nothing to enable me to run a memory test program.  I've ordered some IC sockets for the memory and I'm looking to order some memory chips.  Does the number after the chip type make any difference ie KM4164-XX?  There's ones on ebay with 12, 10, 15 & 12 - the board is full of 10s at the moment.  Will the machine start up with no memory at all, or a limited amount?
Sorry for all the questions.  If there's a troubleshooting guide with this all in there then please point me in that direction!

[Bryce]

Nope, the machine will only start if the entire lower 64K of RAM is fully functional. 4164-10 are the fastest, but -15 are fine too for the CPC.

Bryce.

[ShadowTD]

I really hate it when people don't follow up their own posts.  I'm very happy to say that replacing all the RAM chips (and fixing the trace I ripped out when removing the top chip!) has brought another 464 back to life.  It's also been a crash course/memory jog in soldering, so future repairs might be of a slightly higher quality.  But this works (apart from the tape pause) and will soon get a DDI-5 when the postal situation improves.  Final question - anyone point me in the direction of a source of case screws?

[HarrierAttack]

Awesome, so glad you were able to keep another CPC alive! 
I've been lurking for a while, but your post has prompted me to join, where did you get the RAM ICs from? And did you take the chance to put in a socket for each IC? Apologies if there is a section for parts procurement, I'm new here and still finding my feet.

[Bryce]

The RAM chips can easily be found on ebay. I'd definitely recommend the use of sockets as these chips can be very unreliable and then the next swap won't have you reaching for the soldering iron.

Bryce.

[ShadowTD]

Wow - happy I was able to inspire somebody!
Yes, I socketed the RAM chips - there was no way I was going to risk direct soldering on to the board.  I used the -15 chips I found on eBay, and TBH they were China specials but they were cheap!

[Bryce]

Wow - happy I was able to inspire somebody!
Yes, I socketed the RAM chips - there was no way I was going to risk direct soldering on to the board.  I used the -15 chips I found on eBay, and TBH they were China specials but they were cheap!

China specials?? As far as I know, the 4164 was never produced in China. The more commonly seen ones are:

TMS = Texas Instruments - Made in the US.
HY =  Hyundai - Made in Korea
KM / SEC = Samsung - Made in Korea
MCM = Motorola - Made in the US.
MHB = Tesla - Made in the Czech Republic
µPD / NEC = NEC - Made in Japan.
TC = Toshiba - Made in Japan.

By the time that China got into Silicon fabrication, the 1x64K DRAM market was already over.


Bryce.

[ShadowTD]

Thanks @Bryce - appreciate the info.  I bought these:
https://www.ebay.co.uk/itm/NEW-10PCS-SAMSUNG-KM4164B-15-Encapsulation-DIP-16-64K-X-1-BIT-DYNAMIC-RAM-WITH/113226359003
And I just assumed with the location that I'd be getting some low rent garbage.  Apparently not!
Just need to wait for shipping to open up again so I can buy a DDI-5 

[Bryce]

Thanks @Bryce - appreciate the info.  I bought these:
https://www.ebay.co.uk/itm/NEW-10PCS-SAMSUNG-KM4164B-15-Encapsulation-DIP-16-64K-X-1-BIT-DYNAMIC-RAM-WITH/113226359003
And I just assumed with the location that I'd be getting some low rent garbage.  Apparently not!
Just need to wait for shipping to open up again so I can buy a DDI-5 

No, they are quality parts. Made in Korea in September 1983 (in plant "C" so they must have had at least 3 plants producing them).

Bryce.

[HarrierAttack]

So I don't know if this is the right place to continue this, but;
IRAM for voltage, the first 5 are registering 4.6v whilst the others are giving me nothing!
My gate array is giving me 4.6 but my z80 registers nothing and is stone cold.
I obviously have no output, will replacement RAM and a new z80 give me the start screen? Or is there something else I should be looking for?

[ShadowTD]

I think this might have been the wrong group TBH!  Better people than myself will offer advice, but I learned a lot from watching this video:

https://www.youtube.com/watch?v=EyT7gYs--vo
and it's part 2.  Although it's a 6128 the principles are the same.  It really helps if you have access to a scope so you can see your clocks, data going to the RAM chips etc etc.  My z80 wasn't dead so maybe you have no clock? 

[Bryce]

So I don't know if this is the right place to continue this, but;
IRAM for voltage, the first 5 are registering 4.6v whilst the others are giving me nothing!
My gate array is giving me 4.6 but my z80 registers nothing and is stone cold.
I obviously have no output, will replacement RAM and a new z80 give me the start screen? Or is there something else I should be looking for?

You are getting ahead of yourself here. Start with the power. You say the first 5 read 4.6V (on what pins?) and the rest "give you nothing"? That's pretty much impossible (and they actually take the 5V, not give), so the first thing to find out is whether they really aren't getting 5V or if you are measuring wrong, because if they aren't getting 5V then new ones aren't going to make things any better.

Bryce.

[HarrierAttack]

On the RAM, using a multimeter on pins 8 and 16 (as per this data sheet I found http://www.datasheet.hk/view_online.php?id=1061986&file=0031\km4164b_249647.pdf).
When I tested my power supply it was giving 4.9v , I assumed losing 0.3v was normal. I've taken apart and cleaned the power switch with contact cleaner. The RAM that gave me no reading was corroded in places.

I'm actually enjoying learning more and more about my childhood computer, I only received my CPC on Tuesday having bought it "for parts only" off eBay. It's the first board model I think (big heat sink on the 4007). Later today I'll start a new thread for it (with pictures). This community feels very friendly and knowledgeable, I'm glad to be here! =)

[Bryce]

Ok, 4.6V on all IC's should be enough to get the computer going. Then it's most likely the RAMs are bad, so go ahead and order some (if you intend replacing them yourself). There's no reason to believe that the Z80 is bad at this stage, so I'd hold off on ordering one of those for the moment.

Bryce.

[VintageAdvantage]

On the RAM, using a multimeter on pins 8 and 16 (as per this data sheet I found http://www.datasheet.hk/view_online.php?id=1061986&file=0031\km4164b_249647.pdf).

You cannot test RAM with a multimeter. You need AT LEAST a logic probe (15 $).

[GUNHED]

From the pictures I can asure you that Z80 and CRTC are good. Problems could be aside of RAM also the GA (most likely).

[Bryce]

You cannot test RAM with a multimeter. You need AT LEAST a logic probe (15 $).

Pins 8 and 16 are VCC and GND, he wasn't checking the RAM, he was checking whether they were getting 5V.


I doubt the GA is damaged, they are tougher than you think. I suspect it's just a bad RAM or two.

Bryce.

[VintageAdvantage]

Pins 8 and 16 are VCC and GND, he wasn't checking the RAM, he was checking whether they were getting 5V.


I doubt the GA is damaged, they are tougher than you think. I suspect it's just a bad RAM or two.

Bryce.

ok