Voltage damage to a 6128 plus

Macross_VF1 · 22:32, 13 August 19

How screwed am I in this particular situation? This evening I was going to do a simple test on my 6128 plus. However, since I don't have a dedicated Amstrad monitor for it I brought out my video cable and PSU. I plugged everything in and powered on but quickly realized that I had forgotten to change the voltage on the PSU (it can be set from 5 to 24V) so I powered it off in a panic. So how much damage have I done to it? Right now, with the correct voltage, it doesn't display anything other than a black screen.

The polarity on the plug was correct mind you, just the voltage was set to 9V instead of 5V and it wasn't powered on for more than 2 seconds. Still, I know for a fact that the 6128 plus doesn't have internal voltage regulation so it can't handle much above or below the specified levels.

Token · 23:09, 13 August 19

So, there's nothing on screen (no border at all), does the power led works? Did you tried a game (cart) to see if there's sound? The problem with the PLUS or GX is the ASIC chip, if it's dead you won't fix it. Check the RAM when power up, some chips could be hot.

Macross_VF1 · 23:21, 13 August 19

The power LED does turn on and I do get slight flash of colour on my TV when the computer starts, though it's probably just interference and/or noise from powering it on. I only have two cartridges, one is from a GX4000 (Burning Rubber) and the cart that came with the machine (Burning rubber plus Locomotive BASIC) but neither seems to do anything.

Is there anyway to check for damage? I have a multimeter and I'm not afraid to use it.

Also I'm welled versed in soldering and have the necessary tools, so I could quite easily desolder many of the chips and test them in another, working computer. However, I don't have another plus unit. Would a standard CPC 6128 be similar enough that I could test chips in it?

Bryce · 15:04, 14 August 19

Open it up and power it up (with 5V! this time). If any one chip is getting very hot, you could be really lucky and that chip shorted the entire 9V, protecting the others. However, if none are particularly hot or only the ASIC is very hot, the damage could be worse. If you ave a spare Z80, you could try swapping that out first and see if it gets closer to living, most likely then a border will be visible, but not the correct colours and no text.

Bryce.

Macross_VF1 · 16:59, 14 August 19

I've been doing some simple heat tests when the unit is powered on (with 5V!) and IC12 gets really hot in no time at all. I measured with an IR thermometer and it exceeds 80°C. Some of the other chips also become a bit warmer, like the ASIC and CPU but that's really only to be expected.

So hopefully IC12 took the hit and saved the rest of the computer. If I remove IC13, the rest of the computer should work, right? Albeit with less RAM obviously. Also, I happen to have a chip that I might be able to replace the faulty one with. The original chip is a KM41464AP-12 and the possible replacement is D41464C-10.

Macross_VF1 · 15:26, 15 August 19

It's alive! As stated earlier, I noticed that a RAM chip was hot as fudge. Well, I desoldered them all and put sockets in the IC11 and IC10 positions so I could truly test the chips. One of them is truly dead but the others do seem OK.

However I ran into another snag, the Burnin' Rubber + Locomotive Basic cartridge might have taken a hit as well. I only have two cartridges, the previously mentioned one and a Burnin' Rubber cartridge from my GX4000. The computer works fine with the GX4000 cartridge, but not the other way around. Similarly, the Basic cartridge does nothing in my GX4000 but the Burnin' Rubber cartridge works just fine.

Fortunately, even if the BASIC cartridge have died, it is a relief to know the rest of the computer have survived.

fgbrain · 11:26, 16 August 19

I have another strange issue with my 6128 plus.
With different PSU some keys don't work. You type and nothing is done on screen!!!

I remember that key C doesn't work with one PSU and i can't type CAT...!?!???

Furthermore with other PSU the picture is b&w.

I have no original monitor to check if this problem exist there as well...

Can this be another voltage damage??

pelrun · 15:58, 16 August 19

If it changes when you change the PSU, then it's the PSU at fault. You really need 5v at 2A (or even better, 5.1-5.3v) at the CPC socket, and a cheap or thin cable between the PSU and the CPC can easily drop half a volt.

Cleaning the keyboard sockets on the PCB (use a piece of thin cardboard, it was described recently in another thread here) will help reduce the resistance of the keyboard, so it'll work with weaker PSU's.

If you're using a SCART cable for video, then that also needs enough voltage to tell the TV to switch to RGB mode.

Bryce · 09:12, 18 August 19

Quote from: Macross_VF1 on 15:26, 15 August 19
It's alive! As stated earlier, I noticed that a RAM chip was hot as fudge. Well, I desoldered them all and put sockets in the IC11 and IC10 positions so I could truly test the chips. One of them is truly dead but the others do seem OK.

However I ran into another snag, the Burnin' Rubber + Locomotive Basic cartridge might have taken a hit as well. I only have two cartridges, the previously mentioned one and a Burnin' Rubber cartridge from my GX4000. The computer works fine with the GX4000 cartridge, but not the other way around. Similarly, the Basic cartridge does nothing in my GX4000 but the Burnin' Rubber cartridge works just fine.

Fortunately, even if the BASIC cartridge have died, it is a relief to know the rest of the computer have survived.

The cartidge has two chips. An EPROM with the program and a security chip called an "ACID". Either or both could be damaged. EPROMs are cheap, so I would try burning a new BASIC/Burning rubber image to a new EPROM and swap them out. If the ACID is damaged then there's not much you can do but get a new cartidge. Unfortunately you'll have to do it without a socket, otherwise the cartridge won't close afterwards.

@fgbrain: That PSU is struggling to provide enough current and the voltage is dropping below 5V, so it's just not up to the job. Your CPC is fine.

Bryce.

fgbrain · 19:51, 15 October 19

Quote
@fgbrain: That PSU is struggling to provide enough current and the voltage is dropping below 5V, so it's just not up to the job. Your CPC is fine.

Quote
Some keys dont work. You type and nothing is done on screen!!!
I remember that key C doesn't work with one PSU and i can't type CAT...!?!???

I changed PSU with the Meanwell gs18a05-p1j (as suggested in other threads) and exactly the same happens... I fear its not a PSU issue!

Bryce · 08:30, 16 October 19

Quote from: fgbrain on 19:51, 15 October 19

I changed PSU with the Meanwell gs18a05-p1j (as suggested in other threads) and exactly the same happens... I fear its not a PSU issue!

But you said earlier that you have a PSU that it works with?

Bryce.

fgbrain · 19:01, 17 October 19

No I just remember that with my previous setup with a PC psu everything seemed fine!

fgbrain · 11:37, 20 October 19

@pelrun Hey, you're my saviour!!!

Quote
Cleaning the keyboard sockets on the PCB (use a piece of thin cardboard, it was described recently in another thread here) will help reduce the resistance of the keyboard, so it'll work with weaker PSU's.

yesterday evening I opened my 6128plus and cleaned the sockets as you suggested....
almost by magic, after many moths of pain and suffering, my precious PLUS is now fully operational!!! everything seems to be working fine now !!! THANK YOU AGAIN!!

@Bryce
you were right that my CPC was not harmed - thank god you're right!

ATB to both of you guys!!!

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Voltage damage to a 6128 plus