the power supply in CPC 6128 monitor, but no visible sign. Easy fix?

[old_north]

I have turned on my CPC 6128 with green monitor after several years of it being stored in a dry place. The monitor would briefly show a green line but the computer would not turn on. I measured the voltage on the 5V lead, and it's 0. When I turn on the monitor without the computer being attached, the voltage stays close to 5V with some fluctuations, but if I now attach the computer, the voltage goes down to 0 and wont't rise again.


Opening the monitor doesn't reveal anything suspicious - all the capacitors look perfectly fine, with no bulging or any fluids having been leaked.


I hope I can troubleshoot my problem with your help, perhaps there is a usual suspect that I could check?

[old_north]

I checked the two glass fuses on the GT65 motherboard, and they are both OK. And now I could test the 12V supply and that's OK as well. So it's only the 5V that's broken, but that's a bummer because it's really hard to access. But I shall keep buggering on.

[Bryce]

The 5V regulation on the GT65 is a pretty simple circuit. One or more of the following 6 parts will have failed. As the circuit manages to output 5V until it is loaded, I would replace the two electrolytic caps first to see if that solves the problem. The next most likely culprit (if the problem persists) would be the transistor as it's the part that actually supplies the load current. The actual regulator (L78MG) regulates the voltage, but doesn't supply the current, so it's highly unlikely that it has failed.


Bryce.

[old_north]

The 5V regulation on the GT65 is a pretty simple circuit. One or more of the following 6 parts will have failed. As the circuit manages to output 5V until it is loaded, I would replace the two electrolytic caps first to see if that solves the problem. The next most likely culprit (if the problem persists) would be the transistor as it's the part that actually supplies the load current. The actual regulator (L78MG) regulates the voltage, but doesn't supply the current, so it's highly unlikely that it has failed.


Bryce.

Thanks. In the meanwhile I connected a very good 5V power supply to the CPC 6128, and got nothing. It seems that the machine is dead, as now it has both 5V and 12V supplied, but not even the On light turns on. I didn't expect  this: both the PSU in the monitor AND the computer have both failed. And yet they both worked just a few years ago.

[old_north]

As the circuit manages to output 5V until it is loaded, I would replace the two electrolytic caps first to see if that solves the problem.

You make a great point, and I shall do so, but for now I am preoccupied to get the damn computer to work.

[Bryce]

The power switch on the CPC is known to gather dirt and no longer conduct. Check whether it needs cleaning. The easiest way is to remove the connector on the main PCB and bridge it out. If the PCB is ok, it should boot.


Bryce. 

[old_north]

The power switch on the CPC is known to gather dirt and no longer conduct. Check whether it needs cleaning. The easiest way is to remove the connector on the main PCB and bridge it out. If the PCB is ok, it should boot.


Bryce.

Welp, this is embarrassing (probably): I checked the PSU's 12V, but t his time "from the outside", i.e. I measured the voltage at the plug itself instead of opening the monitor and measure it from the inside. At any rate, I get a random reading, anything between -11V to +couple of V, varying widely and wildly. I have no other recourse but to make a PSU with both 5V and 12V. But I have a question: IF only the 5V is supplied, shouldn't the On lamp/LED light up? That is, even without the +12V?

[LambdaMikel]

But I have a question: IF only the 5V is supplied, shouldn't the On lamp/LED light up? That is, even without the +12V?

Yes it should. Without 12 V, only the floppy drive doesn't work. Like Bryce said - if you tried with know good external 5 V PSU with the right polarity and nothing, not even LED, then its the barrel power socket / jack, or the switch, or...

[old_north]

OK gentlemen, I made it to work THANKS TO YOUR HELP! Indeed, the switch is busted. Looks good otherwise, and no dust anywhere (since I kept the machine in a dust-free enclosure), but something is not right with that switch.

At any rate, I hooked up the CPC 6128 to my 5V PSU and to the monitor, and got the nice, green phosphorus letters.

[old_north]

My further course of action will be to make a replacement PSU so I can connect the CPC 6128 to our TV via a SCART cable... which I should prepare as soon as I get the DIN 6 pole connector from Hong Kong. I have also an RGB --> VGA converter board, so I might just make a pseudo MP2 (but one that contains an RGB to VGA converter instead of an RF modulator).

[robcfg]

If you plan to convert the RGB signal to VGA, you may need a scandoubler device, as the CPC outputs a 15khz signal and VGA requires 30-31khz if I remember correctly.

[LambdaMikel]

"Scandoubler" devices are a little bit antique by now (at least in the classical meaning of the term); unless you plan to use a CRT VGA with that, there is really no reason to sink a lot of money into a ScanDoubler. I'd suggest to get a CPC SCART cable and this device:


https://www.amazon.com/VCANDO-Converter-Adapter-Switch-Scaler/dp/B01N06Q9WH?ref_=ast_bbp_dp


The SCART cable may need to be "active" though:


https://coolnovelties.co.uk/coolnovelties/amstrad-video-cables/258-amstrad-cpc-464-6128-active-rgb-scart-cable.html?search_query=cpc+cable&results=15


Btw, I also have


https://www.ebay.com/itm/Arcade-Game-RGB-CGA-EGA-YUV-to-VGA-HD-Video-Converter-Board-HD9800-GBS8200-DIY/112236179750


and the image quality and responsiveness is not better than the HDMI converter... it's actually worse.

[LambdaMikel]

Check out this thread for
GBS8200
in the MP2 case:

http://cpc-live.com/forum/index.php?action=dlattach;topic=735.0;attach=1865

http://cpc-live.com/forum/index.php/topic,735.15.html



https://youtu.be/xqlrOcGZx38

[Bryce]

Welp, this is embarrassing (probably): I checked the PSU's 12V, but t his time "from the outside", i.e. I measured the voltage at the plug itself instead of opening the monitor and measure it from the inside. At any rate, I get a random reading, anything between -11V to +couple of V, varying widely and wildly. I have no other recourse but to make a PSU with both 5V and 12V. But I have a question: IF only the 5V is supplied, shouldn't the On lamp/LED light up? That is, even without the +12V?

First - Important: The 12V plug has the positive on the outside! So you were probably getting 11V not -11V. If the voltage is jumping up and down, then it's most likely just a break in the spiral cable. Replace it.

Bryce.

[old_north]

If you plan to convert the RGB signal to VGA, you may need a scandoubler device, as the CPC outputs a 15khz signal and VGA requires 30-31khz if I remember correctly.

I bought an RGB to VGA converter on eBay, and I trust that it should work, as I have seen it used on YouTube by a guy who accomplished the same thing I want - used it to connect an Amstrad's RGB to a VGA monitor.

[old_north]

By the way, could anyone suggest a suitable replacement (model, part number...) for the switch that died in the CPC 6128? I don't really care if it looks the part, as long as it fits more-or-less correctly.

[old_north]

Btw, I also have


https://www.ebay.com/itm/Arcade-Game-RGB-CGA-EGA-YUV-to-VGA-HD-Video-Converter-Board-HD9800-GBS8200-DIY/112236179750


and the image quality and responsiveness is not better than the HDMI converter... it's actually worse.

That's what I have, too. I don't plan on investing in more hardware for this project, at the moment. Also.... we don't have a single HDMI monitor in the house, which is quite remarkable in this day and age. Our TV set does, but then, our TV set has also a SCART input.

[old_north]

First - Important: The 12V plug has the positive on the outside! So you were probably getting 11V not -11V. If the voltage is jumping up and down, then it's most likely just a break in the spiral cable. Replace it.

Bryce.

Yes, the cable does indeed have a slight intermittent contact problem. I hope to isolate it and fix it without having to toss the entire cable. It would be a cheap way to preserve the original looks, if I can manage to do this.

[Audronic]

By the way, could anyone suggest a suitable replacement (model, part number...) for the switch that died in the CPC 6128? I don't really care if it looks the part, as long as it fits more-or-less correctly.

Hi You can take the switch apart and clean the contacts Not difficult.


Good luck   Ray

[old_north]

Hi You can take the switch apart and clean the contacts Not difficult.

Thanks but I am not going to do that - for one thing, the switch I have in my CPC is not meant for disassembly as it would require the bending of some sheet metal comprising the enclosure of it. For another thing, if the contacts are corroded and/or otherwise worn, this would be less than temporary. Once the thin contact layer (made of gold and platinum group metals) is compromised, the switch is doomed.

So if anyone would know a suitable or compatible model/part number, that is still my preferred or actually only choice.

[Bryce]

It's highly unlikely that you'll find a switch that fits. The contacts won't be worn or damaged, just tarnished. Users regularly open and clean these switches. There are several tuorials on how to do it. Yes, you'll need to bend those metal tabs back, but it's a simple process and when the contacts have been cleaned it will work again like new for years.

Bryce.

[old_north]

It's highly unlikely that you'll find a switch that fits. The contacts won't be worn or damaged, just tarnished. Users regularly open and clean these switches. There are several tuorials on how to do it. Yes, you'll need to bend those metal tabs back, but it's a simple process and when the contacts have been cleaned it will work again like new for years.

I am actually thinking that it should be rather likely to find such a switch, since I have a very similar one at home... in one of my boxes. Let me try to do my homework on this, before I crack open the failed switch.

I have a question related to the topic in this thread: what is the lowest voltage at which the floppy drive will still work? I found an excellent miniaturized PSU with multiple voltages, but the voltages are wrong, except for the 5V lead: 9.6V and 24V. What would happen if I used 9.6V line with the CPC 6128 (or the 664)? Would that voltage be enough to have the diskettes read and written to?

[Bryce]

As the speed of a motor is usually voltage dependent, I doubt the drive would work at anything less than about 11.5V.
Also remember, that if you lower the voltage you are increasing the current and I have no idea whether all other parts in the circuit are dimensioned for higher currents.

Bryce.

[1024MAK]

I don't know about the drive in a CPC6128, but various other types of floppy drive (5¼" and older 3½) often require the +12V to be within +/- 10% of 12V. So no lower than 10.8V and no higher than +13.2V. This is at the drives power connector. Due to possible voltage losses in the cables and connectors, the output voltage from the PSU would therefore have to be closer to 12V. I would recommend no lower than 11.3V. But even at this voltage, there is no guarantee that your machine will be able to read existing disks.

Mark