Author Topic: [PLEASE READ] GX4000 with original power brick and C4CPC  (Read 15369 times)

0 Members and 1 Guest are viewing this topic.

Offline gerald

  • 6128 Plus
  • ******
  • Posts: 1.150
  • Liked: 821
I've just serviced a GX4000 that failed after using a C4CPC.
To make it short, the original power supply likely failed, ultimately frying the RAM.

The C4CPC consume up to 80mA more than a regular cartridge, a GX4000 is consuming about 400mA + 50mA if it has an RF modulator. This slightly exceed the rated current supply of the Amstrad provided power brick.

Therefore I urge you to stop using your Amstrad power supply with the C4CPC.

Instead :
  - use any regulated DC 9V to 12V 750mA (positive on center pin, GND on outer pin), powering the 11V entry. The 11V is used to power the 5V regulator and to drive the scart switching pin.

  - use the 5V supply input and the DIN rgb output (as if connected to a plus monitor). 5V supply may not work well with GX4000 having a RF modulator when using the SCART plug.

As a general rule, when using the 11V input, using a power supply with a voltage as low a 7.5V will work with the possible exception of the scart switching signal. Using it at lower voltage will minimise the 5V regulator(s) heating.

Failure details :
To fix it, I first replaced the main voltage regulator as it was issuing 7.5V instead of 5V. However, this happened also with the new one.
I then found that the 7805 bypass diode (to protect it when powered from a 5V supply, see attached schematics) failed in a way that feed the 11V on the 5V. So I removed that diode to discover that the RAM chip failed as well.
Once the RAM have been changed, everything worked fine. I've also replaced the diode with a new one.

Now, what happened ?
I suspect the power brick to have failed, or at least had a transient that made the diode fail, which in turn fried the RAM, which roughly protected the remaining part of the GX4000. The GX4000 jumped to 1A with the dead RAM.


Offline gerald

  • 6128 Plus
  • ******
  • Posts: 1.150
  • Liked: 821
The original Amstrad power supply is a 11V 500mA stabilised power supply, using a 220V AC input.
From oscilloscope trace under load, it seems to use a full diode bridge and a 3300µF filtering cap.

Powered with a 230V main, the output voltage is :
  • unloaded : 15.4V DC
  • under load (415mA) : 12.1V DC + 1V AC ripple.
With a 240V main, DC voltage would go at 12.6V, which mean about 3.8W to dissipate for the 5V internal regulator instead of 3W at specified 11V

With a 9V DC regulated power supply, the dissipated power will be reduced to 2W.

Offline dxs

  • CPC664
  • ***
  • Posts: 106
  • Country: fr
  • Liked: 100
Hi, So does that mean that if we have a GX4000 without the RF output we can continue to use the original Amstrad PSU?

Offline CraigsBar

  • Supporter
  • 6128 Plus
  • *
  • Posts: 2.962
  • Country: ie
  • The party ain't over yet
    • index.php?action=treasury
  • Liked: 1000
The original Amstrad power supply is a 11V 500mA stabilised power supply, using a 220V AC input.
From oscilloscope trace under load, it seems to use a full diode bridge and a 3300µF filtering cap.

Powered with a 230V main, the output voltage is :
  • unloaded : 15.4V DC
  • under load (415mA) : 12.1V DC + 1V AC ripple.
With a 240V main, DC voltage would go at 12.6V, which mean about 3.8W to dissipate for the 5V internal regulator instead of 3W at specified 11V

With a 9V DC regulated power supply, the dissipated power will be reduced to 2W.
Mine is now safely on a 9v 1A supply and the original one is packed up back in the box.


Seems to be fine with 9v not 11, my 6v supply does not power it (well the LEDS on the C4CPC and console light, but nothing else) and I don't have a 7v supply.


Craig


Offline Ast

  • 6128 Plus
  • ******
  • Posts: 781
  • Country: fr
    • Amstrad cpc Website of Ast/iMPACT
  • Liked: 451
Hi, So does that mean that if we have a GX4000 without the RF output we can continue to use the original Amstrad PSU?
I've asked the same question to Gerald a few days ago. His answer was it'll be better to change the power supply to not have any problems with the old one.
After that, do what you have to do!
_____________________

Ast/iMP4CT. "By the power of Grayskull, i've the power"

Amstrad Plus :: Découverte de la gamme Old et Plus des Cpc d'Amstrad.
All friends are welcomed !

Offline gerald

  • 6128 Plus
  • ******
  • Posts: 1.150
  • Liked: 821
Hi, So does that mean that if we have a GX4000 without the RF output we can continue to use the original Amstrad PSU?
I would not recommend this, you only get a 50mA margin.

Offline gerald

  • 6128 Plus
  • ******
  • Posts: 1.150
  • Liked: 821
my 6v supply does not power it (well the LEDS on the C4CPC and console light, but nothing else) and I don't have a 7v supply.
6V is too low for the 5V regulator to work as it need 2V margin.
And while the GX4000 may not work at 4V, the C4CPC will : it's a 3.3V device and it regulator only need less than .5V margin to work.

Offline Crazy_Borg

  • CPC464
  • **
  • Posts: 6
  • Country: de
  • Liked: 0
Sadly, we lost a large part of the discussion here because of the database crash.

All of my GX4000 need at least 9V at the 11V input, 7,5V is too low and results in visual corruption / game freezes.

It is possible to power the GX4000 through the 5V Monitor input if you only have a 5V power supply.
To power the RGB chip CXA1145 for the SCART output to work you have to remove the UPC78L05 (IC17) and bridge point LK7 on the pcb, which  it is marked on the pcb near the IC17.
That way you will turn your GX4000 into a french model.
I already modded a regular GX4000 this way some time ago and it worked fine, though I only testet the SCART output.

Gerald noted that the RF modulator may not work because of noise after the above described conversion, but who is using it anyway?
Where you worried just about image stability or about a complete failure of the RF modulator because of it?


words are cheap...words come and go

Offline gerald

  • 6128 Plus
  • ******
  • Posts: 1.150
  • Liked: 821
Gerald noted that the RF modulator may not work because of noise after the above described conversion, but who is using it anyway?
Where you worried just about image stability or about a complete failure of the RF modulator because of it?
Mostly noise. But as you said, who is still using it nowadays ?
Yes, 7V is the low limit. I think the sweet spot is 9V.
Also, the original 11V power supply will run at higher voltage with current 230/240V instead of the old good 220V.

Offline dthrone

  • CPC664
  • ***
  • Posts: 80
  • Country: gb
  • Liked: 39
Is it possible to recommend a suitable replacement brick, maybe with an Amazon (or equivalent) link or something?