Author Topic: CPC6128 voltage question / black screen  (Read 389 times)

0 Members and 1 Guest are viewing this topic.

Offline P5ychoFox

  • CPC664
  • ***
  • Posts: 64
  • Liked: 22
CPC6128 voltage question / black screen
« on: 14:08, 12 June 18 »
I recently bought a 6128 (untested) from Ebay for £25. A good deal as I was only after the 3" drive which I've now got running again in another machine. This left me with the 6128 which I hoped to one day put a Gotek inside, but after some testing I've ran into a problem:

I have a 5v 2a tip positive PSU (producing 5.3v DC) hooked up to the 6128 and though the power light comes on, all I get via my newly purchased 6128 scart cable is a black screen.

The speaker crackles a bit but pressing delete repeatedly gives no beep. When left running the Z80, CRTC & IC201 get warm (especially IC201) but no RAM chips warm.

Despite putting out a steady 5.3v when measuring the barrel, the PSU only seems to be getting 3v to the board, even when measuring directly behind the 5v jack.

I was hoping for an easy fix but now I'm not so sure. I have swapped out the Z80 but nothing changed.

Could it be something simple I've not thought of? Perhaps my power supply isn't up to task?

Offline Duke

  • Supporter
  • 6128 Plus
  • *
  • Posts: 944
  • Country: dk
    • spinpoint.org
  • Liked: 901
Re: CPC6128 voltage question / black screen
« Reply #1 on: 14:46, 12 June 18 »
Could it be something simple I've not thought of? Perhaps my power supply isn't up to task?
Try shorting the power switch, to rule out resistance in it. (ie. replace the power switch cable with a jumper, temporarily).

Offline Bryce

  • The Hardware Guy.
  • Supporter
  • 6128 Plus
  • *
  • Posts: 10.293
  • Country: wf
  • It's not broken, it just hasn't been fixed yet.
    • index.php?action=treasury
  • Liked: 3529
Re: CPC6128 voltage question / black screen
« Reply #2 on: 14:53, 12 June 18 »
Try cleaning the power socket on the back of the CPC.
Bryce.

Offline P5ychoFox

  • CPC664
  • ***
  • Posts: 64
  • Liked: 22
Re: CPC6128 voltage question / black screen
« Reply #3 on: 15:10, 12 June 18 »
Thanks for the suggestions. I jumped the power connector on the board and still got 3.2v on the board, so I cleaned the 5v jack but am still only getting 3.2v reaching the board.

Offline Bryce

  • The Hardware Guy.
  • Supporter
  • 6128 Plus
  • *
  • Posts: 10.293
  • Country: wf
  • It's not broken, it just hasn't been fixed yet.
    • index.php?action=treasury
  • Liked: 3529
Re: CPC6128 voltage question / black screen
« Reply #4 on: 15:14, 12 June 18 »
Then, either the PSU is dodgy and can't really supply what it claims, or there's a serious short circuit on the PCB. I'm tending towards a dodgy PSU, because if the short is able to pull 5.3V down to 3.2V, then it's pulling over 3A and disappating more than 10W which would probably cause smoke.
Bryce.

Offline P5ychoFox

  • CPC664
  • ***
  • Posts: 64
  • Liked: 22
Re: CPC6128 voltage question / black screen
« Reply #5 on: 16:11, 12 June 18 »
Thanks Bryce, that's encouraging. I'll source another PSU and go from there.

Offline Bryce

  • The Hardware Guy.
  • Supporter
  • 6128 Plus
  • *
  • Posts: 10.293
  • Country: wf
  • It's not broken, it just hasn't been fixed yet.
    • index.php?action=treasury
  • Liked: 3529
Re: CPC6128 voltage question / black screen
« Reply #6 on: 16:30, 12 June 18 »
If you have the parts and equipment, you can test the actual capabilities of the PSU you have at the moment. Connect a 10W 2.7R resistor across the output of the PSU, this equates to approx 1.96A and then measure the voltage across the resistor. If it's anything below 5V then the PSU definitely can't supply what it claims.

Bryce.

Offline P5ychoFox

  • CPC664
  • ***
  • Posts: 64
  • Liked: 22
Re: CPC6128 voltage question / black screen
« Reply #7 on: 20:36, 24 June 18 »
I have now received my 5v 3a positive tip power supply and it is providing a steady 5.1v DC which is giving 4.7v to the board once the 6128 is powered on. The computer is now getting enough power to boot and is working perfectly.

Does this power supply sound adequate to power the 6128? Also would it be able to power any other CPC and Plus model CPC?

Offline Bryce

  • The Hardware Guy.
  • Supporter
  • 6128 Plus
  • *
  • Posts: 10.293
  • Country: wf
  • It's not broken, it just hasn't been fixed yet.
    • index.php?action=treasury
  • Liked: 3529
Re: CPC6128 voltage question / black screen
« Reply #8 on: 09:38, 25 June 18 »
If it's really able to supply the full 3A then yes, it would be fine for any CPC (assuming it had the correct plug fitted  - The Plus has a different plug than the Classic CPC). However, the 4.7V would suggest it's either not able to supply that, or you need to clean the contacts on the power socket and the power switch.


Bryce.