Author Topic: CPC6128 voltage question / black screen  (Read 140 times)

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Offline P5ychoFox

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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?

Online Duke

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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

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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

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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

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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

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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

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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.