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(Split from: "CoCo2 26-3134A with 230V~ ???" Thread: viewtopic.php?f=8&t=4947&p=12051#p12051 )

tormod wrote:By the way, at the vintage festival, through the cool guys of steckschwein.de fame, I acquired a switched power supply which seems suitable for feeding a Dragon directly, also the form factor would be perfect for replacing the PSU board. One day I will try it out. http://www.pollin.de/shop/dt/Nzg5OTQ2OT ... 7188B.html

Wow, that's cheap...

It's a "normal" PC power supply in a non-standard form factor, isn't it?

But how to replace the PSU board? There are more things on it, than only the voltage stuff.
Or do you mean to skip the external transformator and put this switching power supply directly into the dragon case?

jedie wrote:Wow, that's cheap...

Ditto, that's crazy cheap!

I remember doing something similar to a D32. The PAL video and sound are available on the motherboard pins, so it is simply a matter of wiring up a new monitor socket. The only things you lose are the RF output and power switch.

sorchard wrote:The PAL video and sound are available on the motherboard pins, so it is simply a matter of wiring up a new monitor socket. The only things you lose are the RF output and power switch.

Oh, really? Than it's interesting, i never used the RF output

It would be a follow-up of this: viewtopic.php?f=5&t=4455&p=9673 (that's one year now and I haven't gotten back to it ). I am envisaging a connector board with matching connectors for both the switching PSU and the Dragon motherboard, and which also includes the new S-video circuitry. I am just a bit worried what kind of RF noise a 2.50€ supply can produce and how it may affect the video signal.

tormod wrote:I am just a bit worried what kind of RF noise a 2.50€ supply can produce and how it may affect the video signal.

Can be, but must not... It's the question: Has it been developed as a cheap product? Or is it cheap, because nobody need it?

Have found some pages about this VOLTEK SPEC7188B PSU:
http://hackaday.io/project/1745-digital ... t-printing
http://www.instructables.com/id/A-Raspb ... rs-server/
http://www.lh-electric.net/projects/6Vsmps.html

tormod wrote:It would be a follow-up of this: viewtopic.php?f=5&t=4455&p=9673 (that's one year now and I haven't gotten back to it ). I am envisaging a connector board with matching connectors for both the switching PSU and the Dragon motherboard, and which also includes the new S-video circuitry. I am just a bit worried what kind of RF noise a 2.50€ supply can produce and how it may affect the video signal.

Interesting thread. I only joined the forum this year and it looks like I've missed out on a lot of stuff. I'm pleasantly surprised by the improvement obtained with the S-Video conversion.

@tormod: If you're going to the trouble of making a PCB, then you may as well include some L-C filters which will help reduce the switching noise passed on to the rest of the system, though the biggest noise producer is the Dragon itself. It really is terrible (like many home computers of the day) and would probably fail modern EMC requirements miserably. When I was a kid I used to tune my AM radio so I could hear the Dragon processing!

That reminds me: On some Dragons I've seen a series of fixed vertical dark lines on the text display with a spacing similar to the width of a character. This is an example of noise synchronised with a display. It can be cured by fitting the missing 100n bypass capacitors to the memory array. Presumably they were left out to reduce cost. I imagine this can also happen if one of the larger caps on the 5V line fails.

IMHO, there are two aspects here:
1. the EMC from the open PSU board (The VOLTEK SPEC7188B PSU is closed, cased)
2. the rest RF noise of the output voltage

That price is absolutely a killing one for a PSU with negative voltages!. Sadly it doesn't provide the -12V needed by a D64, only is a suitable replacement for a D32...

I think the -12v are for the serial port. If that's the case, and you are not using that port, the psu would be a good deal.

robcfg wrote:I think the -12v are for the serial port. If that's the case, and you are not using that port, the psu would be a good deal.

While it's true that -12V is used for serial, there is one more hoop to jump through: The -12V gets dropped down to -5V for the video circuit by a reg on the main board. To supply a D64 with -5V it should be OK to short the reg input to output to allow the -5V through. (But clearly label what was done in case someone else comes along and tries to put the usual -12V in!)