I've got a few Dragon 32s, one with the original internal PSU PCB and two with the internal SMPS supplied by dragonplus (thanks John). The first is powered by the original transformer, the other two by a 12V wall wart. All three use an external RGBtoHDMI (analog) to produce a colour display.
The facts are these:
a) The one with the original components produces a nice clean and crisp display with no artefacts.
b) The two with the internal SMPSs and external wall wart produce a correct display but there is additional dynamic and rapid pixel flickering/speckling all over the screen.
c) If I replace the wall wart with a suitably fused 12V car battery then the two machines with internal SMPSs produce a clean & crisp display with no pixel flickering/speckling.
That rather points to the display circuitry being sensitive to switching noise from the wall wart. I've tried a beefy filter, combining low pass and common mode circuitry after the wart, with no success. I've also tried several wart makes to no avail (most seem to produce around 400mV of switching spikes).
The question is: can anyone recommend a make/model of 12V 2-3A external SMPS that's as quiet as a mouse, or must I build a linear one?
Alan
Wall wart and pixel speckling
Re: Wall wart and pixel speckling
The culprit is probably common mode noise which can be surprisingly hard to get rid of. This will be bouncing relative to the mains (and therefore ground) at the PSU end and turned into a signal at the monitor end.
Ideally you would attempt to block with a decent common mode choke close to the PSU and return the spikes to ground (before the choke) with a couple of small caps. As wall warts are usually double insulated this is easier said than done. Returning the spikes to the supply internally would be an option but you've really got to know what you're doing.
I've had some success with board level DC-DC converters by following manufacturer layout recommendations and filtering the output with a common mode choke and C-L-C pi filter.
Can't recommend a good wall wart I'm afraid, as even the ones we buy at work from the RS PRO range all seem to be as bad as each other. You can actually feel the spikes if you connect either of the outputs to a metal plate. Very disconcerting the first time you experience this!
Ideally you would attempt to block with a decent common mode choke close to the PSU and return the spikes to ground (before the choke) with a couple of small caps. As wall warts are usually double insulated this is easier said than done. Returning the spikes to the supply internally would be an option but you've really got to know what you're doing.
I've had some success with board level DC-DC converters by following manufacturer layout recommendations and filtering the output with a common mode choke and C-L-C pi filter.
Can't recommend a good wall wart I'm afraid, as even the ones we buy at work from the RS PRO range all seem to be as bad as each other. You can actually feel the spikes if you connect either of the outputs to a metal plate. Very disconcerting the first time you experience this!
Stew
Re: Wall wart and pixel speckling
Just a random thought: I've not measured so can't say for sure, but a laptop PSU must be built to a far higher standard than a typical wall wart. At least I've never felt like I'm being electrocuted when using a laptop. If you can find an old 12V laptop supply then that might be worth a try.
Stew
Re: Wall wart and pixel speckling
Thanks for that Steve. Yes, I tread warily with mains PSUs, restricting myself to like-for-like repairs. Very useful info wrt the board DC/DC converters, thanks. Incidentally, the EMI filter I'd used with only partial reduction of the speckles was https://www.ebay.co.uk/itm/155933304852 .
I had tried a DELL 12V laptop PSU but that was just as bad. So, rather than spend a fortune buying a selection of warts I spent a fortune buying a beefy linear mains 12V DC jobbie (just over £100). At least there was a guarantee of success.
Alan
I had tried a DELL 12V laptop PSU but that was just as bad. So, rather than spend a fortune buying a selection of warts I spent a fortune buying a beefy linear mains 12V DC jobbie (just over £100). At least there was a guarantee of success.
Alan
Re: Wall wart and pixel speckling
Apple trackpads. Nuff said.sorchard wrote: ↑Sat Jun 01, 2024 2:02 pm Can't recommend a good wall wart I'm afraid, as even the ones we buy at work from the RS PRO range all seem to be as bad as each other. You can actually feel the spikes if you connect either of the outputs to a metal plate. Very disconcerting the first time you experience this!