Philips NMS-8255 power (?) supply question and modding

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Por RetroTechie

Paragon (1563)

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23-08-2010, 01:51

LT1084CP-5 should be an excellent replacement: precision, low dropout, fixed +5V output, 5A max, and somewhat bigger package than TO220 (read: better contact with heatsink). Pin-compatible & if you're lucky you don't even have to bend the pins to make them fit. Cheap @ Eur 3.85 too, doesn't get any better than that. With backside connected to center pin (=Vout, +5V) no worries about isolating the heatsink. Remember to also get some cooling paste for between regulator & heatsink - or apply same stuff you'd use between CPU & heatsink on PC.

Btw: MRC doesn't seem to accept https URL's :-? maybe that's why your links went dead?

Por JohnHassink

Ambassador (5655)

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23-08-2010, 02:33

Links are good now.

Por Wild_Penguin

Hero (644)

Imagen del Wild_Penguin

23-08-2010, 20:52

Hi!

Thanks for the help RetroTechie! I think I'll get the regulator now (when I'll have time).

@Eugeny (and perhaps others interested):Here are some pics. There's a bit noise, since my pocket camera is quite bad, and there was no good light (and flash tends to make terrible glares, unless you take pictures from an angle). Sorry about that :).

Por Wild_Penguin

Hero (644)

Imagen del Wild_Penguin

01-09-2010, 18:31

Hi!

I changed the regulator, and so far (after a few hours) the +5 voltage seems stable @4.91V (also the other voltages are OK). I still need to do a few hours Space Manbow testing session to make sure the stability issues are gone =)

In case someone else needs a replacement, the regulator LT1084CP-5 in TO-247 packaging was an exact match, there was no need to bend pins. (I believe the only difference between TO-247 and TO-220 is that the plastic packaging is a bit larger and the metal flap does not protrude at all in TO-247). De-soldering the old regulator was a bit tricky since there were some collets (not sure what you call them in English) in the regulator pins. They were sitting kind of tightly on the PCB even after all solder was removed. DO cut the pins of the regulator if you need to change it!

But thanks for your help, RetroTechie, once more!

I still have problems with the PSG-SCC mixing. (should I start a new topic, by the way, for this, since there's no power supply nor modding involved?). To summarize, two of the fixes in here

were in place when I got my 8255, I only changed R428 and R429 changed to elcos, but didn't notice any effect on the sound. In the PDF, there's the mixing circuitry (on page 2). Assuming something is broken because of the circuit being used at 5.9V for... well, no one knows for how long, perhaps it can be guessed with the help of the diagram that what might be broken?

Also, FM-PAC behaves exactly same as PSG... i.e. if there's only Panasonic PAC installed, then the PSG and FM-PAC mixing is OK, but if I put in SCC and FM-PAC at the same time, the FM-PAC is very quiet (as is the PSG and keyclicks). I find this a bit weird, since I thought that the mixing circuitry treats all external (those in cartridges) sound sources the same, but apparently this is not the case...

Can anyone help with this? What components in the audio circuitry are most likely to fail because of the over-voltage?

Cheers!

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