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

Страница 1/3
| 2 | 3

By Wild_Penguin

Hero (644)

Аватар пользователя Wild_Penguin

16-07-2010, 11:28

Hi all MSX'ers!

I recently got a Philips NMS-8255. My first MSX2 machine! =)

It works well, but, I've noticed that the picture brightness level, and PSG sound level varies annoyingly at times (strangely SCC sound level stays same all the time, though I only tested a few times). It's a bit difficult to describe, but the general brightness of the picture gradually (and slowly) gets higher, as does the sound level, then (maybe after 1-4 minutes) there is a sudden drop in the level (and sometimes a mild *pop* from the speakers). While I was playing boulder dash, when this drop occured, the MSX hanged =(. Luckily after a while it was alive again.

At first, I thought that it is a faulty SCART lead, but the hang made me think otherwise... also I checked the lead and it does not seem to be the case... anyone had a similar problem?

I also opened the MSX to see what it looks like inside =). Also to see if there are some clearly broken elcos or something, causing the above problem. I noticed there is a jumper (soldered wire) for 220V and 230V... I believe it was at 220V setting, could this be the cause of the problem above? I didn't look so carefully because I remembered we have 220V here in Finland, seems I was wrong ;-)

Also, I'm thinking about modding the MSX cradually... at first I'm thinking about adding RAM, then VDP9958 and later MSX2+ BIOSes... But I can't find 41464 (or 4464) type memory chips anywhere (these were used in the expansion instructions I could find, and are also the type already on the motherboard). Does anyone know where to order some? I might be able to find some 1MB 30-pin simms, though, perhaps I should use those ;-)

Also, if someone can point me to a place were to get the VDP9958 and MSX2+ BIOSes at a reasonable price, that would be appreciated :-)

Cheers!
- Ville

Для того, чтобы оставить комментарий, необходимо регистрация или !login

By Bastiaan

Champion (333)

Аватар пользователя Bastiaan

16-07-2010, 13:04

check this website:

http://www.bas-ditta.info/verkoopenlinks/onderdelen%20negels.htm

he may have quite some usefull parts for you... :RNFF:

By Repair-Bas

Paragon (1165)

Аватар пользователя Repair-Bas

16-07-2010, 15:46

That is the correct website.

There are all parts you need in stock.

By RetroTechie

Paragon (1563)

Аватар пользователя RetroTechie

16-07-2010, 19:25

Welcome, Wild_Penguin!

About V9958's: eBay is your friend... I bought a couple here. Seller is in China (payment through PayPal), shipping is free but takes a while. I saw some more eBay sellers that ask around the same price, with 5+ or 10+ quantity discounts. I suggest you contact seller first & ask if he/she can include some (turned pin?) sockets for these chips. You can buy those elsewhere, but 64-pin DIP sockets come in different sizes...

Replacing the V9938 with it is pretty straightforward, as long as your soldering skills are good. ._, Full 2+ build is a lot more work.

As for RAM upgrades: I'd go for a simple 512Kx8 SRAM if that's enough. Stick on top of a (EP)ROM, some mods to mapper circuitry, wire it up, presto. Way easier than stacking a pile of 4464's & wiring that up. Or deal with refresh on 256Kx4 DRAMs. For 1 MB+, you can use 1Mx4 or 4Mx4 chips found on old PC simms. I have a tried & tested 1 MB. circuit description for 8250/55/80 machines that uses 2 pcs 1Mx4 chips and very few wires, but you'd have to make your own circuit board. Also schematic for 2 MB. circuit, but no matching PCB design (maybe I used 1 MB. PCB back then with a few extra wires here & there, don't remember)... For internal 4 MB., I might use 4Mx4 DRAMs today, but can't say I have a known-good circuit for that... *_*

By Eugeny_Brychkov

Paragon (1232)

Аватар пользователя Eugeny_Brychkov

17-07-2010, 11:57

It works well, but, I've noticed that the picture brightness level, and PSG sound level varies annoyingly at times (strangely SCC sound level stays same all the time, though I only tested a few times). It's a bit difficult to describe, but the general brightness of the picture gradually (and slowly) gets higher, as does the sound level, then (maybe after 1-4 minutes) there is a sudden drop in the level (and sometimes a mild *pop* from the speakers). While I was playing boulder dash, when this drop occured, the MSX hanged =(. Luckily after a while it was alive again.
Sounds like +12v problem - maybe overheating. Try running machine open, and measure all the motherboard supply voltages. You may also see where components are overheating (but not necessarily "fried") PCB became "dark".

By Wild_Penguin

Hero (644)

Аватар пользователя Wild_Penguin

17-07-2010, 16:10

Thanks for the replies! They are helpful!

Sounds like +12v problem - maybe overheating. Try running machine open, and measure all the motherboard supply voltages. You may also see where components are overheating (but not necessarily "fried" PCB became "dark".

The power issue seems to be solved =). There was a lot of dust, the rectifier diodes (D1050-1053) had kind of a dust-fur coat that probably acted like a thermal insulator =). The PCB side was also darkened around tham. Also, there was a lot of dust everywhere. I removed the dust and changed the rectifier diodes (as recommended alsewhere). I aso noticed that the pcb had darkened around them =).

I took a picture comparing the originals to the ones I got from a local electronic shop, they are huge compared to the old ones=D
img641.imageshack.us/g/cimg3786f.jpg/

As for RAM upgrades: I'd go for a simple 512Kx8 SRAM if that's enough. Stick on top of a (EP)ROM, some mods to mapper circuitry, wire it up, presto. Way easier than stacking a pile of 4464's & wiring that up. Or deal with refresh on 256Kx4 DRAMs. For 1 MB+, you can use 1Mx4 or 4Mx4 chips found on old PC simms. I have a tried & tested 1 MB. circuit description for 8250/55/80 machines that uses 2 pcs 1Mx4 chips and very few wires, but you'd have to make your own circuit board. Also schematic for 2 MB. circuit, but no matching PCB design (maybe I used 1 MB. PCB back then with a few extra wires here & there, don't remember)... For internal 4 MB., I might use 4Mx4 DRAMs today, but can't say I have a known-good circuit for that...

Thanks for this information! Though, can you elaborate on the 512Kx8 SRAM upgrade? Are there instructions anywhere? I could only find 30-pin SIMM and 4464 upgrade instructions (on the MSX info pages). Neither requires a PCB, although a lot of wires at least for the SIMM upgrades :P

One question about the SIMM upgrades: IIRC, there were two types - older and newer type - of 30-pin SIMM's... the newer needed a refresh signal (or somesuch?) from a motherboard, if you used these types on an older (386/286) computer, you'd run into issues (might have been a SRAM -> DRAM change?). Does anyone know, if the SIMM upgrade from the MSX info pages works with also the newer types of 30-pin SIMM's? I'd like to know beforehand, since adding the SIMM's invovles removing the 4464 already on the board, I would not like to find out it doesn't work only after an appempted upgrade :P.

The 30-pin SIMM upgrade seems the most viable for me, since I have some... somewhere, I might have lost them oO

Also, thanks for reminding about ebay, RetroTechie.... although, the cheapest V9958's seem to be 20€, I might go for Bas von Ditta's MSX2+ upgrade set at 45€, as I will eventually wan't to upgrade to that... and the price seems very reasonable for me, as it includes the ROM's and some other necessary components =)

By Manuel

Ascended (19273)

Аватар пользователя Manuel

17-07-2010, 16:37

"Bas von Ditta"! LOL! Smile

Ditta is Bas' wife Smile

By Wild_Penguin

Hero (644)

Аватар пользователя Wild_Penguin

17-07-2010, 17:01

Eh,
LOL! Smile Big smile

As the site is in Dutch, I had no idea what I was talking about =). In addition seems I misread a few letters Wink

By RetroTechie

Paragon (1563)

Аватар пользователя RetroTechie

17-07-2010, 18:47

can you elaborate on the 512Kx8 SRAM upgrade?
The Philips 8250/55/80 machines are easy because you have lots of room inside & direct access to the mapper circuit, it's not integrated in some custom IC. There's a 74LS670 that provides 3 expanded address lines (=128K mapper support), and is easy to mod to provide a 4th bit (for 256K). Add a 2nd 74LS or HCT670, connect a few wires, and voila: 5~8 mapper bits (for 512K/1/2/4M support). Perhaps you can deduce from existing instructions how to add these expanded address bits (it may involve 1 more IC to allow read-back of those bits). After that you just need to add the memory itself, which is more difficult in the case of DRAMs.

SRAMs have more pins to connect, but most match 1:1 with (EP)ROMs inside the machine. Remaining signals are easy to control, DRAMs are a lot more difficult because of timing/refresh/multiplexed address lines. That's why I wouldn't touch DRAMs for <1MB expansions these days.

It's the usual - simple if you understand memory mappers from a hardware point of view, but if you don't you need exact instructions (which I don't have for using 512Kx8 SRAMs btw). Most memory upgrade descriptions were written back when MSX was more popular, when 512Kx8 SRAMs either didn't exist or were ridiculously expensive. oO

By Wild_Penguin

Hero (644)

Аватар пользователя Wild_Penguin

08-08-2010, 11:59

Hi!

After being a bit busy I have had some time to play around with the MSX too =).

Unluckily, it still seems to have some instability problems, that I thought went away since I changed the rectifier diodes... they are still there, but a lot more infrequent. Sometimes, it will run without problems for hours, sometimes after 20 minutes of Space Manbow, it'll hang (i.e. the picture just freezes, as does the sound). Also the picture brightness level still varies, though it's slighter now and not so easy to spot.

I measured the voltages and this is what I get:
-12.01V
+5.78 - 5.94V (!)
+11.86V

I think the lines should be -12V, +12V and +5V (and that's what I was told in IRC...). I think the +5V line is not within tolerance? I have no idea where the fault might be and were to look next. Any suggestions / tips?

Cheers!

p.s. Thanks, RetroTechie for the tips about the memory upgrade... unfortunately I have no experience about designing and/or building memory mappers. I'm not any kind of electronics expert, this is just a hobby for me =P So adding a memory expansion via SRAM without not readily available instructions might be a bit over my heels. Also, I believe the additional memory mapper might not be that useful for me for the time being, I'll first upgrade this one to MSX2+ - after I got this instability problem solved, of course!

By RetroTechie

Paragon (1563)

Аватар пользователя RetroTechie

08-08-2010, 14:47

+5.78 - 5.94V (!)
That's definitely out of range, +5V is usually specified as 5.0 +/- 0.25V (4.75 - 5.25), most regulators stay well within that (4.9-5.1V). I don't think it's healthy to run your MSX at nearly 6V, so better unplug that power from mainboard until fixed...

8255 power supply is very simple, this would be either a cooked +5V regulator, or maybe a capacitor at the output that has gone bad.

The latter is easy to fix, replace capacitor between +5V regulator and power suppy -> mainboard connector. Schematic says C1051 (1000 uF, >5V of course & watch polarity). I suggest you try this first, cheap & easy, and doesn't hurt even if not the problem.

If that doesn't help, regulator may be busted (IC1050, S-3052V says schematic, I think I've seen STR9005 in that location too). You'd need a regulator with +5V output that can deal with around ~5A power draw (not sure it draws that much, but 3A is too weak). If it comes to replacing this, you may consider using a switching regulator instead (higher efficiency -> less waste heat in power supply). But that would depend on your electronics skills since not a 1:1 replacement.

I'm sure Repair-Bas has some spares for these power supplies... in case of DIY, be aware that NMS8250/55 power supplies are not safe to touch! Some parts carry mains voltage so be careful when repairing these!

Страница 1/3
| 2 | 3