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

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Van Wild_Penguin

Hero (644)

afbeelding van Wild_Penguin

10-08-2010, 20:25

Hi!

Thanks for the help again. I also measured the voltages without any load (mb power connector disconnected) - even the 5V line was OK then. I changed the elco anyways (1000µF, 10V).

Still, voltages are OK (all <+-0.1V from ref), but, after connecting the MB connector, the 5V voltage jumps to the mentioned near-6V range (while -12V and 12V are OK)... does this mean that I have a broken regulator? Or, could there be something wrong on the MB?

Someone suggested on IRC to test it using a PC power supply - Is it a good idea? Is the -12V line needed?

Cheers!

Van RetroTechie

Paragon (1563)

afbeelding van RetroTechie

10-08-2010, 22:04

Still, voltages are OK (all <+-0.1V from ref), but, after connecting the MB connector, the 5V voltage jumps to the mentioned near-6V range (while -12V and 12V are OK)
Interesting, that could mean just a few things:

  • Some problem in the feedback / current / voltage sense circuitry of regulator. Since not external parts for the regulator used here, does not apply (read: if this problem = same as 'broken regulator').
  • Current is 'leaking back' to +5V from a more positive voltage enough to raise +5V to about ~6V (only candidate here: +12). Seems unlikely, to exclude this: can you measure +5V output with just the mainboard powered (analog board & power to floppy drives disconnected) ?

One other thing to check: solder connections on that +5V regulator (loose ground pin etc).

I wrote that 3A output current is too weak for +5V regulator in these machines, but I'm not 100% sure about that (anyone know what normal +5V load is for these machines?). It would be nice if you can measure actual load with everything connected. If regulator is the problem, Repair-Bas may have a spare. And local electronic shop should have something that might be used as replacement. For example LM323K or LT1083/1084/1085 series.

Van Wild_Penguin

Hero (644)

afbeelding van Wild_Penguin

10-08-2010, 22:28


Interesting, that could mean just a few things:

  • Some problem in the feedback / current / voltage sense circuitry of regulator. Since not external parts for the regulator used here, does not apply (read: if this problem = same as 'broken regulator').
  • Current is 'leaking back' to +5V from a more positive voltage enough to raise +5V to about ~6V (only candidate here: +12). Seems unlikely, to exclude this: can you measure +5V output with just the mainboard powered (analog board & power to floppy drives disconnected) ?

    I already measured the voltage without the AV board connected (suspecting something similar) and got slightly over 6V. But because you suggested the above, I also removed the disk drives and got 7.4V or thereabouts(!!!) oO and immediately switched the power off. I reconnected the AV-board and the disk drives, and found I'm back at the 5.78V (and the 8255 does floppy seeks, so probably nothing was broken; I have no monitor at the desk I'm working with this currently).

One other thing to check: solder connections on that +5V regulator (loose ground pin etc).

I'll do that tomorrow since it's quite late already here =)


I wrote that 3A output current is too weak for +5V regulator in these machines, but I'm not 100% sure about that (anyone know what normal +5V load is for these machines?). It would be nice if you can measure actual load with everything connected. If regulator is the problem, Repair-Bas may have a spare. And local electronic shop should have something that might be used as replacement. For example LM323K or LT1083/1084/1085 series.

I'll see if I can manage the measurements (I need to get my multimeter somehow in between the 5V line to do that, don't I? Not sure I have the needed connectors right know, but I might came up with something from the closet...)

Thanks for the info about the regulators, though. I could easily find 78LXX series, but those won't do because of wrong pinout... neither could I found any you mentioned in the catalogues, so I might not wan't to start looking for the regulator if there's something else I can do to confirm what is wrong. Of course if it IS the problem and none is available locally, I'll ask Repair-Bas Smile

Van RetroTechie

Paragon (1563)

afbeelding van RetroTechie

10-08-2010, 23:08

Let's hope nothing was damaged during that voltage hike... oO Anyway: has all the signs of a dodgy +5V regulator (as I said: 8250/55 power supply is really simple). I suggest you don't use this power supply any further until fixed, indeed you can use PC power supply temporarily (+5 and +12V is enough).

Don't worry about pinouts, even adjustable types can be used, but you need a regulator that can supply enough current... 3A (or more) linear regulators aren't used much (usually it's 1~1.5A max or a switching regulator), so if you're replacing with a linear regulator, it shouldn't be hard to locate the few possible types that you can buy through local shops. But of course I wouldn't know what shops that are or what they have available (many different types/parts for this job). If you have to buy elsewhere (international), shipping will make things expensive regardless of what/where you order... Shocked!

I'd grab a switching regulator, but that's much more complicated than a bit of re-wiring, so I suppose that's 'out of your league' ? Only other option is find a complete power supply that fits & has suitable output voltages (like ripped from broken DVD-player or similar equipment). Elegant fix but hard-to-find...

Van Wild_Penguin

Hero (644)

afbeelding van Wild_Penguin

15-08-2010, 16:22

Let's hope nothing was damaged during that voltage hike... oO Anyway: has all the signs of a dodgy +5V regulator (as I said: 8250/55 power supply is really simple). I suggest you don't use this power supply any further until fixed, indeed you can use PC power supply temporarily (+5 and +12V is enough).

I tried PC power supply, and it didn't crash (although it run even on the 5.78V sometimes without any problems for at least few hours). I still have volume balance issues when SCC cart was plugged in, so I believe something in the audio circuitry might have gone bad (the PSG can not be heard at all while SCC is plugged, but oddly PAC does not change PSG level; also SCC "crackles" at some rare points, for example at the point in SD-Snatcher introduction, where Gilian is introduced). I have all the fixes mentioned here (C119 was already the right way, i.e. different than print and C120, and R174 is 910Ω. so I only changed R428 and R429 to elcos, but didn't notice any effect in the audio). Need to dig deeper into this once I get this regulator changed...


Don't worry about pinouts, even adjustable types can be used, but you need a regulator that can supply enough current...

I'm a bit lost me here - bear with me I know little about regulators :P What do you mean, why don't I need to worry about pinouts? Rewiring should not be too difficult looking at the PCB, but I might get problems because of conducting heat sink, since it is always connected to the middle pin?

I could get the LT1085 series from here. But it is adjustable - I have no idea how to use those :). How do I adjust the voltage to 5V? I need to add something else to the PCB?

Also I'm confused about the 3052 datasheet, it says "input" is middle and pin1 "output", but looking at the service manual / PCB, the middle pin is connected to the +5V line, and the pin1 to the rectifier diodes... there must be something I don't understand about regulators here? I thought the pin "output" should be connected to the 5V line and "input" to the rectifiers? Maybe there's a fault in the datasheet?

Also, according to the datasheet, 3052 (the original) is a 2A regulator. So a 2A regulator should be enough? Also I measured the current while I connected this to a PC ATX power supply, and got ~1.50A in basic operation, if a disk drive is used, then around 1.56A, at most 1.60A but not over 1.6A. I'm not sure an MSX can use both disk drives at the same time hardware-wise, but even in theory, if both disk drives were read/written to at the same time, the current should never exceed 1.7A.

3A (or more) linear regulators aren't used much (usually it's 1~1.5A max or a switching regulator), so if you're replacing with a linear regulator, it shouldn't be hard to locate the few possible types that you can buy through local shops. But of course I wouldn't know what shops that are or what they have available (many different types/parts for this job). If you have to buy elsewhere (international), shipping will make things expensive regardless of what/where you order... :o

Usually I by my electronics stuff from SP-elektroniikka (page in Finnish only)... they've been most helpful in the past and the salespersons are experienced. Here are their regulators, though a few are also on this page. I tried to look but couldn't find a suitable one from their catalogues :(. If I'd go there with the S-3052V datasheet they'd most probably give me some solution that works anyways - unless they really don't have it. But I haven't had time to visit them (again) yet. From the net I've once ordered from partco. Also this chain opened a store downstairs from my place just recently. So I think I should have no problems getting any kind of parts =)

I'd grab a switching regulator, but that's much more complicated than a bit of re-wiring, so I suppose that's 'out of your league' ? Only other option is find a complete power supply that fits & has suitable output voltages (like ripped from broken DVD-player or similar equipment). Elegant fix but hard-to-find...

Yes, that is a bit too difficult =). Actually, if I'd have the instructions, it would not be too much, but I think it is much more simple to just change the regulator.

Thanks for your help and suggestions!

- Ville

p.s. I also measured the 12V line. 0.22A when no disk drive in use, and if disk drive in use, then 0.3A.

Van RetroTechie

Paragon (1563)

afbeelding van RetroTechie

15-08-2010, 19:38

Also I'm confused about the 3052 datasheet, it says "input" is middle and pin1 "output", but looking at the service manual / PCB, the middle pin is connected to the +5V line, and the pin1 to the rectifier diodes... there must be something I don't understand about regulators here? I thought the pin "output" should be connected to the 5V line and "input" to the rectifiers? Maybe there's a fault in the datasheet?
No you're right - probably means that the 3052 inside NMS8255 is not exact same type as in above datasheet. The STR9005 you also find here, is a low-drop regulator rated at 4A (which looks more like it). Rectifier diodes are 3A or 5A types & they regularly fail. So a 2A regulator definitely won't do.

Also I measured the current while I connected this to a PC ATX power supply, and got ~1.50A in basic operation, if a disk drive is used, then around 1.56A, at most 1.60A but not over 1.6A. I'm not sure an MSX can use both disk drives at the same time hardware-wise, but even in theory, if both disk drives were read/written to at the same time, the current should never exceed 1.7A.
Good to have some actual readings. :) Shows that 2A is cutting it too close, but a regulator rated for 3A continuous may be fine. Looking at that list from SP-elektroniikka, I'd pick the 78T05 @ Eur 3.50 (or preferably, 78T05A). Cheap, fixed +5V output, rated for 3A, 3-pin TO220 package (or similar) so no need to drill extra holes. Drop-out voltage is 2.5V max, so you need input voltage that never goes below 7.5V (which will probably be okay here).

As for pinout: on the board there's 3 solder points on one side, and 5 solder points on the other side (for STR9005). Indeed heatsink is connected to +5V output, which gives you some options:

  • Make some cuts on the circuit board & repair connections, such that heatsink becomes (electrically) isolated.
  • Use isolation sheet + something around the M3 screw used for TO220 packages. Shops have ready-to-use sets for this, if used the regulator backplate has good thermal, but no electrical contact anymore with heatsink.
  • If regulator backplate = GND (as for the 78T05), drill a hole & screw to metal frame of computer. Perhaps with a small extra plate to distribute the heat better, this is a nice option because you can then remove that heatsink, making your NMS8255 a bit less heavy (and case = good heatsink!).

Then you use short & thick wires (preferably color-coded) to connect the pins that don't match up with the board's pinout. You can screw it on a bit tilted, or bend a pin to make at least 1 pin match up, so if using normal heatsink you shouldn't need more than 2 wires. Whatever you do: verify output voltages before re-connecting to mainboard / other circuitry! If okay, verify again after re-connecting, and monitor for a while how regulator/heatsink heats up.

Van Wild_Penguin

Hero (644)

afbeelding van Wild_Penguin

21-08-2010, 21:02

Hi again!

Been a bit busy so haven't had time to attempt the fix or buy the regulator yet. However, I've looked into fixing the power supply, and I'm just wondering a few things:

1) The heat sink is connected (electrically) only via one screw (to the output line). The screw is right next to the +5V line to the MB (you can actually see this on the service manual too). If I electrically insulate the heatsink bu cutting the trace, there will be only <2mm at the narrowest point left for the trace. Is this enough for the 1.7A current? It would be that narrow only for a short while, so it might not matter. But I'm not that experienced with these electronic thingies Smile. Of course I could solder a wire to the connector, but the connector doesn't protrude that much to the print side, so it might be difficult to make a good / durable connection...

2) At least on my power supply, the regulator heatsink is black =). I think it is coated with teflon or something, anyways the surface is non-conductive. Can I count on this, so that I might not actually need to isolate the heatsink? (Probably not, I think that the heat sink will be electrically connected at least via screw holes?).

I think I could get a pin compatible regulator from ELFA. It will be more expensive, but after weighing the work and inevitable cutting of traces on the PCB, I think this is what I'll get. I don't need to worry about the above questions, too. And also, I think my NMS-8255 deserves a proper fix even if it would cost a bit more Smile. Also, if there's something else wrong (in the coming years/decades) it'll be much easier to fix if the PCB is in original shape.

There are two canditates, a 5A and a 3A one (There's a link to the datasheet on the 3A page, both LT108x series, fixed regulators, so same datasheet). The 5A one is in TO-247 packaging, and end-of-life component but still on stock.

Should both of them work? I think so but I think you forum readers might have more experience on this issue :). Which one would you choose for replacement? Can I fit the heatsink screw trough the TO-247 package's hole, it looks smaller on the datasheet :P.

Cheers!

Van RetroTechie

Paragon (1563)

afbeelding van RetroTechie

22-08-2010, 01:47

If you can isolate the cooler without cutting a track entirely, that's definitely the most elegant. If remainder of track becomes too small in places, scratch off a bit of the protective layer (so that blank copper shows) and solder on top to make the track thicker there. Or solder a tiny piece of wire (~1 cm) on top of the track.

If you cut through the track, you can use a short (2~3 cm) piece of thick wire (speaker cable for example) to 'jump over' the cut. If short & thick, it won't move & thus be very reliable if soldered right.

2) At least on my power supply, the regulator heatsink is black =). I think it is coated with teflon or something, anyways the surface is non-conductive. Can I count on this, (..)
Nope! Evil The only safe assumption is that anything that touches the heatsink, gets electrically connected to it.

I think I could get a pin compatible regulator from ELFA.
Which type(s) exactly? You've already determined that a 3A type might do the job. Whether you want a 5A type then depends solely on how much extra money you're willing to spend for a larger safety margin. FWIW: I doubt it matters much, more important is that the regulator has a good heatsink, and keeps having a good heatsink (read: use cooling paste). If done properly, something else will fail next so it doesn't matter if you re-wire a few regulator pins. For anyone who knows power supplies it's easy to see what goes in & out. Smile

Van Wild_Penguin

Hero (644)

afbeelding van Wild_Penguin

22-08-2010, 07:55

I had links in the post above (see the 5A and 3A, after "There are two canditates", might not be obvious), but seems that the forum somehow messed the links Sad (if you copy the links from the 5A and 3A above, remove the extra https after the http: and the links will work). But the types I was looking at are LT1085CT-5 and LT1084CP-5, later one being in the slightly different package. The 3A one is 9,49€, and the 5A one 3,85€. So, a bit expensive, but still nothing too much (but hey, 10€ is not even enough for two beers in the pubs here). I think I'll go with the 5A one.

Thanks for the tips about scratching the laquer... didn't remember you can do that too Smile. If I'll need to go that route, I'll now it is feasible to do, also.

MODS: If you're reading this, can you fix the links? I can't edit the post anymore :-)

Van Eugeny_Brychkov

Paragon (1232)

afbeelding van Eugeny_Brychkov

22-08-2010, 11:43

A lot has been written and discussed in this forum. Can we see hi-res pictures of the power supply - from the top and from the bottom?

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