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North Star Horizon power

Ttpilot

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Okay, I got the tantalum caps and regulators installed, including fixing the broken trace. Now the resistance readings are:

8v to ground: 98.6 ohms
+16v to ground: 164.4 ohms
-16v to ground: 164.3 ohms

I checked for shorts between ground and the caps and regulators. All looks good

I plugged the power to the board, and the +16v and -16v were dead at the connection lugs. If the power leads are disconnected there is a supply of + and -16v, respectively. Another check of the board after disconnecting it shows the same resistance values between the power and ground lugs.

Also, the power doesn't seem to have negatively affected any of the regulators or caps. They still check out okay

IMG_4190.jpeg
 
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Gary C

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How odd.

Given that all you have is a transformer and a rectifier between the mains and the lugs, there is nothing to kill the +16V & -16V lines like there is in switched mode supplies.

Try 200 ohm resistors from +16 to 0 and -16 to 0 (which should only by 160mA) with the lines disconnected and check the voltages maybe ?

Then plug it all back in and measure the +16/-16V at the S100 bus connection.
 

Ttpilot

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How odd.

Given that all you have is a transformer and a rectifier between the mains and the lugs, there is nothing to kill the +16V & -16V lines like there is in switched mode supplies.

Try 200 ohm resistors from +16 to 0 and -16 to 0 (which should only by 160mA) with the lines disconnected and check the voltages maybe ?

Then plug it all back in and measure the +16/-16V at the S100 bus connection.
I have a switching power supply, namely a Meanwell HRP-75-7.5 to deliver the 8v power and two Meanwell RS-15-15s for the +16v and -16v, respectively. The old linear supply has been retired to a box
 

Gary C

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So looking at it, you have two +16 volt supplies and when you connect it to the board, you effectively connect the +ve of one to the -ve of the other ?

is the ground of the supply connected to ground so your just effectively shorting out one supply ?
 

Gary C

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Look at this
1654055526971.png

Ah, capacitively coupled, ignore me.

but when you aren't connected, do you have the + & - of the two 15V connected together and they continue to run ?
 
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Gary C

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Yep, got a couple of them but with the original supplies.

So, connect the -ve of the positive supply to the +ve of the negative supply but dont connect them to the board and see if they still work ?
 

daver2

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I think Gary is on the money.

Switch mode power supplies (in general) don't like being run lightly loaded. The fact that you have 15V with no load made me think that these units were designed to run lightly loaded - but this may not be the case. The manual didn't help here either!

As Gary says - connect one power supply positive to the other power supply negative to simulate what will happen when the two supplies are connected to the motherboard and check to see that you still have volts from the two power supplies when they are not actually connected to the motherboard. In theory, the +/- 15V should still be there. If not, you can't use these power supplies for this role - but I can't see why from the datasheet...

Gary has also made another observation - if the power supply is NOT loaded it could provide volts, but - if it is 'lightly loaded' does it still produce volts...

Gary has recommended connecting 220R resistors from +15V to 0V/GND and -15V to 0V/GND (again, not connected to the motherboard). Make sure you still have volts...

After that, we are running out of options...

I would think the next stage would be to connect one 15V supply up to the motherboard at a time after that to see if just one supply shuts down. If so, we can safely find out what is going on with that supply...

Dave
 

Ttpilot

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I'm a bit confused about how to connect the leads as you suggest. The +16v is connected to the motherboard by + on the ps to + on the board, with gnd to gnd; the -16v supply is connected to the motherboard by gnd on the ps to + on the board, and + on the ps to gnd on the motherboard:

IMG_0538.jpeg
I guess I'm not sure as to how to connect them correctly
 

daver2

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Basically, connect together the two (2) leads of the two (2) separate power supplies that are connected to GND on the motherboard.

From your drawing above, that would be '-' of he top power supply and '+' of the bottom power supply.

Power up the supplies (without them being connected to the motherboard of course) and you should be able to measure +15V (between GND and the '+' of the top power supply and -15V between GND and the '-' of the bottom power supply. The BLACK lead of your multimeter connected to GND in both cases.

You should also be able to measure 30V between the '-' of the bottom power supply and the '+' of the top power supply.

If you don't measure anything, then the power supplies have shut themselves down.

Is that clear now?

Dave
 

DeltaDon

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There's EARTH GROUND and there's neutral. Earth Ground is for safety. Neutral or motherboard ground is part of the current loops for the operation of the circuits. These two "grounds" may connected or the motherboard ground plane may be floating. The green AC cord lead should tie to the metal case of the computer grounding the metal case to earth ground. This may be the only connection to EARTH GROUND in the computer. Or there may be EARTH ground terminals on the two power supplies. These also need to be connected to EARTH GROUND for safety.

The motherboard ground plane neutral maybe connected via standoffs or an existing wire to EARTH GROUND or isolated from it. It doesn't matter as far as the computer power needs or power supply connections.

In any case, the Plus PS should have the + terminal connected to the motherboard's +16vdc input. The Plus PS should have the - (minus) terminal connected to the motherboard's ground plane terminal. Not earth ground!

Real important ---> The negative voltage power supply needs to not have it's - (minus) lead tied to Earth Ground. Period!

If it is jumpered or by design tied to EARTH GROUND it will not work for this use. The power supply's output needs to be able to float and needs to be isolated from EARTH GROUND.

If it floats then you can use it to supply the minus rail on the motherboard. So use an ohmmeter to test for open circuit between the minus output and EARTH GROUND. Remove any jumper to EARTH if installed. If the negative voltage PS has the minus (-) terminal connected to case ground internally then it will most likely short out when you try to use it. You will need to purchase a different PS in this case.

So assuming the output floats, you connect the + (positive) terminal on the supply to the motherboard's ground plane. The same ground plane terminal the PLUS PS has connected it's - (negative) lead to earlier. Yes, the NEGATIVE PS + (plus) lead goes to the same motherboard ground terminal. Then connect the NEGATIVE PS's - (minus) terminal to the -16vdc terminal.

So a total of three terminals on the motherboard are used.

No power supply outputs should be tied to EARTH GROUND directly. If the motherboard has contact with EARTH GROUND it will be done by the motherboard via an existing wire from the motherboard or metal standoffs. None of the two new power supplies outputs terminals should be using the metal case for current paths or tied to EARTH GROUND.

If you measure voltage make sure the negative (black in most cases) voltmeter lead is tied to the motherboard ground plane.

I hope I'm clear with this - good luck.
 

daver2

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So the external power supplies are OK with the 'GND' arrangements (commoning of one PSU '+' and one PSU '-' lead).

I agree with DeltaDon, there is something funny going on somewhere...

Can you now connect the effective GND point of the two external power supplies (the two that are connected together) to the GND points on the motherboard and repeat to see if you still have volts from the power supplies.

To be clear - the '+' lead of the +15V power supply and the '-' lead of the -15V power supply should NOT be connected to the motherboard.

See if that causes the power supply modules to shut down or not. You should still read +15V from GND to +15V and -15V from GND to -15V and 30V from +15V to -15V.

Dave
 

Gary C

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Just realised

The NS board includes a jumper for floating or earthed logic.

If the jumper is installed, then PSU ground will be connected to 0V so you will be effectively shorting it out, have a look and see if C47 is installed. If it isn't, and instead the connection is shorted, that would be a problem I think.
 

Ttpilot

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….
I hope I'm clear with this - good luck.
Thanks. I’m not sure I followed completely. You can’t see it in the photos, but the the AC power supply ground lead is screwed into the computer chassis and doesn’t go on to the ground screws on the power supplies. I’m assuming the ps mount screws provide earth ground via the chassis. I don’t think the motherboard is tied to earth ground; the mounting screws are not connected to anything on the board
 

Ttpilot

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Just realised

The NS board includes a jumper for floating or earthed logic.

If the jumper is installed, then PSU ground will be connected to 0V so you will be effectively shorting it out, have a look and see if C47 is installed. If it isn't, and instead the connection is shorted, that would be a problem I think.
C47 is in place. I measured it at .047uf
 

Ttpilot

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One other thing I just noticed. When the mb is powered up, the 8v is steady. The +16 and -16v lines are not 0, they seem to fluctuate quickly and cyclically, pausing every few seconds at about 7.5 for a split second, then starting over
 

deramp5113

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Sounds like the power supplies are cycling into self protect mode over and over. A 165 ohm load at 16v is less than 1.6 watts, so a resistive load doesn’t appear to be the problem. Could be a diode short to ground in one of the ICs. Put your meter into diode test mode and see what you measure on the motherboard (no power supplies connected) from the +16v lug to ground (put the red lead on +16v lug), and then from -16v to ground (put red lead on ground lug) .

Mike
 

Gary C

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One other thing I just noticed. When the mb is powered up, the 8v is steady. The +16 and -16v lines are not 0, they seem to fluctuate quickly and cyclically, pausing every few seconds at about 7.5 for a split second, then starting over
Certainly seems as if there is some leakage the PSU's dont like and as your not operating them really as designed its possibly not surprising. Really you need a +15/-15V psu that's specifically designed to operate in this mode.

Only thing I can think of now.

Put a 200ohm resistor across each PSU's then connect one +Ve to the other PSU's -Ve and check, then connect this point to the GND connector on the NS motherboard and check.
 
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