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North Star Horizon power

Ttpilot

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Have you tried putting a load resistor on the leads of each 15V power supply as was suggested a while ago?
I did try that today. I clipped a 220 ohm resistor between the -16v lead and the power lug. Not much difference. I also tried a 1k resistor—also not much difference

i could try a more substantial load
 

Dwight Elvey

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I've got a suggestion. Take to low wattage 12V lamps ( find them at an automotive store ). Put them in series between the supply and the load.
Don't worry about the bulbs taking 16V. They should be able to handle that for hours. Most interior lights are about 3 to 5 watt. Most dash lamps are about 1 watt.
We just want to see how much load the supply is seeing.
If the supplies do power up normally take reading of the voltages to ground on both sides of the bulbs.
Dwight
 

Ttpilot

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I put a 1k resistor on both the +16v and -16v power leads. Before I had just tried it on one power lead. Anyway, I got steady readings of +1v on the +16v lead and -0.8v on the -16v. It's not much, but it hints that we might be on the right track. I'll try the 12v light bulbs tomorrow
 

deramp5113

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With the 1K resistor in-line to the motherboard, what is the voltage drop across the 1K resistor? I.e., is there +16v on the power supply terminal and +1v at the motherboard, or did you see the 1v reading at both the power supply terminal and at the motherboard terminal?

Mike
 

Ttpilot

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With the 1K resistor in-line to the motherboard, what is the voltage drop across the 1K resistor? I.e., is there +16v on the power supply terminal and +1v at the motherboard, or did you see the 1v reading at both the power supply terminal and at the motherboard terminal?

Mike
Oh, glad you asked me to check this. With the two resistors in place I'm getting 16v and -16v on the power supply side of the resistors and 6.7v and -2.2v on the other side of the resistors
 

Ttpilot

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I've got a suggestion. Take to low wattage 12V lamps ( find them at an automotive store ). Put them in series between the supply and the load.
Don't worry about the bulbs taking 16V. They should be able to handle that for hours. Most interior lights are about 3 to 5 watt. Most dash lamps are about 1 watt.
We just want to see how much load the supply is seeing.
If the supplies do power up normally take reading of the voltages to ground on both sides of the bulbs.
Dwight
With 2 14v 3.8w bulbs in place of the resistors I get 16v and -16v on the power supply side. On the motherboard side I'm getting -218 and 474 just off the motherboard side of the bulbs (?!). I get 30.3 and -8.8 on the motherboard power supply lugs.

On the S-100 connectors I'm now seeing +25.7 on the +16v (pin 2) and +12.3v on the -16v line (pin 52)
 

daver2

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This is all as confusing as **** I am afraid.

If the sum total of the two voltages is greater than 30 Volts - then reality is wrong somewhere.

Can I suggest just doing this test with one supply on (say) the +16 Volt line? Let's walk before we can run.

Dave
 

Ttpilot

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Still using the bulbs I'm getting 28.8 on the 78L12 input and -0.5 on the output. I get -.1v on the 79L12 input and -.5v on the output. C1 is -.5v on the + terminal and -.7v on the negative. C3 is 28.8v on the + post and -.5v on the - post. C2 is -.5 on the + and -.5 on the -. C4 is 28.8 on the + post and -.5 on the - post.
 

Ttpilot

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Still using the bulbs I'm getting 28.8 on the 78L12 input and -0.5 on the output. I get -.1v on the 79L12 input and -.5v on the output. C1 is -.5v on the + terminal and -.7v on the negative. C3 is 28.8v on the + post and -.5v on the - post. C2 is -.5 on the + and -.5 on the -. C4 is 28.8 on the + post and -.5 on the - post.
I disconnected the -16v power input to the board. I get +16 on the power to the bulb, +58.7 at the +16v motherboard lug. Even better, I get +27.8v on the -16v motherboard lug! I'm about ready to call an exorcist :eek:
 

Ttpilot

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Just out of interest, how are you actually measuring the voltage?

Dave
The mb is perched on a cardboard box at the moment. Ground from the meter is clipped to a ground lug on the mb. The 16v lead from the power supply is clipped to one of the light bulb wires. Another lead is clipped to the other bulb wire and then to the +16v lug on the motherboard. The ground wires from the supplies are all plugged into the ground lugs on the motherboard
IMG_4206.jpeg
 

daver2

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The answer I was looking for is a digital multimeter (I see from the photograph).

Is this set to read DC Volts?

Is your meter set to autoranging and, if so, is it possible that the readings are really mV not Volts? Just trying to cover all bases?

Dave
 
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Ttpilot

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The answer I was looking for is a digital multimeter (I see from the photograph).

Is this set to read DC Volts?

Dave
Yes. I think it's set to that in the photo

I checked the Vcc pin on some of the chip sockets. Most seem to read 5v just fine. The 1488 chips read 3.3 on pin 14. Shouldn't that be 5v? Also, pin 1 on those should be a -Vcc. I get about +6v on those for the 1488 chips
 

daver2

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If I remember correctly, a 1488 RS232 transmitter should have + and - 12 V on it, not 5V.

I am on my phone at the moment, so limited access to the NSH schematics.

Something is right screwy here, or we have slipped into an alternate universe where Ohm's law doesn't apply.

So you have the +8V supply connected as well if you are reading +5V, yes?

Dave
 

Ttpilot

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The answer I was looking for is a digital multimeter (I see from the photograph).

Is this set to read DC Volts?

Is your meter set to autoranging and, if so, is it possible that the readings are really mV not Volts? Just trying to cover all bases?

Dave
It's set to auto. Damn, those higher volt readings were mV. Doh! [sound of forehead being slapped]. So it's +16v at the power supply side of the bulb, 57.9mV at the +16v power lug on the motherboard. Pretty much everything downstream of that on the 78L12 and caps was mV.
 

daver2

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So, the bulb is dropping all the Volts across it. This implies a short circuit (again) between the +16V lug and the GND lug of the motherboard. Unless the resistance of the light bulb is too large to support the requisite current draw for the +12 Volt regulator.

What 'low value' resistors have you got in your 'bits box'?

Dave
 

Ttpilot

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So, the bulb is dropping all the Volts across it. This implies a short circuit (again) between the +16V lug and the GND lug of the motherboard. Unless the resistance of the light bulb is too large to support the requisite current draw for the +12 Volt regulator.

What 'low value' resistors have you got in your 'bits box'?

Dave
I've got a little breadboarding kit that has a batch of different values, including some pretty low ones
 

daver2

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What we need to do is to find a series current limiting resistor that will allow a reasonable current to flow, but not burn out!

For example a 100 Ohm resistor.

This will permit a maximum current flow of 15 V / 100 Ohms = 0.15 A (150 mA).

At 0.15 A, the resistor will dissipate 0.15 * 0.15 * 100 Watts = 2.25 W. So a 0.25 W resistor would burn out pretty quickly if subject to a short circuit on the motherboard!

See what you have got and calculate the maximum current flow and maximum power dissipation to see what will work. I have some rather (physically) large resistors bonded to a very big heatsink for just this eventuality!

Dave
 
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