Northstar Advantage - Screen doesn't display anything

[Robuck]

Hello,

My name is Mark, and I just bought a beautiful Northstar Advantage from ebay. It was advertised as not working, and of course it doesn't. The only thing it does do is beep when I switch it on. I can hear the fan running, and the CRT screen makes the high pitched scream. Sometimes when I press enter one of the floppy drives lights up. I tried reseating the processor, but I'm not sure what I should do next. I've seen a ton of videos about restoring old PC's, and I would consider myself somewhat knowledgable about vintage computers. The only problem is that I'm not at all experienced with actually fixing them. Is there a checklist of things I should be troubleshooting? My goal here is to just get the "LOAD SYSTEM" prompt on the screen. The floppy drives are going to be a whole other nightmare. Any info would be greatly appreciated! I can provide pictures if needed.

 

[cruff]

Are the brightness and contrast (if any) controls set mid-position? Can you tell if the CRT cathode heater is glowing? The "high pitched scream" sound - do you think this is not the normal CRT whine? That may mean a problem with one or both of the deflection circuits. If you turn the brightness up, do you see a line on the screen? Be sure to remember to turn it back down again.

The access attempt of the floppy drive when you press return/enter probably means the CPU is running the BIOS code well enought to attempt to boot from the floppy.

 

[Robuck]

I have tried adjusting the brightness knob with almost everything I change, there isn't a way to tell which direction is brighter. Either way, it still has nothing. The noise sounds like a normal CRT noise, a lot like a normal TV from the era. However, compared to a Macintosh screen, it's extremely loud. Where would the glowing occur? I haven't seen anything glow, but I haven't been looking for it.

 

[cruff]

Where would the glowing occur? I haven't seen anything glow, but I haven't been looking for it.

The glow will be near the end of the neck of the CRT close to where the wire harness attaches to the rear of the CRT neck. If you can't see in through air ventilation slots, you will need to remove the cover. When you power on the unit, the cathode heater should start to glow.

Use care when the cover is off, high voltage will be present, and a CRT will have several thousand volts across it internally to accelerate the electron beam.

 

[Robuck]

Took it apart, there is a piece at the very back of the tube that seems to be glowing. I also tried removing all the cards from their slots, still nothing.

 

[cruff]

The issue is probably in the video drive circuitry, or possibly the video controller. Do you know if there are schematics available for your unit? I've not used a Northstar computer since 1980, so probably can't help much unless there is a schematic I can look at. Do you have an oscilloscope and a high voltage probe for a DVM or oscilloscope? Those could be useful to diagnose it.

 

[JonB]

Check the soldered joints in the video circuit for cracks, especially near the flyback transformer (that's the one with the big fat wire coming out of the top that plugs into the side of the tube). But note, there are dangerous voltages in there even when switched off. The CRT itself is like an enormous capacitor, so do not attempt to remove the fat wire from the side of it. If you are not sure, ask a professional.

If you find cracked joints, reflow them with an iron and a bit of solder. I had this on my Superbrain's video board: https://stardot.org.uk/forums/download/file.php?id=29948&mode=view

 

[Dwight Elvey]

We need to know what test equipment you might have. What ever the problem is, without some test equipment, all we can do is give you some vague guesses.
Dwight

 

[Robuck]

The solder joints look fine to me right now, but I am unable to take a very close look. I don’t have any wires to discharge the tube with and I’m afraid I’ll electrocute myself while taking it apart if I don’t. Here is a pictures of where the flyback transformer meets a board:



And here is one of the rest of the tube circuitry:



As for the schematics, all I was able to find was the original technical manual PDF. Sections 3.6 and 4.1.6 jumped out at me as useful. Thanks so much for reaching out!

 

[Robuck]

I have a volt meter, but no oscilloscope. Vague guesses can be helpful too!

 

[JonB]

You need to look under the board at the soldered side.

I've done this many times, without removing the HT lead from the tube. You just have to be careful and wear some rubber gloves.

Alternatively, if you feel up to it, you can make a "chicken stick" easily enough. Connect the tip of an insulated screwdriver to the chassis of the monitor and (wearing rubber gloves) carefully poke it under the rubber cap of the HT lead. You should be able to feel the metal clip and you may even hear a discharge sound ("CRACK!"). It should be safe to remove the HT lead then.

This is quite a dangerous undertaking, so if you have any doubts, leave it to a professional.

 

[cruff]

You need to look under the board at the soldered side.

I've done this many times, without removing the HT lead from the tube. You just have to be careful and wear some rubber gloves.

Alternatively, if you feel up to it, you can make a "chicken stick" easily enough. Connect the tip of an insulated screwdriver to the chassis of the monitor and (wearing rubber gloves) carefully poke it under the rubber cap of the HT lead. You should be able to feel the metal clip and you may even hear a discharge sound ("CRACK!"). It should be safe to remove the HT lead then.

Then keep a ground clip attached to that lead to keep the HV capacitor from recharging due to dielectric absorption.

 

[Dwight Elvey]

I have tried adjusting the brightness knob with almost everything I change, there isn't a way to tell which direction is brighter. Either way, it still has nothing. The noise sounds like a normal CRT noise, a lot like a normal TV from the era. However, compared to a Macintosh screen, it's extremely loud. Where would the glowing occur? I haven't seen anything glow, but I haven't been looking for it.

Is the sound a high pitched sound or just a low frequency buzz?
Is there a white dot at the center of the screen when you turn it off?
Dwight

 

[Dwight Elvey]

Is the meter you have an analog or digital?
If it is an analog, is it a VOM or VTVM type?
If it is digital, we can make some measurements to see what basic things are working. We will be measuring several hundred volts. The meter will need a scale of at least 500V.
Carefully remove the connector at the back of the tube. Do not wiggle it too much side to side. There is a small glass tube at the center of the socket. If you break it, the tube is history. With age these are often hard to remove.
Once you get the connector off make sure you can probe the contacts in the socket connector.
Because of the high voltages involved, it is best to do hands off measurement. This means setup the measurement probes first and then turn the power on. Wait for the voltage to drop to less then 50 V before moving the probes.
Attach one lead of the meter to a circuit ground.
I wish the schematic had some typical voltages but all that seem to be there are capacitor voltage ratings. We will have to proceed with guesses.
First measure the cathode voltage. This is the yellow wire. My guess is it should be about 35 to 40 volts.
Next we can do the control grid. That is the green wire. I expect that to be negative 100V to 120v some place. It should vary with the brightness control. More negative counter clock wise.
Next lets check the first anode. It is the red wire. It is likely positive 300V to 400V someplace.
Last the focus anode. This is the blue wire. It should be a lower voltage then the first anode. This voltage should vary with the focus control.
Measure these and get back to us.
Dwight

 

[cruff]

I found a field change notices PDF at https://www.hartetechnologies.com/manuals/Northstar/North%20Star%20Field%20Change%20Notices.pdf that contains a description of a failing capacitor on the video board, but the failure mode sounds different from described. It may or may not be informative. It is notice #5 starting on page 17, it says only about 200 units were shipped before they noticed the problem.

 

[Robuck]

haha don't forget that step, or you're toast! I've ordered some alligator clips from amazon and when they get here I will do exactly that.

 

[Hugo Holden]

For God's sake, when I read threads like this it makes me wonder (as Sigourney Weaver said on one of the Alien movies) if IQ's had dropped sharply while she was away.

There is no significant safety risk from the stored electric field energy in the bulb of a typical VDU's CRT.

Do the math. Lets say the capacitance of the internal to the external aquadag around the glass on the CRT was 1000pF (its more likely 500pf for small CRT's) and it happened to be charged to say 15kV, a voltage higher than most. Lets play devils advocate and say the capacitance is a whopping 2000pF and the voltage is 15kV. The stored energy is CV^2/2 is about 220 milli-Joules. It is going to take a heck of a lot more than that to kill you, like about 10 to 100 times more at least.

There is one reason and one reason alone to discharge the anode on a CRT, that is, if you are removing the CRT from the chassis and have to carry it across a room. If you get a zap from the anode connections on the way (unless you are prepared for it) it will frighten you and you will drop the CRT on the floor. There is no need to otherwise do it.

It might be worth pointing out, the the amount of energy required to defibrillate a Heart (alter its rhythm), is in the order of 100 to 400 Joules and that is applied directly on the chest with paddles which are a large surface area plate with a conductive paste applied. This is at least two orders of magnitude higher than the zap from a charged CRT bulb. So get real.

I don't know who started all the urban myths about the dangers of charged CRT bulbs, but none of it is based in reality, yet I see remarks about it all the time.

It is not a wonderful idea to short the anode of the CRT bulb out to discharge it as is recommended on many forums. This is because, at the moment you short it out, the peak currents are very high. This can potentially damage the internal aquadag in the CRT and break its connection to the metal anode cap in there. It is much better, if you want to discharge the anode, to do it with a 100k series resistor and wait a second or two. Then it is less likely you will get a harmless "nip" from any residual charge on the bulb when you remove the CRT's anode cap. Most of the time there is no scientific or logical reason to remove the anode cap. All you need to do to check it is to slip the tip of the EHT probe under there to measure the EHT. If you don't have an EHT probe, while working with CRT's , then shame on you.

 

[Robuck]

Ok then if you’re so sure of yourself, record a video of you being shocked by one and post it. If you don’t, I’ll just assume you’re no longer with us...

 

[Dwight Elvey]

For God's sake, when I read threads like this it makes me wonder (as Sigourney Weaver said on one of the Alien movies) if IQ's had dropped sharply while she was away.

There is no significant safety risk from the stored electric field energy in the bulb of a typical VDU's CRT.

Do the math. Lets say the capacitance of the internal to the external aquadag around the glass on the CRT was 1000pF (its more likely 500pf for small CRT's) and it happened to be charged to say 15kV, a voltage higher than most. Lets play devils advocate and say the capacitance is a whopping 2000pF and the voltage is 15kV. The stored energy is CV^2/2 is about 220 milli-Joules. It is going to take a heck of a lot more than that to kill you, like about 10 to 100 times more at least.

There is one reason and one reason alone to discharge the anode on a CRT, that is, if you are removing the CRT from the chassis and have to carry it across a room. If you get a zap from the anode connections on the way (unless you are prepared for it) it will frighten you and you will drop the CRT on the floor. There is no need to otherwise do it.

It might be worth pointing out, the the amount of energy required to defibrillate a Heart (alter its rhythm), is in the order of 100 to 400 Joules and that is applied directly on the chest with paddles which are a large surface area plate with a conductive paste applied. This is at least two orders of magnitude higher than the zap from a charged CRT bulb. So get real.

I don't know who started all the urban myths about the dangers of charged CRT bulbs, but none of it is based in reality, yet I see remarks about it all the time.

It is not a wonderful idea to short the anode of the CRT bulb out to discharge it as is recommended on many forums. This is because, at the moment you short it out, the peak currents are very high. This can potentially damage the internal aquadag in the CRT and break its connection to the metal anode cap in there. It is much better, if you want to discharge the anode, to do it with a 100k series resistor and wait a second or two. Then it is less likely you will get a harmless "nip" from any residual charge on the bulb when you remove the CRT's anode cap. Most of the time there is no scientific or logical reason to remove the anode cap. All you need to do to check it is to slip the tip of the EHT probe under there to measure the EHT. If you don't have an EHT probe, while working with CRT's , then shame on you.

The danger has always been what you might do when zapped. Still, those with pace makers should always be a little more careful.
Getting zapped and dropping a screw driver on the neck is not a good idea.
Even the running current of the transformer at the second anode is not enough to kill you.
That is not quite so true with the lower voltages used in the first anode. The 400 volt line can provide a more serious shock. Although, created with voltage doubling diodes, a short would have the voltage and current of the lower voltage transformer tap.
Dwight

 

[Hugo Holden]

The danger has always been what you might do when zapped. Still, those with pace makers should always be a little more careful.
Getting zapped and dropping a screw driver on the neck is not a good idea.
Even the running current of the transformer at the second anode is not enough to kill you.
That is not quite so true with the lower voltages used in the first anode. The 400 volt line can provide a more serious shock. Although, created with voltage doubling diodes, a short would have the voltage and current of the lower voltage transformer tap.
Dwight

Well I wasn't suggesting touching the final anode connection with the CRT and the EHT generator running, that is a completely different matter and very dangerous, I was referring to the stored charge and energy in the CRT bulb, with the monitor switched off.

At worst for a large CRT, the stored the stored charge might produce a tiny surface skin burn (like diathermy) and very unlikely to upset a pacemaker. However it could cause some trouble possibly if a person had an internal defibrillator. Many electric fence modules can put out more energy and they don't kill the animals or farmers very often, some put out about 10 Joules per pulse, making the 200 milli-Joule from a charged CRT bulb look anemic. However I regard that as a fairly significant shock and I wouldn't go out of my way to grab an electric fence.

But I do agree that its what you do if you get a small shock that can have the effect, for example dropping the CRT if you are carrying it, or withdrawing your hand rapidly from an area, maybe cutting it on a sharp corner or breaking something.

Most of the time ,there is no need though to go under the anode cap for any reason servicing a monitor unless you want to do an EHT measurement or remove the EHT transformer & wiring. No requirement to discharge the stored energy there with the monitor off, if you don't go under the EHT cap. The charge on the CRT bulb remains isolated there, and isolated from the CRT's base pins.

Somebody started the myth that before you start doing repairs on monitors, the CRT had to be discharged. But this is not the case mostly, unless you want to remove the EHT cap or EHT transformer. And when it is discharged, it should not be done by shorting it out with a screwdriver.

In the early days of tube TV's, the EHT was sometimes checked (TV running) with an ARC test, to determine the EHT without a meter. This is folly in a semiconductor based TV or monitor as a number of components can be damaged. This was discovered in the early days of transistorized TV sets in the late 1950's and early 1960's . I have attached a remark from a Sony TV service manual, 1963.