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Northstar Advantage CRT Schematic

K

kyeakel

Experienced Member
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Jul 27, 2009
Messages
491
Location
Curwensville, PA
I have a defective CRT in a Northstar Advantage, the only technical reference I could find on Dave's Old Computers, has a different CRT schematic than the one in my machine. The manual on Bitsavers has no schematics. The imeadiately identifiable thing is that the schematics have only one 555 timer, my circuit board has 2 and the board has 2 transistors marked Q414 and no Q413. The symptoms are no display at all, there is filament voltage, I don't think there is high voltage. When it died, there was a vertical line in the center of the screen from top to bottom. It then went away completely. There was no smoke, or noise, it just stopped. There are signals on the main board connection to the CRT board.

Thanks,
Kipp
 
Kipp,

Has that monitor been setting around for a good while and maybe subject to excessive moisture? If I had to T/S the thing I'd probably wring out the high voltage. I might really suspect the yoke and/or the flyback. Just a hunch - good luck.
 
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Kipp,
Another couple of things to check are all the plugin connections to the Video Board. I had a Kaypro II
that had a blue-ish tint on the six pins protuding from the Video Board. It had a Horizontal Line
centered on the Display. And don't forget about the possibility of some bad Electrolytic Capacitors.
A good ESR Meter like the Cap Analyzer 88A will test those while still in the circuit.

Larry
 
Agent orange, I had this computer working a few months ago, ran it for a few hours over several days and then put it back on the shelf. I had noted that when turning off, the monitor showed a bright line, transitioning to a bright dot, and then off. So, this may have been an early indication of trouble.

Larry,
I've been trying to follow the signal with a scope, but since I have the wrong schematic it's not leading me in the right direction. Twice I've removed the board and desoldered caps where I thought the signal was missing, only to see that the caps check out ok with my Fluke 117. The cap analyzer looks interesting, and a nice addition to my tools. So, I ordered one.

Thanks,
Kipp
 
Kipp,

I would also check the horizontal output transistor. Be real careful poking around the HV area and make sure you are unplugged when ohming components out. If you have a freeze compound aerosol you might want to give the horizontal output circuit a shot and see what happens. Also, if you can't find a schematic you can troubleshoot the flyback with a generic print (Google). And, don't forget to look at the caps.
 
I'd think a buzzer and a 9 v battery would be useful for
checking to see it the transformer had an internal short.
This is better than blowing output transistors just to see
if it was a bad transformer.
Tinker Dwight
 
Well I've checked the caps, and there does not seem to be a problem. I've traced the signal thru both NE555 timers and have a 4v 10ms pulse on a 1k resistor that is connected to the base of Q401. At the base I have a very low signal < .5v, there does not seem to be anything between the %%% timer and the transistor except the resistor. The transistor seem to have a Motorola M on it. It's labeled as follows:
Msps
8870
EBCK
I cannot seem to find this part. I've down loaded a Motorola transistor data book from 1984, but there is no 8870 in there. Does anyone have the datasheet on this part?
It's a plastic transistor in a TO-92 form factor, and it's face is blue-gray in color.

Kipp
 
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Well I still haven't found the issue, but I did find a cross-reference for the transistor. In case anyone else needs to identify an unknown part, try this link.
http://www.nteinc.com/
typing in sps8870 gives NTE108.
The thing I don't understand is why is there a need for a high frequency amplifier in the horizontal section. Unless it's for the fast rise and fall times.

Kipp
 
kyeakel said:
Agent orange, I had this computer working a few months ago, ran it for a few hours over several days and then put it back on the shelf. I had noted that when turning off, the monitor showed a bright line, transitioning to a bright dot, and then off. So, this may have been an early indication of trouble.
Click to expand...

I think this is perfectly normal, my Advantage does the same. Just due to the various power supplies going to 0V after switching off at different rates.

+1 to suggestion of checking the HOT, good luck with the fix, I will check my video card for how many 555s it has.

Regards,
John
 
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