My OCC2 gives a blank screen when powered on. The composite video output is OK. There is a faint glow in the end of the tube. The voltage readings from the PSU is OK. After some checking I suspect the horizontal output transistor to be defect (Q103, under the heat sink). Its markings is 88245 1070. I have tried to google these numbers to find a replacement, without luck. Does someone know if there is a similar transistor I can use? I also wonder what might have caused the HOT to "die" and what other components that this failure might have affected. Any suggestions will be greatly appreciated.
- VCF SoCal - February 15 - 16 2025, Hotel Fera, Aneheim CA
- VCF East - April 4 - 6 2025, Infoage Museum, Wall NJ
- VCF Southeast - July 2025, Atlanta, GA
- VCF West - Aug 2025, Computer History Museum, Mountain View, CA
- VCF Midwest - Sept 2025, Schaumburg, IL
-
Please review our updated Terms and Rules here
Osborne Executive CRT problem
- Thread starter jmidtun
- Start date
I see two models of CRT displays used. A 5" Motorola or a 7" Zenith. I don't find a schematic for the 7" and the 5" hor out transistor ref# and part# don't seem to match yours. Do you know which display you have?
The 5" display uses what's called a "darlington pair" transistor for a horizontal output which is actually two transistors inside with internal resistors from base to emitter. Does your transistor measure low ohms (short) from
collector (probably a metal tab) to emitter (probably ground) ? Do you have a meter with diode check? Do you have an oscilloscope ?
Larry G
The 5" display uses what's called a "darlington pair" transistor for a horizontal output which is actually two transistors inside with internal resistors from base to emitter. Does your transistor measure low ohms (short) from
collector (probably a metal tab) to emitter (probably ground) ? Do you have a meter with diode check? Do you have an oscilloscope ?
Larry G
Hi Larry. This is a 7" Zenith. I use a meter (with diode check), and the transistor leads a current from B to C, and from C to B with the same resistance. The same with B to E and E to B; no difference. I measured the resistance between C and E as you suggested, and there was hardly any resistance at all. I have no oscilloscope I´m afraid. I found an Osborne field service manual as pdf on the internet, but the chapter about the Zenith was unfortunateldy missing.
Earl
Earl
Progress...I hope; After some fiddling around, I sprayed the CRTs circuit boards with electronic cleaner and assembled the OCC2. To my big surprise the CRT came alive and worked perfectly for about a minute. Then the screen image narrowed horizontally and finally faded away. Now I wonder what this could indicate. Could it still be a transistor issue, or could this be caused by bad electrolytic capacitors?
Stone
10k Member
Check the flyback.
I noticed too the zenith schem is missing. Probably due to some kind of agreement Osborne had with Zenith. Sucks without a schem or scope. Can you post photo's of the component and circuit
side of the deflection board? Also the CRT tube, just whatever you can get w/o taking it apart too much. I want to see if I can match it up with one of the video displays of the Zenith Z-100. The shop
I worked at many years was a Zenith authorized service, not computers, but I recognize zenith part#'s and etc.
Larry G
side of the deflection board? Also the CRT tube, just whatever you can get w/o taking it apart too much. I want to see if I can match it up with one of the video displays of the Zenith Z-100. The shop
I worked at many years was a Zenith authorized service, not computers, but I recognize zenith part#'s and etc.
Larry G
Just saw your last post. Spraying cleaner lowers the temperature significantly. Hopefully it was all dried out before you powered on. Do you smell any odor like burned plastic?
It could be a thermal problem and still could be the output transistor you were suspecting. As Stone suggests, could be flyback but usually shorts out the transistor or smell an odor from it.
Look around the flyback transformer for any cracks or swelling. Photos would still help. Can you power it on when it is apart? Just wondering for troubleshooting.
PS - if a bad capacitor I would think tuning caps in the deflection circuit itself. Look for cracks / swelling in the larger capacitors around the deflection area.
It could be a thermal problem and still could be the output transistor you were suspecting. As Stone suggests, could be flyback but usually shorts out the transistor or smell an odor from it.
Look around the flyback transformer for any cracks or swelling. Photos would still help. Can you power it on when it is apart? Just wondering for troubleshooting.
PS - if a bad capacitor I would think tuning caps in the deflection circuit itself. Look for cracks / swelling in the larger capacitors around the deflection area.
Last edited:
First of all; thanks a lot for taking your time to help me out on this. I did some more digging on the internet and found this page that describes a similar problem with an Osborne Vixen. It has a 7" Zenith DT2 CRT, but its HOT is identified as Zenith 121-1070 and is said to be equivalent to the NTE 379. Since "my" HOT has 1070 printed on it, I hope/suspect it might be the same transistor. It looks the same and has the same pin-positions (C in the middle). I haven´t noticed any smell or cracks in the flyback. All the flybacks input wires measures 11.1 V. I haven´t dared to fiddle with the high voltage connection to the tube. The caps looks ok (no swelling or leaks), but I could be wrong. I can start the OCC2 with the top plastic cover off. I will take more photos of it tomorrow. The one I have added today is from the moment when the screen image started to narrow and fade.
Stone
10k Member
You don't need to see or smell anything unusual for it to be bad.
I read to the end of that thread and sounds like it's a bad hor out transistor and he used a MJE13007 for a good substitute. Darn thread went OT talking about light bulbs or something but eventually the fix
was reported. Chuck recommended the substitute.
was reported. Chuck recommended the substitute.
Thanks for reading it through Larry (quite a few pages there). Next stop is ebay. I think I´ll give the Osborne a rest until I have received and installed the MJE13007. I´ll come back with a report when thats done. Fingers crossed. Cheers.
VERAULT
Veteran Member
Shame he never came back. Id like to know what happened.
cj7hawk
Veteran Member
Well, I'll take a moment to thank you on his behalf. Whether or not it worked, help was given and received. I find the level and quality of help with old computers on this forum is incredible. I knew nothing about CP/M when I came here and, well, I still don't know a huge amount, but I may have never solved my OS problems without help from this forum from people who actually knew what the OS was supposed to do.
The dawn of computing is almost out of living memory.
The dawn of computing is almost out of living memory.
@Culprit welcome to the forum. When you say the CRT doesn't heat, have you opened it up to see if there is an orange filament glow in the neck of the tube ? Darken the room lighting or view under a blanket
to see if there's a filament glow. Also turn brightness and contrast up to check the face of the tube for a raster while the room is darkened.
Larry G
to see if there's a filament glow. Also turn brightness and contrast up to check the face of the tube for a raster while the room is darkened.
Larry G
Yeah, exactly right. I have taken the shrouds off the machine/CRT unit, powered it up, and messed with the Pots/dials. I have also sprayed contact cleaner in the pots. I need to find out which cable is supplying the 12v (I am not good at reading schematics). I know another cable from the PSU is supplying 12v (along with 5v for the floppy drives). I collect computers from this era (along with late 90's 2000s') and have a zenith z100 as well so I was blown away that someone on another forum mentioned that machine. As you might be able to tell, I was alive when CRTs were on their way out.
I have a fully functional Zenith Z100 with a hard drive I installed. Anyway, back to the problem. The Osborne Executive service manual is on Bitsavers. Evidently the CRT can either be a 5" Motorola or 7" Zenith. The Motorola schematic is in the manual but the Zenith page is mysteriously blank. Do you know which CRT you have ? The Zenith or Motorola name is probably on the CRT frame somewhere internal.
same here! i have the aio terminal version I repaired that I got from someone who worked at an afb, i just am waiting to figure out how to write floppies so I can actually use it. The thing weighs a ton, more than the IBM tower of power. Anyways, the crt: I thought I had the zenith one. It was made in March of 83 and is the 7-inch model. Where could I check? I have the housing but just see numbers not a manufacturer.
