restoring a second OSI

[nama]

I recently received another OSI machine. This time a C4P. After my success with the C4-2P I'm confident I can get this working.
Firstly I tackled the CPU board. As I had a complete set of working boards, I just swapped the new board into the working system.
Here is what I have found so far:
- CPU didn't work, but have fixed it now.
- Memory cards seem ok, but need to test individual RAM chips, and fix some broken wires
- 540 vide board totally dead, no picture at all.

C4.jpg (69.9 KiB) Viewed 15513 times

The CPU bord wasn't working, but I could see that it was trying to reset. Cleaning the legs on the socketed Ic's seemed to fix it.
Now I've moved onto the 540 video board. It's totally dead. It differers slightly from my other working 540 bored in that it has what looks like a very large crystal (although that probably isn't what it is) installed. It also has the same regular sized crystal as the working unit. This large component looks like an elongated silver can. It's wrapped in protective tape to stop it from shorting on other boards so I can't see any writing on it. Anyway, this 540 boar is not pulsing at all. I've checked it with a logic probe and it seems that everything is either high, or low, with no pulse seen. Just wondering if anyone would know what I should look for next?

One last question. Unfortunately the cable with the 40 pin headers coming off the 505 bored was removed before shipping the computer, and the headers were crushed in transit, with legs falling off and is totally unusable. Does anyone know where I can get a replacement cable?

Cheers

Philip

[nama]

Quick update:
Naturally I suspect the crystal.
I'm not sure it it's possible to check a crystal with a logic probe, but I did anyway. I looked at the non-working board. One leg is high with no pulse, the other shows nothing at all. I then looked at the working board. One leg seems to show pulsing high and low, the other leg pulsing low. Again, probably not a very scientific way to test a crystal, but certainly it would suggest no pulse from the non working boards Xtal.

P

[nama]

Another update:
I don't have an 11.79648 Xtal, but I do have a 12.000. I gently placed the legs of the 12.000 Xtal agains the legs of the Xtal soldered to the board and the screen jumped to life. A thin strip of compressed random characters on the left side of the screen, Maybe 10 columns in total. Would this suggest that the original Xtal is dead and I should replace it?

Edit:
Just looked at the Sams repair manual. The large component on the 540 board is indeed a Xtal. 3.58Mhz and is used in the color generation circuit. The 11.79648Mhz Xtal in the Sams manual is actually specced as 12.000Mhz. The OSI 540 Manual specs it at 12.08Mhz for color boards, and 11.79Mhz for B/W boards.

P

[nama]

Dropped by Akihabara on the way to a meeting today. No 11.79Mhz or 12.08Mhz Xtals available at a all. Closest is 12Mhz.
Will 12Mhz work?

P

[dave]

12MHz should work fine. CRT's are pretty forgiving on slight timing differences. I have two 540B cards, with 11.765 and 12.0 MHz crystals. Good luck with the restoration. Please continue to keep us posted. If you have some pics, feel free to post them here in the thread!

Best regards,

Dave

[dave]

One last question. Unfortunately the cable with the 40 pin headers coming off the 505 bored was removed before shipping the computer, and the headers were crushed in transit, with legs falling off and is totally unusable. Does anyone know where I can get a replacement cable?

I recently ordered some insulation-displacement 40-pin DIP headers from Digikey, part# HDP40H-ND. I just did a search, and foundthat Jameco has a similar connector, PN# 8200-40-R, at a better price.

Good luck!

Best regards,

Dave

[nama]

Thanks for the advice in the Xtal.
I went to replace the Xtal today and realized that I had left the machine on for the past 10 hours or so. Unfortunately there is nothing to indicate wether it's on or off except for the position of the switch. Anyway, the video bored now displays nothing, were as previously placing the 12Mhz Xtal on the board showed some signed of life. My bad!
Probing the board again seems to show some IC's now pulsing but others not.

[nama]

I recently ordered some insulation-displacement 40-pin DIP headers from Digikey, part# HDP40H-ND. I just did a search, and foundthat Jameco has a similar connector, PN# 8200-40-R, at a better price.

Good luck!

Best regards,

Dave

Great! If I can't find anything in Akihabara, I may just take you up on your offer. Cheers.

[dave]

Thanks for the advice in the Xtal.
I went to replace the Xtal today and realized that I had left the machine on for the past 10 hours or so. Unfortunately there is nothing to indicate wether it's on or off except for the position of the switch. Anyway, the video bored now displays nothing, were as previously placing the 12Mhz Xtal on the board showed some signed of life. My bad!
Probing the board again seems to show some IC's now pulsing but others not.

Don't feel bad. It was going to happen soon anyway; better now while you're in the mood to fix it

[nama]

Good point. Very optimistic of you.
Anyway, I've been probing around for the last hour and I think I may have found something. I've been focussing on the signal coming out of the composite color line on J1 pin 5. Basically this is continually low when probed. There is a 2N5225 NPN transistor that is in the circuit just before the signal exits the machine. The Collector is at 5V, the Base is pulsing low, but the Emitter seems to have no pulse and is flat at 0Volts (GND). My guess is that the emitter should be pulsing. Am I right. Could I have found the dead part?

EDIT: actually looking at the schematic again. Seems that the signal on the Base of transistor is actually the B/W composite signal. I've also tested the B/W composite signal to my monitor and basically get nothing on the screen too, so maybe the transistor isn't the problem and it's something else?

P