SSL Video TR RGB Switching hell

[skipburrows]

Hey All you Uber smart people.

I have a School (CLIENT)with a later model G+ that started having an Issue with video Switching With The TR computer. I have the manual but no extenders for the computer (ARRRRG). So here it goes. When I do A Play Setup command the Red LED on the Video Switcher Distro Card does NOT light up and therefor stays on the "Green screen". This logic line is also referred in the manual an MPX Enable. I think. This is a voltage on Pin 71 on the 82R112 (AKA Video Switcher Distro Card) On Page 2-36 Of the computer manual is says next to Pin 71 82E89 Slot 23 pin 70. When I get on Pin 71 On The Video Switcher Distro Card I have about 4.5 V. and its is present even when I do a play setup. This seems really strange. I would expect to see 5 V when there is NO play setup issued and should be gone when I do a play setup. So whatever feeds Pin 71 on the Video Switcher Distro Card Might Be faulty. However I'm a bit confused where this MPX enable originates from.. The Only Place I see is on Page 4.14 there Is a MPX En on Pin 70 that is fed from (Total recall interface card) T4 Pin 13 (74ls33). To make matters even more confusing, occasionally I loose both primary video outputs (GREEN SCREEN). I rebuilt the internal monitor and it looks fantastic when it doesn't flake out. Same with the B&C output on the computer. It will just disappear every now and then. So when I look at the TR section of the computer back on page 4-14 I see that T4 looks like it feeds some of the driver portions of the Green and sync. Almost like a distribution amplifier (OLD SCHOOL). So I didn't have the parts and found the gate (T4) and ordered. Any other advice would be fantastic if you think I'm on the wrong track. Sorry for the long winded post but this has been a tough one as I've really not worked on the computer. They always have worked for me. Thanks to all for this forum. Really great...
Cheers
Skip Burrows
 

[horizonsound]

Hi Skip,

I don't know the circuit that well, but it looks to be a fast pulse to enable coming in on Pin 71. It is isolated via a CNY17-1 optocoupler (T1). These optocouplers fail regularly. I had to replace all of them on the 88 card.

From there it seems to hold a signal high for 3secs via a 4538B. When it drops low, it send the TR enable signal high (via T10 NOR gate), which triggers a JK flip flop. The output of which drives the TR LED and cross point.

Look at that CNY17-1 first, then follow the chain. Page 2-44 of the service manual.

Anthony.

[skipburrows]

Hi Anthony, I swapped t1 and t2. No change there. I’m still trying to grasp why I have a constant 4.5 volts on pin 71… no matter what I do regarding total recall calls. “Play setup” so not the opto coupler. Could be something further downstream putting that stray voltage on pin 71 but it would require a fault in the opto device? Thanks for the ideas I really appreciate it. The other common issue is the flaky green screen. That would be t4 on the total recall portion? Unfortunately, the desk is 2 hours away and the school is busy so I must schedule diagnostic time.. so please any other ideas are greatly appreciated..
Thanks again for your help……

[horizonsound]

Hi Skip,

The MPX I would expect stays at logic high (5v), and only briefly drops to low for a few milliseconds when enabled. You may not be able to pick it up. Though now reading the circuit description on P 2-40, I may be wrong. If the MPX line is not staying low then you might have an issue "When TR is cancelled, the MPX line goes high" implying it stays low when active.

In that case, you'd be turning the attention to the TR computer (ref 4-14 where the MPX is generated).

Anthony.

[horizonsound]

I ran some tests.....

Pin 71 of the video distribution card will remain low, when TR is active. Returning to 5v once exiting TR.
This pin corresponds to Pin 70 of the 82E90 Total Recall board - the one closest the main processor board.
Interestingly on mine, it was originally connected to Pin 72, but the trace was cut and a bodge wire installed to Pin 70. Check yours has the same, and that the wire has not come loose.

[skipburrows]

Man Anthony,
You are the best…. I can’t thank you enough..
I doubt that the wire is loose as I have that 4.5 volts always there. I am suspecting t4 or something there on that part of the card. I owe you sir.. so amazingly kind to look and pull that card. If I can ever return a favor please ask..
I’ll let you know when I can get back in front of it..
Cheer…
Skip