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Andy_Burn
October 16th, 2010, 08:42 PM
Hey Guys - first post ! :)
I have a Decscope VT-52 which, after repairing the EIA board (works!) I suspect now needs a new Character generator board.
Anyone have a spare or suggestions on how to repair it?

Thanks,
Andy

Lou - N2MIY
October 17th, 2010, 04:52 AM
Andy,

You will have to explain the problem in more detail. The more detailed, the better. If something looks funky on the screen, post pictures. Such a problem should not be that difficult to repair in a VT52. It's all 7400 series TTL. I have not had to work on that problem in a VT52, but I do have two 52s and prints, and manuals (and you can have them too, they all came from the internet). I have fixed other problems though.

I am pretty sure you will not find any spare boards anywhere. You will have to do component level troubleshooting.

Lou

Andy_Burn
October 17th, 2010, 06:46 AM
Lou,
Thanks for the reply.
The Character Generator board originally worked, but gave occasional garbage on screen. A reset did the trick for a short while.
Now, when I type with the Decscope set to local echo, the cursor moves, but no characters. Same with the board totally removed. I cleaned all pins, removed the optional printer interface board and plugged in direct. Same deal, no characters. I will look closer at all the soldering to see if theres a dry joint.
Now then, the EIA board works after some pins were replaced. When I type, I get characters on a linux terminal and the cursor on the Decscope moves when I type on the other machine so it looks like everything else is good.
I have prints and manuals too in pdf, I'll look harder to see if I have schematics which I will have to learn to read. Its going to be a challenge, since its not my day job. It will keep me out of the bar at least :)

Thanks again, Andy

dave_m
October 17th, 2010, 08:32 AM
When I type, I get characters on a linux terminal and the cursor on the Decscope moves when I type on the other machine so it looks like everything else is good.

I don't know anything about the VT-52, but from the symptoms I would look at the video RAM. If the VT-52 has an 80 column by 25 line screen, then somewhere there is a 2K RAM to hold the 2000 bytes of information. This is where I would start to look. If you do not have a scope, start by re-seating the RAM chips if possible or replacing them.

edit: This assumes the problem is in the digital area and not in the CRT area. If you turn up the brightness, do you see any horizontal or diagonal lines? Is there a glow in the CRT tube? Be careful poking around the CRT, there are very high voltages.

Lou - N2MIY
October 17th, 2010, 05:08 PM
Andy,

What do you mean by "a reset did the trick"? What did you reset, did you reseat the cards? Also, you say you fixed the EIA card by replacing some pins. That is odd, because the pins are pretty robust. If somehow yours were all corroded, plugging and unplugging the board a few times should have scraped enough crap off to make a connection. Usually, the 1488 on that board is what gets killed (by someone connecting two DTE devices head-to-head.)

Was this terminal stored in a corrosive environment?

Lou

Andy_Burn
October 17th, 2010, 06:05 PM
Lou,
" a reset did the trick" - Typo! reseat..... yes I originally removed the card, cleaned the pins to be sure, and put it back in. The last time I remember it working, I got a screen full of garbage after it had been running for a while (while testing command codes on the keyboard), and then it stopped working - cursor only after a power cycle.
The EIA card was badly mistreated. The whole machine was forcibly removed from its original home and the card was pulled from its mount and the pins twisted off on one side. I only found this out when I opened her up. Three paper clips, a new serial port and a soldering iron later, and its good. I used a linux laptop terminal to check with a null modem cable.

Inside, theres no major corrosion I can see. Just white gunk on some of the pins, not sure what it is. I plan to remove all the boards and check the seats/soldering. It may not be the board, but the connector on the board perhaps. I just need some spare time to do it before I start component level investigation. She's worth saving :)

There are also three chips hard soldered onto the Rev A Character Generator board 2* DEC 7404N (7550) and 1 white eprom I think, marked El? 7627, 2608 1, CN0030 and 23002B(8?)400.
I'll post some pictures later when I find my camera.

Many Thanks,
Andy

cosam
October 18th, 2010, 01:21 AM
Lou,
Inside, theres no major corrosion I can see. Just white gunk on some of the pins, not sure what it is. I plan to remove all the boards and check the seats/soldering. It may not be the board, but the connector on the board perhaps. I just need some spare time to do it before I start component level investigation. She's worth saving :)

If that white stuff is dry and powdery, it sounds like a bit of oxidation, which is one of the top causes for flaky electronics. Sometimes you can just wipe it off (maybe with a touch of alcohol or similar). A bit of very fine sandpaper will take off any really stubborn stuff. If it's really severe you could replace the connectors. If they're the same as the ones in your photos they're still readily available.



There are also three chips hard soldered onto the Rev A Character Generator board 2* DEC 7404N (7550) and 1 white eprom I think, marked El? 7627, 2608 1, CN0030 and 23002B(8?)400.

Yeah, I think you've got a ROM there which likely contains the actual character bitmap patterns, i.e. which pixels are on or off for each character. The 7404N is a simple inverter and easy enough to test on the bench if the connectors turn out not to be the problem.

billdeg
October 18th, 2010, 08:59 AM
Hi - Make sure S1 and S2 are set correctly, it may be as simple as a dial out of position.
Bill

Andy_Burn
October 18th, 2010, 04:24 PM
Yes Bill,
I've tried setting it to local echo 9600baud (and all the rest) - no joy. This was my first thought.
I'll be busy working for the next week-10 days. Then I'll have a week to strip her right down and put her back together, and drink beer. Heh, the cure for all ailments!
I'll post again when I have more information.
Thanks for the advice guys, it's appreciated :)
Andy