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V T220 Resurrection

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wallc

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Jan 16, 2025
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While cleaning out a storage space, I uncovered a DEC VT220 terminal which I would like to get working. I see several comments about VT220s on this forum and was hoping I could get some advice. If this is not the appropriate place to ask, please forgive me.

The terminal has not been powered on in years and was not stored in a conditioned space. I would like to disconnect the power supply and slowly ramped up the input voltage to check things out, but I don't see a way to disconnect the power supply which is not an analog supply. There are two boards. One DEC called the analog board which contains the power supply and video display circuitry. The other called the digital board contains the logic. I have removed both boards, cleaned everything and inspected the capacitors. I don't see any capacitors leaking. I thought about replacing at least the electrolytic capacitors but haven't decided to do that yet. I have looked at several schematic diagrams and DEC manuals on the internet but unfortunately the ones I have seen are all the same but do not agree with the power supply portion of my terminal.

I would appreciate any suggestions on how best to proceed and suggestions for other places to look for diagram that matches my unit.

Thanks for any advice provided.
 
Sounds like you have the VT220CR with the large transformer mounted on the chassis - it's a low voltage switcher, so if it has a mains input filter as part of the IEC plug, I usually replace those with a plain connector - but you can try it as is - I suggest running it on reduced voltage or via a dull bulb test for a while for the sake of the 10,000oF main smoothing cap and if all looks OK, proceed to mains.

I had one with the 5V PSU adjustment pot going open and resetting randomly - but all fairly standard apart from that.

The -CR drawings are here:


Robin
 
During the last years i got my hands on circa about a dozen vt220 of different models in conditions from bad to good - some have even been stored outside in those metal storage pallets and collected lots of dust and humidity.
I always cleaned them from the outside and then just turned them on, nothing ever happened with some kinda fireworks.
The worked or had the usual issues like no heating or vertical or horizontal amplifier issues, some also powersupply issues.

So, if you wanna be on the safe side do the bulb thing, otherwise just turn it on with mains directly connected and let yourself surprise.
If you have one, you can use an isolating transformer.

Have fun:-)
 
Thanks. I cleaned it outside and inside. I put it back together and have powered it on at half voltage using a variable ac supply. No smoke so far!
 
No smoke but when the VT220 powers on and does self-test I get "KEYBOARD ERROR 4". I tried 3 different keyboards and cables. I have a second VT220 from the storage building so I tried it as well with all 3 keyboards and get the same results. One keyboard is LK201-AA, the second is LK201-BA, and the third is LK201 but I can't read the suffix. Any thoughts? I don't know what "KEYBOARD ERROR 4" signifies.
 
Do you have another keyboard ? LK401 also will do it.
After my experience the keyboards are defective.
Chances are not that high that both terminals are broken.
Did you get both from the same source and have you been told they are working, or in unknown condition ?
 
wallc said:
No smoke but when the VT220 powers on and does self-test I get "KEYBOARD ERROR 4". I tried 3 different keyboards and cables. I have a second VT220 from the storage building so I tried it as well with all 3 keyboards and get the same results. One keyboard is LK201-AA, the second is LK201-BA, and the third is LK201 but I can't read the suffix. Any thoughts? I don't know what "KEYBOARD ERROR 4" signifies.
Click to expand...
I no longer have any VT220s here (although I did find a NIB VT220 documentation set in its custom slipcase the other day), but I have a gaggle of VT420s and a bunch of keyboards they don't like, from the LK201 all the way to the newer LK4xx ones. What I've discovered with them is that they can all trigger the keyboard error message which can be cured by wiggling the handset-sized RJ connector in the terminal's jack. With the LK201 it is a lot easier - it will emit anything from a cluck to a beep when correctly plugged in. I don't know if it is the plug on the end of the keyboard cable or the jack on the terminal mainboard - I haven't had time to check. I'll cut the plug off and put a new one on, since that's a lot easier than changing the jack in the terminal.
 
Thanks Terri. I tried 6 keyboards and 3 VT220s. All the VTs behave the same. 4 keyboards give Error 4 which I notice is the same error I get if no keyboard is attached. One keyboard passes the self test but about 1/4 of the keys don't work. The sixth keyboard passes self test and only one key doesn't work. I am going to see if I can clean the nonworking key on that one. I have another couple of keyboards to clean and try. I was becoming suspicious of the keyoard cables/connectors and will try jiggling them as you suggest.
 
An update. I tried 8 keyboards. 4 give the "Error 4" message. 2 pass self-test but have keys that don't work and 2 seem to be okay. I have 2 more keyboards I can try but one is marked as "Bad" and the other has missing key caps which probably means it was bad and scavenged. All 3 of the VT220s I tested seem to be okay. There are 2 more terminals I can try but I am happy with 2 working terminals and keyboards. Thanks for all the advice.
 
Well, as you can get a keyboard connected and it works, you can say that the keyboard input circuitry in the VT220 is OK.
I have always thought that you can use a variac to slowly ramp up the "mains" voltage, but that is only safe to do in case the power supply is a linear type.
If the power supply is a switcher, the ramp-up method is not advisable. a switcher will try to generate the required output voltage. Power equals the product of (input) voltage and (input) current. Depending on the load, if the input voltage is lower (ramping up), it will result in a higher input current.
If the voltage is low-ish, and there is a "serious" load, the input current could be high enough to cause damage. To be on the safe side, the load should not draw too much current, but that might interfere with the minimum current required to make the switcher work properly.

I might be wrong (and like to hear that), but AFAIK, you should not power a switcher with a ramp-up voltage. You have to run it with the specified input voltage (and keep fingers crossed at a safe distance :whistle:.
Of course, you can take measures to keep the input current on the low side, for example by adding a series light bulb.
 
Thank you, Henk, for the information. Not knowing a lot about switching power supplies I had wondered if your explanation might not be the case, so I watched the current draw off and on while I ramped up the voltage. I did see a current increase at one point from .5 Amps to about .75 Amps that dropped back down as I increased the voltage about 10 Volts. A light bulb is probably a better idea. I'll use that next time. I noticed that once the VT successfully powered up, I could reduce the input voltage from 120 volts to 80 volts or less and it would still operate.

You live and learn or ask for help!
 
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