VT340 works great in colour, miserable in monochrome...

[Roe]

I found a seriously battle scarred VT340 on ebay for fairly cheap, and it arrived today. The case is a mess, but the thing works almost perfectly, bright readable characters and graphics, but only in colour.

In monochrome mode -- which happens when you hit SETUP (F3), the text is so dim I can't take a picture of it. The only way I could get through the setup process was in a completely dark room.

Can anyone give me a hint before I dig out the oscope?

 

[gslick]

Did you try resetting to factory defaults in setup? My VT340 arrived with some altered color schemes saved as the active setup that looked weird. Things looked better after resetting everything to factory defaults in setup. Don't know if someone could have changed the setup settings in such a way to cause what you are describing.

 

[Roe]

Yes, the factory reset didn't make any difference. Is there a different video amp for monochrome mode in this beast?

 

[gewtnet]

My VT340 looks the same in both monochrome AND colour...there's arcing, smoke, and a blank screen in both modes.

I didn't even know the 340 HAD a monochrome mode. You could check http://bitsavers.trailing-edge.com/pdf/dec/terminal/MP02412_VT340_EngrDrws_Feb87.pdf

 

[gewtnet]

It occurs to me that it could be using different parts of the CRT to display the monochrome signal...could also be different brightness/contrast potentiometers.

How old is your particular model? Read over the maintenance guide to hunt around for adjustments inside...failing that it COULD be an aging electron gun.