Discharging a VT100 CRT

[Julian Wolfe]

The whole situation makes a lot of sense now too.  I wonder if the cause of
this wasn't a weak cap.  They do "dry out" after sitting for a long time
like that anyway, which I believe this one had.

Usually with old monitors, I try to leave them on if possible, and just turn
the brightness and/or contrast down so they don't burn in.  That's what I've
done with the VT220 I have and it seems to be much happier as a result...and
bonus with the VT220, because it has its own built in screen saver.

I wish everything I owned was built like this.

-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Allison
Sent: Wednesday, November 09, 2005 5:23 PM
To: cctalk at classiccmp.org
Subject: RE: Discharging a VT100 CRT

>
>Subject: RE: Discharging a VT100 CRT
>   From: "Julian Wolfe" <fireflyst at earthlink.net>
>   Date: Wed, 09 Nov 2005 16:51:13 -0600
>     To: "'General Discussion: On-Topic and Off-Topic Posts'"
<cctalk at classiccmp.org>
>
>Yep, you know your stuff Allison.  C102 and 103 are both 50v caps according
>to the schematic.  Should I replace them with equivalent electrolytic 100v
>caps?  I'm not an electrician, but I've built working stuff with a
soldering
>iron when given the right parts.

Perfectly reasonable thing to do.  Look for other stressed parts while
there.

>If I can build this into a better VT100 and not have to open it again for a
>good long time, it'll be worth my time and trouble now.

I've done a few of them and never seen them fail.  Considering that the 
original lasted typically 2 years at 100% power on you can see it should
last.
FYI: the original problem was a 50V part in a circuit that had 70+ Volt
spikes.
Part was simply mis spec'ed.  I remember seeing 5 refrigerator (large ones)
boxes full of failed boards due to that error.


Allison



>-----Original Message-----
>From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
>On Behalf Of Allison
>Sent: Wednesday, November 09, 2005 1:21 PM
>To: cctalk at classiccmp.org
>Subject: RE: Discharging a VT100 CRT
>
>>
>>Subject: RE: Discharging a VT100 CRT
>>   From: "Julian Wolfe" <fireflyst at earthlink.net>
>>   Date: Wed, 09 Nov 2005 12:43:29 -0600
>>     To: "'General Discussion: On-Topic and Off-Topic Posts'"
><cctalk at classiccmp.org>
>>
>>Well, you were right.  Something did blow on the VT100 video board, and in
>a
>>bad way.  It looks very much like it was that diode you speak of, cause
>>there's a huge burnmark on the board at the contact points of the diode.
>If
>>that's not an "I died" indicator, I don't know what is.
>>
>>Can anyone tell me what diode to buy to replace it?  I'm thinking of
>getting
>>a replacement video board, and then replacing the diode on that, just in
>>case it took something else with it.  The whole "y" trace has a brownmark
>>around it that the diode was connected to, and that has a big cap next to
>>it.
>
>Check the diode first as often they survive.  If it didn't it may be 
>the 1A 1000V device (1n4007 will work) at CR102.   The cap is correct value
>(22uf) but about half the required working voltage so it shorts after a 
>long time. I forget if it was C102 or c103 and the working voltage should 
>be at least 100V but not more than 160V.   or so memory says.
>
>Allison
>
>
>
>