Discharging a VT100 CRT

[Allison]

>
>Subject: RE: Discharging a VT100 CRT
>   From: "Julian Wolfe" <fireflyst at earthlink.net>
>   Date: Wed, 09 Nov 2005 18:12:10 -0600
>     To: "'General Discussion: On-Topic and Off-Topic Posts'"	<cctalk at classiccmp.org>
>
>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.

Like I said DEC made a truckload of these with a 50V cap where a 75V or 
higher cap should have been used.  Most died in the first two years.  
Those that werent used much lasted decades, but failure was enevitable
due to an undervoltage part.

>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.

There were s a few issues with those too, but they were overall good.

>I wish everything I owned was built like this.

DEC tried, often succeeded.  

Allison


>
>-----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
>>
>>
>>
>>
>
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