5 Jan
2026
5 Jan
'26
6:59 a.m.
-----Original Message-----
From: Peter Coghlan via cctalk <cctalk(a)classiccmp.org>
Sent: 05 January 2026 12:47
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: Peter Coghlan <cctalk(a)beyondthepale.ie>
Subject: [cctalk] Re: Identifying This Component from VT100 Monitor Board
It seems a bit odd that the same component
is labelled Q-something in one
instance and CR-something in another. Are the two TO-220 packages the
same component too?
It isn't clear in the Mar82 schematic what the diode package is, it is
listed as optional.
Could someone have fitted the B411 component in the CR406 position in
error?
This theory might be supported if the two TO-220 components on the other
board are not the same.
From what I can tell, the emitter on the CR406 is not connected. So possibly
they used it as some kind of expedient.
It also seems a bit odd that there seems to be a redundant mica washer
under CR406. Is there a missing heatsink? Is there even room for a
heatsink?
What do they measure? Are they a dead short or the
same low resistance in
both directions or something else?
Using the diode function on the DMM I am measuring Q414 as follows
b-e forward voltage is 1.3V,
e-b reverse voltage is 0.15V
b-c is 2.7V (this is open on CR406)
c-b is 0.09V (and the DMM beeps for a short on Q414)
c-e is 1.4V
e-c is 0.17V
This assumes that the package is as shown on p54 of the Mar82 PDF.
For some reason I hadn't spotted that b-c is so different between the two. I
have always suspected Q414 is bad.