5 Jan
2026
5 Jan
'26
4:47 a.m.
I asked this before a few years ago but didn't come to a firm conclusion.
The two parts are shown in this picture:
https://rjarratt.wordpress.com/wp-content/uploads/2023/12/img_20231221_1123…
They are from a VT100 Monitor board. There is no schematic for my version of
the board, but a related schematic is on p58 of this schematic:
https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Feb82.….
They are labelled Q414 and CR406, but the Feb 82 revision uses a different
board and those two parts are in TO-220 packages rather than TO-3 as here..
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?
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.
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?
I have removed both of them and tested them with a DMM. Neither of them
measure as two diodes, they do not measure open circuit in any direction,
What do they measure? Are they a dead short or the same low resistance in both
directions or something else?
Bear in mind that some DMMs of recent vintage on diode settings might not be
expecting the lower forward voltage drop of a germanium diode compared to
a silicon diode.
Regards,
Peter Coghlan.