I think I have got my VT100 basically working now except for the actual
video display. I think there is a problem with the transistor that drives
the flyback transformer. This is Q414 on page 58 of the Feb82 schematic on
BitSavers
(
https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Feb82.
pdf). I have removed it from the circuit and tested it with the diode tester
of my multimeter. It does not test as two diodes, indeed across
Collector-Base the multimeter beeps for a short circuit, and so I am fairly
sure it is bad,
The part is the one with the heatsink as shown in this picture:
https://rjarratt.files.wordpress.com/2023/12/img_20231221_112305.jpg. I have
found some information on it here:
https://www.web-bcs.com/transistor/tc/b0/B411.php but the Feb82 schematic
shows it is a BU407D. My video board is not exactly the same as the one in
the schematic despite having the same DEC part number, but the circuit that
drives the flyback is the same on the secondary side of T403, and includes
the optional diode CR406 in the form of another B411 transistor that is not
connected at the emitter.
I am trying to identify a replacement and could do with some help
identifying one. I can't find a full datasheet for the B411, all I can find
is this
https://www.web-bcs.com/transistor/tc/b0/B411.php, which seems to
match the part I need to replace, and I have also found one for a BU407 (not
BU407D) here
https://www.mouser.co.uk/datasheet/2/308/1/BU407_D-2310257.pdf.
I have tried to find something that meets or exceeds the voltage, current
and switching time specs. I have found a couple of possible replacements and
would welcome opinions on their suitability:
https://www.farnell.com/datasheets/2861437.pdf
and
https://www.farnell.com/datasheets/3033460.pdf (assuming that the switching
times are a typo and they are in microseconds rather than seconds)
Do these seem like suitable replacements?
Thanks
Rob