> -----Original Message-----
> From: Peter Coghlan via cctalk <[email protected]>
> Sent: 05 January 2026 12:47
> To: General Discussion: On-Topic and Off-Topic Posts
<[email protected]>
> Cc: Peter Coghlan <[email protected]>
> 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?
>
No room for a heatsink I suspect. Definitely no evidence of there ever
having been one, it is pictured more or less how I received it.
> 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.
>
> 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.
