> Rob, if you were to put up decent photos (better lighting) of both the
> component side and solder side of the board, we out here may be able to
> double-check some of the issues being raised rather than having to speculate.
> If the B411s are still removed (IIRC, you mentioned earlier you had removed
> them for testing), all the better.=
So many great replies here, thank you to all. I will try to answer everything
in one place.
First of all, the terminal has never fully worked since I have had it.
Initially I couldn't even get a sensible display using the external video
connector, but I have since repaired the Basic Video board and now it works on
the external video connector. As requested, I have taken additional pictures of
the board.
Pictures here:
https://rjarratt.wordpress.com/wp-content/uploads/2026/02/img_20260202_224335815.zip
Note that I replaced some of the electrolytics (I know Tony doesn't agree with
this and perhaps I should have waited before doing this). Also, some of the
components are partially lifted to assist with diagnosis, this was a while ago
when Tony gave me a few suggestions. Finally, one of the yoke wires (brown) is
also broken now, I am not sure when this happened. I am sure it wasn't broken
originally and it may have failed during all my probing and checking for the
schematic. However, I believe it drives the heater and this may explain why I
could never get a glow from the neck of the tube.
Peter asked a number of questions:
- The body of Q414 is connected to HORIZ GND. No signs of modification that I
can see, but I have desoldered it now and I guess any evidence may have gone. I
have taken pictures of the area now as above.
- Certainly the printset shows Q414 as NPN. I am not sure if this "B411" is
really PNP or not. I cannot get sensible measurements with the DMM diode tester
out of either of the two. Although the one in the Q414 position does show open
circuit on the B-C junction when I apply a positive voltage B-C and 0.1V when I
reverse the probes. This at least would suggest it is PNP.
- I cannot see how a BU407D could have been fitted here without it looking very
odd, the footprint on the board is definitely TO3. I haven't got around to
thinking how I might install a BU407D, although I have bought three now ready
to try if needed.
- Regarding the remarks about the CR406 and the mica washer. The footprint is
definitely TO3 as you should be able to see in the photos, were there TO3
diodes?
- Yes you are right, I did mean B411 and not BU411! Sorry just getting confused.
To answer Tony's questions:
- You are right, J3 is the BASE, I confused the terminology and I will correct
the schematic.
- You are also right about Q101, I got it connected the wrong way round, again
corrected.
New partial schematic here:
https://rjarratt.wordpress.com/wp-content/uploads/2026/02/5414131-partial-schematic-1.pdf
Full KiCad project here: https://github.com/rjarratt/Schematics/tree/main/VT100
So many of these questions would be answered if there was another one of these
boards to compare against. Someone must have the same board somewhere
