Adding Z axis input to a BK 2120B oscilloscope

[smbaker]

I want to turn my oscilloscope into a Vectrex game. Like usual I blundered forward and built myself a scopetrex, without bothering to check to see if my oscilloscope had a Z-axis input. It does not!

So, next step, I want to see if I can modify my oscilloscope, a BK Precision 2120B, to add Z-axis input. I think a binary input (all-on / all-off) is probably sufficient. The scopetrex just needs to be able to blank the electron beam while moving between objects in the display. I found the schematic for a BK 2160, which does support Z-axis input, and I found the schematic for my BK 2120B, which does not. Schematic snippets are attached below. My scope lacks the Z-axis circuit, R801, R804, D807, Q802.

So someone please vet me on this approach. All I need to do is add a PNP transistor, base resistor, and a pull up, and I ought to be good. The choice of transistor probably isn't even all that critical -- just something that can handle 24V at about 10mA.

Transistors I have easily laying around are the 2N3906 and MPSA92. Either ought to do?

Scott

[smbaker]

Opened the case and realized I wasn't even looking at the right manual. Attached is the right picture for the 2120-B.

Two interesting things here, First is a header marked "Z-MOD In" that's not connected to anything. I'm not sure this does what I want though. While it would appear to be Z-Modulation, it's not connected to a transistor driver, and it's AC-coupled through a 4700pF capacitor.

The other interesting thing is P650, an unpopulated header marked "BF In". This does run over and (via a 4148 diode) hook up to the base of Q801, which looks like it might blank the beam. Maybe that's where I want to play.

Scott

[james_s]

Z-mod is feeding G1 so that will certainly modulate the brightness, however as you have observed it is AC coupled so you won't be able to blank it for any length of time.

[smbaker]

I managed to come up with a working solution. I added a 2N3904 NPN transistor. Emitter to ground, 1K resistor and 33K pulldown on the base, and 100 ohm resistor from the collector to the junction of R802/R804 in the 2120B schematic. This successfully blanks the beam, though it is a bit of a hair trigger and I have to ensure the scopetrex's brightness is adjusted "just right", else I either get a display that is too dim, or a display that doesn't blank at all.

Scott

[james_s]

You will probably find that it drifts all over the place if it is that sensitive. The gain of a transistor varies with temperature, but you're on the right track at least.