On Sat, Mar 24, 2018 at 03:45 pm, Jeff Urban wrote:
If nothing else I found out a few things. First of all for some reason they
use the shield of the coax cable for the blanking signal. The reason for that
is a mystery. Why they reverse the connection on the left and right sides just
might be so that if someone uses verticals in both sides like for X-Y that
would prevent interference between the two channel switching circuits, though
I am not sure why that would be undesirable.
(BTW you mean the sync signal, not the blanking signal.)
In the 2B67 the Sync levels at pin 4 are +110 V and +125 V. Suppose you used a 2B67 in the left slot and a 3B1 in the right slot while pins 4 were connected in the mainframe ...! Note that in both the 3A1 and 3B1 their internal coax shields are left open at the interface side.
There is the combo 3S76/3T77 which uses the pins 3 and 4 exactly the opposite way. Also their internal coax cables are fully wired to the connectors. Apparently Tek has thought about wrong combinations of plugins.
I decided to put some bias on it to compensate so I put a 1.8 K resistor
across C 263. I think it came out to around 8 mA or whatever and that it
should result in about 300 mV across R 263. (that is memory and could be
wrong)
This works the wrong way, making the base of the PNP transistor even more positive w.r.t. the emitter.
BTW which type silicon transistor did you try as substitute for Q260?
