@Slyka Would you be interested to know why that's connected like that? We've seen similar stuff before. -F
@Felthry Yeah, I'd be very interested!
@Slyka so, the one thing that comes to mind first is that, until relatively recently, the junctions of common BJTs were better zener diodes than cheaply available zener diodes. They aren't designed to as tight a tolerance, but they have low zener impedance (i.e. very steep I-V curve). Though using them that way, you'd more likely leave either the collector or the emitter floating, not tie it to the base.
-F
@Slyka basically, on the assumption that this isn't a mistake, they're bound to be relying on some kind of breakdown behaviour of the device. This is the kind of thing that only the least or most experienced engineers would generally do, nowhere in between
there is also the possibility that it's a mistake, which is entirely reasonable. staying within the designed operating conditions of that transistor, it will do nothing significant
-F
@Felthry Ohh, thanks so much for all the info and ideas!
I've had a similar suspicion that it might have something to do with being able to turn off V23 quicker, but I'm not sure why they wouldn't have just used a diode. They use them everywhere else, so it doesn't seem like an attempt to simplify the BOM.
According to the schematic X1/3 is the "Pre-scan impulse" so something timing/speed related would make a lot of sense here. But then they're only pulling up that output (X2/13) pretty weakly…
I'm gonna sit down tomorrow and actually scope out the waveforms and see if I can learn something from that and also if it isn't just a mistake in the drawing.
Thanks so much again for your help ^-^
@Slyka bear in mind that bipolar devices in general (both bjts and pn diodes) are much quicker to turn on than to turn off! you need to get all the free carriers out of the depletion region to, well, return it to being a depletion region. So a harder turn-off than turn-on is common
-F
@Slyka out of curiosity, what' sthe schematic excerpted from?
-F
@Felthry It's from an old vidicon camera I'm currently fixing, specifically the high voltage section. It's got all kinds of weirdness in it that I'm trying to figure out
@Slyka ooh, high voltage stuff is weird
that makes me think potentially a little more in the direction of zener? because the collector-base junction would have a pretty high breakdown voltage and high-voltage zeners aren't as readily available even today
though the fast turn-off is also a potential thing, i don't know what it's being used for
if you'd like thoughts on any other bits of weirdness feel free to ask! we love this stuff
-F
@Felthry I'm not actually sure if that section (all the way in the top right) is actually involved in any high voltage stuff. It just seems to either connect the cathode to ground or the unregulated 19V supply based on the input from another board which I assume is logic level or at least close to it.
The other main weirdness I'm trying to work out is the high voltage regulation and reference voltage, which for some reason uses the 6.3V heater voltage both for powering an opamp (N3) and one of the inputs to it, which seems a little strange. And since the voltage regulator (N1) is bypassed with a 68Ω resistor (to put less load on it I assume) as soon as you unplug the tube the heater voltage shoots up which leads to a way too high reference voltage being supplied and the HV section going basically unregulated. The reason I'm concerned about that is that I want to add a slow start to the heater cause my tube suffers from really bad heater flash and I don't want it to burn out prematurely, but now I'm worried that reducing the current draw at startup could throw off the HV regulation…
I really appreciate hearing your input on this! ^-^
@Slyka So using the heater voltage as both power and an input doesn't sound odd to us--after all, it's not uncommon to use ground as both power and an input, right?
A resistor across a voltage regulator is... kind of weird. what time period is this from? The schematic says 1997 but this looks like a much older design that's been carried forward, and maybe had the schematic redrawn in 97
we'll need to find some time to sit down and see what's going on here, maybe simulate it
-F
@Felthry the design is from the mid to late 80s, but its from East Germany, and things they built were usually kinda old school even for that time.
I really appreciate your help with this but please don't feel obligated to put a bunch of time into this. Though if you really want to I can also post links to some slightly higher quality PDFs of the full schematics.
@Slyka oh yes! another possibility is that they're using the transistor in reverse-active mode as a diode to speed turn-off of V23. this would be strange, but it would work.
-F