I popped out the transistor pair in the downstream A trigger view circuit and the voltages didn't change, so that leaves out that circuit as causing problems.
So I did like Fabio said and pulled out the preamp transistor pair, and biased the input to the chip with two parallel 100 ohm resistors to ground. Now we're getting somewhere. The input was close to zero (don't remember now exactly) and 1 and 16 were -.755 volts. So it seems the chip works.
Ohmed the transistors but that didn't tell me anything specifically about their condition. So since the B trigger side has an identical pair of preamp transistors, I swapped those into the A trigger side. Voltages were -30 mV on the input to the chip, and -.755 at 1 and 16. Tested the trigger and now I can get a trigger! I also have the A trigger view.
The chip isn't coming out unless it were to turn out to be bad. It's very soldered complete with solder fillets.
So somehow the original preamp transistors are bad, or at least one is. I just need to source them, which so far the internet is telling me they are obsolete (of course). Any ideas anyone?
Manufacturer part number: SF50031, which comes back to 2N5245 N channel JFET, which is obsolete. It's late and I haven't tried to cross reference that yet.
As a side effect the trace seemed to have some ac coupled into it. It hasn't done that before but I'm wondering if it's because the case is off?
Thanks Fabio and everyone else!
I also own a 464 and mine also had a problem triggering... And while looking at your description I looks like deja vu to me.Hello Lorn,
On mine one of the FET transistors of the input circuitry were leaky...(it had about 200kOhms impedance between source and gate, while reverese biased. The good ones measure Infinite, or very high Megohms.
The leak developed a voltage drop on the 1M resistor at their gates and make the output voltage to have a huge offset of... wow! 70mV...(that's 1/10th of what you're getting).
70mV offset was enough to take the preamplifier IC so off that trigger would only work by adjusting the level potentiometer near its end of range and, still, after warming up, the offset would grow just a bit more and then even taking the Level potentiometer to the end was not enough to make it trigger.
On yours it seems the offset is some 0.7V, so no wonder why it doesn't trigger.
Your course of action is exact and removing the FET transistors will allow you to rule-out if the positive offset is coming from the IC, or if it's coming from the buffer stage into the IC.
It's likely, however, that if you just leave pins 2 and 3 opened, as they will get when FETs are removed, you will be fooled by false voltage readings at pins 2 and 3, due to the input pins need some biasing.
So, after you remove the FET ICs, connect a resistor of - say - 51R between pins 2.3 of the preamp IC to ground.
The impedance at the other side of this balanced input differential amplifier is roughly 50Ohm, so biasing pins 2,3 with 50Ohms will make the input perfectly balanced.
In this condition, pins 2 and 3 must read a low voltage, close to 0V. By no means it can be more than 50mV... as I told, my 464 had 70mV and that was enough to screw up the triggering.
Alternatively, you can check the FET input buffer's offset, by taking the IC out and measuring voltage directly at pins 2.3 of the IC socket...
Don't worry, taking the IC out is safe... nothing that's past of the IC will burn because the IC isn't present. (I had this same doubt back then).
Set the Trigger input to EXT, coupling to DC, and ground the Ext. input, so that you're sure that you're not feeding DC into the input buffer, and its output should be very close to 0V.
If the transistors are perfectly matched and thermally even, in theory, the DC offset of this input buffer is exactly ZERO.