Tektronix 475 Triggering

Triggering

Unfortunately the trigger on my scope still doesn't work. There is a trigger delay knob you can use to delay the sweep after the trigger. If I set the scope for auto triggering (so it sweeps anyway even when it doesn't have a trigger) and then use the delay knob I can sort-of kind-of get the waveform to stay still. And it looks really good when I do! I like this scope!

I really want to fix this scope now!

Ok so I pick a mid-point which is just after Q526, I set the trigger source to channel 1 and connect a signal to the scope. I can see the signal in auto mode but in normal mode I can't get it to trigger. There is a signal there but it is very weak (less than 10mV). So then I pick a spot in front of the two JFETs and sure enough I have a good signal there. Ok we have a neighbourhood!

The base of Q526 has a signal so then I turn it all off, pull Q526 and put it in the transistor tester. The transistor tester tells me it is a resistor. I check the BE and BC junctions with the diode range on my DMM and they look wrong.

Q526 is a 2N4258 PNP switching transistor. Not really sure why they picked this. I have some 2N2907s on hand as I was going to replace the one in the channel switching logic. I decided to give it a go and start up the scope again. Ok! now I have a signal at Pin 3 of the U520! But still no trigger :(

When I hit trace view before I got nothing but now I do see the input waveform but it is pretty dirty and big. The voltage and (as far as I can tell) the waveform matches the pictures in the scan of the service manual I have. So why no trigger.

The manual says I should see the trigger signal on pin 8 and 9 but I am not seeing anything. The voltages are a bit off (13.4 instead of 14.6). The voltages on some of the other pins are a bit off too.

As luck has it the scope has two trigger circuits and they are pretty much identical. Oddly the B trigger seems to have the same issue with the source follower circuit so I replace that transistor too. Strangely the output of that chip is also silent even after doing this. I decided I will swap the two chips and see what happens but this has no effect - they are both still not working.

Someone on the Tektronix forum asked what the bias on the input to U520 was like but that looked Ok.

I tried pulling one of the transistors after U520 to see if there was something loading down the output but this had no effect.

When I scope the tunnel diodes I can see the auto trigger pulse happening so I think they must be fine.

I'm a bit stuck. I really suspect these custom Tektronix paraphase amplifier chips (U520 and U720) are stuffed. Any ideas would be welcome.