Tektronix 475 - Doh!!

Short Trace

In the last post I was still trying to figure out why the trace got short when the scope was running in the fastest two or three sweep rates.

In the end I asked again on the Tektronix forum and once they got their head around what the problem was the answer was quite interesting. Essentially what I was seeing was normal!

I thought it was a bit of an ask to swing 100V or more within a few nanoseconds to unblank the display and sure enough it *is* too much of an ask! Apparently this is normal for fast scopes at high sweep rates - there was in fact (is in fact) nothing wrong with the blanking circuit. Someone commented on the fact that the delay line is 75ns which is intended to move the start of a fast pulse far enough into the display so you can see it after the scope has unblanked!

Well I'm learning! It would have been useful to know this however.

B Trigger

In the mean time the tunnel diode turned up so I turned my attention back to the B Trigger. I installed the new diode but because of the damage I did to the differential amplifier when trying to calibrate the scope, the only way I can tell it is working is by looking at the waveform on the diode.

Unfortunately the diode didn't seem to change things that much. The signal on the TD was pretty much a flat line no matter what I did with the trigger slope or level control. The signal still seemed to get lost somewhere around the second pair of amplifiers.

I assumed there was a capacitor gone or a resistor gone high-impedance but nothing I tried was conclusive. Out of desperation I tested the two TD driver amplifier transistors (Q776 and Q786). Weirdly it turned out Q776 was bad! How did I miss that! Turns out it is the same as the one used in the unity gain amp so I had spares. Changing this transistor now meant I do get a small signal on the tunnel diode when I adjust the trigger level but still nothing like the change you see on the A trigger diode.

The more I thought about it, the more I thought that this is not going to work without the hold-off signal coming back from the end of sweep. The trouble is I broke the B sweep before when trying to calibrate the scope and am still waiting on parts.

B Sweep

A couple of days later the JFET transistor pair I ordered turned up. I carefully organised the six leads on the thing and replaced the broken part. I turned the scope on but.. well nothing - no B sweep but now no A sweep either. If I remove the dual FET A sweep works again.

I tried fiddling with the time delay knob and measuring the voltage on the output of the delay pick-off but this looked Ok. I plugged the FET back in, turned it on and noticed that the graticule illumination was dim and when I moved the delay knob it changed. Hmm

There is a transistor configured as a current source for the differential amplifier formed by the dual FET and the dual bipolar transistors above it (Q928). I pulled this out and sure enough it was broken - a dead short in fact. I replaced it and now I have B sweep!

I switch the B trigger mode from 'starts after delay' to trigger on Ch 1 and hey this works! If I fiddle with the trigger level, the B trace moves!

Combinations of Sweep Rates

So the final problem is that some sweep rates don't seem to work. For example, in MIX mode setting the B sweep rate to 50,20,10,5 or 2 us per div doesn't work - the A sweep rate just continues through to the other of the delay point. If you select B Delayed while in these speeds you get no sweep at all.

The odd thing about this is that the B sweep rate is what is used to generate the A sweep in A mode and *all* the sweep rates work correctly in A mode.

Some A sweep rates (in MIX or A INTEN mode) don't work either, The trace will start really late or you get multiple traces over each other. Adjusting the trigger delay cleans this up however which makes me think the circuit for cutting off the B sweep at the end of the A sweep isn't working but I haven't found any fault in that yet.

It doesn't make sense in terms of the switches in the B timing circuit.

This one has me stumped again...  Back to the bench...