Tektronix 475 - A step backwards

Ok it's been some time since I posted anything. I'm still pretty busy with work so I'm a bit limited on time to work on this. I have been working on it but only in small snatches of time and hence I've not had time to update the blog. So here it is a - a big update.

B Trigger

I looked at the B Trigger some more and compared the signals with the A trigger circuit. I tried to trace the signal through the amplifiers following the paraphase amplifier chips to see where it disappears and it does some to around Q776/Q786 but it isn't apparent why. The transistors look Ok so then I thought maybe the hold-off signal wasn't getting through to reset the tunnel diodes.

After lots of head scratching and looking I got desperate and simply swapped the tunnel diodes (I did this before but only one diode). This time I swapped both diodes from the A trigger to the B trigger and when I did the A trigger stopped working! Interestingly B trigger still didn't work but this is still progress.

I'm pretty sure the firing TD is damaged. I went to order a replacement and found they are not so easy to come by. I found this ebay vendor called Tucker that had some but the shipping was a bit over the top. Their website had the same part for a bit less shipping and not having much choice I ordered the part. I didn't see an order confirmation so I checked my emails and found the paypal notification. I noticed the part number was wrong! The part I accidentally ordered was a 152-0368-00 which is a diode for a different kind of scope (not a tunnel diode at all) - the one I wanted was 152-0386-00. Unfortunately they had already shipped it so I had to order *another* one. They said they would accept the wrong part back at least.

Calibration and Oops

Now apart from the B trigger the scope is pretty useable now. I thought if I calibrated it so the timing and voltage measurements were roughly accurate it would be fine until the part arrives. I started going through the calibration procedure and found I could do lots of it but there were a few steps I couldn't as they needed a signal insertion unit or a 200MHz signal generator (I have an accurate 20MHz Siglent unit but that's all).

This went really well - the waveforms look cleaner and now that it all lines up correctly it just works so much better.

At one point in the procedure I was adjusting a pot on the vertical output board which is inside the cavity next to the tube and (stupidly) I was using a metal screw driver (as I don't have a proper low capacitance one) and I bumped the leg of another capacitor! The screen fuzzed for a second but returned and it looked ok so I didn't worry. The next day I was looking at it and I realized the B sweep wasn't working at *all*. In Mix mode you only see the A sweep and in B Delayed you see nothing at all. Damn! I broke it!

B Sweep

I figured out I must have touched the leg of C1072 with 50V on the pot :(

My guess was this affected Q920 or Q984 in the sweep circuit. I quickly put a scope probe on these transistors to see if the sweep sawtooth was there and quickly figured out Q922 was toast. (See below)

Thankfully this was a 2N2369 which I already had from a separate repair. I replaced it and while the MIX, B DLY etc modes still didn't work, when I put the scope on this transistor I could see the sweep was running. In fact all the sweeps were running so why no B sweep? 

I thought maybe Q1086 could be bad but it is deep on the timing board which you can't get to without removing the sweep logic and trigger board. After some faffing about I went for it and removed the board. This went smoothly and I checked all the transistors I could see and all were Ok. I couldn't run it like this unfortunately.

So then I read the circuit descriptions again and realized that the delay after magic depends on this differential amp formed by Q938, Q940, etc. So I had a look at the levels on the output of the delay amp and they looked fine. I already checked the sweep sawtooth was present on Q926B's base so then what? Q940 looked ok as did Q944 an Q946 and even then these are to do with blanking.

I looked at the output of Q938 and there wasn't a signal there.The diodes looked fine. I pulled Q938 and tested both sides and they looked ok (normal bipolar transistor). I pulled Q926 and it didn't look right at all - this makes some sense. It looks like I killed it :( Again this is a special part and thankfully QService has one so I ordered it.

Fast Sweep

At this point I can't work on the B trigger, problem as I need parts. Also, I can't work on the problem where certain combinations of A and B sweep rates behave oddly as ... well I have no B sweep visible. So then what? I could park it for now but I remembered that when I was calibrating the scope the faster sweep rates seemed to 'fade in' from the left.

Slower sweep rates start at the very left but the faster one progressively start later. The photo above shows a 20MHz signal on the fastest (0.01us/div) rate.

Clearly this is a blanking problem. If I turn up the intensity I can see the start of the waveform (as well as the retrace unfortunately).

I looked at the waveform at the collector of Q698. This is the blank signal that goes out to the Z amplifier. The signal looks like a ringing mess!

If I remove the coax the signal looks Ok.

I looked at the signal that drives this on TP588 and it looks fine!

So then I attached channel 1 to the Z axis output on the back of the scope and the signal is practically not there. Even at the 20mV/div setting of the scope it is barely a bump.

So then I dug into the Z amplifier a bit more and even though the input signal and the Z axis output looks broken, the output of the amp seems to be Ok. 

Here the yellow waveform is the collector of Q1352 and the blue waveform is TP588. The signal is pretty clean and the delay between them seems minimal (20ns or so).

So I have no idea why there is a delay :(

Hmm...