Tek 475 chop problem

[Zenith]

I have a problem with chop on a Tek 475.

The backstory is that I've had a Tek 475 for about 14 years and used it a fair bit over that time. It's played up twice and been fixed, but it occurred it would be good to have a parts mule.

Last summer I bought a parts mule, but it was so good it seemed a shame to use it for that. It worked with a few problems, such as the timebase vernier knob had had a blow and the shaft was bent, and the A trigger pot had also had a blow.

A couple of months later I came across a proper parts mule for a parts mule price. No display, but it was complete. I decided to fix the second Tek 475.

I sorted out the timebase vernier problem and changed the trigger pot. I checked the supply lines and set the 50V line to 50V within a few mV. All the supply lines were within the spec in the manual. A heatsink on a preamp hybrid was missing and was replaced with one from the parts mule.

There were problems with the vertical sensitivity and AC/GND/DC switch, sorted out by the careful application of Deoxit. Some of the pots weren't smooth, but that went away with use.

Not much left to do but check and adjust the calibration.

Then I noticed a peculiar problem. On channel 1, channel 2 and alt, everything works as expected. On chop there's interference between the two channels.

Set up a square wave trace from the calibrator on one channel and set the other to DC and on alt there are two independent traces. On chop there's also a smaller square wave on what should be a straight line. Set both channels to GND and on alt each can be moved without affecting the other. On chop, move one channel up a few divisions, and the other moves down a good part of a division.

DSCN2879.JPG

There's no sign of anything like that with the good scope.

I investigated the channel switching circuitry.

The two channels are switched by the hybrid U370.

Pin 1 hi, Pin 12 lo, Pin13 hi, Pin16 lo = Channel 1 selected.
Pin 1 lo, Pin 12 hi, Pin13 lo, Pin16 hi = Channel 2 selected.
Pin 1 hi, Pin 12 hi, Pin13 lo, Pin16 lo = Add channels 1 & 2.

On Alt, alternate sweeps of the timebase select ch1 and ch2 alternately. On Chop the scope switches between channels at about 1MHz.


The waveforms of the defective scope are the same as on the good scope, but the chop frequency is a little lower at 1.8MHz. Voltages and resistor values have been checked and look spot on.

Changing the channel switching hybrid makes no difference and the one from the bad scope works in the good scope.

For those following along here's a jpg of the vertical channel switching circuit.

chop.jpg

Removing the 7400 in the chop oscillator and injecting signals from a function generator does not fix the problem. Signals were injected at pin 11 and pin 6 of the 7400. Altering the phase difference between the two signals made the problem slightly better or slightly worse. Altering the frequency between 1.6MHz and 4MHz makes no difference, which suggests the frequency is not critical.

The chop blank waveform looks the same between the good and bad scope.

I'm wondering if this might be a problem with the z axis modulation circuitry.

Has anyone seen anything like this before?

[MED6753]

I have not seen that problem so I can't offer any advice except as you mentioned check the Z Axis area.

I'm about to dive into a 475A that has no alternate mode. I've narrowed it down to the alternate switch itself but getting to it will involve removal of the Vertical board and most likely both channel attenuator assemblies. Not going to be much fun.

[Zenith]

Thanks.

With a lot of these things, even with a mainly good parts mule, the dismantling and reassembly is a daunting prospect. Hence very little temptation to swap boards between a working 475, a mostly working 475, and a parts mule. Swapping out the channel switching hybrid was also daunting prospect before I did it, but after about a dozen swaps, I whip 'em out and whip 'em back in, with no problems with carefully shaped leads having to be re-shaped.

I suppose I could forget the chop function, because I rarely use it and if I wanted to investigate something that needed it, I have scopes other than a Tek 475 better suited. The same is true for Z axis modulation. I'm glad it's there on many of my scopes, but I've rarely used it. I suppose it fits between flyback suppression, which I've always been glad to have, and a sort of throwaway feature.

Nonetheless, it's intensely irritating I can't solve this problem.

[mnementh]

I assume you've decided that the grief factor of asking on the Tek Yahoo group exceeds your tolerance level?

mnem
I like coconuts; you can break them open and they smell like ladies lying in the sun...

[Zenith]

I'd be reluctant to bother the exalted souls over there with so piffling a problem, concerning a humble instrument like this.

I searched on the web and I couldn't find anything like it.

I've become fed up with it for now. I checked the Z axis input and it seems to work. I'll have another look at it and investigate the z axis modulation further, in a few days time. It's probably a dry joint or a resistor gone high or something. Part of the fun is tracking the problem down and learning something along the way, although it doesn't seem like fun when you hit a brick wall.

[tggzzz]

Oh, I think the TekScopes mob would find this interesting - certainly more interesting that the bog-standard stuff they patiently answer

I would

• see if I can find what's inside U370. Some of the schematics are in the "Xref" service manual on tekwiki. I'll bet the internals are the long-tailed-pair current amplifiers/switches found elsewhere in the signal chain, with the four inputs 1/16, 12/13 connected to the bases
• if that's the case, then the inputs should be equal and opposite; if not then unequal/imperfect switching might occur
• so, I'd use my other scope to look at those voltages on u370 and all four flip flop outputs
• if there's significant imbalance, then I'd suspect the resistors used to shift the voltages (from TTL outputs) have shifted. The values look pretty specific, e.g. 374,634 (and 150, 47) ohms
• I'd also give that area of the board a good clean

Fun, innit. Certainly better than crosswords or so-bloody-doku.

[Zenith]

I found the schematic for U370 and it's as you say. I was fortunate enough to have a couple of spares and so I could do swaps. There appeared to be no difference between U370s in the good scope or the problem scope. The area around U370 was thoroughly cleaned with IPA. Contacts and leads were cleaned with contact cleaner.

I checked the voltage levels driving U370 using a DSO and there appeared to be little or no difference, pin to pin or between scopes. It occurred that for a short time U370 was in an undocumented state - that is with its logic levels not as in the manual. I'm pretty sure neither the manual nor U370 documentation say what the voltages corresponding to the logic levels are.

I checked resistors either by removing chips to make them open ended, or by measuring the supply voltage and doing the calculation for the voltage divider. Nothing unexpected for resistors of the stated precision. The outputs of the 7474 were examined with a DSO and compared with those from the good scope. Other 7474s were tried including LS, ALS with no difference. The original 7474 was replaced as the designers seemed to rely on the specific properties of logic chips and there was no point in possibly introducing an unknown variable. e.g. The chop oscillator uses a 7400 and relies on its input bias current for its action. It won't work with LS, ALS, F, or any other variant and different specimens of 7400 produce different frequencies.

It did occur that since the good scope has a chop frequency of just over 2MHz, and the problem scope 1.8MHz, that could have caused timing problems. The chop oscillator chip was removed and its outputs substituted through the magic of a DDS function generator. Altering the frequency from about 1.5MHz to 4MHz made no difference. Altering the phase between the two outputs, one to the 7474 and the other to chop blank made the problem slightly better or worse, but didn't make it disappear.

Comparing the waveforms from the circuit shaping the chop blank pulse between the two scopes, shows no significant difference. The resistors and voltage levels in that circuit show nothing odd either.

It could be something I've missed, but I'm inclined to look at the Z modulation next.

Yes, so many hours of amusement for such little cost. Certainly better value than going to the pictures these days.

[Zenith]

I'm about to dive into a 475A that has no alternate mode. I've narrowed it down to the alternate switch itself but getting to it will involve removal of the Vertical board and most likely both channel attenuator assemblies. Not going to be much fun.

Yes, part of the problem is the amount of careful dismantling and reassembly you have to do, and some of the parts are very delicate. Some things like the nuts holding the attenuator to the front panel are hard to access. The little plug in connectors are very easily damaged and although I've been very cautious with them, I always give a sigh of relief when they are re-plugged. There are tricks to avoiding damaging things and it's well worth checking whether anyone else has written up the job before tackling it. That way there's less chance of blundering into a pitfall.

The most similar thing to the channel switch push buttons I've done, was having to deal with the timebase push buttons. There was no problem with them, but they had to come out to deal with the damaged timebase vernier. It was fiddly but nowhere near as awkward as the channel switch bank.

Best of luck with it.

[mnementh]

I found the schematic for U370 and it's as you say. I was fortunate enough to have a couple of spares and so I could do swaps. There appeared to be no difference between U370s in the good scope or the problem scope. The area around U370 was thoroughly cleaned with IPA. Contacts and leads were cleaned with contact cleaner.

I checked the voltage levels driving U370 using a DSO and there appeared to be little or no difference, pin to pin or between scopes. It occurred that for a short time U370 was in an undocumented state - that is with its logic levels not as in the manual. I'm pretty sure neither the manual nor U370 documentation say what the voltages corresponding to the logic levels are.

I checked resistors either by removing chips to make them open ended, or by measuring the supply voltage and doing the calculation for the voltage divider. Nothing unexpected for resistors of the stated precision. The outputs of the 7474 were examined with a DSO and compared with those from the good scope. Other 7474s were tried including LS, ALS with no difference. The original 7474 was replaced as the designers seemed to rely on the specific properties of logic chips and there was no point in possibly introducing an unknown variable. e.g. The chop oscillator uses a 7400 and relies on its input bias current for its action. It won't work with LS, ALS, F, or any other variant and different specimens of 7400 produce different frequencies.

It did occur that since the good scope has a chop frequency of just over 2MHz, and the problem scope 1.8MHz, that could have caused timing problems. The chop oscillator chip was removed and its outputs substituted through the magic of a DDS function generator. Altering the frequency from about 1.5MHz to 4MHz made no difference. Altering the phase between the two outputs, one to the 7474 and the other to chop blank made the problem slightly better or worse, but didn't make it disappear.

Comparing the waveforms from the circuit shaping the chop blank pulse between the two scopes, shows no significant difference. The resistors and voltage levels in that circuit show nothing odd either.

It could be something I've missed, but I'm inclined to look at the Z modulation next.

Yes, so many hours of amusement for such little cost. Certainly better value than going to the pictures these days.

A random WAG: Maybe something dicky in a COUPLING switch? I've had one or the other coupling switch play hell with triggering and with ALT/CHOP modes on several older scopes.

mnem
I am MOOP.

[tggzzz]

The general principle with the input channel signals in the 4xx series is that they aren't voltages running "horizontally from left to right". Rather they are differential currents, and all the sections are "stacked vertically" with the input sections "lower" than the output sections. One LTP stage's collector outputs are connected to the emitter inputs of the next stage.

Hence the common mode voltages rise strictly monotonically as you go from the BNC inputs to the deflection plates. That insight can help understanding what the common mode voltages in each stage should/shouldn't be.

I checked the voltage levels driving U370 using a DSO and there appeared to be little or no difference, pin to pin or between scopes. It occurred that for a short time U370 was in an undocumented state - that is with its logic levels not as in the manual. I'm pretty sure neither the manual nor U370 documentation say what the voltages corresponding to the logic levels are.

Without bothering to do the work associated with understanding the circuit, I would presume that the common voltage on the two pairs of inputs is relatively unimportant but unchanging. I would expect the differential voltage to be important; too little and the analogue signal current won't be steered completely from one transistor to the other, and too much and there might be saturation and/or switching effects. Since the LTP is the basis of ECL switches, an ECL differential swing should be sufficient.

It did occur that since the good scope has a chop frequency of just over 2MHz, and the problem scope 1.8MHz, that could have caused timing problems. The chop oscillator chip was removed and its outputs substituted through the magic of a DDS function generator. Altering the frequency from about 1.5MHz to 4MHz made no difference. Altering the phase between the two outputs, one to the 7474 and the other to chop blank made the problem slightly better or worse, but didn't make it disappear.

I doubt the frequency is important, but I would expect the timing could be significant. The two "ECL" signals driving the inputs to the LTPs need to be switch at the same time in order to switch the current in the LTPs.

However, I would expect timing flaws would simply manifest themselves at the transition times. If the change you see is in the flat-top amplitude, then there is some interesting "integration" going on somewhere. I'd look at the common and differential mode voltage on the U370 in and out signals, to check there are no saturation effects.

Comparing the waveforms from the circuit shaping the chop blank pulse between the two scopes, shows no significant difference. The resistors and voltage levels in that circuit show nothing odd either.

It could be something I've missed, but I'm inclined to look at the Z modulation next.

Yes, so many hours of amusement for such little cost. Certainly better value than going to the pictures these days.

Have you looked at the analogue signals entering the u370 LTPs, pins 2,3,10,11? Obviously there shouldn't be any crosstalk visible there; the voltages (both common and differential should be less than those on the control inputs (otherwise the LTP wouldn't function).

Ditto exiting the LTPs, pins 14,16? The voltages should be higher than the control inputs.

[tggzzz]

A random WAG: Maybe something dicky in a COUPLING switch? I've had one or the other coupling switch play hell with triggering and with ALT/CHOP modes on several older scopes.

Or the 20MHz bandwidth limit switch, or the delay line. But it is difficult to see how that would, on its own, change with the chop/alt mode.

Random aside: the HP1740 is notorious for suddenly "losing" HF bandwidth. The solution is to squeeze and fondle the delay line. Seriously.

[tggzzz]

The most similar thing to the channel switch push buttons I've done, was having to deal with the timebase push buttons. There was no problem with them, but they had to come out to deal with the damaged timebase vernier. It was fiddly but nowhere near as awkward as the channel switch bank.

My experiences with a 475 timebase controls and switches, are noted here: https://www.eevblog.com/forum/repair/re ... msg1438311

[Zenith]

A random WAG: Maybe something dicky in a COUPLING switch? I've had one or the other coupling switch play hell with triggering and with ALT/CHOP modes on several older scopes.

Thanks for the suggestion but I can't see how it could affect chop. The signal does go through physical switches in the attenuator and after channel switching, the 20/100/200MHz bandwidth filter selector. They seem to cause absolutely no problems on Ch1, Ch2, add or alt.

The way the channel switching works is by having the channel switching hybrid controlled by logic levels provided by a 7474.

With Ch1, Ch2 and add, the switches directly control the outputs of the 7474.

With alt, the 7474 is fed with logic levels from the sweep circuitry which do go through a mechanical switch, so say, odd sweeps select ch1 and even sweeps select ch2.

When chop is selected, a logic level is provided via a switch to an input on one of the chips making up the chop oscillator, which is enabled producing about a 2MHz square wave. This causes the 7474 to produced rapidly changing logic levels for U370, causing it to switch between ch1 and ch2 at about 1 million times a second.

I've looked at the -5.7V voltages controlled by the channel switchbank with a scope and I could see nothing unexpected.

[Zenith]

My experiences with a 475 timebase controls and switches, are noted here: https://www.eevblog.com/forum/repair/re ... msg1438311

I came across this

https://entertaininghacks.wordpress.com ... -response/

which I found informative, constructive, instructive and inspiring. It is clearly the work of a commanding engineering talent.

[bd139]

Try injecting a square wave signal into one channel but not the other and look at the waveform output on chop when the non stimulated channel is grounded. Then repeat the other way round. That might lead to a few clues.

Also note these channel switches are current driven so measuring voltages might not be as useful as a diagnostic process if they are all say one diode drop from ground. I would do some differential measurements across the 47 ohm driver resistors with two scope probes with SUM+invert...

[Zenith]

Glad to see you back.

I've done it, in fact that's the way I noticed the problem. What's more if both channels are on GND and the triggering set to auto, so two straight lines appear, if the position of one is moved up, the other moves down a noticeable amount, and vice versa.

I'm inclined to try tggzzz's suggestion and check the balance of the Y preamps, but frankly I'm getting fed up with it and I'm inclined to give it a couple of day's break. Maybe it's a siren call and if I worked through the set up/calibration procedure as in the manual, the problem would disappear.

[tggzzz]

My experiences with a 475 timebase controls and switches, are noted here: https://www.eevblog.com/forum/repair/re ... msg1438311

I came across this

https://entertaininghacks.wordpress.com ... -response/

which I found informative, constructive, instructive and inspiring. It is clearly the work of a commanding engineering talent.

I'd forgotten that!

Curiously it mentions using DeOxit Faderlube on cracked open potentiometers, rather than only IPA and D5. I didn't realise I was so sensible.

[Zenith]

Peace and love returned and I had another go at the 475. i really couldn't see any problem with the channel selection circuitry, so I decided to go through the vertical amp procedure properly, as far as I could accommodate the Tek manual with the equipment I have at hand, and there wasn't anything that needed much adjustment - in my view. It appeared to work properly and not need adjustment before, but there was the chop problem. Some of it is very frustrating in that many of the presets that are supposed to do this or that, don't seem to do anything at all.

Having struggled through it and got things as right as I could, I checked for the chop problem and it wasn't there. I assume it must have been because the differential input into the channel switching board was imbalanced, but I'm sure I checked that and it didn't seem so.

That's pretty much the end of the story, apart from that the third Tek 475 bought as a parts mule, is really too good for such a fate, so I'm looking for a fourth. That's TEA.