Tektronix Type 184 Time Mark Generator

[Kidon]

So I picked up this unit from eBay which arrived yesterday.  I was gonna use it to calibrate my scopes sort of properly.
I don’t know if it’s me, but I’m kinda disappointed by it, or maybe I don’t use it correctly...

If anyone knows and/or has one of these, don’t hesitate to weigh in.

The issue is:  Everything works apparently as expected from the 10ns time marker up to the 1us marker with a 1us trigger mark.  After that, it’s virtually impossible to have any scopes trigger the signal.
All voltages are ok, contact has been cleaned.  I’ve carefully inspected the solder joints, and everything seems to be ok.

[tggzzz]

You need to get the manual and follow the service procedure.

Each divider step depends on the output of the previous divider.
Each divider is an analogue circuit, not a digital circuit. Each incoming pulse dumps a glug of charge onto a capacitor. When the voltage reaches a threshold, the capacitor is discharged and that pulses the next divider.
There are trimpots controlling that process, and they need to be tweaked to get a divider stage to work.
You will need a scope to watch the waveforms as you tweak them.

Make very sure you understand which bits of exposed metal are at >100V - and don't touch them!

[Kidon]

You need to get the manual and follow the service procedure.

Each divider step depends on the output of the previous divider.
Each divider is an analogue circuit, not a digital circuit. Each incoming pulse dumps a glug of charge onto a capacitor. When the voltage reaches a threshold, the capacitor is discharged and that pulses the next divider.
There are trimpots controlling that process, and they need to be tweaked to get a divider stage to work.
You will need a scope to watch the waveforms as you tweak them.

Make very sure you understand which bits of exposed metal are at >100V - and don't touch them!

I take it you have encountered that issue before.
As far as I could check, the crystal is calibrated.  Although I don’t have a lab-grade frequency counter, all the markers that work are absolutely dead on on my 485 and 475A, as well as on my other scopes (7623, 465, 465B, 466, etc....  I guess that must mean something, unless they are all not calibrated, and exactly in the same way...

I have a printed version of the manual albeit not a very good copy.  I have a feeling this is not gonna be easy....

As for the 100V, I never ever touch anything inside an electronic device; particularly one with high voltage capacitors, let alone a live one.  I do a lot of tube amps builds and repairs, so I know better than to put my hands where they don’t belong.  Don’t ask me how I know!   

[tggzzz]

You need to get the manual and follow the service procedure.

Each divider step depends on the output of the previous divider.
Each divider is an analogue circuit, not a digital circuit. Each incoming pulse dumps a glug of charge onto a capacitor. When the voltage reaches a threshold, the capacitor is discharged and that pulses the next divider.
There are trimpots controlling that process, and they need to be tweaked to get a divider stage to work.
You will need a scope to watch the waveforms as you tweak them.

Make very sure you understand which bits of exposed metal are at >100V - and don't touch them!

I take it you have encountered that issue before.

I resurrectected my 184.

As far as I could check, the crystal is calibrated.  Although I don’t have a lab-grade frequency counter, all the markers that work are absolutely dead on on my 485 and 475A, as well as on my other scopes (7623, 465, 465B, 466, etc....  I guess that must mean something, unless they are all not calibrated, and exactly in the same way...

Your scopes are 2% instruments. It would be remarkable to find a crystal that was 2% off. Anything derived from that will either be as good as the crystal, or not work.

I have a printed version of the manual albeit not a very good copy.  I have a feeling this is not gonna be easy....

Take it one stage (literally!) at a time.

Firstly make sure that all the front panel controls are in the correct position; some controls prevent other controls from operating as you would expect, because one part of the circuit steals power from other parts. Get it so that you see a 1us output.

Then there are three main possibilities: the low voltage transistor PSU, the big front panel switches need cleaning, or some of the trimmers need tweaking. Cleaning switches can be done with either IPA or a DeOxit, or both.

The schematic shows the waveforms you would expect to see. Start at R140. If that is right, look at R154. If that isn't right, tweak R146. Then repeat for other stages.

As for the 100V, I never ever touch anything inside an electronic device; particularly one with high voltage capacitors, let alone a live one.  I do a lot of tube amps builds and repairs, so I know better than to put my hands where they don’t belong.  Don’t ask me how I know!   

Make sure you have a *10 probe, not a *1/*10 probe! Your problems lie in the 12V domain, not the 120V domain!

[0culus]

I recently repaired and aligned an ebay 184 as well. Good advice above...get the manual and study it. This instrument is rather complex and most of the circuits depend on the functioning of others. Mine had a bad Nuvistor in V40 which disabled all the sine markers.

I adjusted the crystal to 5 decimal places with a GPSDO reference....this is way overkill for analog scopes. Close enough is fine.

[Kidon]

A little update.  I adjusted the right cap, and it is now perfectly fine and triggering properly, instead for one thing (so far):  The pips are not straight on any scope I use (see picture taken on a 466), they lean to the left, and I didn’t see any reference in the manual to adjust/compensate for that.  Maybe I missed it...

I offset them a bit so it’s more visible in the picture.

After calibrating a 2225, a 2215, and a 2335, and precisely measuring a frequency I randomly (not particularly on the mark, ie. 14.657Khz) set without looking at it on my function generator, I did calculate a 0.5% accuracy.  I repeated the process across various ranges from 100Hz to 10Mhz, and the accuracy was always between 0.5% and 0.8%.  I know it’s nerdy and overkill, but in order to do that, I actually used a magnifying glass to precisely position the beginning of the period, and then look where the end of that period crossed the horizontal middle.  I’d say it’s not bad.

[srb1954]

More likely the left leaning pips is a geometry problem with your scope. Check your scope calibration manual to see if there is any provision for the adjusting the geometry parameters of the CRT.

For a correctly adjusted scope it shouldn't be possible for the leading edge of a signal to lean left unless, of course, your signal generator has a special function to send signals backwards in time.

[tggzzz]

A little update.  I adjusted the right cap, and it is now perfectly fine and triggering properly, instead for one thing (so far):  The pips are not straight on any scope I use (see picture taken on a 466), they lean to the left, and I didn’t see any reference in the manual to adjust/compensate for that.  Maybe I missed it...

Good; it is always worth RTFM!

That cannot be a problem with the 184; a "backward leaning" trace implies time going backward!

Have a look at the scope "trace rotate" control. Usually it is a trimpot on the front panel.

[Kidon]

Good; it is always worth RTFM!

I did read the manual.

That cannot be a problem with the 184; a "backward leaning" trace implies time going backward!

Have a look at the scope "trace rotate" control. Usually it is a trimpot on the front panel.

Yes, I know where the trace rotation is.  And they all seem to be fine when positioned on the center horizontal line with the scopes I used...  That’s why I thought it might be something else...

[tggzzz]

That cannot be a problem with the 184; a "backward leaning" trace implies time going backward!

Have a look at the scope "trace rotate" control. Usually it is a trimpot on the front panel.

Yes, I know where the trace rotation is.  And they all seem to be fine when positioned on the center horizontal line with the scopes I used...  That’s why I thought it might be something else...

What happens when you use the vertical position control to move the trace up and down. That should give clues to the source of the problem and its cure. But don't forget to compare the non-ideality with the scope's specification, which is probably 2%.

[srb1954]

A full alignment of a CRT generally requires more than just the front panel trace rotation. The trace rotation is intended for correcting any tilt in the trace due to variation in the position of the CRT relative to the Earth's magnetic field and it alters the X and Y deflection simultaneously.

Trace rotation doesn't correct any orthogonality problems with the CRT i.e. are the X and Y deflection plates at exactly 90degs apart.

Check your manual. Higher quality scopes will have a procedure for checking/adjusting various CRT geometry issues.

[Kidon]

This is what happens when I set the trace rotation for straight up pips.  See what I mean?  (Still on the 466)

Both of you guys are most likely 100% right.  The scopes I used probably don’t have their CRT properly aligned....
I’ll check if the same problem occurs on the 475A, which I know has been properly calibrated not long ago.

[Kidon]

I’m still pretty amazed at how accurate these thing are.
That’s approximately 0.1% accuracy at around 2Mhz!!!

[tggzzz]

I’m still pretty amazed how accurate these thing are.
That’s approximately 0.5% accuracy at around 2Mhz!!!

5000ppm is awful for a crystal. Even before ovens have stabilised, mine is ~15ppm out, i.e. 0.0015%

If that is a standard frequency counter, they can have difficulty setting the threshold for waveforms with a low duty cycle.

I suggest you select DC coupling and manually change the threshold so the count just stops - then set the threshold to be halfway between those limits.

[Kidon]

I’m still pretty amazed how accurate these thing are.
That’s approximately 0.5% accuracy at around 2Mhz!!!

5000ppm is awful for a crystal. Even before ovens have stabilised, mine is ~15ppm out, i.e. 0.0015%

If that is a standard frequency counter, they can have difficulty setting the threshold for waveforms with a low duty cycle.

I suggest you select DC coupling and manually change the threshold so the count just stops - then set the threshold to be halfway between those limits.

?? ?? ??

I haven’t touched the crystal frequency (Yet).  I don’t have a lab-grade counter to do that.  Last time it’s been calibrated was in the 80’s (according to the sticker on the machine).

I thought 0.1% is pretty good considering those scopes are supposed to have 1%-2% tolerance.

Yes, this is an older and simple signal generator from the 80’s I guess.  Although not a bad one by any means.
I’ve got another one, but it’s a Chinese thing....  It does the job sometimes.

[tggzzz]

 I thought this thread was about a 184. If not, please let us know what you are looking at.

[Kidon]

I thought this thread was about a 184. If not, please let us know what you are looking at.

It definitely is.  You brought up the signal generator issue.  I was just measuring accuracy against what I have.

[k7sparky]

This is what happens when I set the trace rotation for straight up pips.  See what I mean?  (Still on the 466)

Both of you guys are most likely 100% right.  The scopes I used probably don’t have their CRT properly aligned....
I’ll check if the same problem occurs on the 475A, which I know has been properly calibrated not long ago.

I think what you are seeing is the pulse decay sloping off as the cap discharges.
You missed the initial pulse.

I found this thread going through my 184 again.

Another note:
Be careful about lifting the countdown board.  There are 8 ea #30 of smaller wires coming up through the board from the square delay line mounted under it.

[jdragoset]

Artek Media offers this one for a very reasonable price.
For some reason, when I click to enlarge any of your pix, it always shows the enlarged view of the last pic.
A Metrix counter showing 1.0000 MHz
 

[jdragoset]

Tektronix Tek 184 Time Mark Generator Operating & Service Manual with Schematics
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