Tektronix TG501 - Out Put problem

[dan3460]

I was jinxed by Mark  , I pulled my modules to start a calibration on a recently finished scope and my time mark generator didn't work.   This generator was dead when I got it a few years ago, I repaired and has been working with no problem until now. Last time that I used it was about 2 months ago. Is probably my brain that is not working but I can get past this.

I have included the part of the schematics were I having the problem. This is just at the end of the signal gets passed to the marker out BNC. I have disconnected the cable that ties J468 and P468 to isolate the problem. At CR470 to the side of J465 I get the marker (picture DSC_1149) at the other side I get picture DSC_1150, this is a .1mS markers, is like is a capacitor there. With a lower speed about 1mS the bottom drops to 0V. I go to a higher speed I end up with a flat line at about 3.5V. Just in case the other side of CR468 is sitting at 0V with no signal there.
Because there are very few parts there I thought that CR470 was acting as a capacitor, I changed the diode but did not solve the problem. I used a 1N4148, the Tektronix Cross Reference page calles for a 1N4244, but I don't think that is the problem.

Is anyone experience something like this?

[MarkL]

CR468 and CR470 can only pull the voltage up at J468.  By your scope traces, that appears to be happening.

But what's not happening is the voltage getting strongly pulled back down.  The 50ohm terminator that should be present on the output of the generator, either inside the scope under test or on the BNC cable, is responsible for this.

Since you've disconnected J468, you have no pull-down resistor.  Or if you do and you still get traces like this, check for continuity between the junction of CR468 and CR470 and the output.  Could be the relay or a bad connector.

[SoundTech-LG]

Yup...  that 50 termination is critical! Low or no output without it!

[dan3460]

Thanks, now that you mention that I remember the T at the end of the cable but don't remember if I put a 50 ohm termination. 

Got flustered and started to chase this rabbit without thinking. I'll reset my connections and test again.

[dan3460]

Indeed Mark and SoundTech you were 100% correct  , I did not have a terminator on the gear. Sometimes your mind takes you to a complete different path and you start to chase ghosts, this is embarrassing, even the instructions that I was following for calibration of the 2465 tells you clearly to put a cable with a 50ohm termination, I looked at that T several times and I did not see that there was NO terminator.

[SoundTech-LG]

It can be quite confusing, as you will have output at the lower repetition rates, but by the time you're up in 2nS, or 1nS, nothing... absolutely no signal apparent. Add the terminator... Voila! You wouldn't think "loading" the generator would bring up the signal, but not the case! It's a gotcha to remember.

[LY1]

Hello,

I have Tektronix TG501 time mark generator too.

I have one question about his outputs: When I set 5nS (5nS button is pushed), I get output from "Markers output bnc". Thats is all correct.
When I set 2nS (2nS button is pushed), I get output from "Markers output bnc" 2nS (500Mhz) and I get output from "1nS only" 1ns (1Ghz), but only 2nS button is pushed.
When I set 1nS (1nS button is pushed), I get output from "Markers output bnc" 2nS (500Mhz) and I get output from "1nS only" 1ns (1Ghz), but only 1nS button is pushed.

It seems that 2nS and 1nS buttons are not working correctly.
I can push 2nS or 1nS button but I get output from "Markers output bnc" 2nS (500Mhz) and I get output from "1nS only" 1ns (1Ghz) independently what button is pushed.
I think that if I push 2nS button I do not have 1ns output from "1nS only" bnc connector, and when I push 1nS i do not have 2nS output from "Markers output bnc".

As a testing source I use frequency counter.
Can any body to make a small test with Tektronix TG501 time mark generators?
Its interesting that you will have from output from "1nS only" when 2nS button is pressed.
And what will be readings on "Markers output bnc" when 1nS is pushed.

[tggzzz]

... this is a .1mS markers,...

No, it isn't - unless you are calibrating conductance. It is a 1ms marker. (Siemens vs seconds).

[dan3460]

Sorry for confusing S vs s, but the picture clearly shows pulses that are 100µs apart, which is .1ms, not a 1ms as you pointed out.

[tggzzz]

Sorry for confusing S vs s, but the picture clearly shows pulses that are 100µs apart, which is .1ms, not a 1ms as you pointed out.

The S vs s thing just irritates me; usually it is clear what is meant. Then there is the non-SI Samples/s

The ".1ms" is a bad habit, because it is easy to misread. "0.1ms" is better practice. On schematics where dust/dirt/blemishes can be an issue, a 1.5k resistor is better shown as 1k5 to avoid mistaking it for 15 ohms.

[LY1]

I have 1 nS output. 2nS or 1nS button is pushed. Can any body tried this?

This is good or my generator is not working correctly?

[tggzzz]

I have 1 nS output. 2nS or 1nS button is pushed. Can any body tried this?

This is good or my generator is not working correctly?

It looks like your counter is measuring frequency, not pulse width.
It looks like the frequency is the same in both pictures, so it is unsurprising the meter shows the same value.

[LY1]

I have 1 nS output. 2nS or 1nS button is pushed. Can any body tried this?

This is good or my generator is not working correctly?

It looks like your counter is measuring frequency, not pulse width.
It looks like the frequency is the same in both pictures, so it is unsurprising the meter shows the same value.

Pulse width is good.

In the first photo is pushed 2nS button (I am geting 1nS output) I think this is not good.
In the second photo is pushed 1nS button (I am geting 1nS output) this is good.

[tggzzz]

I have 1 nS output. 2nS or 1nS button is pushed. Can any body tried this?

This is good or my generator is not working correctly?

It looks like your counter is measuring frequency, not pulse width.
It looks like the frequency is the same in both pictures, so it is unsurprising the meter shows the same value.

Pulse width is good.

In the first photo is pushed 2nS button (I am geting 1nS output) I think this is not good.
In the second photo is pushed 1nS button (I am geting 1nS output) this is good.

How can you tell that from the photos?

If you are measuring frequency, these two waveforms with different pulse widths will show the same value for frequency:
100000000
110000000