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SC504 INT SYNC queries


EJP
 

1. What is the internal switch behind the SC504 indicator lamp for? Is this the switch referred to at the bottom of p.2-5 of the Specifcation and Performance Check section of the manual? and if so can someone expand on the rather cryptic description?

2. I'm looking into this area because the INT SYNC signal I am providing at 15B/14B of the backplane isn't triggering. It seems to be getting through correctly, because if I rotate the Triggering Level control rapidly through its top dead centre it momentarily syncs, lights the SYNC light, and freezes the display, but there is no position of this control I can find, however slowly, that will keep it that way. The plugins I'm using to provide the SNC vary from two SG505s to an SG502 and an AF501. The main SG505 I use has been adjusted by me to provide 200mV at the front SYNC output with this backplane connection in place, which sucks the level down a bit due to the 50-ohm termination, but even setting it as high as 350mV doesn't fix it, and this is about as high as it can go.

I've checked both 24R loading resistors at this input, they are OK, and all these oscillators can trigger everything in the place via their front panel SYNC outputs.

Any clues?

EJP


EJP
 

I realized after posting this that the behaviour when totating the Triggering Level control rapidly isn't conclusive evidence for the presence of INT SYNC, and in fact it does the same thing when there is no oscillator plugged in at all, i.e. when there is definitely no INT SYNC signal at all. So I need to investigate whether the signal really is present at INT 15B/14B. Doing that today. It's complicated by the presence of a slot barrier, which I have to remove in order to get an extender cable in there.

EJP


Stephen
 

On Mon, Aug 24, 2020 at 03:33 PM, EJP wrote:

It's complicated by the presence of a slot barrier, which I have to
remove in order to get an extender cable in there.
Or just cut a slot in your extender... 🤷‍♂️


 

On Mon, Aug 24, 2020 at 09:41 AM, EJP wrote:


1. What is the internal switch behind the SC504 indicator lamp for? Is this
the switch referred to at the bottom of p.2-5 of the Specifcation and
Performance Check section of the manual? and if so can someone expand on the
rather cryptic description?
The switch is used to determine the function of the variable time base setting knob.
In the default position (backward), the knob serves to reduce timebase speed from the calibrated position.
In the other position (towards front), the knob provides a variable holdoff function. Holdoff blocks triggering for a certain time after the trace finishes. It is used when the periodicity of a complex signal is significantly longer than one full time base sweep. It thereby prevents triggering on alternate parts in a complex signal, preventing "misfires" showing up as double triggers.

Raymond


 

On Mon, Aug 24, 2020 at 09:41 AM, EJP wrote:


2. I'm looking into this area because the INT SYNC signal I am providing at
15B/14B of the backplane isn't triggering. It seems to be getting through
correctly, because if I rotate the Triggering Level control rapidly through
its top dead centre it momentarily syncs, lights the SYNC light, and freezes
the display, but there is no position of this control I can find, however
slowly, that will keep it that way.
Stupid question: You put the Trigger Source selector on Int, did you? And did you try appropriate trigger coupling settings, like both AC triggering (AC in) and DC triggering (all CPL out)?
Time bases trigger when the level is crossing any trigger level/source during setting. By adjusting the knob you are varying that level setting through its range so your conclusion that it gets through from where you want is wrong, although it shows it is able to trigger, given the right conditions/settings.
Did you check continuity (Ohm meter) from 15B to the common point where all triggers enter the Trigger Switch Board?

Raymond


EJP
 

Stupid question:
There are no stupid questions ;-)

You put the Trigger Source selector on Int, did you?
Yes.

And did you try appropriate trigger coupling settings, like both AC triggering (AC in) and DC triggering (all CPL out)?
Yes, tried them all.

Time bases trigger when the level is crossing any trigger level/source during setting. By adjusting the knob you are varying that level setting through its range so your conclusion that it gets through from where you want is wrong, although it shows it is able to trigger, given the right conditions/settings.
Agreed: my conclusion was wrong. I have subequently established that the trigger signal does indeed get around the backplane from the oscillator slot to 15B, but from there it goes via coax to the scope's trigger select switch, with the centre connectors concealed under the board that I cannot get at. The external and channel-linked and line triggers all work perfectly, certainly no issue there.

Did you check continuity (Ohm meter) from 15B to the common point where all triggers enter the Trigger Switch Board?
I haven't located that point yet, but I will certainly check it when I do.

Thanks also for the explanation about the internal switch. At least it has nothing to do with this issue.

I've also succeeded in connecting the CH 1 and CH2 INT signals around the backplane to the AA501, so I can finally get rid of some spaghetti at the front linking AA501 Function and Monitor Outputs to CH1 and CH2. However there is mention on p.5-6 of the SC504 manual of having to disconnect a coax connecting 28B CH1 Trigger Out to 16A CH2 INT IN to do this, but being unable to find any such cable, or any evidence of any continuity between these points in any input selection configuration, I went ahead anyway and it worked fine. Maybe my SC504 is later than my manual, or earler.

EJP


EJP
 

The mystery continues. I have signal around the backplane to 15B, and I have 24 ohms of R from 15B to the input end of R3220, which is the point at which all the SYNC inputs meet, the 24R being R3320; and, just for completeness, 68+24R from 15B to the other end of R3220. And when SYNC INT is deselected it goes open-circuit of course. So all those jumpers and switches in that circuit are working correctly.

So I'm now wondering again whether it's a level problem; or else a short when the SC504 plugin is actually inserted somehow.

EJP


 

On Wed, Aug 26, 2020 at 06:08 AM, EJP wrote:


The mystery continues. I have signal around the backplane to 15B, and I have
24 ohms of R from 15B to the input end of R3220, which is the point at which
all the SYNC inputs meet, the 24R being R3320; and, just for completeness,
68+24R from 15B to the other end of R3220. And when SYNC INT is deselected it
goes open-circuit of course. So all those jumpers and switches in that circuit
are working correctly.

So I'm now wondering again whether it's a level problem; or else a short when
the SC504 plugin is actually inserted somehow.
- As you suggest yourself, you could have a DC level or amplitude problem. Triggering by presenting the signal at the external trigger input may give some info.
- Doesn't pressing Trigger View give any info?
- Is External Trigger Balance adjusted ok (R3527 on the Trigger Switch Board)?
- If you don't have an extender, it should be safe and easy to temporarily solder a wire to R3220 and take that out of the box to watch it on another 'scope (if you have one)

Raymond


EJP
 

Raymond

Excellent suggestion, and the balance was indeed a bit out at 50mV, but setting it to 0.0000 volts didn't help.

R3220 is under the board you do this adjustment to, so I would have to solder a wire to it anyway. I will do that next.

Re Stephen's suggestion to cut a slot in the extender, my original Tek extender already had several plugin-family slots cut in it, including this one, lookng to have been made that way: but my beautiful Dan Meeks extender doesn't, only the TM500 family slot, and I'm not keen on starting down that path.

EJP


EJP
 

Thanks to all for the help. After testing at TP3136 and getting nada, and then ditto at TP3520, I twigged that there was no signal getting through at all with the TM515 power unit in the frame, although there had been when I had it out, and of course that led immediately to discovering 14B/15B shorted together *by* the frame when put back together, because I had used pins to make the connectors, and these fouled on the extreme RHS of the RHbay with the RH flat bar of the frame. But nowhere else. So I removed the pins and soldered the wires straight into the holes and bingo. A 12 year mystery solved.

Fun for all the family.

EJP


 

On Thu, Aug 27, 2020 at 09:07 AM, EJP wrote:


A 12 year mystery solved.
Congratulations, Esmond and thanks for informing us!

Raymond