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Old 15th Nov 2019, 11:57 pm   #1
Tim
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Default Oscilloscope delay line question

Hi gang.


In reference to this thread:
https://www.vintage-radio.net/forum/...=159953&page=2


a wire has broken off the input to one of the delay lines on my OS3000. My main question is this:
The delay line does what it says, and introduces a delay to the timebase so the first part of the trace is visible. In my case about 95 micro seconds.
Presumably this can be achieved by making the delay line a precise length. therefore would it cause a problem if I cut , say 2-3mm of the core to find the end of the broken wire?
Would that amount of reduction in length be significant? Would I have to do the same to the other delay line?
I have zero experience of delay lines etc, as I don't think they generally give much trouble, so please help. I've spent enough time on the wretched thing, I cant give up now!


Many thanks
Tim
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Old 16th Nov 2019, 10:13 am   #2
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Default Re: Oscilloscope delay line question

I suspect your delay line is 95 nanoseconds rather than microseconds but that is just nit-picking.

The precise value of the delay time is not at all critical, it is just that it has to be long enough for the trigger and time base circuitry to start the sweep before the signal reaches the Y plates. A 95 nsec delay in ordinary coax would be something like 70 feet long and the delay lines have to be specially constructed with two wound centre conductors to slow down the speed at which the signal propagates. You just need to make sure that you don't do too much damage in removing the material you need to make a new connection. I have to admit I have never actually opened up a delay line so proceed with care.

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Old 16th Nov 2019, 10:17 am   #3
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Default Re: Oscilloscope delay line question

Yes Roger, 95nS. It was very late! Thanks.
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Old 16th Nov 2019, 11:46 am   #4
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Default Re: Oscilloscope delay line question

You shouldn't have any problem with even taking a few cm off the length. What the line is compensating for is hardly precise and the line is made long enough to give plenty of margin for triggering speed.

HP scopes used two zig-zag tracks on a very long piece of very thin PCB. screening foils were added and the PCB rolled-up! Some added inductance from coiling the inners of twinax like line, or by increasing dielectric constant slows the line down so less length is needed. Good matching is needed to avoid mutilating waveforms.

David
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Old 16th Nov 2019, 3:44 pm   #5
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Default Re: Oscilloscope delay line question

Thanks chaps.
The delay "unit" look a bit like co-ax, but there are two "lines" inside, enclosed in a sort of gel stuff. It's measuring s/c at the moment. Presumably phase is important, in other words it has to go back exactly as it is now?
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Old 16th Nov 2019, 4:55 pm   #6
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Default Re: Oscilloscope delay line question

Yes, you need to get the phase right or the Y deflection will be reversed. The Y amplifier is usually a pair of opposite phase, DC coupled amplifiers each driving one of the Y deflection plates. I don't have the manual for your scope but usually the delay line is resistively terminated in its characteristic impedance so you will measure 50 - 100R to ground from each of the conductors and double that between the pair of inner wires. You also probably have a common base transistor as the input to the next stage so in one polarity you have a forward biased junction as well. If you really have 0R either to ground or between the inner conductors then something is seriously amiss.

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Old 16th Nov 2019, 6:29 pm   #7
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Default Re: Oscilloscope delay line question

If you get the phase wrong, you can always reverse it somewhere more convenient, like individual contacts onto deflector plate pins out the side of the CRT neck.

David
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Old 16th Nov 2019, 7:08 pm   #8
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Default Re: Oscilloscope delay line question

Quote:
like individual contacts onto deflector plate pins out the side of the CRT neck.
That would make the shift work backwards, or is that upside down.
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Old 16th Nov 2019, 7:36 pm   #9
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Default Re: Oscilloscope delay line question

Rats!

Actually, maybe not. Shifts get added in earlier in the chain so that with a beam switch for dual trace you get to have individual shift controls. Trigger gets taken off ahead of the delay line, ahead of the beam switch (unless you want mixed triggering)

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Old 16th Nov 2019, 9:34 pm   #10
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Default Re: Oscilloscope delay line question

Delay lines can come in various forms. I had abig 2200 (Marconi, gertting forgetful) and that had two big panels at the rear populated by Cs and Ls. There were others similar. Reference was made to 70' of twin coax. I would guess that two 35' lengths of single would also do, with the two "balancing out".
Telequipment used a block (say 4" x 2" x 1/2" with finely coiled up two contra wound coils. If you fail in successfully repairing your D/L, I think I have a spare block somewhere, though it is untested. Probably just needs continuity and insulation checking).
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Old 17th Nov 2019, 12:15 am   #11
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Default Re: Oscilloscope delay line question

Hi Les. I have been a little worried/curious because my D/L is only about 18” long so I wonder if the “cores” are actually coils. I’ll have to be very cautious stripping back the inner core insulation. I have a thing that can make a circumferential cut, but I’ll still have to be careful.
Thanks to everyone for their help so far.
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Old 20th Nov 2019, 2:42 pm   #12
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Default Re: Oscilloscope delay line question

Thanks to everyone who has helped with this.
I have managed to strip about half an inch off the insulation of the centre core and find the broken end. I now have enough to solder to, and with the help of croc-croc leads and PP3 battery connected to a Y input have established the correct connections.
The line itself is a screened plastic tube containing a pair of thin uninsulated wires( bi-filar wound?) wrapped around a stiff inner core.
I would like to seal the end of the line with something to hold the bare wires in place during the handling and soldering process.
Silicone sealant springs to mind. I understand it is a good insulator, but all I have is the standard acid cure variery( with the vinegary smell-Acetic acid presumably).. Would this be suitable?
I might be able to manage some paraffin wax.
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Old 20th Nov 2019, 6:56 pm   #13
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Default Re: Oscilloscope delay line question

Or possibly super or hot melt glue. Actually I also have some Epoxy
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Old 20th Nov 2019, 10:10 pm   #14
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Default Re: Oscilloscope delay line question

I use a copper coloured (NOT copper containing) Loctite product (5920??) but it has shot up in price recently.
CPC do a clear (transparent) low odour silicone sealant. Either would do. leave 24 hrs to set, then go ahead and solder.
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Old 20th Nov 2019, 10:27 pm   #15
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Default Re: Oscilloscope delay line question

Delay line.
I would hesitate over using a silicone sealant which uses acetic acid as the curing agent. It will surely corrode your wire, just as badly as if you used a good plumbers flux full of zinc chloride to solder it in place.
Either select a non-acid silicone sealant (they are made, but need searching for), or use your hot melt. or a dab of paraffin wax or a scrap of Scotch Tape, the non perishing sort but not common Sellotape, where the adhesive absorbs moisture and perishes.

The delay line is acts as a transmission line where the wire is wound in a spiral around a central core to slow down the propagation rate along the line and to increase the inductance to match the capacitance.
Usually these lines are low impedance (usually 93 to 200 ohm) and have to be carefully matched at each end.
Set up by instructions in the service manual on setting up the high frequency pulse shape of the Y amplifier.
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Old 21st Nov 2019, 9:40 am   #16
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Default Re: Oscilloscope delay line question

Don't use silicone sealant (at least not the standard bathroom stuff) it will corrode everything in sight!

Hot melt is the way to go.
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Old 21st Nov 2019, 8:08 pm   #17
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Default Re: Oscilloscope delay line question

Thanks folks. Found some time last night when SWMBO was in bed to connect the delay line and Bosc the 3000’s up and running again. I still have to sort out the vertical output transistors, ( as one is connected on fly leads from a breadbord) but hopefully a piece of. Veroboard should provide a mounting solution for the replacements.
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Old 21st Nov 2019, 10:51 pm   #18
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Default Re: Oscilloscope delay line question

The Loctite sealant I sited earlier is OK. I have used it on all sorts of stuff, including sensitive electronics stuff over the last 10 or more years. Likewise the low odour transparent sealant from CPC which I used even earlier.
I still prefer the Loctite, but as I said, very expensive now. I think I have seen tubes in excess of 20.
It should seem unnecessary to point out the obvious of NOT using the "vinegar smelling" versions used much in the building game.
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