How to make calibration signal lead for oscilloscope?

[Luxman1050]

Hi guys I need to make up a 500mV calibration signal cable.
I need two male bnc connectors connected to a single 4mm plug. I assume need 50ohm coaxial cable? I have plenty of that.
Looked everywhere but can only find single bncs with two 4 mm plugs which I have a few of also with crok cclips here.
Anyone made one of these up?
Any help most welcome.
Cheers Chris.

[Radio Wrangler]

I'm sorry, I don't understand.

Most oscilloscope inputs are very high impedance ( 1 Megohm and a few tens of pF is typical)

The calibrator circuits are usually designed with a view to seeing high impedance. 50 Ohm coax is rather capacitive and will spoil the risetime of pulsed calibrator signals.

Usually the purpose of the calibrator is to allow the risetime compensation trimmer of a scope probe to be set. The calibrator is not for calibrating the oscilloscope, it is for calibrating whatever cable/probe/etc that you use to connect the scope to whatever you are investigating.

There's another thread going at the moment on the restoration of a Bradley oscilloscope calibrator - this is what it takes to calibrate an oscilloscope itself.

David

[Luxman1050]

Thanks for getting back to me.
All it is is to do with first time operation testing. Nothing to do with testing other equipment.
See attached pic section 10.
Hopefully you will understand what I mean.
Cheers Chris

[Radio Wrangler]

Oh, that's easy. Just about anything will do and give you a good enough signal to help you find your way around the controls.

Y controls are easy, shift and volts per division do what they say on the can. AC/DC coupling is obvious. If the AC/DC switch has a gnd position it's useful to flick to that from DC to see where the shift has put zero volts.

X shift positions the timebase scan to start from the left edge of your graticule as default. Timebase time/division is easy. If you have a holdoff knob, leave it at minimum. If you later get confused traces writing over each other, try increasing holdoff delay and you may get the picture clearing up.

It's the trigger controls that you really have to think about and find your way around
'Norm' triggering gives you no runs, no picture until you have triggering set up so that the timebase gets triggers.... difficult when you can see nothing!
'Auto' triggering is a modification of the above. If it hasn't had a real trigger in some time, it says what the hell and fires off a sweep anyway. So you see something to help guide you in setting triggering. If your triggers really are infrequent, auto becomes a nuisance, so you switch to norm, so all the sweeps you see are real triggered sweeps.

It takes practice, but it'll stand you in good steak. You see a lot more information and clues than you'd get from a multimeter.

David

[Luxman1050]

Nice one thanks David glad it all made sense in end.
Yes never used one before but luckily have complete operators manual plys all the service sheets but was bit confused re the lead.
Don't think been used much as it came complete with the accessories except the make up your own lead and on switching on screen nice and clear with lines and floating dots moving fine both channels.
Anyhow ill have a play.
Cheers Chris

[Chrispy57]

Hi - sounds like you already have the original scope leads from your 2nd post, so the cheap and cheerful way to get the cal signal on he scope for familiarisation would be to just put a 4mm plug, or DVM lead into the cal socket, and touch the tip of the scope probe to it. The earthing crocodile clip of the scope probe is already earth referenced to the scope.

Many scopes have a tag or pin of some sort to clip the scope probe on instead of a 4mm socket - makes life a bit easier. In "calibration" terms it also makes sense to use the actual scope probe itself rather than a different made-up lead to attempt "calibration".

If you don't have a 4mm plug/lead then even an M4 screw or bent piece of solder in the 4mm socket will do at a pinch just to get the waveform on screen to play with the controls. Enjoy!

Cheers
Chris

[Luxman1050]

Thanks Chris
Yeah think it has those clips in the probes packet go on tips of probes.
Nice one thanks again guys not as complicated as initially thought ��
Cheers Chris

[Ed_Dinning]

Hi Chris, there is a good booklet on the web, called the XYZ of oscilloscopes, By TEK and an ideal intro

Ed

[Luxman1050]

Hi ya Ed
Thanks for that ill have a look.
I need some spare valves and was gonna email today.
All best Chris

[Craig Sawyers]

It's the ABC of probes. Tek produced that in a number of editions over the years, but there is a good copy here http://w140.com/tekwiki/images/d/d6/60W-6053.pdf

Also in the Concepts series

http://w140.com/tekwiki/images/6/62/062-1146-00.pdf
http://w140.com/tekwiki/images/1/19/062-1120-00.pdf

Lots there as light bedtime reading ;-)

Craig

[Luxman1050]

Blimey Craig thats great 👍 thank you.
I do like a read trouble is I've also got a WW11 TF 108A wating on manual for that big old beast mains input never seen one like it as had to change caps out. Two coils and 6 y caps pic attached.
The scope I think good bit of kit bit basic but suit my needs once I've had a few bedtime reads 😴 ha.
Also managed to get for couple quid a 500v wind up milli ohm metre come in handy as sometimes had low readings under 20 ohm not getting readings. All working fine. Also been offered by guy who lives nearby a military jungle proof power supply waiting on some pics sounds interesting. So all going on keeps me going. Sorry guys going off track here ew.
Cheers guys

[Radio Wrangler]

Ah, you've got a Telequipment D67. That model was the standard lab scope when I did mu batchelor's. They are a good, workman-like oscilloscope, and sufficiently modern for what you learn with it to remain useful with anything later.

Remember what I said about triggered timebases? Well this scope has two. This puts it a notch above the most basic models.

In normal use, you only use one timebase and everything drives normally. But when you want to look at some event happening in a short section of your trace, you can bring the second timebase into play.

Assuming you have the main timebase running normally and a stable picture, you poke a button which tries to get the second timebase running, you set a faster speed (less seconds/division) on the secondary timebase knob and the bit of your trace which covers where the secondary timebase runs is brightened up, that multi-turn knob with the little scale adjusts the delay before the secondary timebase triggers, play around with it and the bright bit of trace will move across the screen, allowing you to see where the fast, secondary timebase will show, once you switch fully over to it.

So you adjust to get the bright bit over the part you're interested in, and then poke the next button to switch to the secondary timebase. You now get the area you were interested in across the full screen, and you read the time scale (seconds/division) from the secondary timebase knob.

The original motivation for these dual timebase scopes was for liiking at individual lines in the waveform of an analogue TV signal, and also for looking at pulsed sort of things like PCM telephone systems and Radar.

You won't need the dual timebase function for just fixing radios. So you can just find out how to be sure you have it turned off for the time being. If you want to work on tellies (or even more so CCTV and cameras) then you'll start to need it. It's worth learning how to use it because then you have a more versatile tool when you run into something more challenging.

It's an excellent choice for a first scope. Plenty for you to get going with, and enough beyond that to show you a comfortable learning curve to the more complicated ones.

Plenty of people will tell you that you don't need a scope and they can do everything they need with a multimeter. This is most likely true. What it misses out on saying is that the multimeter only approach often relies on a lot of experience in interpreting readings in various places. The scope may not be necessary, but it's an awful lot easier. It gives you a lot more information and it SHOWS you what is happening.

In ye olde days, hobbyists couldn't afford a scope in their workshop, and in radio/tv repair workshops most Dicks and Smithys worked away with multimeters, though there might be a scope kept carefully in a cupboard for special occasions. Often only the boss was allowed to use it.

That was then, this is now. Surplus scopes are affordable and people can take advantage of being able to see more. Watch an electronics professional at work and you'll see him go in with a scope as his first look instrument. It's not as precise as a meter, but it gives more insight and much more quickly. Multimeters get brought out when special accuracy is needed.

So there are two camps of electronics people, multimeter first, and scope first folk.

As you can guess, I'm firmly in the scope-first group. As a kid in the sixties I bought a tube from Jim Fish in Huddersfield, and set about building my own. Later I bought an AVO CT38 valve voltmeter. Many years later I found myself on a team developing a new scope model at HP! Gottem in the blood you might say.

That Marconi sig gen looks like good fun. Be careful in your choice of capacitors around the tuned circuits. The originals were likely silver mica high stability parts and may well have been OK. It's the paper ones used for power supplies and decoupling that are likely the main troublesome ones.

David

[Luxman1050]

Thanks David
Yeah I did notice that you can superimpose the signal your getting to the signal it should be or should say waveform which personally as you say at least you can see if there's a glitch somewhere. I think VTMs okay but cannot as previously said see what's going on.
I'm lucky to have got the manual as it has everything about this scope as well as all the schematics if needed but I think this has been looked after. Like say came with the front plastic cover, both leads in their original plastic folders with paper inserts showing how diffetent connections work and a large heavily padded carry case. I think only one probe has been used.
So I'll have to get a cheap telly �� �� and have a play around there. Suppose a valve TV be good?
Sig gen was a bargain again got all paper work for it and managed to get hold of the operators instruction manual.
So more fun ha. Apparently he wanted to me to see it working but having terminal cancer I have to keep my distance from people and def no no going in properties. Anyhow he said oh had her up and running and just blocked out radio 2 signal ha. Not sure how that works?
Yes the electrolytics def need changing usual bulging not looked properly at paper caps but what I've seen seem okay. I will change the crystal valve CV253 as it's the oscillator and I'm sure they drift over the years and being over 70 years old speaks for itself. Already done the filter on mains input. Odd one pic posted on here.
I will get round to scanning all docs have on it for other members if they happen to have one. The copy around think there's one not very eligible especially the schematic and components list.
Yes there are alot of they call special alongside capacitor numbers. Should think there fine.
Still lots to get on with and if you think of a type of television worth having a play on let me know so long as it's cheap and cheerful.

[G8UWM-MildMartin]

I was confused, but think I've worked it out now after an evening out:
The signal generator is not a TF 108A but a Marconi TF801A.
The crystal valve CV253 is not an oscillator, as Gunn diodes were not developed until 1962.
Nor is it a quartz crystal. It is a very early (1946) semiconductor diode originally intended for use as an X-band (microwave) mixer.
Some info here, including how not to test them:
https://www.silicon-ark.co.uk/datash...dling-1945.pdf

[Luxman1050]

Its CV253 mixer crystal valve 1946 it's situatated in a screened box within unit and marked as X2 on schematic to avoid damage from RF fields.
Don't recall anyone saying quartz crystal valve? I'll have a read through the posts. Yes it's a TF801A again don't recal it being called TF108A? My bag L63 is the oscillator
Pic attached where it is situated.

[Luxman1050]

Originally Posted by cdm1christopher

Blimey Craig thats great 👍 thank you.
I do like a read trouble is I've also got a WW11 TF 108A wating on manual for that big old beast mains input never seen one like it as had to change caps out. Two coils and 6 y caps pic attached.
The scope I think good bit of kit bit basic but suit my needs once I've had a few bedtime reads 😴 ha.
Also managed to get for couple quid a 500v wind up milli ohm metre come in handy as sometimes had low readings under 20 ohm not getting readings. All working fine. Also been offered by guy who lives nearby a military jungle proof power supply waiting on some pics sounds interesting. So all going on keeps me going. Sorry guys going off track here ew.
Cheers guys

Called typo error all do it

[Radio Wrangler]

Consumer tellies and scopes don't mix easily because of live chassis and that opens the whole can of worms of earthing and isolating transformers and it took long enough for the last thread on that to fizzle out.

A late telly with an isolated switch mode supply might be a better choice, or a monitor, or a CCTV camera. Those would be good for messing about with.

David

[Luxman1050]

Ah well cctv cameras pretty cheap used so prob an avenue to persue.
Yeah rember as kid getting bolts of those valve TVs on numerous occasions it took me a while to just leave the ****** things alone but they just fascinated me all those glowing bulbs. I also remember my Dad cheap scate using bolts for fuses in plugs. It was a very safe house electrically yeah right. I can rendering knocking out the whole street on numerous occasions with his make do mains input fuses. Not really sure what he was doing. Bought some electric blankets secondhand needless to say bare wires exposed so bedtime could at times be a bit shocking.

[Radio Wrangler]

My aversion to AC/DC live chassis lasts to this day. I won't touch them in either sense of the word.

It's one of nature's little miracles that the barrel of a 1/4 inch jack plug fits as a substitute for a 13A plug fuse. This handy coincidence is used by roadies throughout the realm.

David

[Luxman1050]

Well posted for a TV or cctv camera and got"You'll have to be more specific about the TV. Colour or B&W? Portable or large screen? 80s fake wood or 90s black plastic". So do I have to be that specific David?