(solved) Z modulation on super-cheap analogue oscilloscope

[hexreader]

I plan to buy Digimess MO10 oscilloscope tomorrow.

this one:  http://cpc.farnell.com/digimess/huc70-01-mo10/oscilloscope-10mhz-digimess/dp/IN02283?anyFilterApplied=false&searchref=searchlookahead&ddkey=http%3Aen-CPC%2FCPC_United_Kingdom%2Fc%2Ftest-equipment%2Foscilloscopes%2Fanalogue-oscilloscopes

I would like to add Z modulation, but I do not know how.

Any useful hints on how please?

schematic is attached below:


....  and yes, I know this is a terrible scope, but I want a definitely working cheap scope that is simple and easy to work with and purely for fun. I have better scopes for real work.

My suspicion is that any disturbance to brightness (Z modulation) might upset other circuitry, since the scope is sure to be built down to a price, not up to a spec.

All thoughts and/or opinons very welcome.

Many thanks...

EDIT:  I kind of have a sense of how most of the circuitry works, but CRT theory is a bit of a mystery. I am used to large +ve EHT attracting electrons to the front of the tube. Large negative voltage on heater seems odd, but presumably equivalent.

I see that C117 appears to pass fly-back blanking to CRT, but I sense that this will not help me too much, .... but maybe it would?

[xygor]

That's (C117) what you want.  Z-mod works on the grid, same as blanking.  Stay away from the grid since, as you noted, it's high negative voltage.  Sum your z-mod into the base of v117.  I would probe it first maybe on another scope to get a feel for what's going on there.  There is some feedback from the blanking circuit to the sweep and you don't want to affect that by your signal going back through R106.  (It takes more time for me to follow the schematic than what I want to spend.)

EDIT: I wouldn't worry too much about feedback as R106 is driven by a low impedance source, /Q from the FF.  Also the C117 capacitive coupling will cause the brightness to change with the duty cycle of the z-mod signal.  You'll have to live with that unless you want to do a much more extensive modification.

[hexreader]

Thanks xygor,

Am I right in thinking that a parallel capacitor to C117 (with some sort of added circuitry) will only provide a very short blanking signal at best?

Presumably a higher value capacitor for longer blanking pulse will drag down some other vital voltage? or am I making it up?

[xygor]

I don't think you need to mess with C117 at all.  The signal at the grid will be up to 100V p-p (1 to 100V on the left of C117, and a large negative voltage modulated by 100V p-p to the right)

[xygor]

Oh, I get what you're asking. If the blanking interval is constant with sweep rate, then you may need to parallel the cap.  I was thinking the blanking interval scales with the sweep rate in which case the capacitor is already sized for the longer interval.  I think you're kind of screwed though because the diode V119 and the time constant there is optimized for whatever the blanking signal is.  It won't hurt it to try.  It will just make the blanking not work right or the added z-mod to be non-linear.

[hexreader]

Thanks xygor 

Perfect answer.

I sensed that adding Z modulation was wishful thinking, but your replies confirm my suspicions.

Will buy the cheap-nasty scope just the same, but at least I don't need to expect miracles now.

Your input is very much appreciated.

Many thanks

[xygor]

Maybe get a not-so-nasty used Tek scope.  But it won't come with that new scope smell, so I understand.

[hexreader]

Old Tek makes sense, but I need new smell and low EHT (1.3KV) - and small size for minimal waste of bench space.

Custom modules and high voltage frighten me. I have been bitten by high voltage before

At the end of the day...   this is a toy for fun and education, not a professional tool.

I am 60 years old and miss analogue scopes - but not so much that I want to use one for any real job


EDIT:   I guess next question is how to 2nd scope around safely with RIGOL DS1054Z scope on MS10 that contains unexpected -1.3 kV  without spending a fortune on fancy high voltage probes?

[xygor]

Searching for the tube, I found this.

http://www.spectrum-scientifics.com/v/vspfiles/pdf/Properties_of_Electrons_Apparatus.pdf

Lot's of good info if you want to get into the CRT theory.  It uses the very same tube for the experiments.

[xygor]

EDIT:   I guess next question is how to 2nd scope around safely with RIGOL DS1054Z scope on MS10 that contains unexpected -1.3 kV  without spending a fortune on fancy high voltage probes?

a.) don't probe the tube. or
b.) stick a HV resistor in series with the cheap probe.

[hexreader]

Thanks xygor,

Very interesting link two posts up - will read fully later.

Time to find some high value resistors and find safe ways to add them to a probe - and gain enough courage to use them.

Much appreciated.

[vk6zgo]

Old Tek makes sense, but I need new smell and low EHT (1.3KV) - and small size for minimal waste of bench space.

Custom modules and high voltage frighten me. I have been bitten by high voltage before

At the end of the day...   this is a toy for fun and education, not a professional tool.

I am 60 years old and miss analogue scopes - but not so much that I want to use one for any real job


EDIT:   I guess next question is how to 2nd scope around safely with RIGOL DS1054Z scope on MS10 that contains unexpected -1.3 kV  without spending a fortune on fancy high voltage probes?

The 'scope you are looking at is the horizontal format version of the little Jaycar one I bought a few years back.

139.20 Pounds Sterling  seems like a "ripoff" to me----that corresponds to  $A240.80, for what is basically, the same device I bought for $A139.00 .
OK, inflation does happen----but, in general, the prices of test equipment have consistently fallen over the ensuing years.

That said, it isn't at all a bad little 'scope, & I used it for a lot of jobs which, without any kind of  Oscilloscope, would be a lot harder to resolve.
Of course, it wasn't any thing like what I was used to with work 'scopes.

What does happen to these, is that the knobs break after a while, the vertical input BNC connector starts to unscrew, which makes the wire on the back fall off-----all easily fixable.
The tube HT switchmode supply croaked on mine, but it was also fixable without much hassle.

I eventually bought a Tektronix  7613 from a Hamfest,
Forty or so years old, & it hasn't failed yet-----& it cost the same as the little 10MHz Jaycar thing.

I'm even more ancient than you, & I've only used one digital 'scope for real work.
Even then, when an old  analog with one faulty channel became available, I dropped the DSO like a "hot spud"!

Of course, modern DSOs are a lot better than those early beasts.

[hexreader]

Thanks vk6zgo,

I can live with stuff breaking after a while. It might be fun to fix, or just time to bin the thing, ... or maybe convert to some other pointless purpose. My main criteria is that it all work well when I buy it, which is hard to be sure of when I buy second hand.

As for a rip-off then maybe, but I see no cheaper NEW analogue scope. Considered better spec scopes, but size gets bigger, the price triples and EHT voltages get to be really scary.

This is purely for fun, education and reminiscing, so practicalities don't count for too much.

I have Rigol MSO1104Z-s for hobby MCU development, which is way more than I actually need, but nice to own.

EDIT:   Important safety note :   I am well aware that CRTs are dangerous and that high voltages kill.  If there are any children following this thread, DON'T DO THIS AT HOME!. Stick with low voltage equipment

[hexreader]

Searching for the tube, I found this.

http://www.spectrum-scientifics.com/v/vspfiles/pdf/Properties_of_Electrons_Apparatus.pdf

Lot's of good info if you want to get into the CRT theory.  It uses the very same tube for the experiments.

Wow - that PEA001 suitcase looks really cool and fun. When I am a millionaire (ain't gonna happen) I will invest $2200 to buy one. - or maybe I can turn my mo10 into one

I will add it to the growing pile of projects that I will never get around to completing ....