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Old 27th Sep 2018, 8:17 pm   #1
RubberGloves
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Default Mullard 1 inch oscilloscope. (Pupil or Studentesque)

Good evening folks, I'm very pleased to have found and joined your forum, so a big hello to you all.

I was wondering please if anyone has any circuit information for an old oscilloscope that I'm trying to get working?

It's been in my family since about 1977 when dad had a dabble in electronics and I was six! I guess he bought it second-hand for peanuts in Chadwick's or Worthington's in Stoke-on-Trent, and from memory never had it working.

It's using Mullard DH3-91 1 inch crt and two separate EF80 valves. I'm a beginner, with some knowledge from an Electronics HNC from the dim and distant. I've made a promising start but ground to a halt due to lack of available data and lack of available knowledge!!

Good news is, I powered it up on low voltage AC and then mains, and the HT came up to about 350Vdc which I guess is about right?... All heaters light too. With the two EF80 valves removed the crt beam spot is central on the crt face - I injected about 20Vac onto the Y plates the other night and the crt seems good showing Y deflection.

Bad news is that with the EF80 valves fitted the trace is off the screen and I am now getting out of my depth, and very out of my depth without a circuit diagram!! I did manage to have a probe about with a scope to look for the X timebase ramp, but could only find a sine wave that could be a red herring and nothing like a triangle ramp. Hmmm... So could have a Y-amp problem or timebase problem, or both!

The unit is built on a good quality 'kit-type' metal chassis. Front panel controls are Y gain, Sync, TB range 1-2-3-4, TB fine and Brilliance On/Off. Front panel has UHF input socket and 6.3Vac 4mm output socket. Rear panel has 4mm sockets for HT, 6.3Vac again, Earth and two more unlabelled 4mm yellow sockets that seem to be Xin or Xout or both!

I would be really very grateful if anyone would be able to help me out with some circuit information or advice.

Thank you for reading folks, Rob.
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Old 28th Sep 2018, 2:18 am   #2
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Default Re: Mullard 1 inch oscilloscope. (Pupil or Studentesque)

There's a two pentode miniature 'scope using the 1CP1 (US version of your tube) in August 1960 Popular Electronics.

https://www.americanradiohistory.com...nics-Guide.htm

For full details.

It may just be similar enough to help, though it uses EF91 type valves.

There was a Mullard student scope, but it used triode pentode and had an inverter type PSU.
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Old 28th Sep 2018, 7:09 am   #3
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Default Re: Mullard 1 inch oscilloscope. (Pupil or Studentesque)

Hi Rob

It seems as if your timebase has stopped working.

What kind of capacitors are used?

It going to be something like an open circuit resistor or a leaky capacitor most of the time.

Some types of capacitors notebly wax paper capacitors are known to get very leaky.

A couple of pictures would be very nice

Cheers

Mike T
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Old 28th Sep 2018, 7:36 am   #4
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Default Re: Mullard 1 inch oscilloscope. (Pupil or Studentesque)

Practical Wireless March 2005 - Radio Basics column has most of what you need from a quick squizz, including the circuit diagram.

Circuit diagram also here:
https://www.instructables.com/id/Mul...-Oscilloscope/

The mention of two seperate EF80's in the OP's post puts the above info in doubt, although all searches for Mullard Pupils Oscilloscope ends up at the same circuit diagram (on various different sites).
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Old 28th Sep 2018, 6:39 pm   #5
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Default Re: Mullard 1 inch oscilloscope. (Pupil or Studentesque)

Evening chaps and thank you for your advice.

Yup - am struggling a bit and keep getting stuck in internet searches that all end with the Mullard Pupil's Scope circuit diagram on the Instructables website.

Seems like the scope I have with the two EF80 pentodes is slightly more developed than the Mullard Pupil's scope and less developed than the Mullard double beam scope. Hmmmmm! Maybe it's not even a Mullard design - it is somewhat similar schematically to the scope helpfully suggested by Herald 1360 although I guess mine is a British design.

Here are some photos. Not sure what type of caps those black ones are, or whether they are notoriously bad or good! They are 0.1uF and 0.05uF.

All further clues are very welcome.

Cheers chaps,

Rob.
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Old 28th Sep 2018, 8:11 pm   #6
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Default Re: Mullard 1 inch oscilloscope. (Pupil or Studentesque)

Hi Rob, see if you can post a picture of the chassis.
As far as I remember the pupils scope was a 2" tube. There was a practical TV design that used a VCR97, 6" tube, but had 2, EF50/80 valves for Y amp and TB.

There were lots of circuits about using 1" tubes, including one for a TV (with a few more valves).
Carry on the good work, you have made a good start with the fault finding.
Try leaving the TB valve out and fault finding on the Y amp first, then, when that is working, start on the TB.

Ed
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Old 28th Sep 2018, 9:43 pm   #7
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Default Re: Mullard 1 inch oscilloscope. (Pupil or Studentesque)

First step is to replace ALL those black Hunts capacitors with modern ones with same capacitance value.
It is not a "Triggered" scope, with automatic lock to the incoming waveform, but instead the time base runs at a frequency determined by the T/B speed setting, fine tuned by the "Sync." control to make the display stationary (in horizontal direction).
Regarding the spot being off the screen, with most 'scopes, you put your testmeter (on say 100v range) across the two Y pins, or X pins, and expect a range of (say) -40v to + 40v, with central spot at around 0v. However, this is almost certainly NOT like that, and voltage more likely to go from maybe +30v to + 100v (could be much higher), with the trace central at about +65v. With no Y input (shorted out preferably), there will likely be an internal preset allowing the spot to centralise at (in the suggested case above) +65v.
But with those caps replaced, it will probably just work.
Les.
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Old 28th Sep 2018, 10:20 pm   #8
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Default Re: Mullard 1 inch oscilloscope. (Pupil or Studentesque)

Hi!

I think you'll find it's the British National Radio School "Learnakit" Design of oscilloscope, simplified down to a single EF80 for the vertical amplifier, and a single EF80 timebase, based on the Miller–Transitron circuit with screen–grid/supressor–grid coupling!

The DH3–91 and it's American Equivalent, the 1CP1, is a "loctal" (B8G) based minature CRT with asymmetric deflection (one x and one y plate is internally connected to the third anode) and has automatic focus and brilliance control, needing only a single 680k cathode resistor to set–up the bias.

If you search "Learnakit" you'll find a diagram of the original full design, I'll see if I can draw the basic DH3–91 1 inch version out for you when I've time to spare!

The "Learnakit" has the 4–way T/B switch and the single gain control described by the OP and I'm fairly certain they used the same chassis as well!

Incidentally, several people have come on here in the past and said they've got the original B.N.R.S. "Learnakit" oscilloscope notes and course experiment notes, how about getting busy with a scanner sometime? There's a number of other Members wanting these notes!

Chris Williams
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Old 29th Sep 2018, 10:06 am   #9
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Default Re: Mullard 1 inch oscilloscope. (Pupil or Studentesque)

Could this possibly be the circuit?
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Old 29th Sep 2018, 4:19 pm   #10
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Default Re: Mullard 1 inch oscilloscope. (Pupil or Studentesque)

There is a slight error on that - the diode type is BYX10
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Old 29th Sep 2018, 4:50 pm   #11
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Default Re: Mullard 1 inch oscilloscope. (Pupil or Studentesque)

Hi!

That was the full valved design with the larger 3" CRT on as supplied by the B.N.R.S. for their "Learnakit" Oscilloscope Based course, however a few simplifications necessitated by the auto–focus nature and internally connected Y plate of the DH3–91 are needed, and the draughting of a revised circuit diagram for completeness is in progress (up till 2 a.m. drawing!!) – I'll post it as soon as I've completed it!

Chris Williams

PS!

Can anyone who's got one of these to hand tell me the value and voltage–rating of the dual–can electrolytic C12a/C12b that was supplied please – for some strange reason it"s not shown on the B.N.R.S.'s drawing – I always thought it was twice 32 @ 500V – can anybody confirm this?
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Old 30th Sep 2018, 9:20 am   #12
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Default Re: Mullard 1 inch oscilloscope. (Pupil or Studentesque)

Thank you very much indeed chaps for all the fantastic information.

The scope does indeed look like a simplified Learnakit scope. The Y amp appears to be identical and probably the time base generator too.

I will have more of a look at it during the week. I'll have a look at the caps and might buy a cap tester too. I'll start with the Y amp as suggested - I did vaguely try this last week, but I had incorrectly presumed that the valve with the aluminium screen was the ramp oscillator. It's not, it's the Y amp, so I'd taken the wrong valve out - what a RodneyPlonk!...

Thanks again and will let you know how I get on.

Rob.
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Old 30th Sep 2018, 5:23 pm   #13
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Default Re: Mullard 1 inch oscilloscope. (Pupil or Studentesque)

With many old radios, some will advise "Change all wax caps", whilst others will say "You should not change them all, it does not help with a proper repair".
However, in this case, I suspect nobody with any oscilloscope experience would disagree with "CHANGE all those black Hunts capacitors". Don't waste time measuring them. On the Telequipment S31, much more sophisticated but probably similar vintage, Brown Wima capacitors were used. Exactly the same advice, gleaned from experience, Change ALL of them. All a capacitance tester will likely tell you is that they have either gone short circuit, or their values have gone up considerably. In fact that "increase in value" is the tester unable to measure capacitance, instead being influenced by the increasing leakage whilst on its way to becoming short circuit.
Les.
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Old 30th Sep 2018, 8:43 pm   #14
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Default Re: Mullard 1 inch oscilloscope. (Pupil or Studentesque)

Thanks for the advice regarding the capacitance meter Les - I've just ordered a full set of caps and will change all of them. I'll give the capacitance meter a miss for now too!

I dug out my old Avo 8 earlier and it seems that the caps are leaky (I tested a newer high voltage cap that I had lying around and that tested well on the Avo). All caps except one are made by R.T.C. and the larger one by Hunts - I guess the R.T.C. caps are just as suspect as the Hunts?...

Good news today - injected roughly 1V pk-pk (50Hz) into Y amp today with ramp-gen valve removed and it looks like Y amp works. Got about 60V pk-pk on the anode with full Y-gain set and this gave roughly full screen Y deflection too (1 inch pk-pk!!). Also swapped the valve for the valve out of ramp-gen and got about 80V pk-pk so looks like both valves are ok.

Plugging in the ramp-gen valve causes the trace to exit the screen horizontally to the right - fingers crossed for the big cap change!

Cheers chaps,
Rob.
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Old 30th Sep 2018, 11:32 pm   #15
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Default Re: Mullard 1 inch oscilloscope. (Pupil or Studentesque)

Quote:
Originally Posted by RubberGloves View Post
I guess the R.T.C. caps are just as suspect as the Hunts?...
Yes. Hunt's just happened to be the dominant UK supplier back then, so get the same accolade as Hoover or Tannoy. It's the paper dielectric that's proved to be problematic. Some varieties fare marginally better than others, but all are suspect by now. The long term properties of early plastics didn't help either!
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Old 2nd Oct 2018, 8:51 pm   #16
RubberGloves
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Default Re: Mullard 1 inch oscilloscope. (Pupil or Studentesque)

Hi Chris 55000,

I just realised that my scope has a double 32-32u cap as you have described.

It's a Plessey cap in a 'silver' ali can. Working and surge voltage are printed but obscured by the clamp - I'll try and have a look when I get a bit further with the scope (don't want to disturb the cap and clamp at the mo as don't want to create any more variables!).

Cheers Chris, Rob.
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Old 3rd Oct 2018, 5:50 pm   #17
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Default Re: Mullard 1 inch oscilloscope. (Pupil or Studentesque)

Hi!

Cheers Rob!

I've marked it up in my diagram as 32uF + 32uF at 550V working, 625V surge which will be the most likely!

My diagram is almost completed & I'll be posting it on here in a day or two's time!

Chris Williams
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Old 3rd Oct 2018, 10:57 pm   #18
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Default Re: Mullard 1 inch oscilloscope. (Pupil or Studentesque)

Wahoooooooooooo!!!

'tis workin'!!!!

Changed a few caps to no avail. Then changed cap linking ramp gen valve anode to crt X plate and away she went

Most caps were a bit leaky, measuring about 120k on Avo. This particular cap was particularly bad at 90k and was the culprit.

All good on TB2, TB3 and TB4 settings. TB1 setting is missing right hand 25% of scan - my scope does not have the 'width' pot shown on the BNRS circuit diagram which may largely explain this.

Just waiting for delivery of a couple more caps. Will try and borrow a sig gen and post some more info soon.

Thank you ever so much to all of you for your help. And Les, yes, change all caps!

Rob
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