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Old 9th Jun 2023, 7:34 pm   #1
agardiner
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Default Sinclair / Thandar SC110A Oscilloscope

Hi,

So I have started restoring this little scope that was recently kindly donated by gingpeakin. So on arrival there was just a spot and the timebase was not running. The Y amplifier appears to work, although I have not yet conducted any proper diagnostic checks. I have stripped the unit down and got the timebase working again. The front panel has yellowed quite badly and I am going to retrobright it. There were just a couple of loose wires in the battery compartment which had a foam pad glued into place. This has been removed and a new C cell battery box ordered. All controls were crusty and intermittent, but a good dose of switch cleaner has resolved that. Next step is to go through operational checks to see what else might be needed.

I do have a couple of questions though for anyone that owns one of these. I think this unit is quite high hours, as it was originally a BBC outside broadcast scope. All pictures I have seen of these scopes seem to show a really bright trace. Currently my trace seems quite low to me, although the service manual shows an EHT of only 485V which is quite low for the tube spec. Can someone confirm if the trace should be nice and bright? I note you can still get replacement CRTs if needed.

Also, (with the scope out of the case) I note that when you shift the trace using the X control, you cannot move it off the screen. It 'dissolves' into a spot on either side. Is this also normal or is my X amplifier failing to drive properly? The Y control does allow the trace to be moved completely off.

Many thanks.
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Old 10th Jun 2023, 8:02 am   #2
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Default Re: Sinclair / Thandar SC110A Oscilloscope

Yes, the brightness on these is usually more than acceptable, although whatever brightness there is to start with is always going to be reduced by the blue filter placed in front of the tube, the tube phosphor itself being white because the tubes were originally used in Sinclair mini TVs. The trace was always quite 'thick' even when adjusted for best focus.

I have owned mine from new and when I first had it, I was working on city centre CCTV systems and it had quite a tough life riding around in my toolcase.

In truth it was never a great scope but it was supremely portable and better than no scope at all when I did need one.

I can't answer your question about the behaviour of the horizontal shift just now. If no-one gets back to you sooner I will fire mine up next week to have a look.

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Old 10th Jun 2023, 10:12 am   #3
agardiner
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Default Re: Sinclair / Thandar SC110A Oscilloscope

I know I am missing something somewhere, , but I cannot quite figure out how the tube is driven. I have read and re-read the manual, but it is a bit disjointed. How can the EHT be a negative value? It is marked as -540V which ends up at around -485V on the CRT. Surely for the electrons to be attracted, the cathode would have to be at least -1000V for this arrangement?

What am I missing?
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Old 10th Jun 2023, 10:46 am   #4
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Default Re: Sinclair / Thandar SC110A Oscilloscope

That apparent paradox is very easily explained!

In order for the X and Y plates of an electrostatic deflection CRT to be conveniently and safely driven from transistor circuits, it is necessary for the final anode and the deflector plates to be at a low positive potential with respect to chassis earth!

In order for the CRT to have its specified acceleration voltage, it is therefore necessary to drop the cathode/heater, grid and 1st anode BELOW earth by the amount of the accelerator voltage specified by the CRT maker, hence the e.h.t. is a large NEGATIVE voltage relative to earth!

Always remember that CRT electrode potentials in data sheets are quoted relative to cathode, not earth – so, if we have a CRT requiring 450 V third anode potential, as in the SC110, we connect A3 to earth and supply the cathode from 450 V negative, and in order to provide the negative intensity control voltage for the grid, the bottom end of the intensity control is returned to a point of greater negative voltage below cathode potential, hence –525 V in this example, so the grid is between –25 V and –75 V with respect to cathode!

This explains why the focus anode is also at a large negative voltage in most Oscilloscopes – for a typical dual beam workshop/laboratory type instrument with an e.h.t. of –1200 V, the focus anode will typically be –700 V to –900 V relative to earth, but +300 to +500 V relative to cathode!

Incidentally, the data–sheet for the CRT, a Telefunken D5–100WB, recommends a minimum acceleration voltage supply of 1000 V, so Sinclair are operating the CRT well below the minimum voltage specified by the CRT maker, so the trace intensity is not going to be as bright as the CRT is capable of – the maximum rating of the CRT (relative to cathode) is 2000 V!

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Old 10th Jun 2023, 1:25 pm   #5
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Default Re: Sinclair / Thandar SC110A Oscilloscope

Thanks Chris for the explanation. I think I get it, will read a couple more times and then have a lie down to think about it!

Best regards,
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Old 10th Jun 2023, 2:08 pm   #6
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Default Re: Sinclair / Thandar SC110A Oscilloscope

High Adrian,

I have just checked mine, when the x shift is adjusted the trace collapses to a spot at the extreme ends of the trace.

The display is quite bright on mine. I don't know if these tubes were intended for tv or oscilloscope use, if they are intended for tv use the accelerating voltage could be reduced for use in an oscilloscope to increase sensitivity as the brightness would not be required.

Dave
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Old 10th Jun 2023, 2:36 pm   #7
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Default Re: Sinclair / Thandar SC110A Oscilloscope

Thanks for checking Dave. And I presume this spot at the end of the trace is just inside the viewable area when the scope is in its' case? Mine is before the ends of the tube, but that is with the unit on the bench rather than in the cabinet.

The CRTs were originally intended for TV use in the microvision. I think the brightness is OK, but lower than perhaps it originally was.

Assuming the unit is indeed high hours, would the CRT suffer from cathode poisoning given the low EHT? It is not phosphor wear as the screen is still even and no burn in.
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Old 10th Jun 2023, 3:24 pm   #8
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Default Re: Sinclair / Thandar SC110A Oscilloscope

The trace ends are only just visible when in the case. The low EHT shouldn't cause cathode poisoning but if used at high brightness the beam current would be high could that lead to stripping? Have you checked the +5V rail? This is the only supply rail that is regulated.

Dave
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Old 10th Jun 2023, 7:36 pm   #9
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Default Re: Sinclair / Thandar SC110A Oscilloscope

If this has been out of use for a long time, the trace may well brighten up with more regular use.
The CRT is a mono-accellerator type, and all mono-accellertors operate with a big negative cathode voltage.
The more common tubes having a PDA (post deflection accelleration) have negative cathodes, plus positive final anode voltages, usually about five times the magnitude of the cathode voltages.
The higher the total deflection voltage, the finer the focus will usually be, and the brighter the trace.
Regarding the Y circuit; most Y-amps have balanced voltages on the deflection plates. As one goes positive,the other goes negative (well, less positive) giving good deflection linearity.
The SC110 has a single ended deflection system, so a bit odd if you are not aware.
Les.
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Old 10th Jun 2023, 8:03 pm   #10
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Default Re: Sinclair / Thandar SC110A Oscilloscope

It's pretty standard to have the deflection plates close to ground potential, to make DC-coupled X and Y amplifiers easier with transistor circuits. High performance tubes may run a post-deflection accelerator (PDA) several kilovolts above ground. But, as said, the cathode, grid and focus anodes are run strongly negative. In many good quality scopes, the grid and cathode are rum at about 2kV negative.

This leaves the problem of controlling the difference between grid and cathode in order to adjust brightness and to allow retrace time to be blanked. With triggered timebases which can wait indefinitely for a trigger to happen, the blanking needs to be DC coupled and the driving amp needs too suit transistors.

So, many scopes have DUAL -2kV supplies, matched to each other, and the groundy reference of one is offset from the other to control the grid-cathode voltage. There are other ways, but they have to come to the same thing.

This makes Z-mod and blanking a bit messy, but it makes the Y and X amps a lot easier and brings a net performance advantage and cost saving.

David
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Old 10th Jun 2023, 11:56 pm   #11
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Default Re: Sinclair / Thandar SC110A Oscilloscope

One point worth noting is that some CRTs, such as the D14–180GH used in the Gould OS4000, Telequipment D65 and the 1975 Wireless World 50 MHz Oscilloscope Designs uses an auxiliary blanking electrode which either repels the electron stream towards the first anode structure when negative to the first anode, but when at the first anode potential, allows the electron beam to pass through the gun unhindered – this electrode obviates the need for a separate floating e.h.t. supply at C.R.T. grid potential for unblanking purposes, since the sweep unblanking voltage from the timebase can be directly coupled to the blanking electrode at low voltage.

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Last edited by Chris55000; 11th Jun 2023 at 12:02 am.
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Old 11th Jun 2023, 1:18 am   #12
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Default Re: Sinclair / Thandar SC110A Oscilloscope

The blanking electrode approach saves a lot of trouble and works great for blanking the trace outside timebase run time, but the dual EHT method allows reasonably linear brightness modulation which extends the scope's capabilities into greyscale (Greenscale?) picture display.

David
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Old 11th Jun 2023, 8:34 am   #13
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Default Re: Sinclair / Thandar SC110A Oscilloscope

Thanks everyone for the interesting discussion & suggestions. My next step is to go through each stage with the manual, starting with the power supply. If everything checks out OK, then I might consider replacing the tube.
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Old 16th Jul 2023, 6:28 pm   #14
agardiner
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Default Re: Sinclair / Thandar SC110A Oscilloscope

OK, an update on this project.

The power supply has been thoroughly checked and all supply rails are correct. I have measured the EHT using my main oscilloscope and this is also correct, as is the 10V blanking supply.

I was also able to borrow another CRT from a microvision and there was no difference. Likewise, my CRT worked OK in the microvision, so I believe the CRT is fine.

Now, I am not expecting miracles, as I know this scope uses a low EHT, but the trace really seems very dim to me and certainly when compared with any photos of one of these scopes working online. I attach a couple of photos of my scope running on the bench, the close up one with the blue filter.

What does anyone think? Does mine look right or do you think there is a fault and if so any idea what I should be looking for next?

Thanks all.
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