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Old 30th Dec 2011, 4:02 pm   #1
Anthony
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Default Cossor CDU110 'Scope: Brightness Probs.

Is any one familiar with this instrument? The trace on my work-horse (15 years with me) has been progressively more out of focus for the last few weeks. I have cured it by replacing a couple of 2.7Meg resistors which had gone high adjacent to the focus control.

Although there is now a nice sharp trace, the trace intensity is now very poor. The 'Brightness' control kicks in suddenly from nothing and then has little effect on the brightness. Cathode voltage appears fine at -1kV but I cannot achieve the manual's spec. of 0.45-0.65 volts across R50 by tweaking the 'Set Beam Current' 5K wire-wound pot (RV50). R50 and RV50 are spot on. +100V at one end of RV50 is also spot on. Nothing else obviously wrong except burning on the board (long-standing) around one joint of R51 which itself reads fine. Have rewetted that joint. Also recapped C50 to no effect.

I must confess to not understanding how the 'Brightness' control works on this insturment. Have not (yet) attempted to measure the +3kV EHT.

Any ideas? Will be thankfully received.

Anthony
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Old 31st Dec 2011, 1:13 am   #2
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Hi Anthony, Check all supply rails to make sure they are okay then you need to measure the EHT - as you are hinting at.

Peter
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Old 31st Dec 2011, 7:32 am   #3
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Peter. Many thanks. Will report back. Anthony
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Old 31st Dec 2011, 1:20 pm   #4
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

When I said 'Brightness', I meant I do not understand how the 'Beam Current' control works. Apologies.

Following Peter's helpful suggestion have checked all rail voltages and EHT - all good. Also the Grid voltage seems to vary sensibly with the Brightness Control, going from -1126 to -849V. The trace appears at about -999V but further decrease of the negaive voltage makes little difference. That does not seem right?

Have recapped C64 and C65 to no effect. The 'Beam Current' setting measured across R50 is still out of spec. I cannot get it above 0.35V whereas the manual shows 0.45-0.65V.

Could it be the tube itself?

Any suggestions again gratefully received. Thank you.
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Old 1st Jan 2012, 11:14 am   #5
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

CDU110. The circuit in the manual is singularly unhelpful. It is drawn almost as if to make understanding the circuit difficult. I should not worry too much about the beam current setting, as long as you have a nice bright beam (which you haven't!). It (RV50) moves the base of the "cathode-focus-feedback to EHT oscillator-earth" chain up and down within 100V, thus limiting the maximum range of the brilliance control. Most other manufacturers use the "setEHT" control to achieve the same purpose.
I suggest your problem is either:
1. The top of that chain R66-focus-R64-RV51-R55 is low. It should be about 1kV. The sharp focus you now have obtained may be just luck for the particular cathode voltage you have.
2. The grid supply is low. The brilliance comes off the top end of the transformer winding, point e. The bottom end of the winding, point d floats on the bright up circuit, emitter of VT59. I suspect the extra resistor chain R85-R86 is a feedback mechanism to correct for main voltage variation, or to filter the grid supply, and shouldn't have much effect on your problem.
But have you checked the bright up voltage, at Vt59 e, or R77/R81. I would expect it to be around +60v for bright trace, going to +20v for minimal trace. Switch to XAmp to avoid confusing the issue by TB brightup pulses, when you should have a steady +56/60V.
I have found that brightness problems are usually either the focus or brilliance resistor chains going high (under voltage stress), or the brightup high voltage transistors failing (as VT57,58,59 in this scope).
You have measured the EHT voltages. Your curious results for the grid may be due to current taken by your voltmeter. It is normally bad practice to measure the grid voltage, as it is at very high impedance, and the current taken by your meter will alter the circuit conditions and so voltages drastically. An Avo on the grid will easily destroy the tube (that is why there are the two diodes from k to g -MR44-45). A 10M digital will confuse. A 100M probe will be satisfactory.
3. Can you tell us what the brightup voltages are. That may be the key.
Do let us know what you find.
You may be interested to know that the power supply has failed on my CDU110, eventually traced to failure of the dual electrolytic C10/C11. It needed reforming, as the positive plates of the two parts of the capacitor had shorted together, but one operates at +20V, and one at +0V, and so shorted out the -50V rail regulator.
When this is repaired, I will measure a few voltages for you.
I attach the circuit of the EHT supply so that people can follow the discussion, and contribute. Bill m0wpn
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File Type: pdf CDU110_EHT circuit.pdf (111.4 KB, 94 views)
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Old 1st Jan 2012, 1:22 pm   #6
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Dear Bill
Thank you so much for this magisterial assessment of the problem. I need to go through it carefully and will definitely get back to you and others. Many thanks. A good start to 2012 I hope. Best wishes for the New Year. Anthony 2E0LFT
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Old 2nd Jan 2012, 7:29 pm   #7
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Sorry for the delay in replying. Decided (in frustration with the problem) to put it away for 24 hours to recover. My results should appear in italics below.

CDU110. The circuit in the manual is singularly unhelpful. It is drawn almost as if to make understanding the circuit difficult. I should not worry too much about the beam current setting, as long as you have a nice bright beam (which you haven't!). It (RV50) moves the base of the "cathode-focus-feedback to EHT oscillator-earth" chain up and down within 100V, thus limiting the maximum range of the brilliance control. Most other manufacturers use the "setEHT" control to achieve the same purpose.
I suggest your problem is either:

1. The top of that chain R66-focus-R64-RV51-R55 is low. It should be about 1kV. The sharp focus you now have obtained may be just luck for the particular cathode voltage you have.

I have double and triple checked the resistors on the focus and brilliance chains and indeed the original focus problem was due to R64 and 65. The rest are depressingly good.

Measured with my homebrew 100Meg probe the top of this chain is -943 volt so O.K.

2. The grid supply is low. The brilliance comes off the top end of the transformer winding, point e. The bottom end of the winding, point d floats on the bright up circuit, emitter of VT59. I suspect the extra resistor chain R85-R86 is a feedback mechanism to correct for main voltage variation, or to filter the grid supply, and shouldn't have much effect on your problem.
But have you checked the bright up voltage, at Vt59 e, or R77/R81. I would expect it to be around +60v for bright trace, going to +20v for minimal trace. Switch to XAmp to avoid confusing the issue by TB brightup pulses, when you should have a steady +56/60V.

This is interesting. The voltage at eVT59 is +89.3V with my DMM or +82.6V with the 100 Meg probe (On X-Amp). What also seems a bit odd is that it does not vary at all with the Brilliance control (?).

I have found that brightness problems are usually either the focus or brilliance resistor chains going high (under voltage stress), or the brightup high voltage transistors failing (as VT57,58,59 in this scope).

I have measured the P-N junctions in VT57,58 and 59 and they are all rectifying. Something at least.

You have measured the EHT voltages. Your curious results for the grid may be due to current taken by your voltmeter. It is normally bad practice to measure the grid voltage, as it is at very high impedance, and the current taken by your meter will alter the circuit conditions and so voltages drastically. An Avo on the grid will easily destroy the tube (that is why there are the two diodes from k to g -MR44-45). A 10M digital will confuse. A 100M probe will be satisfactory.

You caught me out! I did use a DMM to measure the grid voltage but even I would not have used an AVO. I remeasured with my 100 Meg probe but (sadly) pretty much the same results as with DMM, that is, -866V with Brilliance fully CW (brightest), -1117V fully CCW and 975V when the trace appears with little or no change as it goes fully CW.

3. Can you tell us what the brightup voltages are. That may be the key.
Do let us know what you find.

As above. You say 'voltages' - did I miss one or more?

You may be interested to know that the power supply has failed on my CDU110, eventually traced to failure of the dual electrolytic C10/C11. It needed reforming, as the positive plates of the two parts of the capacitor had shorted together, but one operates at +20V, and one at +0V, and so shorted out the -50V rail regulator.
When this is repaired, I will measure a few voltages for you.

That is a tough one.

I attach the circuit of the EHT supply so that people can follow the discussion, and contribute. Bill m0wpn

Given that the actual CR tube voltages seem resonable I still wonder whether the problem is not the CRT itself but very much hope it is not.

Can you interpret the bright-up results?

Sincerely, Anthony
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Old 2nd Jan 2012, 10:47 pm   #8
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Quote:
Originally Posted by WME_bill View Post
I suspect the extra resistor chain R85-R86 is a feedback mechanism to correct for main voltage variation, or to filter the grid supply, and shouldn't have much effect on your problem.

Bill m0wpn
Hi Bill, I've a feeling that these resistors set the time constant to give a similar decay of voltage on the grid as the cathode ie. to minimise the grid- cathode voltage, at power off.

Antony, have you checked RV56 for an open circuit track / wiper?
You say you have around +90V on eVT59, as long as this does not vary, then it's probably ok. Even if it isn't, this would cause a different (bright-up) problem.
In fact it should not vary, but the negative end of the "variable 1000V battery" should with adjustment of the brightness control.
Can you clarify what you mean about the operation of the brightness control "coming on suddenly and then no effect"?
As Bill says, be very careful taking voltage measurements around the CRT grids.
Rob.
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Old 3rd Jan 2012, 4:01 am   #9
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Rob - thank you very much for your input.

I'll reply as below:-

Antony, have you checked RV56 for an open circuit track / wiper?

Yes both are good. Track is 2.37 vs. 2.2Meg. Wiper gives full and smooth resistance change. (RV53, Focus, also OK).

You say you have around +90V on eVT59, as long as this does not vary, then it's probably ok. Even if it isn't, this would cause a different (bright-up) problem.
In fact it should not vary, but the negative end of the "variable 1000V battery" should with adjustment of the brightness control.
Can you clarify what you mean about the operation of the brightness control "coming on suddenly and then no effect"?

Fully CCW trace is blanked (Grid at -1117 (measured with 100Meg probe)), turning it CW, the trace appears suddenly (corresponding to -975V) and further clockwise rotation of the Brilliance control has little effect. So the Brilliance control is more of an 'on-off' control than continuous although the grid voltage variation is continuous.

As Bill says, be very careful taking voltage measurements around the CRT grids.

Point well-taken

Again, I really appreciate your thoughts. I feel a bit stumped at this point. Comparison voltages with a well-behaved model would be helpful. I do not understand the eVT59 voltage readings.

Anthony
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Old 3rd Jan 2012, 9:59 am   #10
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Hi Anthony,
thanks for the reply.
If eVT59 is sitting at +85V, this seems about right for bright-up condition. You say you do not understand the eVT59 readings, do you mean you don't understand how the circuit works, or why you are getting the reading you do?
Once you get to the point where the trace does not get any brighter, this is probably due to MR44 + 45 being forward biased and clamping g1 at around +1V wrt cathode.
Can you connect your dmm between g1 and cathode to confirm this? Connect the meter whilst switched off and use good insulated test leads, do not handle the meter or leads whilst carrying out this test.
If you're getting a normal reading, say -100 to +1v as you adjust the brightness control, then something else is limiting the brightness.
Is the +3kV supply holding up?
Regarding the trace you can get, try displaying a high amplitude, high frequency sine wave to try to fill the screen up as much as possible. How does the brightness compare to just displaying the straight line with no input?
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Old 3rd Jan 2012, 11:43 am   #11
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Hi Anthony,

In TV sets (CRT types) this kind of low brightness problem is often caused by low emission from the heater.

This can be caused by either a low emission tube (I know that you don't want to hear that !!), or by low heater volts.

Have you checked that the heater is nice and orange glowing ?

Have you checked the voltage across the heater ?
This may be tricky to do if the voltage is not DC or 50Hz AC.
In TV's this was often derived from a winding on the Line output transformer and was really pulsey, but had an RMS voltage of 6.3V or whatever.

The circuit that was posted did not show the heater voltage circuit, so I can't comment on this.

It is possible that low emission would also account for not being able to set the voltage on RV50.

Regards,

IanF397.
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Old 3rd Jan 2012, 12:01 pm   #12
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

The power stages for 'scopes don't bear much resemblance to those found in television practice as a rule.

Refering to the circuit which Bill has provided in post number 5 this one is no exception. The heater voltage will be derived from a transformer winding (I can't recall if it's AC or DC) and should be easy to measure. Scope tubes are very forgiving of lower than ideal heater voltages and in over 30 years of working with them I've found very few low emission 'scope tubes.

That said a quick check of R72 which ties one side of the heater to the cathode might be a good idea if you haven't already done so (I'm at work so don't have time to read back through all the posts).
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Old 3rd Jan 2012, 3:31 pm   #13
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Smile Re: Cossor CDU110 'Scope: Brightness Probs.

CDU110. This is getting most interesting. Particular thanks to Rob "glowinganode" as well. I would agree with his advice.

1) I am afraid I was wrong in what I said about the two resistors R85, R86 from brilliance to brightup amplifier output. Thinking afterwards, the two capacitors C64,C65 are to transmit the fast rise front of the brightup pulse quickly to the tube grid. The rest of the pulse comes more slowly after the capacitance of the transformer has charged. You can sometime see this on any waveform using a very fast TB scan, when the trace brightness fades a bit (say after 2cm) and then recovers.
The resistors are merely to divide the voltage between the two capacitors equally. They are only rated at 630V, presumably as much cheaper.

2) The voltage on the slider of the Brilliance control should vary smoothly from -1kV to -800V. Where are you measuring? Brilliance control end of R84, or grid end? Try disconnecting R85/C64 from R84. That will eliminate that area. If the tube grid voltage is sticking at -975V, it suggests that something is pegging the grid at the other end of R84 to that voltage. Have you considered the operation of the two cathode-grid diodes. They should be non conducting, with cathodes (=grid) negative of anodes(=k).

3) At least with your 100M probe, you can poke about the tube grid and cathode voltages without risking the tube. The fizzing will make you very careful anyway. Do you know the tip from Tek and HP to check calibrate your HV probe against one of the other voltage rails which you can measure without the probe. Very useful, and saves worrying about absolute accuracy of the probe.

4) Brightup pulse. I did mean both the voltages.
a) with blanked trace, as you get during TB flyback, or when not scanning, that is not triggered = +20V or so.
b) with brightened trace, as when switched to X amp, or during the TB sweep = +85V.
Can you inspect the voltage change with another scope. Otherwise, your Avo DC reading would change when you start the TB sweeping (free run), as the average voltage will change for the different mark-space ratio of the brightup pulses at different sweep speeds.

5) That circuit does confuse. Surely the HV across the brilliance control chain does not vary with the brilliance control setting. Neither does the negative end of that chain move up and down with the control. But the whole grid HV circuit floats on the output of the brightup amplifier R77-R81. That will not change with the brilliance control, but will move with the brightup pulse- up and down by 60V or so. The cathode supply is firmly held near 0V, via the Set Beam Current control RV50. The difference between the two supplies gives the brightup or beam blanking effect.

6) Tubes are pretty tough, so I would rather agree with Brian Pateman. Don't worry too much about heater voltages at this stage. The tube heater is fed with 6.3V 0.3A 50Hz AC from the mains transformer. So IanF397 can take heart. Bear in mind it is floating at -1kV.
But if it is the tube, you are in trouble, as it is a rare tube, uniquely made by Brimar in the earliest days of transistorised oscilloscopes, when the limiting factor was the voltage rating of the X output amplifier transistors, with a more sensitive X deflection (10.5V/cm) than Y deflection (13.5 V/cm).

7) The tube type is D13-33. Not D13-30 which I had marked in error on the circuit diagram posted. Only alternative is the later Brimar D13-46.

8) The pda voltage should not vary so much as to give this effect. Reduced pda will give a much dimmer, and much larger trace. So trace size is the clue to trouble in the pda. Usually it works, or doesn't.

9) I shall have to get my CDU110 working. Sorry, this seems a very long post. Bill

Last edited by Station X; 3rd Jan 2012 at 3:36 pm. Reason: Added some white space.
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Old 4th Jan 2012, 6:44 pm   #14
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Firstly a thank-you to those who kept awake enough to read and particularly comment. Really grateful. May I reply 'in order'.

Glowing Anode: it was 'why I am getting these readings'. Please see later in relation to your very relevant comments about G1/Cathoge voltage. Yes 3kV (actually 2.7kV) does hold up irrespective of Brightness setting. Good suggestion re: displaying a 'full-screen' waveform - it makes little difference to brightness.

IanF397. Thank you. I had not checked the heater! It is, in fact, 6.14V AC. In truth I cannot see the heater glow but that is probably the shielding. For the moment I do not want to remove it but will if critical. I am jumpy abot too much mechanical intereference at this stage.

Brian. R72 is fine (pity)

Bill.

(2)Very much liked your idea to disconnect RT84/C64 from R84 and to start again. In fact, Brilliance voltage measurements were made at the 'Grid end' since this is safer from the point of view of access and slipping probes. The cathode-grid diodes (which I had not checked!) were fine on a low-voltage measurement.

(3) Aware of the Tek/HP tip and all measurements in the spreadsheet attached calibrated against the 100V rail. It does make a significant ~10% difference vs. theoretical.

(4) Sadly this IS my scope. I have several ancient WW2 ones I have never dared power up.


Looking in a darkened room (!) the trace is brightest when it just appears with CW rotation of the Brilliance control and then slightly dims with further CW movement.

To shed a bit more light on the problem I measured all CRT voltages with R85/C64 disconnected from R84 (Bill's idea) (and in X-Amp too) at 3 Brilliance Settings (Word-file attached). These static measurements show (I think) 2 oddities:-

(a) the Brightness does not follow the Grid/Cathode voltage and
(b) there is a marked interaction with the Focus voltage on A2. ?a 'steal' effect via the transformer.

I think (a) points towards a tube problem but cannot really explain (b).

Any thoughts? A voltage check vs. a functioning scope would be good.

With every best wish and thanks, (apologies for liberal exclamation marks)

Anthony
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Old 4th Jan 2012, 11:06 pm   #15
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Looks like lack of regulation of the -ve eht supply, floating positive as you try to draw current.
Check MR46, R67 & 68, C56 & 57.
Rob.
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Old 5th Jan 2012, 9:56 am   #16
Anthony
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Rob
Thank you. These two resistors are fine but I have no way of testing the EHT caps or selenium rectifiers safely.
I wonder whether you and other interested experts think it would be worth me replacing all the EHT caps and the EHT rectifiers?
For silicon diodes presumably I'll need to add in surge limiting resistors to each diode? Would 100K be reasonable? I do not want to damage the Txfmr.
It still puzzles me that the brightness does not increase as the Grid goes more positive wrt Cathode - could that be secondary to the overall EHT fall-off?
Best
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Old 5th Jan 2012, 4:30 pm   #17
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Ah, sorry Anthony, I didn't see the second page of the document.
Maybe the tube is getting low on emission.
Do you have a variac with over wind on it so you can increase the heater voltage and see what difference that makes?
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Old 5th Jan 2012, 5:01 pm   #18
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Hi glowing anode,

What exactly are you suggesting here ?

If you mean power the whole scope from the variac then surely all of the scope voltages will increase.
Even if they are all regulated then you may stress the mains transformer by applying too many volts to it.

If you mean attach the variac directly to the heater terminals, then there are TWO points that should be considered.

1) Most variacs are NOT isolated from the mains.

2) Another contributor suggested that the heater is running at -1000volts, was it?

I think that this suggestion might prove to be a bit dodgy.

IanF
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Old 5th Jan 2012, 7:50 pm   #19
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Ian, I was suggesting he did what you suggested in post 11.
The EHT supplies are stabilised.
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Old 5th Jan 2012, 10:29 pm   #20
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Anthony, I've got a Thorn V3151 5" round pda tube you can try if you want to. I'm afraid I don't have any data for it, but I'm sure someone here will.
regards,
Rob.
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