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Old 3rd Jan 2012, 5:31 pm   #13
WME_bill
Octode
 
Join Date: Oct 2007
Location: Exeter, Devon, UK.
Posts: 1,550
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 5:36 pm. Reason: Added some white space.
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