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Old 4th Jan 2012, 8:44 pm   #14
Anthony
Hexode
 
Join Date: Sep 2006
Location: Hertford, Herts. UK.
Posts: 338
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
Attached Files
File Type: doc CRT Voltages.doc (67.0 KB, 92 views)
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