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Old 6th Jan 2012, 10:29 am   #21
Anthony
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Rob
That's really kind of you. How amazing this forum is!
I am hoping to do the EHT recap and diode replacement in the next week and will get back to you if that fails. Quick question - can you suggest a value for additional series resistors for the silicon diodes to decrease any surge since they are replacing selenium - is 100K sensible?
Best wishes
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Old 6th Jan 2012, 11:47 am   #22
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

I don't think a series resistor is strictly necessary, the surge currents are a lot lower than a mains derived psu. However it may save the tx winding in the event of a rectifier flash over. 100k maybe a little high, try 47k, but use a physically big resistor for the voltage rating.
Rob.
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Old 6th Jan 2012, 10:04 pm   #23
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Anthony.
1) I am still suspicious of the brightup voltages. Have you measured them, as I suggested in #4 & 5 of my long posting of 3 Jan. Your spreadsheet is most interesting, meticulous work, but what were the sweep/brightup conditions. Were the readings taken on XAmp setting, or with some trace and unknown speed (ratio of sweep to holdoff+ blanking unknown), or with no trace, and the brilliance increased to give a spot.
2) I shouldn't worry too much about internal resistance of replacement EHT diodes at this stage. Once you have tried new one out, and if it works, then put 22k or 47k in series. Normal metal film or carbon, it has very low voltage across it. But a few minutes with a greater charge current will not make ceramic capacitors fail. Anyway, most EHT diodes are much higher internal resistance than 1N4007. I assume you are going to try BYX10 (1600v PIV). Otherwise, another trip to RadioSpares for a TV rated one, eg BY176, BY713, or strip from a scrap TV.
3) Your 4b of 4 Jan. Greater brilliance = higher cathode current will surely only affect the focus if the HV oscillator is not regulating. What is the effect on the cathode supply EHT voltage, say at R67/C57/R66 of varying the SetEHT control RV51, or the brilliance itself. Then measure the voltages through the regulator stages, VT50 c,VT51 c,VT52 e, while varying the Set EHT RV51. All BFY51/2N2297/U3180, so not very difficult to replace as a trial.
4) Your voltage measurements for all the electrodes could be consistent with the EHT voltage dropping under load of greater cathode current.
5) The different in G/K voltage with no trace (188V cf 180V), which disappears when you have a trace could be the brightup pulse.
6) I will now devote some time to getting my CDU110 working, so that we have a direct comparison.

Rob (Glowinganode). I am interested in your reference to Thorn V3151, 5" round pda tube. Usually Brimar/Thorn used V** numbering for a prototype. Can you let me knowverall length of tube, any side connections, can you see a spiral inside down the bell from the screen. I would quite like to work out what tube it became. (eg V3174 = D13-51 mesh, side connections, 6x10cm.335mm overall.)

At worst, you may have a tube problem. But that is still some way off I suggest.
Bill
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Old 6th Jan 2012, 11:39 pm   #24
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Hi Bill, yes it's got a spiral, side connections and an Oliver and Randall mumetal shield.
It came from a prototype scope my Dad built, I'll try and prise some more info for you, and will certainly post some photo's of the crt.
Rob.
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Last edited by glowinganode; 6th Jan 2012 at 11:41 pm. Reason: upper case on a proper noun
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Old 7th Jan 2012, 10:11 am   #25
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

See new post here to save going off thread.
Rob
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Old 11th Jan 2012, 1:49 pm   #26
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Hi Anthony,
I don't know how you're getting on, but you're very welcome to bring the scope over so we can at least test the crt seperately and eliminate it one way or another.
Rob.
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Old 11th Jan 2012, 8:20 pm   #27
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Dear Bill and Rob (particularly) but also to all interested of course


We had a minor domestic crisis over Christmas which has meant I have had to swap the soldering iron for sandpaper and a paint roller. I enormously appreciate your help but won't be in the land of the living until the week-end. May I give you some sensible answers after that?
Best
Anthony
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Old 11th Jan 2012, 10:15 pm   #28
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Anthony, you have my deepest sympathies, having just been through the same myself.
All the best,
Rob.
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Old 12th Jan 2012, 11:01 am   #29
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

I know it seems obvious, but have you checked the EHT regulator is operating.(VT50 - VT53). That could be the cause of all your troubles. Bill
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Old 17th Jan 2012, 5:03 pm   #30
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

I hope that at least some of those previously interested are still viewing. I am slightly losing the plot, I must confess due to my ignorance of the EHT regulator VT50 chain and the Brightup circuitry (eVT59). I need a bit of hand holding hence this longish post.

(As an aside the voltage measurements I reported on the spreadsheet were made as shown top left on X-Amp and R85/C64 disconnected from R84.

All EHT Caps and diodes have been replaced to little effect with a trace but now reasonable brighness with a spot. An odd finding is that the trace will now completely dim if the Brilliance control is turned fast ccw but then 'reovcers' in a second or so.

I realised I had never responded to Rob's question:- '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? '

Answer: The effect is to drastically dim the trace. Is this a ‘Brightup’ (see below) or EHT defect?

Bill has asked me check the EHT regulator VT50 chain - but what measurements should I make Bill? What am I looking for? Not least I am (as ever) a bit jumpy about having a loose PCB swinging about with all that EHT nearby. The measurements will need to be made one-by-one 'no hands' so I need a plan (if I understood more I would have one).

I do now have some Brightup voltages checked at eVT59 (Thank you Ian). This swings about 50V on almost a square wave at about 1/10 (approx.) the sweep speed. That looks respectable to me (?).

Isn't this all starting to look, as Bill has suggested, a failure of EHT regulation? As soon as you want more beam current it cannot give it.

Anyone still awake?

Best wishes

Anthony
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Old 17th Jan 2012, 7:52 pm   #31
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

CDU110.
1. Bright up voltage swing of 50V seems sufficiently likely that you can assume the BU Amplifier is working.
I would though be interested on the voltages with full BU (Mode Switch = X Amp) and no BU (Mode =Trig), as I suggested in point 1 of #23 of 6 Jan2012, and my earlier posting.
2. A single voltage reading of BU with a trace is of limited value for fault finding, as it has a variable mark-space ratio depending on the time base settings of scan time and recovery time. That is why I said compare the two readings with carefully defined conditions.
3. EHT regulation testing. See point 3 of mine posting #23 of 6 Jan 2012.
You can access sufficient test points from the component side of PCB G, so there is no need to have it swinging in the air. First monitor the voltage at link between C55 / C57 on the ceramic strip, and see if it changes as you alter the Set Eht control, (with no trace to avoid confusing the issue), and as you alter the brilliance (with a trace).
4. If the voltage doesn't change, then you have found your trouble,and will have to test Vt50 - Vt53 stage by stage. But you can get to one end of the resistors easily enough, the collectors will be the can, as they are not isolated. Again no need to probe underneath the board.
5. Monitor each voltage around the stages of the regulator, and see if reasonable and alters if you move the SetEht control. You will not get much variation at low impedance points such as the emitter Vt53. The two zeners Z50 + Z51 should have a constant 33V across them.
6. All this circuit lies between 0 and +50v, and is reasonably clear of the EHT rectifiers and capacitors on the lower part of the board. So not really uncomfortable to work upon.
7. Your report of a dimmer full scan in response to Rob's (Glowing anode) suggestion sounds like poor EHT regulation. Bright up shouldn't have that effect.
Your own findings that brightness recovers after a quick change suggests EHT voltage changing after a sudden reduction in current demand by the tube and then various capacitors charge up again. Bear in mind that the tube current may be only 10 microamp anyway.
Bill
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Old 17th Jan 2012, 8:00 pm   #32
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Bill
You have given me a lot to think about and to do. Forgive me if I did not pick up on all your previous suggestions. Will work on it.
Kind regards
Anthony
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Old 18th Jan 2012, 1:55 pm   #33
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

I have an idea what might be happening.
There are 3 eht supplies; +3kV, -ve cathode and -ve grid.
The regulator maintains the cathode at a constant voltage, and makes the assumption that the other two eht supplies are closely enough coupled to track.
I can't remember if you replaced the -ve cathode supply rectifier (MR46) or not, but anyway bear with me.
Suppose MR46 had gone high resistance, the transformer would need to be driven harder to maintain the correct voltage on the cathode.
This would have the effect of increasing the -ve grid supply (and the +3kV supply), cutting the crt off and reducing brightness.
Have you checked the winding for continuity between g,h and j?
Rob.
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Old 22nd Jan 2012, 11:31 am   #34
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Brightness problem now solved but trying to work out exactly why.
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Old 22nd Jan 2012, 10:00 pm   #35
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

While trying to measure the EHT regulator voltages it quickly became apparent that the C55 to C57 connection was open. Restoring it by bridging the two capacitors brought everything back.

(Subsequent tinkering led to burning smells traced to my dripping a tiny solder short across C57 (not visible without moving C57) and burning out one of the EHT diodes and the limiting resistor I added. After sorting this out the unit was good again.)

All I can think is that I carelessly removed the C55/C57 connection when I replaced one of the EHT caps which was itself faulty. Though I am a bit surprised because, as is good practice, I always checked the response to changing each EHT capacitor /diode.

The beam current is now in spec. The brightness control works well (in fact C55 is still original). The trace is fine on real signals.

This is not very satisfactory - can anyone confirm where the C55/C57 connection is made on their unit?

I must give abig thank you to all who helped me particularly Bill who suggestions led me to the target.

Any comments?

Anthony
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Old 24th Jan 2012, 11:27 am   #36
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Well done Anthony, glad the 'scopes up and running.
I wasn't so far out suggesting MR46 then! Shouldn't there be a 47k resistor between C55 and C57?
Rob.
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Old 24th Jan 2012, 4:41 pm   #37
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Quote:
Originally Posted by glowinganode View Post
Well done Anthony, glad the 'scopes up and running.
I wasn't so far out suggesting MR46 then! Shouldn't there be a 47k resistor between C55 and C57?
Rob.
Rob
Thank you for all your help indeed!
I think the 47K you note is between C56 & C57?
Best wishes
Anthony
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Old 25th Jan 2012, 12:25 am   #38
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Ah yes, I'm with you. Excuse my fuzzy eyes.
Rob.
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Old 25th Jan 2012, 10:46 am   #39
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Default Re: Cossor CDU110 'Scope: Brightness Probs.

Anthony. Congratulations to have solved it. Logged down in my list of possible faults on that model. Bill
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