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Old 21st Feb 2021, 4:01 pm   #1
KevinClarke
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Location: Stafford, Staffordshire, UK.
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Default Telequipment D67A

Hi,
Hope someone can help, I need some advice on my Telequipment D67A.
It has developed a weird fault in that at switch on the screen glows bright immediately. This occurs before the tube heater has warmed up. On disconnecting the base connector ie heater and cathode circuits the tube still glows at mains switch on. It’s that bright that I have to switch off pretty quick to avoid phosphor burn. On switch off the glow is still bright and it takes about 20sec to disappear. I have checked the +12v -12v 115v rails and all are good I have also checked for ac ripple on them and all is good.
Any advice would be greatly appreciated.
Thanks
Kev
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Old 21st Feb 2021, 5:48 pm   #2
G6ONEDave
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Default Re: Telequipment D67A

Have you checked the graticule display illumination control, it's probably set to max illumination. This is totally seperate to the actual crt trace brightness control. The over brightness cannot be coming from the crt phospher when the crt base panel is removed.
Dave
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Old 21st Feb 2021, 7:08 pm   #3
MotorBikeLes
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Default Re: Telequipment D67A

Kevin, I once had an identical fault on a D67A. Obviously if you get an instant bright screen at switch on, it is not normal thermionic emission since the CRT filament can not reach full emission instantly, so must be screen fluorescence of unknown cause.
I puzzled over my fault, asked knowledgeable pals and with one exception nobody had ever heard of this. The odd chap had heard of it, but never seen it and knew of no cause.
Whilst hunting for clues, there was a sudden CRACK, and a much reduced horizontal scan.
I replaced both X output transistors, and the bright fluorescence never returned. It is obviously some excited fluorescence, and I suppose it must somehow be leakage between one of the X-plates and another electrode deeper in the CRT, maybe across to the PDA spiral. (It does have a pda spiral I think). Of course the Crack COULD have been something in the CRT getting "blown away"
Les.
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Old 21st Feb 2021, 9:09 pm   #4
WME_bill
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Default Re: Telequipment D67A

D67 Initial bright screen.
Have you tried removing the EHT supply to the PDA. As it is mains operated there is no oscillator transistor to disconnect, or a fuse as in other models, so you will have to disconnect the connector on the tube. Use an insulated screwdriver to loosen and spring it out of the socket on the side of the tube, but be careful, as the capacitances of both the tube and the EHT supply will hold a nasty charge for a very long time. Discharge both with a 1Mohm and then a short circuit or leave it overnight.

I assume the glow is diffuse and spreads over the whole screen. I suspect some discharge within the tube between the PDA or the PDA spiral to the mesh screen G6 which is setting the phosphor aglow, rather like the discharge in a fluorescent tube. It should not exist if you remove the PDA voltage. (Will not harm the tube for a few moments, and will give a much smaller and dimmer trace).

Check carefully that G6, the central connector on the neck of the tube, is connected through point 210/179 and sitting about 35v positive of earth, and perhaps a 5 to 10 volts negative of the Xplate shield G7 on the base pin 11, point 210/182.
G7 is used to adjust the geometry of the X display and varied between 0 and +115v by R309. The manual para 4.3.7.2 shows how to adjust R309 to check the geometry.
These should all be second order effects, but I would confirm that the voltages on G4, G5,G6,G7 are somewhere between 0 and 115V.

I have used the description of these grids/shields as given in the manual, for CRT type Philips 95447 (a special contract number) or the same tube from the Valvo data for D14-160. The Mullard data for their version of the same tube D14-162 uses a different nomeclature, but is easier to read as it is in English. All tube varieties are phosphor type GH, with graticule 09.
The CRT data is available on Frank Philipse's site, or else I can post copies.

I won't mention gassy tube as that would probably give a more general glow over the whole body of the tube. Look for this in a darkened room, but use a torch and make sure your nose, glasses and hands are positioned well clear before switching the torch out.

I don't want to be a bore, but get a very clear idea of the high voltage points before you start to work around the PCB and tube base. Draw a line on the circuit diagrams to show the division.
I often wear rubber gloves and use one hand only. If gives me confidence if nothing else.
Don't have anything sort of balanced or propped up. The big risk working with high voltages around is that something will topple and you will try to catch it, or that you receive a minor jolt from say the 115v rail, and jerk your hand to somewhere much more uncomfortable.
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Old 21st Feb 2021, 10:16 pm   #5
KevinClarke
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Default Re: Telequipment D67A

Hi Guys,
Many thanks for your time, advice and support, I will try your recommendations over the next couple of days and provide more feedback,
Cheers
Kevin
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Old 22nd Feb 2021, 12:08 am   #6
KevinClarke
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Default Re: Telequipment D67A

Quote:
Originally Posted by WME_bill View Post
D67 Initial bright screen.
Have you tried removing the EHT supply to the PDA. As it is mains operated there is no oscillator transistor to disconnect, or a fuse as in other models, so you will have to disconnect the connector on the tube. Use an insulated screwdriver to loosen and spring it out of the socket on the side of the tube, but be careful, as the capacitances of both the tube and the EHT supply will hold a nasty charge for a very long time. Discharge both with a 1Mohm and then a short circuit or leave it overnight.

I assume the glow is diffuse and spreads over the whole screen. I suspect some discharge within the tube between the PDA or the PDA spiral to the mesh screen G6 which is setting the phosphor aglow, rather like the discharge in a fluorescent tube. It should not exist if you remove the PDA voltage. (Will not harm the tube for a few moments, and will give a much smaller and dimmer trace).

Check carefully that G6, the central connector on the neck of the tube, is connected through point 210/179 and sitting about 35v positive of earth, and perhaps a 5 to 10 volts negative of the Xplate shield G7 on the base pin 11, point 210/182.
G7 is used to adjust the geometry of the X display and varied between 0 and +115v by R309. The manual para 4.3.7.2 shows how to adjust R309 to check the geometry.
These should all be second order effects, but I would confirm that the voltages on G4, G5,G6,G7 are somewhere between 0 and 115V.

I have used the description of these grids/shields as given in the manual, for CRT type Philips 95447 (a special contract number) or the same tube from the Valvo data for D14-160. The Mullard data for their version of the same tube D14-162 uses a different nomeclature, but is easier to read as it is in English. All tube varieties are phosphor type GH, with graticule 09.
The CRT data is available on Frank Philipse's site, or else I can post copies.

I won't mention gassy tube as that would probably give a more general glow over the whole body of the tube. Look for this in a darkened room, but use a torch and make sure your nose, glasses and hands are positioned well clear before switching the torch out.

I don't want to be a bore, but get a very clear idea of the high voltage points before you start to work around the PCB and tube base. Draw a line on the circuit diagrams to show the division.
I often wear rubber gloves and use one hand only. If gives me confidence if nothing else.
Don't have anything sort of balanced or propped up. The big risk working with high voltages around is that something will topple and you will try to catch it, or that you receive a minor jolt from say the 115v rail, and jerk your hand to somewhere much more uncomfortable.
Hi Bill,
Again thanks for your advice on this, Please will you post me the crt data sheets you mentioned.
Thanks
Kev
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Old 23rd Feb 2021, 4:38 pm   #7
WME_bill
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Default Re: Telequipment D67A

D67A Telequipment oscilloscope tubes used.
I have been asked to post the tube data sheetsof the CRT types used.
Philips contract 95447, or Valvo D14-160 or Mullard D14-162. All apparently identical tubes with similar Telequipment part numbers. I attach them.
I do not show the various phosphor types or the graticule layout variations.
I have posted details of phosphor descriptions in the past. Do a search. It causes much confusion, as there are many different descriptions: American, old Mullard, older Mullard/Philips/European, modern Mullard/Philips/Brimar, GEC old and modern, Cossor, Ediswan etc.
wme_bill
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Old 24th Feb 2021, 12:28 pm   #8
KevinClarke
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Default Re: Telequipment D67A

Quote:
Originally Posted by G6ONEDave View Post
Have you checked the graticule display illumination control, it's probably set to max illumination. This is totally seperate to the actual crt trace brightness control. The over brightness cannot be coming from the crt phospher when the crt base panel is removed.
Dave
Hi Dave,
Thanks for the info but it’s not the graticule illumination that’s faulty or turned up high.
It is the phosphor instantly glowing as the glow stay on on the screen for about 30seconds when the unit it switched off.

Quote:
Originally Posted by MotorBikeLes View Post
Kevin, I once had an identical fault on a D67A. Obviously if you get an instant bright screen at switch on, it is not normal thermionic emission since the CRT filament can not reach full emission instantly, so must be screen fluorescence of unknown cause.
I puzzled over my fault, asked knowledgeable pals and with one exception nobody had ever heard of this. The odd chap had heard of it, but never seen it and knew of no cause.
Whilst hunting for clues, there was a sudden CRACK, and a much reduced horizontal scan.
I replaced both X output transistors, and the bright fluorescence never returned. It is obviously some excited fluorescence, and I suppose it must somehow be leakage between one of the X-plates and another electrode deeper in the CRT, maybe across to the PDA spiral. (It does have a pda spiral I think). Of course the Crack COULD have been something in the CRT getting "blown away"
Les.
Hi Les,
Thanks for the info and it’s good to know that it’s not just me that gets weird faults. I’m suspecting at this stage that it’s a tube pda spiral fault.

Quote:
Originally Posted by WME_bill View Post
D67 Initial bright screen.
Have you tried removing the EHT supply to the PDA. As it is mains operated there is no oscillator transistor to disconnect, or a fuse as in other models, so you will have to disconnect the connector on the tube. Use an insulated screwdriver to loosen and spring it out of the socket on the side of the tube, but be careful, as the capacitances of both the tube and the EHT supply will hold a nasty charge for a very long time. Discharge both with a 1Mohm and then a short circuit or leave it overnight.

I assume the glow is diffuse and spreads over the whole screen. I suspect some discharge within the tube between the PDA or the PDA spiral to the mesh screen G6 which is setting the phosphor aglow, rather like the discharge in a fluorescent tube. It should not exist if you remove the PDA voltage. (Will not harm the tube for a few moments, and will give a much smaller and dimmer trace).

Check carefully that G6, the central connector on the neck of the tube, is connected through point 210/179 and sitting about 35v positive of earth, and perhaps a 5 to 10 volts negative of the Xplate shield G7 on the base pin 11, point 210/182.
G7 is used to adjust the geometry of the X display and varied between 0 and +115v by R309. The manual para 4.3.7.2 shows how to adjust R309 to check the geometry.
These should all be second order effects, but I would confirm that the voltages on G4, G5,G6,G7 are somewhere between 0 and 115V.

I have used the description of these grids/shields as given in the manual, for CRT type Philips 95447 (a special contract number) or the same tube from the Valvo data for D14-160. The Mullard data for their version of the same tube D14-162 uses a different nomeclature, but is easier to read as it is in English. All tube varieties are phosphor type GH, with graticule 09.
The CRT data is available on Frank Philipse's site, or else I can post copies.

I won't mention gassy tube as that would probably give a more general glow over the whole body of the tube. Look for this in a darkened room, but use a torch and make sure your nose, glasses and hands are positioned well clear before switching the torch out.

I don't want to be a bore, but get a very clear idea of the high voltage points before you start to work around the PCB and tube base. Draw a line on the circuit diagrams to show the division.
I often wear rubber gloves and use one hand only. If gives me confidence if nothing else.
Don't have anything sort of balanced or propped up. The big risk working with high voltages around is that something will topple and you will try to catch it, or that you receive a minor jolt from say the 115v rail, and jerk your hand to somewhere much more uncomfortable.
Hi Bill,
Many thanks for your help re above, I have tried everything you’ve suggested but unfortunately it’s still lighting up the the workshop!
Suspect a faulty tube totally discharged eht everywhere and then checked all the crt connections on mega ohms for readings but it all looks ok. The voltage’s all check out ok. So can only conclude that it could be a faulty crt?
I can’t think of anything else to try
Cheers,
Kev
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Old 24th Feb 2021, 9:27 pm   #9
MotorBikeLes
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Default Re: Telequipment D67A

Kevin, just one thought. Look carefully at the CRT electrodes and see which might withstand high voltages, eg Focus, EHT etc, and try again (cold and fully discharged of course, with a dead short EHT to chassis for a while before testing) then try for inter electrode leakages using a 500v (or better, 1000v) Megger. You may get leakage, you MAY get fluorescence. If you get any leakage or the fluorescence, try charging a big, HV capacitor fully with the megger or why? then discharge across the suspect electrodes.
If the CRT is BER, you can not do any harm, except to yourself if lacking caution.
Les.
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Old 25th Feb 2021, 6:06 pm   #10
KevinClarke
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Default Re: Telequipment D67A

Cheers Les for the fresh ideas, I’ll give them a try ��
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