GOULD-ADVANCE O2260 SCOPE - NEED REPLACEMENT TUBE - Page 1
Forum section: electronics / repair

author: rgarnett1923
author_rank: Contributor
timestamp: September 09, 2018, 06:56:50 am
content: Hi All,Does anyone know where I could get a tube for the above scope? It is a Phillips/Mullard E14-100GH split beam.RegardsRob

author: Chris56000
author_rank: Frequent Contributor
timestamp: September 09, 2018, 02:28:55 pm
content: Hi!Is it simply missing, physically broken or can't you get any illumination on it?There are many faults on oscilloscopes that can make it seem the CRT is useless when this is, in fact, not so!We can help you fit a near equivalent if need be, by the way – Gould parts–mules come up regularly on fleabay!If your original is physically intact, have you had a look to make sure it's heaters are alight? Electrostatic instrument CRTs are fairly robust and reliable providing it's not been in a lab displaying things 24/7/365!Chris Williams

author: rgarnett1923
author_rank: Contributor
timestamp: October 06, 2018, 12:08:03 am
content: Hi Chris,The scope electronics are all good. There was a problem with the EHT regulator, the previous owner had put the power transistor in back to front i.e. swapped emitter and collector. One of the voltage quadrupler caps was loose and because there was an insulating fluid in the EHT box the cap connections were intermittent. I repaired the EHT board and refilled the EHT assembly with a high quality clear epoxy as the original setup was prone to leaks. After I did this all the tube voltages were OK and within spec from the tube data sheet. The EHT could be set exactly and was very stable. I have a pintek EHV probe good to 39 kV to do these measurements. The problem with the tube is that the two spots are oval with the long axis in the horizontal direction. I have made a couple of other posts which provide photos of this and other details. The two spots also appear flat on the top/bottom of the spot closest to the screen center. It's almost like something has happened to the beam splitter plate so that it is casting a shadow on the spots and diffracting them. The problem is you cannot see the gun assembly apart from the cathode and grid as the tube neck is coated with aquadag.I realize it is very unusual for tubes like these to develop faults, but with the time-base turned off and with no vertical input the spots cannot be focussed to a round shape. And as I've said all the voltages appear OK. I thought the deflection plate mean voltages might be off, but these were spot on, (sorry the pun). I checked the plate voltages in differential and there was no ripple on them at all under the no- timebase/vertical signal condition. I checked the timebase with it on and it was excellent, very linear and noise free.When I saw the problem initially and with the EHT way off due to the regulator, I was confident that it would be the tube power supplies, but alas I cannot fault them. All of the tube controls, brightness, focus, geometry and astigmatism as well as the vertical and horizontal positions work as expected with the exception that the spots cannot be focussed into fine circles.I would really like to get another tube of the same type/model as I am sure that this will fix the problem.Best RegardsRob

author: drussell
author_rank: Super Contributor
timestamp: October 06, 2018, 12:51:56 am
content: Heh... when even Sphere is out of stock, good luck even finding a used pull.I suppose you could call them there at Sphere and see if they know anywhere on the planet that might have one if you're really certain that the CRT is your issue...http://www.sphere.bc.ca/test/crts2.html

author: Chris56000
author_rank: Frequent Contributor
timestamp: October 07, 2018, 09:22:25 pm
content: Hi!This might be a fault in the X amplifier or even a spurious signal getting into the X output stage from elsewhere due to faulty HT or LT line decoupling somewhere!What you can try is to short–circuit the X1 and X2 deflector plates of the tube with a 100R resistor rated at about 3W rating, and then turn the intensity down until the spots are only just visible, then try the CRT setting–up adjustments again!If still no luck, check the circuit diagram to see if there's an auxiliary blanking grid in the tube – strange things can happen to the spots if this is floating or at the wrong potential!You could also check the CRT heater/cathode is correctly returned to the EHT–ve supply (has the series resistor gone high?) – I wonder if there's a fault in the insulation of the 6.3V winding of the mains transformer (you can get little dc–dc converter modules that will safely withstand the EHT voltage between their input and output sides – if you can find a safe means of wiring one in!) – another possibility is a fault in the unblanking circuit somewhere!I must admit it's a rare fault in CRTs to come across as almost always it can usually be adjusted out or found to be a definite fault like excessive ripple on power–supplies etc., in a preceding amplifier – it only needs a slight trace of mush in either the TB, X or Y amplifiers to elongate the spot in either direction!The best place you'll get a compatible CRT from would be another Gould, Philips or Telequipment scope of same size and bandwidth – Frank's Electron Tube Data has data for hundreds of instrument CRTs, and if you do want to try a substitute, anything with a similar V/mm deflection sensitivity, p.d.a. acceleration voltage and physical size can be used as a starting–point!I'll study the circuits and come back to this one later!Chris Williams

author: jmelson
author_rank: Super Contributor
timestamp: October 08, 2018, 08:44:35 pm
content: Quote from: rgarnett1923 on October 06, 2018, 12:08:03 amIt is possible that one of the deflection amps is bad, and is deflecting the beam into the deflection plate, and it is partly bouncing off and making multiple spots.If you are getting illumination of the screen, an awful lot of the tube needs to be working.Jon