Tektronix 2465B problem - Page 1
Forum section: electronics / testgear

author: wallis
author_rank: Contributor
timestamp: May 25, 2014, 05:15:06 pm
content: Hello All,Hope somebody can point me in the right direction I have a Tektronix 2465B that I thought had a text display problem as it turns out the problem is with the horizontal display.The point displayed on the CRT be it a wave form or a dot in X-Y mode is stretched horizontally see attached pictures.Anybody have a starting point for me to solve this? What ive done to date is fitted working A5 card. Fitted working U800.Kind RegardsWalter

author: SeanB
author_rank: Super Contributor
timestamp: May 25, 2014, 05:27:55 pm
content: Check capacitors in power supply, one or more is open circuit, so what you are seeing is ripple superimposed on the supply rails. Check the diodes as well while you are there.

author: wallis
author_rank: Contributor
timestamp: May 25, 2014, 08:08:43 pm
content: Hello Sean,I see you from my home country, 14 long years in the UK and I'm missing sea and sunI Digress....back to the problem.I'm fortunate to have a working Tek 2465B so took the power supply module out and tried it in the problem scope no change to the fault so we can rule the PSU, u800 and A5 card out.What do we try next?Kind RegardsWalter

author: MadTux
author_rank: Frequent Contributor
timestamp: May 26, 2014, 02:09:21 am
content: What about working backwards from the deflection plates? Use 10x probes and the other working 2465, disable the readout (makes signal on deflection plates more complex) and look how the waveform on the deflection plates look like (sawtooth + some kind of noise).Then work backwards from there and look at every amplifier stage. Somewhere you'll find some leaked cap, bad chip or something else, that distorts the signal. If you have a third scope, you can compare signals between bad and good 2465, which should make it easier to find the bad part.

author: MarkL
author_rank: Unknown
timestamp: May 26, 2014, 05:30:02 am
content: Can you get the readout characters to focus? If not, there might be a problem with the high voltage supply board. The first photo looks like an abnormal amount of astigmatism.But I agree with the suggestion to verify the signal on the horizontal plates first.

author: wallis
author_rank: Contributor
timestamp: May 26, 2014, 06:04:05 am
content: Hello All,I do have a third scope so can compare signals will work back from the deflection plates.I can't focus the readout characters are there but also stretched horizontally.Kind regardsWalter

author: tautech
author_rank: Super Contributor
timestamp: May 26, 2014, 07:28:04 am
content: As SeanB posted, checkallPSU rails and ensure they areallwithin specs.Check HV circuitry is within spec.I suspect your problem is with the CRT HV or possibly sweep and or flyback as a result of out of spec PSU.

author: wallis
author_rank: Contributor
timestamp: May 26, 2014, 09:11:59 am
content: Thanks MadTuxGood tip to look at the wave form on Test point 62 and 63 without the readout both the good and the faulty are identical at 62 and 63.I presume this would indicate that all is well prior and eliminates the horizontal out as a problem?Kind RegardsWalter

author: wallis
author_rank: Contributor
timestamp: May 26, 2014, 03:53:18 pm
content: Result swapped A9 high voltage card and this sorted the faults.Now where to look on the A9 to repair ? Before I removed the card I compared all the reading at J191 and two of the values pin 7 and pin12 had different readings. I will start there.Many thanks to all who helped. Will let you know what I find faulty on the A9 card.Kind RegardsWalter

author: Bryan
author_rank: Frequent Contributor
timestamp: May 26, 2014, 08:21:24 pm
content: Do you have the service many for the 2465B, not a lot on the A9 and the schematic and flowcharts show it to fairly simple to diagnose. Shoot me a PM and I can send you the service manual

author: wallis
author_rank: Contributor
timestamp: May 27, 2014, 06:33:43 am
content: Hello Bryan,I do have thehttp://www.ko4bb.com/Manuals/Tektronix/Tektronix_-_2465B_Oscilloscope/Tek_2465B_2467B_service_OCR_with_schematics.pdfBut the A9 board is different to the unit shown in the schematic on pg 434. Here is a picture of the faulty one and the good unit both also have slightly different boards. Good unit has 670-7277 12 and the faulty one 670-7277 09.If your manual has mine in please send along.Many ThanksWalter

author: MarkL
author_rank: Unknown
timestamp: May 27, 2014, 03:48:04 pm
content: This version of the service manual references your 670-7277-09 version in the parts list:http://bama.edebris.com/download/tek/2465bfull/Tektronix_2465b.pdfIf the schematic still doesn't match the board you have, look through some of the other 2465/A/B and 2467 service manuals. It appears Tek re-used the same HV power supply but with different revs in the various models. Also check the "Manual Change Information" near the back. Some of the manuals describe specific changes between different versions of the HV supply.For debugging, I would start by checking the voltages being supplied to the various plates on the CRT. USE SUITABLE PROBES! Some of the voltages are around 2kV. And since there's something already wrong with the board, the voltage might also be higher than what you're expecting.

author: Larry F
author_rank: Newbie
timestamp: May 28, 2014, 08:47:11 pm
content: You can get this outcome if the focus and astigmatism controls are set incorrectly. Any circuit issue that effects the voltage applied to the electron gun assembly will cause an out of focus problem. The elements form an electronic lens that first columnates the beam and then converges it. A fleck of material on the assembly inside the CRT can cause these voltage to become out of spec. This can sometimes be repaired by charging a capacitor to a few hundred volts and discharging that across the shorted elements. You can check for this shorted condition with an ohmmeter at the CRT pins. Mostly you can check this out be seeing if both focus and astigmatism peak correctly within their range.Alternatively, as mentioned above, any AC signal superimposed on the deflection plate circuitry will also have this effect. Often circular displays occur if both horizontal and vertical plates are effected. If these voltages are derived from the flyback they could have high frequency noise on them due to filter cap or HV rectifier failure. Your x-y display hints of flyback chopper noise in the supplies. Check out the filter capacitors, HV rectifiers, the solder connections and check for correct value with a capacitor checker. Concentrate on tests you can do with power off. Be aware this circuitry can maintain dangerous potentials long after the instrument has been switched off. Ground them with a plastic-handled screwdriver with the power plug disconnected from the wall before probing around. Some instruments will have a grounding switch that grounds high voltage when the cover is removed. If you operate one of these with the cover off, you'll cause additional damage.