National Oscilloscope  VP-5260A Repair Manual - Page 1
Forum section: electronics / repair

author: EENG
author_rank: Contributor
timestamp: January 18, 2012, 01:04:16 pm
content: HalloI have an old National Oscilloscope VP-5260A. The horizontal sweep works. The vertical scan does not work on both X and Y inputs. Does any one know where I can access a repair or step by step fault finding procedure to restore this oscilloscope?

author: amspire
author_rank: Super Contributor
timestamp: January 18, 2012, 01:23:18 pm
content: So you have a horizontal trace, but Ch1 and Ch2 do not work. Is that correct?Can you get two traces, and can you move them up and down?

author: EENG
author_rank: Contributor
timestamp: January 18, 2012, 01:43:45 pm
content: Current setup is:Sweep Mode: AutoWhen MODE switch is on Channel 1, I get a horizontal line which can be moved up and down by CH 1 vertical defection button onlyWhen MODE switch is on CH2 X-Y, no lineWhen MODE switch is on ALT, I get a horizontal line which can be moved up and down by CH 1 vertical defection button onlyWhen MODE switch is on CHOP, I get a a square wave which has a 5us time period. The amplitude (Volts/division) setting of this square wave cannot be adjusted when I tweak the V/div button on CH1 or CH2

author: EENG
author_rank: Contributor
timestamp: January 18, 2012, 01:46:11 pm
content: Small error...."defection" should have been correctly spelt as "deflection"......this is the button that adjusts the vertical position of the trace

author: amspire
author_rank: Super Contributor
timestamp: January 18, 2012, 01:57:16 pm
content: So in the ALT mode, there is only one line and only the Ch1 position control works. The Ch2 position control does not do a thing?In Ch1 mode or ALT mode, can you get any sign of the Ch1 input working. If you connect a probe to the CAL pin, and you make sure that CH1 is not set to GND, can you get a 1 KHz square wave?

author: EENG
author_rank: Contributor
timestamp: January 18, 2012, 02:01:59 pm
content: When I set the MODE to CH1 and connect a wire from the CALIB output to the input of CH 1, I get no square wave. I also checked that CH1 was not on the GND position.

author: amspire
author_rank: Super Contributor
timestamp: January 18, 2012, 02:06:29 pm
content: And the questions I asked about CH2 ?

author: EENG
author_rank: Contributor
timestamp: January 18, 2012, 02:19:10 pm
content: CH 2 position control does not do anything

author: amspire
author_rank: Super Contributor
timestamp: January 18, 2012, 11:23:38 pm
content: It would obviously be great if you could find a repair manual or schematic. National was a brand name used by Panasonic, so the Panasonic VP-5260A is the same thing as your scope.This site may have a schematic if you register:http://www.radiomuseum.org/r/panasonic_oscilloscope_vp_5260a.htmlThe fact that in ALT mode you do get a CH1 trace and no Ch2 trace suggests a few things :-The scope's horizontal and vertical drive circuits probably work fineBoth of the channel stages seem to be faulty.It is likely the CH2 output is permanently stick high or low - off the screen anywayCh1 output is the correct value, but for some reason, no signal gets from the input to the output.I would open it up and see how accessible the input channel board is. Be careful working inside the scope. Capacitors in the power supply may have some nasty residual charge, and the scope tube anode (the high voltage cable that probably connects to the side of the tube somewhere) is probably storing a charge of thousands of volts. Just don't try and unplug that cable.You will have almost identical circuits for Ch1 and Ch2 and if your lucky, they will have used identical board layouts for both. It will make it very easy comparing one channel to the other.First look for any obvious physical damage - burnt components, exploded or swelling electrolytic capacitors, burnt switch contacts. If someone has put 1000V into an input, there may be widespread damage and you may have to ask yourself if it is worth repairing.If it all looks OK, you could grab a multimeter, put it on the diode test range, and start testing all the transistors. Compare the readings in Ch1 to CH2. Ch2 could have a shorted transistor, and if so, that will be easy to find. You can check other components using the other resistance ranges on the multimeter. Remember, any difference between the two channels is a big pointer to the problem.If you cannot see anything and you do not have a circuit, you could try and start working out the circuit of one of the channels if you are keen. The thing is if you can post even a partial circuit, with some DC voltages, we may be able to help. Photos are always welcome here - we can't get enough of them, so when you open the scope up, you may as well take some photos and post them. Pictures that show readable part numbers in IC's, transistors, etc and any text on the board's sikscreen are especially useful.There could be a power supply problem, but if there were, I would expect to see both channels behaving the same. Just check the power supply components too, but as I said, just watch out for voltages on the high voltage electrolytics. They usually drop down to zero with the scope off, but don't assume that.Richard

author: EENG
author_rank: Contributor
timestamp: January 19, 2012, 08:04:30 am
content: Thanks for your reply.I will try to tackle this in a logical step by step fashion. I will also try to take photos for clarity.I like your suggestion on the overvoltage theory i.e. maybe the previous owner put in a huge voltage into one of the inputs (most likely CH2). This was my first hunch.

author: EENG
author_rank: Contributor
timestamp: January 19, 2012, 10:18:03 am
content: Attached are some fotos in a pdf fileFig 1. "This is the view of the oscilloscope from the frontFig 2. This is the view of the internal circuitry with the top cover removedFig 3. This is the view with the bottom cover removed .This is where the Vertical amplifier main board resides.Fig 4. "Voltages on visible wires.jpg". Voltages of incoming wires that join onto the Vertical amplifier main boardWhatis interesting to note is that if you look at "Bottom Cover Removed (i.e. Fig 3 above), on the top left hand side of the Vertical amplifier main board, the silkscreen refers to +24V for the white wire and -24V for the red wire. I dont measure a negative voltage and certainly not up to 24VIs this a clue to the problem i.e. the voltage supply to the board is not correct?Also, just to ensure conformity beteen CH1 and CH2, both V/div nobs were set at 10V/div. Both CH1 and CH2 were set to DC input mode.

author: EENG
author_rank: Contributor
timestamp: January 19, 2012, 10:34:00 am
content: A small error in my posting,rechecking the board with my multimeter the +24 and -24V are actually testpoints which are located behind the green resistor ...zoom into figure 4 top left had side...and the voltages measured are +22 and -22V.....so this seems OK.

author: amspire
author_rank: Super Contributor
timestamp: January 19, 2012, 11:39:36 am
content: Quote from: EENG on January 19, 2012, 10:34:00 amI will have a good look later, but the circuit is all discrete transistors, and is really a pretty simple circuit. You have a very good chance to fix this.All the transistor pairs are differential pairs - the first pair would be junction FETs. The rest will be transistors - usually NPN, but there can be a PNP pair. Set the inputs to GND. All the different pairs should have identical voltage on each lead of the pair - so if the left transistor has -4.5V on the emitter, -3.9V on the base and 15V on the collector, the right transistor of the pair should have the same.And also the voltages on Ch1 should be close to the voltages on Ch2.Also it is really worth looking for shorts on transistors with the power off. It would only take a few minutes to check every transistor on that board, and it will save heaps of time if a transistor is dead.Richard

author: EENG
author_rank: Contributor
timestamp: January 19, 2012, 01:22:53 pm
content: Hallo RichardI followed your suggestion to do a comparative analysis between the voltages on the transitor pairs on the CH1 and CH2 side.I noticed that the voltage on the base of transistor Q207 was non existant. I looked underneath the board and I noticed that the track was corroded. I placed a jumper wire as per the attached diagram and now I have restored CH2 trace. CH2 now works and can be moved up and down with the vertical adjustment nob.I am not out the woods yet because I when I turn up the gain of CH2 to 50mV per division, I dont see the trace thicken up or oscillate when I connect a wire at the CH2 input. Usually oscilloscopes do this when one plugs in a wire i.e. they pick up surrounding 50Hz ac hum.I also notice that Ch2 has like a ripple on it which leads me to suspect that a smoothing cap has dried out.I will keep on looking.

author: amspire
author_rank: Super Contributor
timestamp: January 19, 2012, 01:39:55 pm
content: That is good progress.

author: EENG
author_rank: Contributor
timestamp: January 19, 2012, 02:30:28 pm
content: I just pulled off the Verical Amplifier main boardA lot of the tracks on the bottom are corroded. Next step is to solder bridge the broken tracks with wires.

author: Bored@Work
author_rank: Super Contributor
timestamp: January 19, 2012, 05:43:20 pm
content: Quote from: EENG on January 19, 2012, 02:30:28 pmEhm, if it would be mine, my next step would be to give the PCB a very good cleaning first.

author: EENG
author_rank: Contributor
timestamp: January 20, 2012, 10:01:00 am
content: I cleaned up the tracks and soldered wires over eroded soldered tracks on the Vertical Amplifier board. Attached is a pdf document with a picture of the final result....a working oscilloscope!Thanks Richard for your advice......and the help from my colleagues at work. Its such a satisfying feeling to see an ancient device up and running again. Just comes to show....a little bit of effort an perserverence does pay off.

author: amspire
author_rank: Super Contributor
timestamp: January 20, 2012, 10:16:19 am
content: Looks like it is working great. Thanks for sending the photo.Richard

author: Drirr
author_rank: Regular Contributor
timestamp: January 20, 2012, 10:52:18 am
content: I have original original manual and service manual, you can download it here.http://www.ulozto.cz/9241546/vp5260a-rar

author: CharlieD
author_rank: Contributor
timestamp: January 05, 2024, 05:15:27 pm
content: Hi, I need the service manual of this Scope, and looks like the provided link in the post # 19 is down, someone could provide a link or send me the service manual trough another way? Thanks in advance