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Tektronix 2440 Oscilloscope Repaired
My good friend and old colleague Frans brought in today a Dual Channel Tektronix 2440 Oscilloscope that short circuited on the Wall Outlet by also destroying the internal 4A 230V Fuse when the Power Button on the Scope front was pressed.
So he asked me to have a look if it was fixable. I myself own a 4 channel 350MHz Tektronix 2465a Scope that did cost me a lot of money to get it. And since my friend got his Scope from a good friend of his for free, I think he is very lucky to own one too!
I therefore instantly checked powering on his Scope by using the well known Bulb trick (a 75Watt Bulb inbetween one of both 230Volt AC power cable wires) to safe the Scope from further blowing up parts when it indeed short circuited again. And the burning Bulb proved that there still was something short circuiting probably inside the Power Board.
So next photos show how I opened the Tektronix 2440 Scope to inspect the cause of the Power problem. And why the Scope didn’t turn on except for only some blinking of the 3 yellow GPIB leds on top of the scope CRT tube and also visible on the front of the Scope. And my friend told me that the Scope supposed to have some Network cable problems also, why the previous owner probably disconnected the internal power plug (normally connected on the Power Board to J166) to Flatcable connector J150 going to one of the side boards.
Above photo shows the top Board of the Tektronix 2440 Scope that has to be opened to be able to inspect the Power Board underneath.
Above Photos show the black plastic holders on which the Power Board was attached. Only one metal Torx screw on the left of the board had to be removed to be able to extract this Board for inspection. Plus one long screw that was attached to the Power Board Metal in the middle from a hole in a board on the side. And on the right of the Power Board the 2 x 2 input wires for the 230V and 115V AC input selector had also to be removed. (Plus a few other flat cables were carefully loosened making it easier to open the Top Board).
On above Photo the for inspection removed Power Board is shown. I thought to find some short circuiting or exploded Capacitors but there were none! After some further inspection I noticed that the Rectifier Diode Bridge was shorted between pins ~ and – . So I removed the Bridge. On the right side of this Power Supply Board (or the Low Voltage Supply Board as Tektronix calls it) is the Voltage Output side were + and – 8V,15V and 5V can be measured. On this side there also is a 10 Volt Reference Voltage. And a unregulated 5V Voltage (5Vd) that is secured by a 10A Fuse (F296). All Power Board Fuses were still intact. So only the Scope Main Voltage Fuse (4A Fast 230V AC) and the Greatz Fast Recovery Diode Bridge were Blown. Also on this board Right under there is a 4 + 5 Pins connector holder for the High Voltage CRT Board. But only the J102 Flatcable connector on the right top side caused problems when connected to all other boards. The AC input selector of the Scope was very simpel and worked like this: The 230V – 115V AC selector only connected two wires P70 and P80 on the board when in 115V mode. In 230V AC mode (and in 115V AC mode) two other wires P30 and P60 connected the 230V AC voltage input wires directly to the Board after the attached input Fuse and the On/off switch were activated. So only in 115V selector mode P70 and P80 were bridged on the Supply Board together also.
On above photo you see the place were the Fast Recovery Bridge Rectifier was connected (CR510 in the Service Manual 3A 600V, page 476 in schematic and page 395 on the component list). Above the second Big Electrolyte Capacitor on the right. Many Service Manuals including for the Tektronix can be found on http://www.textfiles.com/bitsavers/pdf/
The Fast Recovery Diode Bridge for Tektronix Model 2440M only, is a type RKBPC606 600V 3A (6A according to Manufacturer!)
Since I had no Fast Recovery Rectifier Diode Bridge I replaced the RKBPC606 by a KBL405 that easily can handle 4A at 600V (and about 200 Amps in short bursts!). Also to be able to quickly find out if it was wise to continue my repair.
On next photo the Bad original Fast Recovery Diode Bridge is shown, after removing it from the Power Supply Board.
On previous photo another view on the for inspection from the Tektronix 2440 extracted Power Board.
And above photo shows the 2440 Scope front with the 3 mentioned GPIB Leds and the Power PushButton just left from the first BNC Channel input connector.
Because I couldn’t exactly pinpoint were to find the problem why the Power Supply kept working in Bursts, I had removed all boards that were connected to Power Supply Connector J102. And after that the Power Supply worked uninterrupted as also the big loud 12V Fan did.
So it seemed that the Power Supply Board was okay now after replacing the Bad Diode Bridge. But to be sure I checked all regulated and/or unregulated Output Voltages while adjusting the input AC voltage with my Variac from 0 to 240 VAC. (This time without any 75Watt Bulb inbetween). And all checked out to be Fine! So the Power Board had previously short circuited but now wasn’t the problem anymore.
I also had checked the High Voltage PCB for the CRT tube and couldn’t find any shorted components or problem. And I already noticed that when it was connected to the earlier mentioned Powerboard 4 + 5 Pins, it did not cause any problem.
So when I finally a day later brought the Tektronix 2440 Scope back to Life, I first only left the smallest Flatcable connector of the Biggest Board with all Hybride Chips unconnected from the Power Supply. And noticed that some of the Leds on the Front started working. (triggering mode Leds). First with my Variac to a maximum Voltage of only about 140 VAC. And later on also on the
230 VAC Mains. And only when I finally got all Connectors Powered by the J102 connectorplug on the Power Supply Board (or the Low Voltage Supply Board) also the CRT Tube became visible. (Which made me very happy!)
Next the Datasheet of the 600V 4A Diode Rectifier Bridge I used to repair the Tektronix 2440 Scope.
(I had to bend the Leads a bit to Fit in the Power Board. But it was quite easy).
I guess it now only is re-calibrating time to get the scope in usable condition again. Because the internal Memory Chips on the Board with the 3.5 Volt Lithium (Black Keeper Battery) are constantly powered at 3.5 Volt but maybe do not hold the right data anymore?
And I read on YouTube that some people had to reset or re calibrate their Scope entirely to get rid of the jumping Characters and wrong Settings on the CRT Scope Tube.
And I hope for my friend that it is not the very expensive Hybride Board in the Tektronics 2440 500MS/s Scope that is the cause of this. For now my job is done. Another valued and expensive device repaired! Hope my friend is happy and that he, or one of his Radio Amateur Hobbyist, will be able to Calibrate or Re-Configurate the Scope to almost new again.
So much for now. Hope to write some more next time !
Albert van Bemmelen, Weert, The Netherlands.
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Note: You can read his previous repair article below: