Tektronix 2440 Oscilloscope Repaired- Part 2
After my last article on a Tektronix 2440 Oscilloscope Repair was published:
That had a Big Short Circuiting (Low Voltage) Power Supply that blew up the Wall Outlet Main Power Fuse, and its 600V 3A Fast Recovery Diode Bridge plus the Scope Mains input Fuse. I had to find the exact reason why the Scope still didn’t behave as it was supposed to. (The scope Screen was very jumpy that lateron turned out to be in Horizontal direction only).
Above photo shows the Powering Up of the Tektronix 2440 Scope.
Although I brought the 2440 Scope Back to Life after fixing the dead Power Supply it probably still needed new Hybride parts. That was at least my first Impression. Or maybe a new Hybride Board. Because only the Intensity CRT tube potentiometer and the Scope Buttons did work. And it also had something to do with some problem on the 5Vd line from the Power Supply Board to the Hybride Board. Although the original F269 5Vd fuse (10A) was okay, just as all other Board Fuses , it seemed to be short circuiting this line somehow. And maybe also was the cause why the Diode Bridge got defect. The short was not on the Power Board itself! I checked this by connecting an external 5V Power Supply to 5Vd on the Power Board. And only when connector J102 to the other Boards was attached on the Power Board there was a very low Voltage of about 2.25 Volt. (I opened the Fuse F269 connection to 5Vd first of course!). And without J102 connected there was a stable 5.20 Volt on the Power Supply from the external Power Supply! The transformer coil AC voltage for the 5Vd line seemed with 9 V probably a bit too high. The dual rectifier diode for the 5Vd line to the F269 (10A)
Fuse was still fine however! So I did Fix the dead Power Supply but probably needed replacement Boards or new Parts to Fix the Tektronix 2440 Scope. And that all depended on the Price Tags. The 5V line voltage on the A12 Processor Board, measured over the TTL components, was Fine! So I was almost certain it couldn’t be this Board that was causing problems. Knowing the fact that these old but reliable Scopes were becoming very affordable by the day, it was only a question of waiting for the right moment to buy spare parts.
The Sytem I/O Board seemed to operate fine too! Because the Loudspeaker on it (that also was Powered from the +5Vd Power line) just like the 555 chip U274 that generated the sound, and U754 a 74LS273 that activated the three GPIB Leds mentioned in the article, all worked splendidly.
And I also checked following parts on the LV Power Supply Board : U265, U155 (optocouplers), U189AB, C455 (electrolyte 6,3 Volt), U829, U834, U470, U840 and Q295. And they all were perfect! Also I noticed that U100 and U150 were rather cold compared to the other Hybride Components on the A10 Board. And only the Voltages on the J166 connector to the CRT HV Board on pin 1 and 2 were out of spec. With being around -22,5 Volt. But because pin 2 is the -12V line to the Perfect working 12 Volt FAN it doesn’t have to be any real problem. And because I checked most if not all parts on the Power Supply it was not a big problem that also the previous mentioned Unregulated +5Vd with being about 6,7 Volt (unloaded) was a bit higher than the given 5.12 Volt in the LV Power Circuit. Because it had to be a small Power Supply problem because all tested parts were fine!
I then found new evidence that hopefully helped to fix my friend’s 2440 Oscilloscope.
Photo above shows the A11 Board with the removed Bad PMI DAC10 chip. Chip U142 is the other DAC10 responsible for the Vertical deflection on the CRT tube.
Not soon after I found the probable cause of the bad working Tektronix 2440 on the A11 (Display+ more) Board: A bad working PMI DAC10 chip (got very hot!) that had to be for the Bad working horizontal CRT screen deflection. After I removed this about 30 year old chip the CRT produced a very stable and clean vertical line of scope data. That changed its data when I pressed some of the Scope Front Buttons.
Above photo shows the result after DAC10/U250 was removed. No Horizontal deflection.
So I ordered 1x PMI DAC10 chip on Ebay from a seller in the UK for 19.80 euro’s (about 13.99 English Pound). And I was very curious if the scope would operate as new again after the U250 DAC10 chip was replaced. These chips are also at least 30 years or so old. So I’m happy to have found one. The other DAC10 (U142 on the same Board) is for the Vertical deflection and must be perfect because the vertical written data line on the Tube was nice and clear !! After questions from other Jestine’s Repair Blog Readers, I had to make a follow up on the previous article, depending on the fact if the Scope would work after the DAC10 was replaced. So I had to wait until this component got here.
Next photo clearly shows that the CRT Brightness easily can be adjusted to much higher Intensity.
After these facts were noted, I also checked if the FAN voltage on the J166-pin2 to the HV Supply CRT Board was within spec. Because the open/unloaded Voltage of -22.5 Volt was just too high. But luckily it was okay because I measured exactly -13,7 on the Black Fan wire when the cable to connector J166 was connected. REMARK! What I didn’t know at that earlier time was that the Scope’s LV Power Supply gives wrong higher Output Voltages when no correct Loads are connected at all. (So I had checked all important components that were involved in regulating the 5Vd output on the LV Power Supply for nothing. But that was also when the Bad DAC10/U250 still was causing a short on the Power Supply’s 5Vd line and constantly interfered with the Scope’s screen output. And why even an external 5V Power Supply collapsed to only about +2.25V when connected to the 5Vd line. Although the energy overconsuming Bad PMI DAC10 probably doesn’t use any +5V for powering itself. And probably never an unregulated 5Vd voltage either).
But after this problem was solved, I finally measured last but not least, that the 5Vd Power line was exactly 5.13 Volt. (Was earlier unloaded a bit too high to with about 6.7 Volt measured).
Next photo shows a Close-up of the CRT screen when less data is displayed. Only the bottom lines are vertical showing up.
And the photo that follows shows the CRT screen when it displays a lot of data lines.
To be sure that a new DAC10 would work, I also measured the Voltages on the DAC10/U250 pcb socket. V- (on pin 3) was exactly -15V, V+ (on pin 15) was +15 so perfectly Fine! (a DAC10 works upto Max. Voltage of +/-18Volt!).
Next Picture shows the first page of the DAC10 datasheet:
Above picture shows the 18 PINs layout of the PMI DAC10 extracted from the datasheet.
So I was very certain the Tektronix 2440 Oscilloscope would be as new after replacing U250, and the already replaced CR510 Diode Bridge and the 4A/250V Mains Fuse. And the Short Current that dropped the 5Vd output Voltage level was now completely gone. All Voltages on the Power Supply Board were now within exact Specifications ! (But again only after the Power Supply J102 was
connected to all Boards!! Open Voltages on this Board are Higher!). So the LV Power Supply Board is completely working as a brandnew Board again ! Replacement costs of this complete Tektronix repair is only about 25 Euro. With the fantastic Help of the terrific Tektronix Service documentary that is Free available on the WWW.
PCB copper connections/track Layouts are however never given. Only the Schematics with Scope Diagrams. But it is very clear visible and understandable Information. So any PCB/Board has to be in splendid condition. A Very Big applause to the fantastic Tektronix Scope Designers!!. (Keep in mind that all Tek Service Manuals often also have a Manual Change Report at the end of their Manual that give Diagram Changes depending on the Serial Numbers of the Manufactured Scopes. Right after the standard Replaceable Electrical Parts List, the Replaceable Mechanical Part List, the splendid TroubleShooting Diagrams and of course the in their function very well catagorized Schematics).
To give an example of a change that has been made to this 2440 oscilloscope serie : The old Diode Bridge CR510 on the LV Power Supply is in newer models replaced by 4 single Fast Recovery 600V Diodes.
And knowing these Tektronix 2440 Oscilloscopes still cost at least around 300 to 400 US dollar today, my friend has a real bargain after my repair if the scope is re-calibrated. Although this Tektronix 2440 Oscilloscope has a Bandwidth of 300 MHz, also remember that presumably all of these Tektronix Scopes easily are capable of measuring frequencies that are twice their Maximum Bandwidth. At least that’s what I keep reading on several Forums about these great Scopes. My own recently in Texas bought Tektronix 2465A has no GPIB interface like this 2440 does. And my friend’s 2440 Scope is build around 1988 because that year can be seen on many components on the pcb Boards. (And that makes it about 2 years younger compared to my own 2465A 4 channel 350 MHz Tektronix Scope- that had already 48873 working Hours on it’s internal Diagnostic Counter and was switched on a total of 1596 times in about 30 years. The Lithium Battery in my Tektronix 2465A was dated 2486. Which I already replaced about 3 months ago with the help of the splended replacement article Published by Robert Calc on Jestine’s Repair Blog about 6 months or so ago).
Next photos show the result of the 2440 Tektronics CRT screen after I replaced the Bad DAC10 U250 by a new one. As the built-in Self-Diagnostics now clearly shows there are still some Failures found at Start-Up. But now the Scope really works again and I’ve obviously also Fixed the CRT display problem to enable further Diagnostic. Because without a working screen it is almost impossible to find what is wrong with the Oscilloscope’s internal electronics. It would be a too complicated task without the Self-Diagnostic Built-in Test.
As Diagnostic clearly shows, there’s nothing wrong with the Eprom content of the Roms in the 2440.
This 2440 Oscilloscope even has at least 4 CCD (Charge Coupled Devices) Memory modules on Board, besides the regular RAM chips. And probably one or more modules now fail. According to the 6 presented Fail Messages on the CRT.
But the 2440 also has AUTOFOCUS on the HV CRT Board in a 20 pins IC (U227 in the HV Supply & CRT Schematic) with tektronix List number 155-0294-00 (on Ebay about 18 US dollar). And that works perfectly! My own 2465A Oscilloscope doesn’t use a PMI DAC 10 for the Vertical or the Horizontal Scope Deflection. And probably no CCD devices, since the 2440 Tektronix is a Memory (hence the 500MS/s) Oscilloscope which my 2465A isn’t. But my 2465A uses another and totally different U800 chip that gets at least 50 degrees Celsius according to the Tektronics Forums.
After I played a bit with the Front Buttons on the outside of the CRT, I got following message (see next photo) that all selections would be locked apart from Repeat. And that I now had to look into the Service Manual. Since only Calibrating costs about $ 600 US dollar and I do not have other replacemant parts like CCD-Modules, I guess that here my 2440 journey ends. At least until I find new replacement parts.
Summarizing I can only advise any reader to also try to repair any decent Tektronix Oscilloscope, even if these devices are of some age, because they are very nicely build and the service manuals are very helpfull. Of course as long as spare parts are still available! Or buy at least a Tektronix Scope that does not fail on the Internal Build-in Diagnostic Test. Which presents itself at Start-up/Powering Up. Like the Seller had assured me before I bought my 2465A Tek. Ask the Seller of the Oscilloscope
at least what this Diagnostics Test tells him, and how many Hours it already had operated in its electronic lifetime. Because that’s something that also gives any Tektronix something extra over any other Oscilloscope!
I hope you all had just as much fun with this repair as I had!
Until another repair, or a new interesting electronics article.
(And yes Frans, I will shave but another time, I had to fix your Oscilloscope first! LOL)
Albert van Bemmelen. Weert, The Netherlands.
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Note: You can read his previous repair article below: