Tektronix TDS3054B PSU repair

[TaylorD93]

For a few years now i have used my Tek TDS3054B as my reliable old faithful scope on my work bench but I was always aware that for such a small scope with it plugged in but "off", i suppose in a Standby mode, the Top of the scope always got remarkably hot.

Until recently i just kept it off for as long as possible, but knew i was on borrowed time with it. Recently i started getting 80+MHZ ringing every 3uS or so. Reading a few threads online it led me to believe that the PSU was on its last legs. Now not being a fan of Noisy (although admittedly more efficient) Switching PSU's, I decided after looking around to just replace the module. For the sake of time/convenience. However having now done that i dont know if it was quite worth it. But here's a little story i suppose.

The Tek Service manual states it is a NAN40-7615 PSU. This is now Obsolete, but the manufacturer states that an LPS44 model is a replacement, in stock at RS UK, so i ordered one up and turned up in no time.

I stripped the scope down and found the following snags, which anyone else who might chose to do this repair should be aware of;

1) The Line Input connector Polarity is inversed.
2) The Line input connector is moved closer to one side of the PCB, this makes fitting it difficult as the Line solder almost touches chassis.
3) The Earth connection for the PSU is moved to the other long edge of the board, the metalwork it is screwed to, does not have a direct connection to Mains Chassis/Earth, so is now floating.
4) The 15V Output connector has moved a few mm, which makes fitting the output connector trickier but not impossible.
5) Additional Heatsinking on the PSU board, means the Fan cable has to be routed slightly differently (no biggie) & Option tray foulled.

In the end, i did get it to fit and works fine, however i will break these points up into multiple posts

Fix for issue #1 - The Line connector, can easily be de-pinned and polarity swapped.

[TaylorD93]

Issue #2 - I stripped the rear "stretcher" plate and found that if the PSU was screwed down to its normal location, the small clear insulator plastic fitted to the new LPS44 would get crushed between the LINE conductor solder pin and the chassis.

The stretcher plate is held on with a few screws, the Coax cable unplugs from the BNC, so other than removing the ribbon and IEC inlet, it all came apart ok.

So i marked off the a bit on the tab, cut off with a hacksaw and file. Washed it thoroughly before it even went back near the scope so there was no swarf and test fitted. Did add some Kapton tape to the Input terminals too just for the sake of it.

[TaylorD93]

Issue #3 - The short support bracket under the PSU, closest to the Display of the scope, isnt actually directly connected to earth on the old PSU or the mating boards, it just pushed into a friction fit in the battery/smaller PSU board. Now that the Earth connection on the LPS44 is this side, it doesnt have a mains chassis/earth connection.

So i found a small area inside the rear plastic cover, where there was clearance for a M3 screw head. with some approximation i marked it out on the metal work between the Dsub shroud and the Window for the Serial Number.

Note that the LPS44 documentation/manual says that any earth wires should be no less than 1.5mm^2 and less than 7" long. my initial wire was only 0.75mm but it was a proof of concept i later replaced with a 2.5mm^2 earth wire, going from a FASTON crimp to a M3 ring crimp.

Now i was happy that the PSU was suitable earthed.

[TaylorD93]

Issue #4 - 15V Output connector, all that needed to be done with this is to twist the cable as such that the wires laid down its side rather than the "top" of the connector, it meant that the wires sat nearly directly ontop of the D2PAK devices on the connecting board, however if not the wires foul against the nearby electrolytic caps.

[TaylorD93]

Issue #5 - Finally due to the additional heatsinking, there were actually two issues, the plastic locking tab on the "option" tray had to be trimmed slightly to fit as the new PSU fouled on it, and also the Fan cable which historically had been routed near PSU board level then under the Dsub ribbon, now had to be routed above as the cable wouldn't fit that well.

[TaylorD93]

So overall it is now fixed. But would i fix it this way again? hell no! Admittedly all changes ive made can easily be removed and reverted back to the old board. But i think it took much longer than needed. So perhaps this thread can help others in the future.

Just a pleasant reminder that although a manufacturer may say something is a replacement, it doesnt always mean it is a direct form/fit/function replacement.

Learn from my mistakes... just fix the old board 

[m3vuv]

Why didnt you repair it rather than replace it?

[TaylorD93]

Because I was led to believe that the "replacement" would have just dropped in. 4 screws, 2 connectors and done.

Unfortunately it didnt turn out that simple, but the PSU board was £72 to the door, so decided it probably wouldnt have done any harm to upgrade, the new one has much better heat sinking across the board which was one of my concerns in the first place.

[m3vuv]

are you going to fix the old one?,had issues with my hp 54503a scope psu,couldnt get a replacement tho so did a full recap,that fixed it. ,i hate smps tho!

[TaylorD93]

Yes i plan to fix the old one and use it for another project. 15V at 2.6A with passive cooling, I can drop it down to 12V for another project. So it wont go to land fill

[YetAnotherTechie]

Hello
Does the new lps44 power supply also draw 6.37W in standby (plugged in, but scope off) like the nan40 did?