Tektronix TDS540 tales from the repair bench

TDS540 Repair and Restoration


400MHz, 1G/S, 4 Channel digital storage oscilloscope


Like many techy nerds, I have a bit of a soft spot for Tektronix 'scopes (I have 4 of them in the workshop now, of varying age and capability) and picked up this TDS540 some time ago on eBay in a 'working but with faults', condition.  The power-on self test showed this familiar result, with two test failed:





These big, heavy TDS series 'scopes are well known on the second hand market and some can still command a hefty price. The early ones in the series (generally with no letter suffix - like mine), are notorious for suffering from damage due to acid corrosion from leaking electrolytic capacitors. This one was no exception.

In good working order, they are a pleasure to use (if a bit noisy), with an intuitive and easy to use menu system and controls. I'm a big fan. So my hope was to follow the standard approach to repair and recap the main boards, give it a good clean and get it back to working condition.

There are lots of tales showing, especially the acquisition board, being re-capped and then run through a dishwasher (yes, really), to restore them to working condition. 

My example was clearly suffering from the leaky caps. A lot of the power-on self tests failed - as you can see from the error log below-  and the general functionality of the channels was so bad, they were unusable, including a slow DC drift on the trace.











The first job was to remove and replace the many surface mounted electrolytic capacitors. The simplest way to remove them I have found (thanks to Mr. Carlson's Lab on YouTube), is to simply twist them off with a pair of pliers. Sounds brutal, but they snap off quite easily, leaving behind the small tags that solder them to the board. The pads then need to be cleaned up thoroughly. That distinctive smell of leaked electrolyte was very evident when cleaning the pads. With the caps off, the board needs a thorough wash. I gave it a good scrub with soapy water and a hose down. Never had the corouge to put it in a dishwasher....a recommended method I understand.

With the new caps fitted, I still found many errors reported at start-up. Ah well. 

The symptoms my unit showed were:
- a slow vertical drift on the horizontal position on all channels
- poor or no triggering
- a lot of artifacts on a waveform, probably due to missing data

Close inspection showed the trigger area of the board had suffered particularly badly from corrosion, and checking voltages, I found several chips were missing some of the supply rails. With these fixed things improved a little.  I also tracked down the vertical drift to some of the Op-Amps (I think in the signal input area, can't remember now). These were removed, the area cleaned with IPA (again) and new chips fitted. With that done, the vertical drift issue was sorted. This is a high impedance signal area, so is very sensitive to contamination, and I suspect some of the acidic damaged had worked its way into the opamp chips themselves.  This whole process was very time consuming, checking chip voltage rails and signal path traces. But in the end, the basic functionality of scope was restored - apart from the missing trace data artefacts.





I figured a lot of the error messages in the log that I was left with were due to memory faults in the SRAM chips - a known weak area. The originals are 24-pin 2k x 8 and are quite expensive, but the board is also tracked for 28-pin 8k x 8 chips, and these are much cheaper (Mouser part 913-AS6C6264-55SCN). So I decided to start replacing the memory with these, bank, by bank.

For the TDS540 the memory locations map out as:

0x7300000 is the start of acq memory on Channel A, Signal Channel 1
0x7340000 is the start of acq memory on Channel C, Signal Channel 3
0x7380000 is the start of acq memory on Channel B, Signal Channel 2
0x73c0000 is the start of acq memory on Channel D, Signal Channel 4

The screenshot below seems to be telling me that Channel 3 has memory faults. So I set about replacing the full set for Channel 3.




To cut a long story short, each time I replaced a set of SRAM chips, the fault moved on to another channel, until inevitably, I had replaced all of them. Definitely a job where you need some experience in reworking SMD parts. 


This very handy resource is also helpful in pinning down which memory chips are the faulty ones:
https://groups.io/g/TekScopes/topic/tds540_problems/7652169?page=2


And....after finally fitting all new SRAM chips, I was greeted - at last - with a self-test PASS!  :)



I did also replace the Dallas NVRAM module with a new part (Farnell No. DS1245Y-70IND+), copying the content across with a TL866 chip programmer. Pretty straight forward, although desoldering the old module was a bit tricky - not to be rushed.

An GPIB Interface


A bit late in the day, I did get around to sorting out an interface to communicate with the 'scope over the GPIB interface.  I had looked at this early on, but baulked at the price of these things. And then I stumbled across the open source project AR488 that creates an interface using an Arduino. Brilliant. So a big shout out to the guys who created these, thanks a lot.

https://github.com/Twilight-Logic/AR488

The schematic is very simple and can easily be breadboarded - as you can see from mine.





There are many options for what software to use, but I just used putty running on a windows machine. Worked fine.

A few tips that might be helpful:  
the interface by default does not echo typing and does not output responses from the device. So issue these two commands first and it will (otherwise it looks like nothing is happening):
++verbose
++auto 1

If you need to check what address your scope is set to, try this:
++fndl all

Then use this command to set the interface addresss (10 in my case):
++addr 10

You can then read the error log one line at a time with:
:ERRLOG:FIRST?
:ERRLOG:NEXT?

And clear it if you wish with:
:ERRLOG CLEAR


Enabling Options

See this thread for more info:

https://www.eevblog.com/forum/testgear/enabling-options-in-tektronix-tds500-tds600-and-tds700-series-scopes/

PASSWORD PITBULL
WORDCONSTANT:ATPUT 327689, 1 // 2F

WORDCONSTANT:ATPUT 327686, 1 // 1M








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