Tektronix TDS520B Attenuator Swap

[sorenkir]

Hi all, it's my "Scope Fixing Saturday" !
 
I have recently acquired a used Tektronix TDS520B DSO (1997 vintage).  My first 500MHz oscilloscope (among a quite large collection of lower bandwidth scopes: Tek 2445, TDS210, 475, Fluke PM3380B, Hameg HM205-3, Metrix OX8022, etc. – one can never have too many scopes !).
No leaky electrolytic problem with this series, only the limited life NVRAM sword of Damocles !
 
It suffers from a strange issue:
Sometimes, input impedance of CH1 is 170ohms in 1Mohm mode and 38ohm in 50ohm mode as if a 170ohm resistor was in parallel with the input. Apart from this, attenuator behaves correctly.
The strange thing is that this fault disappears when I apply some DC voltage (12V for example): after that, the input impedance is back to normal (1Mohm or 50ohm) until powered off.
I cannot explain that with the schematic of the input attenuator, maybe some sort of fault in M753 or a defective relay (5 relays):
 

 
Input attenuator is a ceramic hybrid (H2462) beyond repair.
By chance, TDS520B has 4 inputs with 4 identical attenuators, although only two "live" channels can be displayed at a time.
So, last Saturday, I decided to swap CH1 attenuator with the much less used AUX2 one.
 
Dismantling the scope is a pleasure: just slide the cover, unclip the front panel bezel, 3 connectors and about ten screws later, the acquisition board is out.
Then, ten more screws hold a massive metal shield where the attenuators can be found:
 

 
Laser trimmed resistors, thick film resistors and custom chip:


 
Relays "on stilts":



 
Attenuators are not soldered to acquisition board. Contact is made by a black plastic plate with small gold(?) wire balls into holes. This "connector" is made by Cinch (# 380-02-20-049):
 

 
Swapping is done quickly (just the front BNC to unsolder/resolder) and reassembly straightforward.
 
On power-up everything seems to be OK. Only a small offset on swapped channels between trace indicator & trace. Fortunately, no need to recalibrate to remove the offset: the Signal Path Compensation procedure fixed that.
By the way, I am looking for the "TDS 700A Adjustment Software" which is the calibration software for this scope according to the service manual. If somebody has this floppy disk somewhere and can make a zip archive  …
 
What is funny is that the problem seems to have disappeared also on AUX2 channel (ex CH1) attenuator (Murphy, where are you ?) !
 
Michel.

[curiosu]

Hi, I have the same problem with a TDS 724A, in my case the attenuator is completely toast  being broken, do you thing if I live the Aux2 floating the scope will pass SPC to be at least a bit uesfull ??
Also did you happen to get the calibration software, could you please share ?

 Cheers and thanks, MC

[pmcouto]

You can find TDS700A Adjustment Software here:

https://www.eevblog.com/forum/testgear/tek-tds-3054b-vs-tds-744a/

[curiosu]

Many thanks, lots of interesting stuff on that topic, too bad that the search function of the forum is a bit anemic .

[tautech]

Many thanks, lots of interesting stuff on that topic, too bad that the search function of the forum is a bit anemic .

This is a well know bug in the forum.
But use the "Search" button by the "Home" button, this works perfectly.

TDS700A Adjustment Software gets this thread first and the one pmcouto linked second.