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.