Last week I scored a Tek 2465A 350Mhz scope on Craigslist for 100$ -- true it was having the TEST 05 FAIL 44 message. Since that was something I have seen and fixed before I could not resists and I bought the thing even that I had and Tek 2465 300Mhz sitting on the bench. At that price ... even if I had it for spare parts.
Long story short, after replacing only 3 electrolytic caps on the LV power supply the 2465A is baaaaack to life. It was the usual suspect, the -8V supply. BTW the 2465 had the exact same issue when I got it
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. EVen more after the fix the 2465A is bang on on both vertical and horizontal calibration on all 4 channels. I cannot see any issues. SO .. verdict ... "that's a winner" !!
But, and here is the annoyance - there is always a BUT , the 50 ohm coupling is dead, flat as GND on both CH1 and CH2. On both channels if I use the 1M coupling on DC or AC signal is spot on, everything works a treat ... if I switch to 50 ohm is like I connect the input to GND .. goes 100% flat. So, started going over the attenuators and channel preamps to figure out how the thing works. I am trying to confirm my theory before I go any further:
1) Both CH1 and CH2 have identical attenuator circuits. Nothing more than a bunch of relay, resistors and caps plus the actual 10x and 100x attenuators. Since the attenuator chips themselves are common on both 1M and 50 ohm I doubt there is any issue with these. The only thing left that can obviously short to ground is the 50 ohm resistors and the clamping diodes. Even the diodes I doubt because these are again common onthe signal path .. so the only culprit I can think of is the 50 ohm resistors to be fried.
2) The signal path after the attenuators is common for both 1M and 50 ohm coupling. Since the 1M path works a treat it means there is nothing wrong with the preamps on both channels.
Given 1 and 2 .. all I can say is the 50 ohm resistors. But on both channels ?!!!! WTF?! That puts my thinking in doubt. Can have been there such a moron that has managed to fry both 50 ohm inputs
Wonder is anyone has seen this before; I never touched such voodoo electronics things like input attenuators in oscilloscopes. I guess what I said above is right but better double triple check before I cut the wire. I can live easily without the 50 ohm coupling but if it can be fixed I wold like it fixed.