This was supposed to be an easy repair, and therefore not worth blogging about. But as it turns out, it might get interesting after all. (The ‘scope is not repaired yet)
The “Leidse Makerspace” owns a LeCroy 9450 350Mhz DSO. When they moved to their new location I temporarily got this oscilloscope. Not just for use or storage, but also to attempt to repair it. One of its channels was not working, sometimes it even wouldn’t display anything.
The display problem quickly turned out to be a loose connector. With this connector loose it would display nothing it al, or if it did work, it would sometimes glitch out like this:
While at other times it would display normally:
Probably just some transport damage, as after refitting this connector the problem has not been back. Now, on to the more interesting problem: channel 1 did not work: it did not respond to an input signal and it had a huge offset. After using the vertical position adjustment knob to bring it into view it would sometimes even oscillate on it’s own, showing needle-like pulses. The ‘scope also wouldn’t trigger on channel 1. Channel 2 functions fine, so it is still a usable 350Mhz DSO. However, 2 channels would be a lot nicer, and fault-finding is one of my hobbies. So… Time to remove the covers.
The easy-est way to measure in the analogue front-end of this oscilloscope is to turn it upside down, remove the bottom cover, remove the 12 screws holding the shielding, and remove the shielding.
After this, an input signal can be followed, measuring before and after each subcircuit.
(I don’t have a picture of this, but I do have a picture of the 9450_7 front end, removed from the ‘scope, and from my annotated copy of the schematic)
Close- up of the area of the board I’m looking at:
Somewhere here, the signal got lost. As you can see there is another module inserted through the pcb, this is the HHZ406. It turned out the signal entered this module (an amplifier), but nothing sensible got out. view from the other side of the board, also showing the relays. (Those metal cans)
This module does not look like it can be repaired, nothing is obviously visibly broken, and those “black blobs” don’t look promising either, because these usually cover (custom) semiconductors directly bonded to the PCB.
It also looks like it cannot be bought anywhere. Too specific, too custom… (If you know where to get these, or work for LeCroy and have spares, or if you would like to reverse-engineer them, let me know.)
But this story does not end here. This scope has 3 HHZ406 modules. One for each channel, and one for external trigger.
So I swapped the modules for external trigger and channel 1. After this, the signal got to the output of the 9450_7 board. When using the same V/Div settings on ch1 and 2, and feeding them the same input signal, the signals here would be identical.
The story does not end here either, however. Channel 1 still does not work. It does respond to an input signal and the ‘scope does trigger on this channel now, but the signal is not displayed properly. It has needle-like pulses on it. These pulses move when changing V/Div settings on the ‘scope or input signal amplitude from the signal generator. (Video: http://www.youtube.com/watch?v=wFei5CZ6lqA)
There is another defect lurking somewhere in the 9450_3a ADC boards, because when I swap them, the problem moves to the other channel (Ch2). Measuring on these boards is harder because they are not easy to get to while the ‘scope is operating, unlike the analogue front-end (9450_7).