LT342 Repair

[andy1]

Hi, I had the possibility to get from work an old Lecroy LT342 scope which is broken and I was wondering whether there is any chance of repair.

The scope powers on and everything works softwarewise perfectly but I guess the problem lies in the input circuitry because it gives calibration errors for practically everything (GAIN10 OFFSET10 GAIN05 OFFSET05 GAIN100 OFFSET100 etc cal failed, BWL, VGA curve cal failed, hfe amp cal failed, digitizer cal failed) changing the settings works and I can hear the relays clicking inside.

I can also see the traces sometimes if I happen to get the offset correctly as it might be outside the visible range, also (propably) the calibration signal on atleast one channel is visible as there is faint square wave so I think the ADC is giving something out and even on the other channel I see low level noise which looks ok for non-connected input.

The scope was most propably damaged by high voltage pulse to one input (~6kV but relatively high impedance) as there was clicking noise from relays right after such propable event and the scope has not worked since (this was several years ago). The scope is in very good condition otherwise so I would be very happy to get it working for higher bandwidth than my 100MHz Hantek.

EDIT1:
So I had some time and managed to test it a bit more, I got both channels visible when I set the channels to 10V/div and swept the offset. Test signals from my signal generator on both channels were visible so the ADCs and data acquisition seem atleast partially ok. Also external trigger input works and triggering on both channels with AC coupling (Unknown offset so DC coupled triggering difficult to find but I guess it works).

Power supply voltages/ripple seems ok.

I'll continue probing the front end and check the stuff from the service manual so hopefully It is just offset problem.

EDIT2:
As I continued looking at the signals I noticed there seems to be attenuation and offset on the calibration output signal, the CAL output DC voltage is offset about -0.21V and the amplitude is only to -0.8 to +0.7V (The settings go +-1V). This might have something to do with the analog switch (74hc4051 ) as I the input looks ok. Will continue testing.

EDIT3:
Also I noticed that if the channel coupling is set to Grounded I can see the calibration output signal on the channel (both the square wave with the changeable amplitude/frequency and the DC value). It is a bit of hit and miss to get the channels visible as the calibration done on the startup and later might change the offsets completely.

EDIT4:
SUCCESS!! Atleast partially, as I traced the internal calibration signal I noticed that the analog switch hc4053 was conducting between two inputs/outputs where it should not and I lifted pin 3 of 16IC4 and now the scope started with no errors and channel offset is at 0, I will be replacing the analog mux at some point when I get the parts but this was really strange breakage considering it is not directly connected to inputs (or atleast there are several other stages before it)

[perdrix]

Sounds to me like no harm would be done my stripping down the input attenuator assembly to see what's up (or not).

Dave

[tautech]

 
Don't you just love fixing a scope with another scope. 
Well done.

Photo?

[digsys]

Excellent progress. Did you find the service manual? I'm guessing you must have...

[andy1]

Yes I got the service manual from http://www.ko4bb.com/manuals/index.php?dir=LeCroy

Here are few bad pictures of the unit, they must have thought about the repairability with these units as it was really nice to get to the inputboard with the scope powered and the cables etc. looked like they were designed so that you can have it on the side as can be seen on the first picture.

I guess the next thing is to maybe get a floppy->sd emulator to easily save waveforms and maybe change all the encoders on the front (couple have some issues and they seem to be quite cheap)