Tektronix TDS 310 repair help

[Helios]

I have some problems with my Tektronix TDS 310 and unfortunately the other TDS-related threads on the forum did not help me enough to fix it.

The scope boots up fine and passes the powerup checks but the Cal. initialisation fails. I have attached the error log for further information.

All measurements are fine up to 100mV/div except for some offset between ranges. When switching into <100mV/div the input signal is gone on both channels. I have read that there can be some problems with the relays on the input but I could not notice anything abnormal (coil voltage, contact resistance etc.).

The only fault I was able to find is the input resistance. When the range is >50mV/div, the resistance is 1 Meg on both channels. When switching to 50mV/div, the respective relay clicks and there is a voltage of 6.7V on the input BNC. What could be the cause of this and how could I fix it? Bad/leaky caps perhaps? Any help is greatly appreciated.

[wn1fju]

I have a TDS350 which is similar and the problems I had were similar to yours:  1) the baselines would shift around, 2) the traces would vanish off the screen at 50 mV/div and below, and 3) there was approximately 7 volts on the input BNC connector.  Incidentally, as soon as I put a load on the BNC, the 7 volts dropped way down.  All this happened only on channel #2 in my scope.

My conclusion - and I know this isn't good news - was that the input preamp hybrid was bad.  I managed to find one fairly cheap on eBay and that fixed the problem.  The hybrid doesn't look like a circuit that is easy to repair - there is all sorts of buried stuff if I recall.  Perhaps someone else on the forum that has actually fixed one successfully can chime in.

[Helios]

I will try to do some measurements on the preamp circuit. Replacing its components sure does look quite tricky. I guess someone must have overloaded ch1 and then continued to ch2 since both channels are affected.
Thanks for narrowing down the problem!