Repair of an HP 5334B counter - Part 2 - finish

Back from a long break when I did not have much time to tinker with electronics stuff. But back with full steam, I picked the 5334B from the backlog to finish first.

I traced the signal path for channel A, and very quickly found that the Q8 jFET was dead. It is a 2N4416, which I was able to source easily. It was cheap, so I made sure that I buy more than one, which later proved to be a wise decision. After replacing it, channel A immediately sprung to life. So channel A is working.

But not finished yet. While channel B seemed to work on initial inspection, it had much reduced sensitivity and bad offset with the trigger level. This was more challenging to troubleshoot. The way the trigger level setting works is that there is a comparator (U22), which always compares at 0V. The input signal is given an offset, which will counterbalance the signal, so that the desired trigger point falls on the 0V level. Normally the trigger level is between -5V and 5V and thus the offset is from 5V to -5V. In reality the offset has other value, because of the gain on the input stage. The problem was that this offset only moved in the negative region, it would not go above around 0.7V.

I had to trace some signals. The trigger level from the front panel pots goes trough a number of analog switches in the DAC section. Fortunately, that was ok. I saw that the full range swing is present when it reaches the amplifier section. There however, something doesn't work. After some pondering and measuring voltages I was able to exclude most of the possibly bad components, with the exception of Q9, the same jFET that was bad in channel A. It was the last component that could have produced the fault and while I couldn't definitely say that it was bad, I had a spare and it was easier to replace than to do more measurements. This promptly fixed the fault!

So in both channels the jFET was bad! The one in channel A was completely dead and the one in channel B was sort of half bad, still enabling some level of functioning. The replacements are the ones with the number 138 on them on the above picture. Note the Teflon standoffs, they are used to provide more isolation to the gate.

Looking at R113, which was burned, I couldn't really figure out what happened. Since that resistor cannot really be overloaded by DC from the input, I suspect either some botched repair, like accidentally shorting something or maybe some brutal high frequency input overload. But I'm not able to tell why these components failed. 

Nevertheless, I performed the adjustment steps in the service manual, since the input stage had been  repaired. All steps completed correctly, so I'm happy to have yet another nice universal counter after a quite challenging repair. One can never have enough frequency counters!