I repaired one of these a couple of years ago. Same issues with the output transistors. Ended up finding some NOS in the US. Easy fix and working as it should. I held on to it for about a month after realising just how unstable the VCO frequency generation was. I could not live with it and ended up finding an older but significantly better HP 3325. Karl
I had to do the calibration on mine. I was more than shocked when I started reading the acceptable ranges and the compromises needed to get it in spec, getting one range in tight tlerance meant another range horribly out. It was only then that I noticed the +-5% on most freqnecies and amplitudes and the relatively poor stability over time I ended up with in about 1% up to 20MHz, then getting worse to about 4% by 50MHz - pointless for my use! I am not hanging on to it. I use a Siglent Arbitrary generator that is "patched" to the max 120MHz for most purposes and it is pretty good.
Putting in fuses, because I couldn't find my zero ohm resistors sounds like something I would do while troubleshooting something in the final output section that's tripping the crowbar over current protection. Especially if what's tripping it is my clumsy probing to measure voltages.
Error 42 here. -24V rail fuse blown. Good lesson to learn to check both sides. Since yes the schematic does not tell you it uses fuses. Was labeled as "W16 with 0 ohms". 5 blown transistors on the output amp. Some failed as shorts others failed open. Seems like a very common issue with the 8116A and the 8112A from what I hear. Wonder what the deal is? Pretty sure it worked in its day though. Another common thing to check, make sure its "original". I think one of the transistors were replaced with a guessed equivalent before it got to me. So there's that as well.
Just checked my scribbled on service sheet and it was E21 which is a VCO slope, but I suspect the error can surface in a number of ways if some power is not available. It was U101A that I refer to, which is pretty much the first section described n the E21 process - this provides a fixed -1.25V for a variable voltage source that is then buffered by U101B, it was of course not providing it! Now had I spotted the power problem earlier...
Re the output transistors, I've seen the design they used in several other devices by HP, and one failed transistor did tend to take out a couple of others and also fried the current limiting resistors. I think the design is not particularly robust in the face of a short circuit if it is being used for high voltage output the current through the resistors on the final transistor is just going to be too high whithout a 50 ohm output impedence to limit it. off the top of my head about 2A which is going to cause transistors (and depending on duration) the resistors to fail pretty fast
Good reminder information. Thank you.
Thanks
I repaired one of these a couple of years ago. Same issues with the output transistors. Ended up finding some NOS in the US. Easy fix and working as it should. I held on to it for about a month after realising just how unstable the VCO frequency generation was. I could not live with it and ended up finding an older but significantly better HP 3325. Karl
I had to do the calibration on mine. I was more than shocked when I started reading the acceptable ranges and the compromises needed to get it in spec, getting one range in tight tlerance meant another range horribly out. It was only then that I noticed the +-5% on most freqnecies and amplitudes and the relatively poor stability over time I ended up with in about 1% up to 20MHz, then getting worse to about 4% by 50MHz - pointless for my use! I am not hanging on to it. I use a Siglent Arbitrary generator that is "patched" to the max 120MHz for most purposes and it is pretty good.
Putting in fuses, because I couldn't find my zero ohm resistors sounds like something I would do while troubleshooting something in the final output section that's tripping the crowbar over current protection. Especially if what's tripping it is my clumsy probing to measure voltages.
Yes, I prefer to power it from external current limited supply.
Error 42 here. -24V rail fuse blown. Good lesson to learn to check both sides. Since yes the schematic does not tell you it uses fuses. Was labeled as "W16 with 0 ohms". 5 blown transistors on the output amp. Some failed as shorts others failed open. Seems like a very common issue with the 8116A and the 8112A from what I hear. Wonder what the deal is? Pretty sure it worked in its day though. Another common thing to check, make sure its "original". I think one of the transistors were replaced with a guessed equivalent before it got to me. So there's that as well.
Just checked my scribbled on service sheet and it was E21 which is a VCO slope, but I suspect the error can surface in a number of ways if some power is not available. It was U101A that I refer to, which is pretty much the first section described n the E21 process - this provides a fixed -1.25V for a variable voltage source that is then buffered by U101B, it was of course not providing it! Now had I spotted the power problem earlier...
Re the output transistors, I've seen the design they used in several other devices by HP, and one failed transistor did tend to take out a couple of others and also fried the current limiting resistors. I think the design is not particularly robust in the face of a short circuit if it is being used for high voltage output the current through the resistors on the final transistor is just going to be too high whithout a 50 ohm output impedence to limit it. off the top of my head about 2A which is going to cause transistors (and depending on duration) the resistors to fail pretty fast