Don't do this
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- Опубліковано 6 жов 2024
- This repair took far longer to diagnose than it should if I followed my own advice... So Annoying! I normally just take a few "sanity" measurements around devices on the board but this time I must have been distracted and tested the power supply and decided the voltages were all fine... And it came back to bite me...
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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