Good Day :-)
Im new here and have a question about the repair of a HP5316A universal counter.
I have a fairly good understandig of basic DC circuits like Cars wiring as well as Mains circuits. I have no formal education in electronics but was able to lern a lot from professional people I know and the internet. I'm considering myself pretty save in the vincinity of mains voltage so please keep the safety advises to a minimum. Im currently studying mechanical engineering in Germany and want to dig a little deeper in electronics, especially the stuff that happens faster than i can think.
To do that Im trying to gather some testequipment. Perferably cheap and broken.
I currently have:
HAMEG 204-2 20MHz Scope (works with a few bugs)
Phillips PE1542 Power supply (succesfully reapaired broken current sense on Ch.3 )
HP3312A function genarator (works, succesfully repaired some electromechanical issues and a FULL BRIDGE RECTIFIER)
HP3478A Multimeter (needs preventive maintenance, memory battery and RIFA caps)
PM6612 80Mhz Counter Timer (works)
a lot of basic tools and handheld multimeters
And obviously the HP5316A
I got it as broken for almost nothing. I know that it failed calibration or a saftey check in 2003 and went in to the repair pile of a guy who sadly passed away bevore he could do anything with it. Now its mine.
When I first tested it it worked perfectly fine. At least everything I could check.
But suddenly the gate time control knob stopped working. I figured out, that it has another gate time preset poti at the back and can switch back and forth between both. As far as I know that function is only useful when the counter is under GPIB control. Then it can be programmed to youse either of those.
I had nerver anything connected to the GPIB port.
U7, a (de-)multiplexer IC (1826-0501) had switched from the front to the back knob. And I figured out, that it got the signal to do so. This signal came from U8 a CMOS latch as the mauals calls it (1820-1466). This ist where my journey ends. I can not find any information what this is, what it is supposed to do and so if it is doing what it is supposed to do.
The critical Signal path between U8 and U7 goes first trough a jumper bridge J5 (I think just for troubleshooting, Thanks HP!) and is pulled up to 5V by a 100k Resistor direct next to the pin of U7. When the Jumper is connecting U8 to U7 I measure about 0.5V at U7 and the rear knob is active.
If I break the connection at the jumper, the input pin of U7 gets pulled up to 5V and the front knob is active.
I hope you can follow me along.
My questions:
How can this happen? It just randomly switched from front to back and stays there. A failed part? A "ghost signal" that switched ist?
What is 1820-1466? I was unable to find anything in crossreferencing sheets.
And what does a CMOS latch do? All I could find was latch up effect wich seems not to be what im searching for.
How do I proceed? I could try to keep figuring out the signal path witch gets increasingly difficult. It might also be useful to talk to it via GPIB and just tell it to switch back to the front knob and see what happens. Unfortunately I dont speek GPIB, english is already difficult enought for me.
Im really sorry if some of the questions are super dumb. And I would highly appreciate if you could help me figuring that problem out.
A link to the Operating and service manual:
https://www.dennlec.com/images/hp-5316a-op-service-manual.pdf
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