HP3468A Rom contents of U502?

[Kjelt]

Hi,
I came across a defect HP3468A. It won't power up, powersupplies are OK (15,-15,5), display shows some random activity for 1/4 second (probably just the display powering up)
No characters however, no relays clicking nothing, dead silence.
Found out that the micro gets reset every 1,3 seconds.
Looking further and making sure the uC was not bothered by the reset circuit, I found that U507 is not getting clocked by the uC.
So I suspect the U502 ROM (Mostek MK 36752) being defect.

Can not find anything on the internet except another person with the same problem 2 years ago. Does anyone here know how I could get the ROM contents of U502 , and also a good (pin compatible) replacement part? Unfortunately U520 is positioned right under the plastick power stick,so a socket with rewiring is probably out of the question.
It looks like a 8092 byte eprom with 24 pins is also unusual (2764 has 28 pins).

[atw60444]

Do you have a copy of the service manual? http://www.sphere.bc.ca/test/3468a/03468-90008-svc.pdf
Is the 6MHz oscillator running? Is the Micro sending address data out, and is the address being latched by U508?

Jon

[Kjelt]

Do you have a copy of the service manual? http://www.sphere.bc.ca/test/3468a/03468-90008-svc.pdf
Is the 6MHz oscillator running? Is the Micro sending address data out, and is the address being latched by U508? 

Hi Jon, yes the 6MHz is running, ALE is working (see my original post) U508 I have to check , TBD. Thanks for thinking with me.

[Kjelt]

Is the Micro sending address data out, and is the address being latched by U508? 

Well something fishy is going on there, look at the screenshots. These were taken on U508, ch1 is the clock (pin 11) and ch2 is 4D and in the second picture 4Q.
It looks like some part is pulling the latched output low, if the clock rises you see the input data  (high) and after the clock goes down and the input should be latched it still goes low, don't know what is going on there, unfortunately I have no logic analyzer to look at the entire picture, might want to visit a friend of mine whose lab has more equipment.

[atw60444]

Sorry, I didn't see your reply until now. Are the Q lines definitely being pulled low, or floating? I wonder if you can prove U508 is actually latching the outputs somehow?
 Doesn't seen to make sense that Address lines into the other chips on that bus are getting pulled down. They're all inputs after all.

Jon

[Kjelt]

I have to investigate further, it could be normal since it latches at the downgoing flank of the clock and there the signal is low.
But I have so little time at the moment due to work, a course and a family that also needs some love and care that the solder irons stay cold for the moment

[TerraHertz]

I have to investigate further, it could be normal since it latches at the downgoing flank of the clock and there the signal is low.
But I have so little time at the moment due to work, a course and a family that also needs some love and care that the solder irons stay cold for the moment

If you're wondering if a signal is being driven or floating at points in the waveform, there's a simple trick. If you have a signal generator, set it to triangle (or sine if you don't have triangle), 2Vpp, offset about +2V from ground. Frequency can be anything, say 10KHz. Now feed that signal via a 4K7 or 10K resistor to the circuit point you're looking at with the scope.
Any intervals in which the signal is floating, will look like filled-in blocks on the scope (assuming the scope behaves anything like an analog scope's persistence.)