HP54600B with spike problem with and without signal

[sacha]

Thanks Tim, you're giving me a lot of advice to think about, in any case I think the best way is to try to understand the cause before intervening with the removal of the components. At the moment I only have the new Ram available. As soon as I can I will take measurements using OE/WR as trigger. I wonder what changes in the reading when using pin 20 (S) as the trigger of this memory?

these are the links to the videos on youtube
I don't know if they can be useful because I didn't use the trigger on the pins recommended by Tim

https://youtube.com/shorts/J2vYJLIUjlc?feature=share
https://youtube.com/shorts/Ifw9fT1vDFY?feature=share


below you can see my latest little one who arrived today :-)

[T3sl4co1l]

Looks nice and periodic, but who knows what the edges are doing.

To clarify: you need a range under 100ns/div at least, to even begin to resolve the edges.  A 100MHz scope may not be sufficient to resolve it, either.

Tim

[sacha]

Hi, I made the measurements with the trigger on pin 22 and I see clean signals on the rising and falling edges. But on pin 20 I see this signal.

https://www.youtube.com/shorts/3bgqQN2jaMc

[T3sl4co1l]

The other pins look fine, sharp rise, no wobblies?

Whatever's driving (or not) might be open-collector/drain style, and the waveform is normal; it only matters how long, and when, /S is below threshold, and the rise might not matter.  You'll have to trace back the source and see what type of driving pin it is.  It is definitely not a normal waveform for a CMOS driving pin.

Tim

[sacha]

I don't see those noises on the address pins or even on the in/out pins. I'll try to understand where that signal is coming from

the photos you see were taken on pins 9 and 11 of the memory

[iMo]

First tell us what are the voltages of your signals, it should be 5V aprox. Next pls indicate the names of the pins (instead of numbers), like DQ4, /WR, /CS, /OE, A11 when you talking the U33 for example.
The ramp on your last shot is suspicious. The U33 memory is 85ns access time (so the system clock in that part of system will not be higher than 10-12MHz), afaik, so you should see all signals well on your 100MHz scope.

When you see such a slow ramp you may try (as a test or a quick workaround) to wire a 1k pullup (against the 5V) on that signal, sometimes it may help.

PS: in order to look at the memory sigs you may, for example, to trigger your scope on the /CS or /WR or /OE edges (on the channel1) and look at all other signals (channel2), one by one.

[T3sl4co1l]

I'm guessing it's 2V/div, 5V supply, and the ramps are open bus (with weak pull-ups, but nothing reading during so it doesn't matter).  But this would be a good place to have multiple channels showing the strobe signals.

Another tweak that may be diagnostic: loading the signal (particularly the suspicious pin) with a resistor of say 1k, down to even a couple hundred ohms, pulling to VDD or GND, or with a capacitor of some 10s of pF, and see how much is necessary to cause data corruption.  Most likely, all the other signals will be well-behaved (tolerant down to, say, 200 ohm or so, or up to 30pF or more), and the suspect signal fails with much less (or in the pull-up case, potentially improves).  You will see the effect of these loads on the scope trace.

Tim

[sacha]

Thanks for the replies, your help is very important in finding the cause of this fault. all the tests done refer to U33 which is powered by 5V. All the signals we see are around 5vpp. I used the video number in reference to the pins of U33, photos or  video 1 = pin1, etc.... For the images you see I used the OE pin (22) as the trigger. And the suspicious shape is seen on the CE pin (20). During the tests I noticed that by touching only pin 20 with the oscilloscope probe the spikes attenuate. and I see that my probe set x10 puts 15 pf in parallel on pin 20. I will do as you said to put resistive loads on +5V and 0V and see what happens.

Opening a similar oscilloscope I realized that it has a RAM with a lower access time (70nS) I could try to replace U33 with a lower access time than the current 85nS.

The photo I posted describes the Memory I saw on the second Oscilloscope, the same but with U33 with an access time of 70 nS


Thanks again, I'll let you know as soon as I've tested it.

[sacha]

I attach what I found online as schematics for this HP 54600B

[sacha]

I attach what I found online as schematics for this HP 54600B

I apologize, but I couldn't find the complete manual 

[fmashockie]

Another general recommendation: you should join the HP/Agilent/Keysight test equipment group on Groups.io.  It is a group for discussing HP/Agilent/Keysight equipment. 

[sacha]

This scheme is also very similar to the 54600b

[sacha]

Hi  fmashockie,

Sorry but I can't see the page, try sending me the link.

[fmashockie]

It is a private group so you have to request an invite, but you should at least be able to search through chat/forum history before getting approved.  And then you can make posts of your own. https://groups.io/g/HP-Agilent-Keysight-equipment

[sacha]

Hi  fmashockie,Thanks, I'll try to ask.