Help with HP 8116A

[idpromnut]

Having a 8116A myself (but it has a different front panel, I will need to throw a photo of mine up), I am interested in the inner workings of this beast   I am following along at home, so if you need me to check anything, just toss me a request 

[George_Race]

Thanks for the offer, not sure at this time what you could do to help.  As I am sure you already know, I am working with Mark to get to the bottom of the problem my 8116A is having.

How is your front panel different, I did not realize they made different models, physically, of the 8116A.

When you get a chance, a picture would be nice to see.

I am really enjoying trying to find the problem with this one.  Hopefully one of these days it will spring to life!

George

[MarkL]

I'm a little in the dark about the 2440 since I can't find an operations manual for it (only the service manual plus pictures from ebay).

It sounds like the scope is not triggering on "Ext Trig 1".  With "Ext Trig 1" connected to pin 19 of U2, and Ch1 and Ch2 connected to nothing, is the "Trig'd" light on?  It should be.  And if you remove the signal from "Ext Trig 1" does "Trig'd" go out?  Make sure you have the trigger source set to the external trigger input and levels set correctly.

To set external trigger in the service manual it says:
  Select TRIGGER SOURCE and push the EXT menu button.  Set A EXT SOURCE 1|2 to 1

The trigger level should be around 1.4V for TTL, and DC coupled.  And not using "AUTO LEVEL", if the 2440 has that.

And, yes, "Add/Invert" is the same as subtract.

And a verification: If "Trig'd" is on, and with Add/Invert on, connect Ch1 to pin 19 (and Ch2 to nothing).  You should get the square wave.  Now disconnect Ch1 and connect Ch2 to pin 19.  It should be a mirror of the signal since it's inverted.  Now connect them both to pin 19.  It should be a flat line.  If you turn off Invert but leave Add enabled, the signal should be doubled.

Does this help?  If everything is triggering properly, a non-functioning U2 would give you seemingly random traces (meaning input is not equal to output).  It could also be something else interfering with the bus on the other side of U2, but one step at a time...

And, thank you, idpromnut.  What vintage unit do you have?  Your front panel is probably different because it does not have burst option 001.

[idpromnut]

Yes, I noted that there are fewer buttons on the front panel of my unit, and yes, it is a version without the burst option. I will snap a couple of pics tonight and post 'em (along with the year).

[idpromnut]

As promised, some vintage p0rn circa the 16th week 1988, built in America:

[George_Race]

Hi there idpromnut:

Interesting user name!  Back many years ago, when the FCC decided that auto CW ID was the newest thing in commercial radio, I designed, built, and sold a bunch of auto ID units for business band radios.  And I even designed and built a Prom Burner to program the call sign into the chip.  That is a lot of years ago and I still have the Prom Burner, and about 40 finished copies of the complete circuit board just waiting for the prom to be burnt, and installed in a very nice silk screened panel case.  Have not had the heart to just throw them away!  I even hand etched the circuit boards!  Was the first on the market, but it only took about a year before the major names in radio come out with their own versions and my initial design went by the way side very quickly.  But I started with a production run of 100 units and think that I sold about 60 of them.

On your 8116A, I did not think about the Burst Option making it physically different.  I really wanted that feature and went looking for one with it on.  Nice pictures a very nice clean looking instrument.

Mark, will try the U2 test using Pin 19 with external trigger and both probes, just to see if I can get the flat line there as well.  But as I indicated, that scheme works perfectly on the built in calibrator, so I do believe that I have all the setting correct in the scope, just as you have outlined.  Will do some more test and get back to you.

If you would like the operator manual for the TEK 2440 I can send you an indexed scanned PDF version.  It comes with a 4 volume set of manuals, number one being the operators manual.  Or if you have a need, I can forward all 4 volumes of the documents.  It is a pretty nice digital scope, and you can pick one up for a really reasonable price on eBay.  As I recall, it was about $10K new a few years back.

Don't know if you may be interested, but I found a really great "paper" on how signature analysis works, and full details on using the 5004A in a lot of examples from the 70's/80's.  The best explanation I have seen, but of course I am a very green novice on signature analysis.  Soon to become a lot more proficient, or at least so I hope.

Guess that is about it for this time around,

George

[idpromnut]

@George_Race:  thanks! I actually encountered "IDPROM"s when working with some old Sun workstation equipment years ago and ended up manually poking bytes into them to revive a couple of dead ones.

Indeed, I had not heard of signature analysis before this thread, neat read!

[MarkL]

Hey George,

We never looked at the data bus on the *other* side of the transceiver from the CPU.  We've only looked at the CPU data bus and the latch bus.

Take a minute to probe D7 through D0 with just regular triggering and the unit in normal (non-free run) mode to make sure there's nothing obvious stuck high or low.   That side is on U2 pins 9 8 7 6 5 4 3 2.

If there is, we can forget the input/output test and start hunting.   (Any stuck bits would show up on the input/output test too, and is likely what you're seeing if you're sure you have the triggering right.)  There's a lot of chips that sit on that bus, about 11 in your version, so it could be an interesting hunting expedition.

If nothing is stuck, it's still worth doing the input/output comparison on U2.

[MarkL]

On the 2440 User Manual, that would be great if you want to PM it to me, or put it somewhere for all to share.  It might come in handy so I know what capabilities we have available for further debugging.

You could also consider submitting your scan to an archive site like http://bama.edebris.com/manuals/.  They only have the service manual.

It looks like a nice scope with a lot of features for its time.  Do you happen to have the Word Recognizer probe?

[George_Race]

Hi Mark:
Nope, nothing is being held high or low on the data out side of U2.  It is on a big data bus with a bunch of chips, all of which have the same data stream, just a mixed up mess of stuff that you can't lock on to with the scope.

I have gone chip by chip all over the board.  When I find something either high or low I check the schematic to see why.  So far nothing had jumped off the board at me.  I have a lot of hours into this thing, probably should have spent another couple of hundred and bought one that was working.  Gosh, then I would never have had this experience of trying to find the problem in this one.  That is really the part of the process that I like.  But sure would like to make a breakthrough one of these days.  And on top of that, I would not have had the opportunity to meet you along the way.  And I would consider that a big loss!

I looked at the TEK 6407 and did a lot of reading about it and it's use.  From what i read, it is one great big ball of wires, and the end results are nothing like having an HP Signature Analyzer.  From what I have read, I don't think it really went over very well with the service techs.

George

[MarkL]

I looked at the TEK 6407 and did a lot of reading about it and it's use.  From what i read, it is one great big ball of wires, and the end results are nothing like having an HP Signature Analyzer.  From what I have read, I don't think it really went over very well with the service techs.

Got the manual, thanks.

I have a 6407 for my Tek 2465 (analog) scope.  You're right, they don't compare to the signature analyzer, or really to a full logic analyzer.  They're good if you don't have those and you've managed to hone the problem down to an exact single shot condition you want to trigger on.  That's rare.  I think I used mine 2 or 3 times over the 10 years when the 2465 was my main scope.  Just wondering if that was in your arsenal for debugging.

As I recall, I was in our local Tektronix sales office and I mentioned the 6407 during some small-talk.  The sales engineer later came back with a bag that had one in it with all the grabbers and said, "Here, take it."  I guess it wasn't the great accessory they thought it would be.

[MarkL]

I think looking at every chip on the board for high/low/signal is only going to build frustration.

There may be other problems you might find with that method, but we need to get the CPU executing code first.  Your earliest observation was that the address lines are clean square waves of halving frequencies all the way up to A15.  This indicates the CPU is incrementing through the full address space and not doing anything useful.

There's not many components needed for the CPU to run.  It would include the CPU, the ROMs, possibly the external RAM (the CPU has some internal RAM), and a little bit of glue logic.  In the 8116A, there's also transceiver U2 that sits between the CPU and ROMs, so it's in the critical loop too.  We need to first make sure those are all working as best we can.  You've tried a new CPU, and the signature analyzer will verify the ROMs (although I think they're probably fine, otherwise the CPU would be doing *something*).  The external RAM might need a closer look, but the unit is supposed to test it before using it, so it's not a prime suspect at this point.

And then there's other components that are not needed for the CPU to run but they can interfere if they're not staying off the bus.  Those may be harder to find since everything's in parallel.

Do you not want to test U2?  Was the setup unclear or unsuccessful?  Were you able to look at all the pairs?


Side note: I noticed in my 8116A in free-run mode the RPT and OFS indicators are on like yours.  Nothing else is on.

[George_Race]

Hi Mark:

I just got your recent post, and here is my quick reply! (I was in the middle of posing it when your posted your!)

I went back through all of our posts from the begining of the file.  Way back in some of the early posts I pointed out that CPU Pin 31, which is D2, was setting high all the time with the other data lines all having a really noisy signal.

I did this because this morning, without any intureptions, I scanned all the data lines on U2 and noticed that Pin 16, D2 was setting high while the other data pins have the noisy signal on each of them. 

Now the interesting thing!  On U2, Pin 4 which is D2 on the A side of the device has the same noisy signals as all the other A side data pins.

Now that I would think is a problem, U2 must be defective as it's A side and B side are totally different.

U2 pin 16 is connected directly to CPU U1 Pin 31, and I varified it with the ohm meter.  Seeing that we have a great looking clock pulse on U2 G Pin 19, my conclusion is that it is that the internal gate system between pin 3 and pin 16 is blown open!  The R1 pull up is holding the pin high!

Now to find a 74LS245N!  Ebay here I come, again!

What do you think Mike, can it be this easy or have I missed something here?

And Mike, you have been saying you suspect U2 had a problem for some time now!  Hopefully you are correct.

George

[MarkL]

Ah, interesting.

But you had also reported that D2 was showing some pulses in post #19, and explicitly called it out since it was somewhat related to LD2.

I think it's a pretty good bet U2 is dead.  But to be sure you need to synchronize your observation to when it's supposed to be enabled.  Again, back to the U2 test.

I like to be as sure as I can before pulling chips off a board since there's always a risk of damage.  If DIP replacement is no sweat for you, go for it.

It looks like 74LS245 is still widely available, even from major distributors like Mouser and Digikey, for under $1.  Why take a chance with some second-hand pull from ebay?

If I had one I'd send it to you, but I only have an HCT version.

[George_Race]

The U2 test failed on the suspected open pair.

Got that all figured out this morning as well.  In compare mode the scope adds a third trace between A & B that shows the results. (Ext trigger CH1 & CH1 Invert)

All on clock pin 19 shows straight line!

I am bothered by the fact that U2 pairs are not a straight line like it looks on Pin 19.  There appears to be random noise on the Data Lines and the input/output does not cancel all the noise.  Maybe this is normal and it would interesting to have you do the same test on your U2 and report the findings.

I am going to take some quick pictures to show you the results I am talking about.  Will post them here in a bit.

I do think we are finally making progress and I have learned even more!  The input output comparison with the 2 channel scope is a new trick to add to my old bag of tricks!  Thanks for that one.

On the parts, for less than $10 I have 4 chips and 20 low profile socket adapters coming.  Chips are NOS so are not pulls!

George

[George_Race]

Me again Mark, here is what is going on with U2:

This is the clock comparison:
The clock looks really rolled off due to the fact that I parelled the two inputs probes and the Ext Trigger.  Kind of loaded down the rise time!

Is a nice 1 MHz Clock.  The ringing is from my long scope ground lead!



This is how a good pair compares:



This is how the bad pair compares:




This is the second set of traces that show the proper timing of the pins!
Thanks to Mike pointing out the error of my ways!

George

[MarkL]

Yeah, that's quite a bit of capacitance on the clock, but I get your point.

But that is an unreal amount of noise on the data lines!  No, my U2 looks nothing like that.

Something is going crazy on that side of the data bus!  Do they all look like that?

[MarkL]

Wait a minute... Your second capture shows it on both sides of U2.  So it *is* on both data buses.

Do they all look like this now?  You had a couple shots from early on and this noise wasn't present (before you tried the new CPU).

And it's common mode noise because it almost cancels itself out.  It's like U2 is oscillating.

What does the U2 pin 1 look like?

Is there anything odd on U2's power pins, 10 and 20?

Does the noise look different if you look at it with only one of the probes?

Do you have averaging turned on?  (Needs to be off.)

[George_Race]

Yes they all look like that!

The scope traces have been changed!  Shows just how ease it is for a novice digital scope user to be doing things incorrectly.  Thanks Mark for setting me straight with your observation and questioning of my traces.


Another observation.  In the Troubleshooting Section it says for free run you need to move the jumper to the free run position and open A2W2 to force NMI High, and then short Reset to Ground. 

I do not have an A2W2 jumper, but do have a W2 jumper on the board.
Do I need to open my W2 as part of the free run process?
 
One side goes to Pin 12 of U28, the other to U10 & U11 Pins 18.  Both Pin 18's are marked on the schematic to go to U28 Pin 1.   So in reality, the jumper W2 ties U28 Pin 12 to U28 Pin1 which in turn is connected to U10 & U11 Pins 18.

From what I can see, U10 & U11 are RAM.   U28 is an added Xistor Quad PNP package.

W2 breaks the connection between U28 Pins 1 & 12. (Not In Any Schematic)
I don't see what this has to do with Forcing NMI High.

My processor board is a 66506.  But, it has many of the changes that appear on processor board 66513.

All of this above I have put together from Section B of the Manual, Backdating.

There are several Backdating pages that apply to my processor board.  Here is a list of the PAGE NUMBERS that apply.  If you have the 66506 board you should print them out and keep with your Processor 3A Schematic sheet.

B-29
B-30
B-45
B-46
B-47

Hopefully this will help others in the future who are trying to trouble shoot their Function Generator.

I guess the next step will be to replace the defective chip U2 and get back here with the results in a few days.

George

[MarkL]

I don't think it's coming from the CPU since it's happening on both sides of U2.  U2 is providing digital isolation between the two buses, and your signals almost cancel each other out in the analog domain.

Free run mode disables U2 via pulling nEnable (pin 19) high.  When you say the noise disappears, do you mean on both sides of U2?

Averaging can make unsynchronized waveforms look pretty strange depending where the transitions are occurring, although admittedly I don't think I've ever seen that kind of result.  Are you using averaging?

What acquisition mode have you selected?  The signals themselves *are* changing during those "noise" periods, but not nearly at the frequency being shown.  Maybe there's something I'm overlooking about the 2440 acquisition modes that's making it look like that.

Does it look the same if you clear the trace and do a single-shot acquisition?

Just trying to figure out if the signal is really doing this.  It's quite bizarre.  I'll look through the 2440 manual.  And there's always the analog scope.


The A2W2 jumper would be on the A2 board.  And the signature analysis needs it to be out otherwise the numbers might not match or you might have unstable numbers on the analyzer.

[MarkL]

Hey George,

Am I seeing it right that you're at 5ms/div?

If so, that's way too slow.  You're undersampling.  That would explain a lot.

You got a good waveform in #1 because of aliasing.

[George_Race]

As always Mark, thanks for helping me to better understand what I am doing!

The three previous trace pictures have been changed, I think you will agree they are finally correct.

The problem is still an open pin 16, as the third trace shows.

What do you think Mark, do I have the traces correct this time?

Sure do appreciate you holding my hand throughout this process!

George

[MarkL]

The traces are clean now and the third screen shot clearly shows the input is not equal to the output.  This is progress.

On the third shot I notice that the Ch2 voltage level is only half of what it should be during the last enable pulse.  Which pin is Ch2 connected to?  It looks like it's struggling get to a logic low and not making it to the threshold (0.8V).  It might be why Ch1 is still a logic high during that interval, but we don't know which direction the transceiver is driving (that'll be next).

If you can manage to capture the level of U2 Direction (pin 1) during one of those half-voltage events that would be useful.  Leave Ch2 where it is, and still using the Ext 1 trigger, probe pin 1 with Ch1 and just keep hitting the single shot button until you get one.  You wouldn't need the Add/Invert for that.  I think we're done with the Add/Invert trace now that we've zeroed in on the bad bit.


And a note:  For debugging logic, you should use DC coupling on both Ch1 and Ch2 so it's clear what the absolute voltage levels are in the absence of any transitions.  Right now you have AC coupling on Ch2 (the "~" symbol).  And I think you're not triggering on the Ext1 input in the third shot, although you did manage to capture the problem which is fine.


PS - It's too bad you took down the "noisy traces"!  Those were great examples of what can happen with aliasing, and the aliased enable signal in particular was perfect.  It totally fooled me!

Also it's Mark, but if Mike wants to join the party, come on in!

[George_Race]

Mark, Mark, Mark!  Sorry about calling your Mike!  I hate it when someone does that!  And I sure did.

I still have the other pictures in the file, do you want me to forward them to you?

This is the very first time that I have put the digital scope to good use, so I just have never learned to do things properly, kind of hit and miss up to now.  So I don't always get all the parameters correct.  But thanks to a good teacher, MARK, I am learning.  And my nose is in the users manual as well!

Got the signature analyzer today.  That is a really neat instrument.  Am in the process of going down the chips on the processor board.  So far all the signatures are exactly as prescribed in the lists.

I wont be in trouble until I start doing signatures on the chips that have been replaced by others on the board.  But the Backdating sheets show the new signatures, so will see what happens when I get that far.

Now to get back to playing with the new tool, looking for signatures!

George

[MarkL]

Good luck with the new analyzer!  Let us all know what you find.

Seems I still have your screen shots in my browser's back button.  I've re-attached them below so others can understand the previous comments.

For those not wanting to go back and dig:  These are all under-sampled, aliased traces.  The actual frequency of the first signal is 1MHz.  The "noise" in the second two are a result of sampling across multiple occurrences of the waveform where the waveform was changing from cycle to cycle.

Put another way, the scope's sample rate was sufficiently slow that it was sampling successive points from different cycles of the waveform.