Help with HP 8116A

[MarkL]

George,

If the latest round shows nothing of interest, I have another test for you.  This one is a little off the menu.

I'd sure like to see VMA and R/nW do something to convince me the CPU is looking at incoming instructions.

Instead of having free run mode execute 0x5f instructions (CLR B), we could force it to execute 0xdf (STX).  STX is a Store Index Register direct.  This should cause VMA to have pulses as well as R/nW.

To do this, you would need to unsolder one end of CR1 which will allow bit 7 to be a one and turn the 0x5f into a 0xdf.  Put the unit in free run mode and look at VMA and R/nW for pulses.  You should also verify that the data lines are indeed set to 0xdf.

The random writes may cause some bizarre behavior in the rest of the unit.

Or... wait until you get your new CPU and try that instead.  Just thought I'd throw this out there if you're interested.

[George_Race]

Good evening again guys:

To answer some of the questions ask, first, yes pin 5 VMA is setting high as is 34 R/W

Here is what Pin 37 E looks like:


Here is U15 Pin 6:


And on the last test, I lifted one end of CR1, move the jumper to free run.

R/W remains high, no pulses.

Here is now what VMA looks like:


Not sure when the new CPU will be here, either Saturday or Monday for sure.

And I though that finding the problem was going to be really easy!
Shows just how wrong you can be I guess.

Have a nice evening,

George

[George_Race]

I have a question.  I have been going to buy a signature analzer for some time, but just have not gotten around to it.  I think that this would better help me to troubleshoot HP equipment, which I tend to buy and repair!

I like both the HP 5004A and 5005B.  I am leaning toward the 5004A because it takes less bench space.  I can't seem to find a great deal of difference in the two instruments, other than size.

Would either be suitable for my purpose?

Thanks,
George

[MarkL]

Between the two I would say the 5005B is a nicer instrument because it has the built-in DMM functions.  Either will do the job.

I bought a 5006A on ebay just the other day for the same reason (not even here yet).  I've always been able to fix older equipment issues without one, but because the service manuals are written around signature analysis it could be a great time saver.

Perhaps others have direct experience with those two models...

[MarkL]

But back to the scope traces (and thanks for the photos, they're very informative)...

STX direct is a 5 cycle instruction with 1 cycle of VMA low.  The duty cycle on your trace is exactly that.  Looks great.

STX direct should also have 2 cycles in a row of R/nW being low.  You're not seeing that.  We need to find out why.

First, please confirm that 0xdf, the STX instruction, is on the data bus (in the modified free run mode).  All data lines should be high except D5.  If not, transceiver U2 might be bad.  To confirm, U2 pin 19 should be high and this should be disabling all outputs.

If you are getting 0xdf on the data lines, either the CPU is bad or something is forcing R/nW (pin 34) high.  What I would do is try to isolate pin 34 by unplugging the CPU, bending this pin slightly, and plugging it back in leaving pin 34 hanging out.  Some of the older ceramic packages won't tolerate this, so use your judgment and don't do it if you think it's going to break the pin.

If you can isolate the pin, hook the scope up to the pin directly and see if you get a duty cycle of about 40% on it (again in free run).  If you do, next steps would be to find who's holding it high.  If not, a dead CPU comes to the top of list.

Also, on your scope traces, that's a really ugly U15 pin 6 (signal: nEVMA).  It looks like it's either driving too much capacitance, or the high side output driver is blown.  We should come back to that, but let's finish with the CPU first.

[George_Race]

Thanks Mark, sounds good.

I am also looking at the  Sony/Tektronix 308 which is similar and from what I read does the HP signatures as well.

Any thought on that one?

Thanks
George

[MarkL]

Also, on your scope traces, that's a really ugly U15 pin 6 (signal: nEVMA).  It looks like it's either driving too much capacitance, or the high side output driver is blown.  We should come back to that, but let's finish with the CPU first.

Ah, I didn't notice U15 is a 74LS12 which open collector, so it's probably ok.  But that sure is a long rise time for a 1MHz digital signal...

[George_Race]

CPU arrived today.  Out working in the machine shop so will not get back to the basement work bench until later today.

Will make the swap and see if anything has changed that we have been following.

George

[George_Race]

Hi Mark, I am starting to have problems telling the trees from the woods!  I have looked at so much and tried so many things I am kind of lost as to where I am in the troubleshooting.

CPU arrived today!  Installed it and immediately saw a big difference.  Several of the front panel lamps came on, including ERROR to the left of the numeric display.  But nothing on the numeric display at all.  They were on for about a minute and then everything went back to like is has been with the two indicators RPT and OFS on.

Here is the big change!  The next two shots are of CPU pins 33 & 32.
All the CPU data pins have varying width signals, except for 31 which is still sitting at 5 volts, no data.
Hopefully this will give a new clue as to another path to follow.




I bought an HP 5004A on ebay today, should be here by the middle of next week.  Just maybe doing signatura analysis, I will be able to come up with some more clues along the way.

When I pulled one end of CR1, and went to free run,  All of the data pins on the CPU had that ugly waveform, like U15 Pin 6 in the last batch of pictures I sent.  (CPU Pin 31 stayed high!  No waveform)

I am at a point where I just seem to be going nowhere.

So what do you think is the next step.

George

[MarkL]

Hi George,

Sorry to hear the new CPU didn't work, but it was a worth a try since you already ordered it.

Since the new CPU didn't solve the problem, I would put the old one back in.  There's obviously still something wrong unless the second CPU was also bad, which I'm going to assume for the moment isn't the case.  The reason to put the old one back in is that we don't want to start introducing new variables, especially with something as central to the troubleshooting as the CPU.

But for all we know, if something killed the first CPU, the second one might have now suffered the same fate.

I'd like to go back to the modified free run test from a couple of posts ago and see if we can find out why R/nW is not pulsing low.  That needs to be answered.  Can you isolate pin 34?

And you did say it's still high with the new CPU.  It would seem more likely now that something is holding it high, maybe even shorted to the +5V rail since the CPU is the only one who should be driving it.  It only goes to a few other chips.

One check for this would be to power the unit up without the CPU and see if you can use a 100ohm (or so) resistor to manually pull the R/nW low by grounding one side of the resistor and touching it to socket pin 34.  Watch the voltage on the pin to see if anything's fighting you.  It should be nearly pretty close to 0V.

[MarkL]

Actually, re-reading your previous post you *didn't* say 34 was high.  Sorry about that.

Can you check R/nW pin 34 with the new CPU in modified free run?  If still high, the "manually pulling it low" test would be useful.

If it's pulsing now leave the new CPU in there.

[George_Race]

OK Mark, getting back to basics and doing what you ask.

Removed one end of CR1.  Put Free Run Jumper to Grounded position.

Here are pictures of what is on Pin 34 and Pin 5.

And, D7, Pin 26 is now low.

These are back on the old processor as well. 




As a side note, when I turned on the unit, with the processor removed, Almost everyting on the front panel lit up.  Does that rule out U22 as being bad?

Ready for the next test,

George

[MarkL]

So, to re-cap here...

Everything is as it was before: It's the old processor, in free run with CR1 lifted, and NOW there's a pulse on R/nW?  That's inconsistent with the previous behavior.  And I would note it's also the wrong length pulse for the STX Direct instruction.  It should be a 40% duty cycle.

And D7 shouldn't be a constant low with CR1 lifted.  R1 should be pulling it high, for at least part of the cycle while the CPU is doing an instruction read.

I don't want to be hard on the CPU or the socket, but can you check the values of D0 though D7 without the CPU?  They should be all high except D5 when in modified free run mode.  Maybe even try checking this several times with power cycles in between.

And with the CPU in, it would also be interesting to know what pin 1 (R/nW) and pin 19 (nEN) are doing on transceiver U2.  I'm really starting to suspect U2.  Pin 1 should be R/nW, and pin 19 should look like ~E, probably with a slow rise time like we saw on U15 pin 6.  We may need to do some pin-by-pin verification to make U2 is off the bus when it's supposed to be, and passing data in one direction or the other when it's supposed to be.  (This will take some scope state triggering and not just looking at pulse shapes.)

And on U22, it's a good sign all the LEDs and segments turn on.  At least it says the output drivers are working.

[George_Race]

Hi Mark, your recap looks good.

Here is how D7-Pin 26 looks with CR1 pulled and in Free Run Mode.
I put the scope in Auto and this is what it is finding.

Is a pretty small signal!

Next is a picture of U2 Pin 1:

Next is a picture of U2 Pin 19, it is setting high.


Next I pulled the CPU for the following readings:

U1 Pin 26-D7 is Low, all other pins are high.
In order to get D5 Low it would be necessary to open CR2.  And that makes D5 go Low, all other pins High.

With both diodes open, all data lines are setting High.

And last is a picture of the front panel with the CPU removed.



The CPU pins are starting to get fatigued and the CR1 and CR2 pads are starting to look bad.

I would like to put the diodes back in correctly and clean up the board, if you think we are done with that phase of the testing.

Thanks,
George

[MarkL]

Hi George,

A couple of things are not making sense.  If CR1 is in, and you're in free run mode, it should pull D7 down.  And if CR2 is in, and you're in free run mode, it should pull D5 down.

So, feel free to solder the CR1 and CR2 back in.  That test is not helping us except it did prove R/nW can move.

Before you put the CPU back in, and with CR1 and CR2 soldered back in, check to make sure all data lines except D5 and D7 are high.  D5 and D7 should be near 0.2V since they are being held low by Schottky diodes CR1 and CR2.  EDIT: This is in free run mode.

If you're up to it, we can check the function of U2 but you will need a scope that can do pattern or state trigger on at least 2 inputs, with a 3rd channel available for probing.  What do you have?

[MarkL]

Hi George,

This odd behavior with CR1 and CR2 affecting the opposite lines was bothering me and I popped the top off my 8116A to look at it.

Using a continuity tester, I found that CR1 is wired to U1 pin 28 (D5) and CR2 is wired to U1 pin 26 (D7).  The component labeling on the schematic (or the board placement, if you prefer) is flipped.  Thanks, HP.  But this is why I asked you to verify that 0xd7 was on the CPU data bus pins.  It's important to double check that changes have the intended effect.

At any rate, since these bits were flipped, the CPU was executing 0x7f instead of 0xdf.  The 0x7f instruction is a CLR Extended.  That is a 6-cycle instruction which results in 1 VMA low pulse and 1 R/nW low pulse per cycle.  That matches the traces you have in post #36.  And the frequency is 1MHz/6 = 167kHz which is right.  In post #26 the frequency was also 167kHz, which it shouldn't have been, but I missed that and I was just eyeballing the duty cycle.

I still suspect U2, and this is what I wanted to check in post #22.  A fairly complete check can be done using pattern/state triggering, but because the enable input is periodic (and predictable), I think it could also be done using a very basic scope with as little as an external trigger input and 2 analog channels.  I will try it first on my 8116A and post back.  No soldering needed.

[George_Race]

Good Morning again:

Yes, when CR1 and CR2 are in, D7 & D5 are pulled down by the diodes.  And the level is in the area of mV's.  That shown in the last set of pictures that I sent.

I am looking at the function of U2.

In free run mode it is in Isolation Mode.  Enable, Pin 19 is setting High

In normal mode Enable, Pin 19 is being driven by U15 which has the rolled off square wave with the ringing at the bottom of the trace.

So data is being transfered across U2, but the timing due to the distorted waveform from U15 may be messing up the data selection being transfered.

Hopefully this makes sense!

Anyway, I am going to see if I can discover what is rolling off the U15 signals.

I really think that U2 is working as it should be, but may be putting bad data out on the data lines.

I have the diodes back in place and the board de-fluxed and looking good.  Also have reseated the original CPU and everything is back to were we started.

George (just got your last post, will look the diode connections to see if mine is the same!  I have two scopes, an HP 1725A Analog, and a TEK 2440 Digital.  Both are dual channel.)

[MarkL]

U15 output pin 12 (driving U2 Enable) could be an interesting investigation.  It looks ok on my 8116A, attached.  And my U15 pin 6 has the same fall and rise shape as my pin 12.

You had an earlier trace in post #26 that showed pin 6 has a very slow rise.  I think you're saying pin 12 looks like this too.  If it's on both output pins, perhaps U15 is ill?  Are the pullups R19 and R20 ok (seems like a long shot they'd both be bad)?  Take a look at the power and ground pins on U15 with the scope.  Anything weird?

You have a lot of ringing on the falling edge on all your screen captures.  I was chalking that up to a long ground lead on your probe.  Reduce your ground lead length and try to use the ground right on the chip you're probing to see if it's an artifact or really there.

[MarkL]

Speaking of probes, are you using a 10x probe to look at these signals?  If you're not, one possible reason you're seeing such a slow rise time is that you're loading down the open collector outputs with a lot of capacitance.

[MarkL]

If you feel so inclined, here's a way to check U2.  It's not the greatest, but it will find any major faults.

Because U2 is a bidirectional transceiver, no matter which way it's driving, the input should always equal the output while nEnable is low.  And because we know exactly when nEnable is low since it's just ~E (a stable clock), we can subtract input from output in this known window and it should be pretty close to 0V.

Here's how you can do this on your 2-ch scope (all done in normal, non-free run mode):

Set up the external trigger input to trigger on the falling edge of U2 nEnable pin 19 (which is ~E from U15 pin 12).  Also probe it with Ch1 so you can observe it at the same time.  Use the vertical cursors to mark the window where nEnable is low.  This is the window where input should equal output.  See U2_pin19.png.

Next, you can optionally probe the U2 Direction pin 1 (which is R/nW from the CPU) on Ch1.  It should look like U2_pin1.png, but we saw before it wasn't changing on your unit.

Now set up the scope to display Ch1 minus Ch2 and turn on display persistence so you don't miss any intermittents.  Attach Ch1 to U2 pin 2 and Ch2 to pin 18.  This is one of eight data paths through the transceiver.  I've attached photos of what the scope channels look like individually U2_pin2_pin18.png, and with subtract turned on U2_pin2_minus_pin18.png.)

Between the cursors is where you should look for compliance.  At the beginning, signals are still settling and they could be anything.  But by the end at the right-hand cursor, everything should have settled down and the signal should be around zero volts.  If something is stuck high or low in either direction it should be very obvious as a positive or negative trace.

Check the other 7 transceiver pairs: pins 3-17, 4-16, 5-15, 6-14, 7-13, 8-12, 9-11.

[George_Race]

Hi Mark:

Putting the probe on 10X and using the short probe GND wire, my U15 pin 6 & 12 look exactly like yours!  So much for that self created problem!

On U2, I am going to take a look and see if I can follow your procedure.  Looks easy enough.

In case no one has every told you, you are a very good mentor/teacher!

Do appreciate all your time and "hand holding"!  I am 79 years young and still learning!

Incidentially, will have an HP Signature Analyzer next week, it is on the way.  That may be how this one gets fixed.  I see another steep learning curve in the future!

George

[MarkL]

Hi George,

You're welcome and thanks for the compliment!  I've done engineering roles in both.

I'm always up for a troubleshooting challenge.  It was clear from your first post you were familiar with the equipment and ready to tackle a repair, and IT WASN'T A POWER SUPPLY PROBLEM.  Sorry if I offend anyone out there, but power supply problems are boring.

You can't help to learn something by diving into these old repair manuals that provide detailed theory of operation along with the schematics.  Today's service manuals of "If Error 123 is Displayed, Replace the Main Board" just doesn't do it for me.  I usually just send it off and let them do it.

My signature analyzer should be here on Tuesday.  I have nothing broken, so I'll probably just poke around my 8116A since the cover is already off.  Never needed one or used one.  We'll see...

[George_Race]

Yes Mark, I am always up for a challenge, but I must admit that this one has turned out to be a lot more complex than I expected, but that is the challenge, and I do need a function generator.  This one looks like new, but obviously has a bit of a problem.

On the analyzer, I too am looking forward to getting it in hand and seeing what it does.  In reading through the troubleshooting on the 8116, there are a lot of tests just on the processor board.   And then looking at the changes log, at the end of the file, there is going to be a lot of signatures that are incorrect because some part has been changed or removed in one of the versions.  So now knowing what updates have been done is going to be another challenge to keep up with.

I wish that I had a complete accurate schematic diagram.  I have gone through the  changes list, one by one, and marked or printed out the change sheets pertaining to my processor board.  But still do not have a complete version diagram of my particular board.  It seems to have bits and pieces of several updates and mods that were probably added in production over the years it was manufactured.

I would think that somewhere there is a complete correct schematic and parts layout file for this particular processor board, but I can't seem to find it anywhere.

If you have any resources for manuals, here is what I am looking for.  Microprocessor Circuit Board 08116-66503 E-2317    And it was built in Germany.

Would love to find a manual in PDF that encompasses that particular Processor Board.

Guess it is about time to call it a night.  Will chat again soon hopefully,

George

[MarkL]

I have a paper copy of the service manual and it's the same as the one on-line.  For early models like yours, I agree the information is scattered and extremely inconvenient.

I think the manual part number you want is 08116-90001 E1182.  That seems to be the earliest printing in 1982.

This one on ebay may be what you want (paper copy).  Read the paragraph in the third photo about the serial numbers to which it applies:

  http://www.ebay.com/itm/251597762190

I think what it's saying is that instead of "Backdating", the change section is moving forward in time.  The manual starts with full information up to serial number 2334G02896 and then there's a section of changes after that.  At least that's my interpretation.

Let's hope the signature analyzer can make quick work of this.  But until then if you still want to try other testing I'm game for it.

[George_Race]

Hi Mark:
Tried to do the U2 dual trace thing but so far cannot get it to work the way your pictures show.

I use external sync to channel 1 from pin 19 of U2.  I then probe ch-1 Pin 2 and ch-2 pin 18.  Setting are for CH-1 invert CH-2 add, should result in a base line as the two cancel common waveforms. (Should be the same process as Subtract which I don't have as an option)  No matter what I do it does not what to sync anything on the screen.

The same process, using the calibrator, results in almost a perfect flat line.  But maybe that is different in some way. Talking about the ch-1 invert and ch-2 add settings.

If I external sync ch-1 and look at pin 19 with ch-1 probe it locks right into a nice square wave with just a very slight roll off, like your picture shows.

Found out this morning that my analyzer will not be here until probably Friday by UPS.

So what am I doing incorrectly?  Must be  something!\
George