Topic: HP-80 Display problem

There’s currently no HP-80 forum, so hopefully the administrator will relocate this post appropriately.

I now have two HP-80 carcasses to play with, one a recent cheap buy from eBay.  The old unit has a known bad main PCB, but the main PCB of the new unit is showing some signs of life.

The new main PCB works with the display board of either unit, but there is one common display glitch:
The "a" display segment (top horizontal segment) is turned on for all locations across the display.  Except for this (sometimes intermittent) problem, the main PCB seems to function normally, showing good arithmetic operations.

It seems almost like a bad connection somewhere, but I don't know enough to track down the source.  Can anyone assist?

Thanks!

Re: HP-80 Display problem

Hi, hptyro,
This problem has been identified,
see :
Segment A (end of the page)

If the segment 'A' is propagated by the STEP signal, the cathode driver does its job.
A Vcc voltage (pin 11) is touching disp2 (pin 3) on the anode driver (or worst the A&R is doing tricks).
Could you take a photo of the display and post it?

Cheers,
Jacques.

Re: HP-80 Display problem

jlaporte wrote:

A Vcc voltage (pin 11) is touching disp2 (pin 3) on the anode driver (or worst the A&R is doing tricks).

As mentioned before, the display glitch is the same on two different display boards.  So, doesn't that point back to the main PCB as the source of the problem?

Re: HP-80 Display problem

Hi,

Sorry, I missed that.
Effectively, it points to the A&R chip (display driver) that puts a permanent TTL level 1 on disp2.
Could be easy to check on a scope.
Please, last chance, check the connector between the 2 PCB, around disp2 (pin4 - A&R side)


In the case it is the A&R, the only cure is 'organ transplant'.
But the A&R can come from any Classic.
The only difference between the 35 and the 80 is the number of ROM chips (3 vs 7).
I have rebuilt a 55 unit with the A&R, C&T and Anode Driver of a 35.

Could be easy to find smile

Cheers
Jacques

Re: HP-80 Display problem

jlaporte wrote:

Please, last chance, check the connector between the 2 PCB, around disp2 (pin4 - A&R side)

I think the A&R chip is OK.  But, flexing the main PCB ever so slightly in the area near the disp2 output (lower right-hand corner of board) causes the display glitch to come and go at will.  I touched up solder connection at A&R pin9 and also inspected the trace from pin9 to output pin4.  There are no obvious cracks.  Yet, the glitch persists!:(

So, question is... what kind of "mechanical" failure on the main PCB can cause all the "a" segments to light up, and then just as quickly to disappear?

Re: HP-80 Display problem

Hi Steve,

We progress.

To innocent A&R, put a '1' on display ; that digit does not transmit the signal for segment 'a (only 'c' -disp0- and 'b' + 'dp' on disp1).
On the scope there must be no signal on disp2 pin 9.

Steve wrote:

But, flexing the main PCB ever so slightly in the area near the disp2 output (lower right-hand corner of board) causes the display glitch to come and go at will.

Do that with the scope probing A&R pin9 : it should "disambiguate" the problem. If the image on the scope is ok (no 6v pulse), then it is a mechanical problem.

The problem could be a bad (old) solder (if your equipped, pump them all and then resolder the chip).

I can reproduce this 'glitch' easily bringing a 'spike' positive voltage on line disp2.

I can't say more remotely : if you have a video cam handy, you can try to show me your 'flexing action' :
- do you touch the A&R chip,
- or just the PCB? and where?

Look this pic, the trace disp2 takes a relay on component side : check continuity.
Check also the hole in the pcb for disp2 (no green traces??) ; clean the path with vinegar+water solution on an old toothbrush.

Check the continuity from A&R pin 9 to anode driver with a ohmmeter or better with a scope.
And displace the display pcb probe, in direction of the connector, while you do your 'flexing' until you'll find the spot that causes the discontinuity OR the short circuit with a voltage.

But the same problem on 2 different display PCB would be curious.
However I've seen things stranger.
Concentrate on the link : the disp2 hole in the main PCB and the connector pin.

Very interesting case.
Your close to the solution,
Take care,
Jacques.

http://jacques-laporte.org/forum/photos/disp2.JPG

Re: HP-80 Display problem

Hi Jacques,

Good news...I think you've nailed the problem!:)

Ohmmeter checks showed open circuit between pin4 on connector board and pin9 on A&R chip!  The problem is either a bad solder joint or a broken trace on the back side of pin9, which is difficult to see under the corner of the A&R chip.

When powered up, I could gently flex the PCB tab (marked 1343 in your pic) just enough to cause the glitch to come and go.  That was apparently enough to make/break the bad connection at the nearby A&R chip.  It must be the case that if pin4 on the connector board goes "open circuit" then all the "a" segments must light up.  Do you agree?

Anyway, I need to study that pin9 connection a bit more to assess the best fix.  Suggestions welcome!

Many thanks!

Steve

Re: HP-80 Display problem

Steve,

If I understand well, main PCB at rest : you see the 'glitch'
Pushing the '1343' corner make it disappear?

Try to make a bridge (with a wire) between the A&R pin 9 and the disp2 hole.
If it fixes the problem, the bug is between : bad trace or bad solder probably.

Cheers

Re: HP-80 Display problem

jlaporte wrote:

If I understand well, main PCB at rest : you see the 'glitch'
Pushing the '1343' corner make it disappear?

Yes, that's correct.  And there's more good news...the culprit turned out to be some corrosion and/or a poor solder joint on the back side of pin9 of the A&R chip.  I carefully cleaned that area, applied soldering iron heat to front side of the pin while feeding solder to the back side of the pin plus a short section of the trace.  That produced a solid joint, now showing full continuity between pin9 and connector hole4, and the glitch is GONE!:)

And, again, it must be the case that if pin4 on the connector board goes "open circuit" for any reason then all the "a" segments must light up.  Do you agree?  If so, that might be a helpful diagnostic tool, since if any of the A&R chip's disp outputs go "open circuit" then the affected segments would all light up across the display.  That would immediately suggest a bad solder joint or broken trace somewhere between the A&R disp output pin and the connector hole (or perhaps somewhere further downstream).

Many thanks for your diagnoses and suggestions!  There's now at least one more HP-80 kept out of the landfill!

Steve

Re: HP-80 Display problem

Happy to read this, Steve.

You have done a good job.

I hope to read you soon with another project!
Cheers,
Jacques.