HP 8116A All Lights and -188.8.8 On Screen

[mkidd9106]

There's a lot of great info on fine tuning or dealing with error codes for The HP 8116A that I hope to eventually use.  However, I have a one that upon my original power on had nothing outside of the fan running.  After fixing a power supply wiring issue to CPU board I now get all light and -188.8.8 on the screen.  I've seen this same display status on other units listed for sale.  Is there a common issue or specific condition that causes this status?  This function generator has more capability than I will ever use, but it would be nice to get it back to an operational state.  Guidance or pointers would be appreciated.

Power Supply Voltages are pretty close as measured on main board:
5.184V
24.03V
15.034V
-5.25V
-14.97V
-22.8V

Thanks

[mkidd9106]

After finally finding the 8116A service manual, I learned the display will enable all lights during startup self test.  After a about 30 seconds the unit is functional.  When doing voltage checks the display was turned away from me, so I missed that it was operating.  Now I will calibrating and begin to use!!

[MarkL]

The unit should be booted and ready in 3 or 4 seconds.  If it's taking 30 seconds, something is not right.

[coppercone2]

I have seen this kind of behavior for a wave tek unit that responded well to percussive maintence

[mkidd9106]

MarkL, thank you for the response.  Here's what's happening.  If the unit is turned off for extended period (24 hrs) the initialization to complete diag is about 20 sec.  Once warmed up (>10 minutes) it's almost instantly (1 sec).  I don't have the battery installed, but not thinking that would be the issue. However, I think you're correct about an underlying issue. What I have noticed when calibrating I have expected output when amplitude is 1v or above. Dropping to millivolt setting I loose the signal.  I assume this a unit issue and not user error on my part. 

[MarkL]

It doesn't sound like you're doing anything wrong.  Without a battery, the unit will not remember the last settings and will just use defaults.

It's possible you might be facing separate problems for cold and warm, but I think more likely it could be the same root cause.  I would try to tackle the booting issue first, and start with the processor circuitry.

As a prerequisite, besides the voltage on the supply rails which you already checked, take a look at each rail with a scope, or a DMM on AC volts.  Make sure there isn't a significant amount of AC line ripple on any of them.

Then I would take a look at the /RESET line, on pin A3U30.19.  After power up, it should go to TTL high almost immediately and stay there.

If all is good there, it might be worthwhile to spend some time with a can of freeze spray and try to pinpoint a component or board area that's sensitive to cold temperature when booting.  You can use an inverted can of air duster if you don't have freeze spray.

If it's a mechanical problem (as coppercone2 hints), you could also gently (!) twist the A3 and/or A2 boards while booting to see if there's some effect on boot behavior.  You could also poke at various board areas with a wooden stick.  It could be a bad solder joint, or fine crack in a trace. 

The service manual employs a signature analyzer for go/no-go checks on the processor and related circuitry.  Do you have access to one?  If you use Sigrok, it supports signature analysis, but to be honest I haven't tried it.

How about an EPROM programmer?  It could be used to check the contents of the EPROM(s).  EPROMs can experience bit rot.

And what serial number do you have?  There are a huge number of design changes in these units listed in the back of the manual, all organized by serial number.


Also, if it's not too late, I'd recommend you mark all the adjustment pots with a fine Sharpie before moving them.  These units can be a real bear to calibrate and it can be useful to get back to where you started.

EDIT: Small clarification.

[mkidd9106]

Wow!!  You know this unit well.  Those are all great suggestions that I will try.  A little history...in the early 80s I completed my Electrical Engineering degree, but immediately went in the corporate IT industry.  Outside of the engineering discipline, most of my EE skills were never used.  I've recently retired and wanted to establish an electronics workbench to start restoration projects on vintage audio devices.  I was able to pickup a used lot of test equipment (oscilloscope, 2 function generators (this HP and a GW), Tenma power supply, HP 339A distortion analyzer, Panansonic FM/AM signal Generator, Matsushita stereo modulator, and other misc RF devices from a estate sale of a retired electrical engineer.  Trying to get this older equipment up and running has been great for getting me use to reading schematics and using the test equipment again.  I have a feeling this site is going to be a great reference for help.  One question, I should remember this, when using my bench DVM or scope for measuring/calibrating outputs, should I be placing some type of load on the output?  I have a 50 ohm bnc load adapter, but the 8116 service manual references various db loads.

Thanks again for your guidance!

[MarkL]

I have a somewhat similar history.   After graduating, I spent 20 years doing networking and not EE type stuff.  I eventually came back to it and enjoy working on this old test equipment that I learned on long ago.

The output of the 8116A should be loaded with 50 ohms during the calibration steps when the step calls for attenuators on the oscilloscope.  I think that's probably all the steps without going back and reading each in detail.

The reason they call for attenuators and adjusting the scale factor on the oscilloscope is because HP chose a ridiculously over-powered 20GHz scope for the calibration, the HP 54121T.  It's a very fast scope, but has a very limited pk-pk input range of +/-320mV combined with a maximum offset of +/-500mV.  So, to measure most of the range available on the 8116A output, you need to insert attenuators to prevent destroying the front end on the scope.

Why they chose the 54121T, I don't know.  You really don't need a 20GHz scope to adjust an analog 50MHz function generator.

You can simply use a 50 ohm feed-through terminator on the input to your scope, or if your scope has it, set the input to 50 ohm termination.  Then set up the vertical sensitivity as specified in the calibration step.

When using internal 50 ohm termination, some scopes limit the vertical range and/or will detect an overload on the terminator.  This might happen on calibration steps which call for the highest output ranges on the 8116A.  If this is the case for your scope, for those steps you can put a 20dB attenuator in-line (and keep the internal 50 ohm termination enabled), and multiply your readings on the scope by 10.  A quick verification of the exact dB of the attenuator could be done so that accuracy doesn't suffer (e.g., compare readings on a lower output with and then without the attenuator).

EDIT: fix 541211T --> 54121T