HP 4194A Impedance Analyzer - A6/7/8 boards

[findus]

Hi there,

I saved an old, broken HP 4194A from work for nostalgic reasons (been using the instrument for years building NMR low temperature probes during PhD).
Yesterday I finally had the time to look at it:

  - fan spins, one beep, two LEDs on the front come up
  - no second beep, no CRT, no response to anything

All voltages on the power supply are fine, the 10 MHz ref output looks good
However all eight POST LEDs on the A6 and A7 boards light up simultaneously and are stuck that way. No description for this state in the service manual.
The troubleshooting part suggests to jump W5 and W6 on the A6 board to use the onboard 20 MHz clock for the MCU. However those jumpers are missing on my board?!

Any ideas? 

[findus]

One more thing: the A6 board has had U58 (SN74BCT2244N) replaced in the past (rather unaesthetic).

[Tomorokoshi]

I found the jumpers in the schematic of the service manual on page 347 of the Keysight service manual 04194-90100. However, it's too compressed to read.

My Artek copy has it on page 8-78.1, also page 347 of the PDF. It clearly shows those jumpers selecting between the two clocks. They are inputs to the logic selection circuit.

http://artekmanuals.com/manuals/hp-manuals/

It looks like W5 and W6 in the manual were renamed to W3 and W4 on the board. Perhaps there was a revision that is not right with the documentation.

U58 is in the clock distribution circuit, so that area is definitely worth a look.

[Tomorokoshi]

From the manual.

[fenugrec]

  - fan spins, one beep, two LEDs on the front come up
  - no second beep, no CRT, no response to anything
However all eight POST LEDs on the A6 and A7 boards light up simultaneously and are stuck that way.

That's the exact same symptoms as when a 4195A has a dead RAM backup battery; when this happens one of the CPUs is held in reset. Not sure how similar it is to a 4194, but you could definitely probe the CPU !RESET pins (or testpoints if you can identify them, I'm sure they're somewhere).

[findus]

Thanks a lot! You guys have been really helpful so far! 

Jumpers W3 and W4 didn't change anything.

The backup battery sounds promising: the system spent years as backup in our basement.
I will check the state of the reset pin as soon as the rest of the family is asleep 

[findus]

I haven't checked the state of the reset pin so far... but:
If I replace the RAM backup battery - what is the correct procedure to initialize the instrument?
I couldn't find a single reference to the battery in the service as well as the regular manual.

[fenugrec]

Not sure - I believe the backup RAM holds fairly non-critical data, but in the case of the 4195 the info is actually in the user's manual p. 4-57 "User math, user-defined functions, sweep end functions; HPIB settings and plot scale;  standard value for calibration" . This last item is also detailed on the same page so I imagine it should be possible to re-enter the values manually.

I'm assuming the 4194 and 4195 have a lot in common, but could be wrong.

If you think your backup RAM holds irreplaceable data, it should be fairly easy to rig up a second battery with a diode while you  desolder the original battery.

[Tomorokoshi]

From the 04194-90011 Operation Manual, see pages 3-127 under "SAVE/GET keys" and 3-136. It seems to only be used for storing test configurations. Note that it takes 48 hours to fully charge the battery. It may be that it just needs to be recharged. Not sure what the best approach is there.

The battery circuit is on the same sheet as the clock test circuit in the service manual. A6U60 is the reset monitor, and it takes in the voltage level of the battery. It is on page 347. This is HP 1826-1273, which cross references to TI TL7705CP, which seems to be this:

https://www.digikey.com/product-detail/en/texas-instruments/TL7705ACP/296-1801-5-ND/277446

Working through it seems the logic in the case where the battery is discharged makes it drive the 4 reset lines from U40 to be the same state as /M-RES, which is test point 20.

[fenugrec]

Ah, it appears the 4194 and 4195 are very similar in that area. I troubleshoot and analyze that section of the circuit in this small video :
https://www.youtube.com/embed/0VhyDC1DTCk

[findus]

You guys are great! Thanks for the advice and the video.
I will take a closer look on the reset circuit on Sunday.

So far I only managed to briefly replace the old battery (1.2V -> dead) with a 3V lithium battery plus diode (about 2.4V, no NiCd or NiMh at hand).
No change so far. Have a nice weekend, I'll keep you posted.

[findus]

After watching fenugrecs video I had to try the LED battery replacement: it works!
Well, still no CRT  but the POST LEDs on A6 and A8 run up to DS5 and DS6 and the front panel seems to work.
So I'll be debugging the video circuit on Sunday. Thanks again!

[Tomorokoshi]

Nice to see progress on that. Great video on that battery issue.

[Sonic-power]

Hi Findus

Found the thread and I wanted to say THANK YOU SO MUCH. I had the similar problem with my old 4194, and now after replacing the battery its up running.

[ArjenCNX]

Hey there fellow techies,  first post! 

I have recently bought an old  HP 4194A,  upon arrival and inspection i found it gives two error codes,  one is bridge unbalanced, other is AC current overload. my guess being that some klutz has blown up the front-end of the impedance analyzer.

As many of you will know this aint such an easy device to fix, so i'm asking on the forum if anyone has repaired this issue before, and what a possible remedy could be.   today i have spend the better part of the day to go over the service manual and re-adjusting the voltage / signal levels over the unknown ports and internally over the test pins.  here is what i found:

1- PSU seems intact, all voltages well within spec
2- CRT works, be it a bit dimm
3- device starts up fine, logic boards seem in order for as far as i can tell.
4- adjustments in the manual on fine-tuning the bridge balance have been completed, several times, with success, i can adjust A23 board TP 15 TP 16 to the desired voltages / signals and have the correct signal on Lcurr port. 

only thing that is left is to check the sensing circuitry, could be a faulty op-amp maybe? or a protection diode?  i need to find schematic first as it would help allot, and my  spectrum analyzer is of the analog kind, and that doesn't help, so i cannot complete all tests, since i cannot really measure Decibels with my DOSX2024   

any tips, tricks, etc are most welcome, this device will be used for medical research (skin / tissue impedance VS inflammation, etc. ) so once its fixed, it will benefit more people that just myself.

[alocam]

Hello, I had a chance to get a used HP4194A, and it looks like J5 cable is missing. Does anyone know the wiring for this 50 pin centronics connector? I also wonder whether the manual offered by Artek is fully readable, as the one I downloaded from Keysight is far from readable.
If anyone happens to have the pinout, as well as describe the type of cable used (twisted, shielded, etc), I would very much appreciate to receive feedback on such subject.
I hope that the unit arrives in decent shape, although I purchased it in the far east and I live in Argentina, so chances are it will suffer transport damage.

Best regards, and thank you!
Alberto

[Tomorokoshi]

I couldn't find anything on the J5 cable in the manuals. So I measured mine out. The housing is shielded to the shielded cable. All 50 pins are straight through.

I spot-checked an old 50-pin SCSI cable. It was also shielded and the pins appear to be straight through.

The schematics and line art in the Artek manuals are definitely clearer and legible. Some of the scan is a bit blurry, but it's legible and the text capture generally works, so you can search through them.

[alocam]

Thank you so much for taking the time to check the cable!. I look forward to receiving the unit by the end of this week, and I will try to manage things to get a SCSI cable to replace the missing J5. Hopefully, it will work. In the meantime I'll check the Artek website and eventually order a copy of the schematics.
Best regards!

[alocam]

Hi, today I finally received my 4194A, and the J5 cable is indeed missing. Just to make sure I do not misunderstand you and make a mistake I would regret  , you mentioned that this is a straight cable, pin 1 goes to pin 1, pin2->... pin 50->pin50, did I get it right?
Thank you!!

[Tomorokoshi]

Hi, today I finally received my 4194A, and the J5 cable is indeed missing. Just to make sure I do not misunderstand you and make a mistake I would regret  , you mentioned that this is a straight cable, pin 1 goes to pin 1, pin2->... pin 50->pin50, did I get it right?
Thank you!!

Yes, on the original HP J5 50-pin cable it was 1 to 1, 2 to 2, ... 50 to 50. I checked all 50 pins in addition to the housing shield. Note that this was with holding the two connectors in the same view at the same time. I can take a picture of that tonight.

The SCSI cable was only spot-checked, but for the pins I checked they were straight through also.

[Tomorokoshi]

I finally got to checking all pins on the SCSI cable. They were all strait through like the original J5 cable.

So I replaced the J5 cable with the SCSI cable. Everything worked normally.

Attached is a scan of a 10 pF mica capacitor.