HP 4195A Repair and Calibration - Page 1
Forum section: electronics / testgear

author: Mrt12
author_rank: Regular Contributor
timestamp: January 25, 2017, 10:05:34 pm
content: Hi,I would like to do a small repair on my HP4195A. I am pretty sure I can isolate and fix the fault. However, afterwards, I also would like to do a backup of the calibration data in the EEPROM and do a new calibration.a) how is it possible to back up the EEPROM? I didn't find any useful information in the service manual.b) for the calibration: fortunately, many adjustments can be made manually by trimmers and tuning screws. However, the last few calibration steps need a HP desktop computer which controls the instrument over GPIB, and a HP BASIC program. I don't have either of these, but I think if I had the HP BASIC program or if I at least knew what that program does I would have a chance to implement something similar using a Python script and a Prologix GPIB adapter.Does somebody still have this old software, or knows at least what the software exactly did?Many thanks in advance.

author: TAMHAN
author_rank: Frequent Contributor
timestamp: January 31, 2017, 10:51:46 pm
content: Sorry for being a little slow. I have the Maintenance Manual and the Operators Manual for the unit. Let me know what you seek!Tam

author: Mrt12
author_rank: Regular Contributor
timestamp: February 12, 2017, 09:40:38 pm
content: Just to let you know: I have managed to repair the 4195A :-) measured the voltage for the voltage controlled bandpass filters. Then I found out that the voltage for the source filter was very noisy and had some offset. I tracked the failure down to the DAC which generates this voltage. Fortunately, the DAC is still available today (as a replacement part from MAXIM). I ordered one, soldered the new DAC into its place, and now everything works fine :-)

author: TAMHAN
author_rank: Frequent Contributor
timestamp: February 14, 2017, 03:40:42 pm
content: Hello,this is very interesting. When I got my 4195A, its former owner reported having replaced a DAC too. May I ask which part it was?Tam

author: Mrt12
author_rank: Regular Contributor
timestamp: February 16, 2017, 07:49:39 am
content: It was U18 on the A51 board in the measurement unit. This DAC produces the control voltage for the voltagate-controlled tracking bandpass filter A44. I found the problem by checking the SELECT signals for the bandpass filter, which looked okay. Then I measured the TUNE signals with the scope, triggering on SELECT, and found out that the trace generated by A51 was very noisy (see attached file 4195_BPF_H). The DAC is an AD7523. Unfortunately, ADI does no longer manufacture these DACs, but MAXIM offers a replacement type, called MX7524. With that in place of the old AD7523, the trace looks fine, as it should :-) my next project is to properly calibrate the whole machine.

author: TAMHAN
author_rank: Frequent Contributor
timestamp: February 19, 2017, 10:02:02 am
content: Hello,big thanks. Now I know - your description matches what the seller told me about a "hard to reach board".Tam

author: samofab
author_rank: Regular Contributor
timestamp: February 21, 2017, 02:08:02 pm
content: I think we can mark this DAC as common problem for these units.(board was hard to reach)

author: aba
author_rank: Newbie
timestamp: March 18, 2017, 07:25:00 pm
content: HiI have an HP 4195Aa that I did not use for sometime. I switched it on yesterday and the screen remained black, with alarms on the processing board A6. On the front, only the LED ext ref and lock are on. These symptoms seem to be caused by a failure of EPROMs in A6, according to some.On the A6 board, there is a 2.4 V NiCad battery. It seems that when this battery fails, the board remains in RESET status and the unit cannot start.When I leave the unit on for some time, it starts when I switch it off and on. So the issue seems to be related to temperature. When I remove the A6 board, cool it and put it back in the unit, it does not work. The unit works again after it has been on for a while. However, there is no signal trace.Is there someone with a similar experience. How to get the data for the EPROM ?Aba

author: TAMHAN
author_rank: Frequent Contributor
timestamp: March 19, 2017, 04:13:49 pm
content: Check the battery. What you describe, to me, looks like the boards battery being dead. Maybe some current trickles into it while "on", and so you can reboot.Re the EPROMs: if your machine works, the answer to that is clear. An EPROM reader, my friend. And yes, I know it is a nuissance - I need to do it for my two Iwatsu scopes too, but didnt get around to it yet ;(