Help me understand this Hp3457a AC board circuit.  - Page 1
Forum section: electronics / repair

author: Simon Spiers
author_rank: Regular Contributor
timestamp: December 05, 2018, 04:00:30 pm
content: I am plugging me way through a non working HP 3457a multimeter with a non working AC section. I have found one fault with the shift register not controlling the comparator u315 correctly, resulting in the outputs not being programed correctly.I now have what looks like a working AC board apart from it still fails cal with error 256. This refers to a flatness error. I am now wondering if u311 is like a capacitive feedback network controlled by another shift register (316)?I have performed the flatness adjustment with the trimmer cap c332 and this passes the diagnostic test. But there is clearly something still wrong with the frequency response of the circuit.#so anyone know if U311 is acting as a filter network? You will see it is not directly coupled to the signal path but looks like it is capacitively coupled around the drains of the fets despite there being no coax.Dave tore one of these apart in one of his videos a few years back and liked the meter a lot.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 05, 2018, 04:34:25 pm
content: There seem to be slightly different circuit versions. The plan show is hardly readable - for such pictures the .PNG or TIFF format with compression is usually better than JPEG.Anway U311 seems to be an DAC that simulates a variable capacitor (some 10-150 pF) to ground, e.g. in parallel to C307.If the capacitors have drifted too much, one might be outside the adjustment range. Of cause this might also happen due a different defect (e.g. broken JFET or leaking LM339, bad contact at K302) that acts a little like an additional load.One way to test the frequency flatness is to apply a reasonably steep rectangular signal (e.g. the adjustment signal from the scope) and than check with the scope where and how the signal comes out. Well adjusted it should be a clean rectangle again at TP301 and TP302 output of U301a.

author: Simon Spiers
author_rank: Regular Contributor
timestamp: December 05, 2018, 05:35:14 pm
content: Kleinstein thanks for you reply.Would I expect to see any state changes on the input of u311? It seems some of the lines never move?Your help is helpful for me understanding how this circuit works.Here is a better close up view

author: Kleinstein
author_rank: Super Contributor
timestamp: December 05, 2018, 05:41:53 pm
content: The inputs to U311 should stay essentially constant. The HW version shown here seem to use 2, maybe 3 different cases - so 2 or 3 different settings depending on the range (e.g. 3 V,30 V, 300 V) selected. Chances are some data lines will stay the same between 2 or 3 numbers.The values could change a little when the flatness adjustment is called.

author: Simon Spiers
author_rank: Regular Contributor
timestamp: December 05, 2018, 05:55:32 pm
content: That is very helpful, I will have a look again this evening. I take it you have worked on one before then?

author: coromonadalix
author_rank: Super Contributor
timestamp: December 05, 2018, 09:25:13 pm
content: there is a youtube video of an 3458 i think or a 3456 or 3457, damn just saw it few days ago, it talked about some problems and the last one was the flatness error, an 8 pin dip was the problem ...... will try to find back the link

author: Simon Spiers
author_rank: Regular Contributor
timestamp: December 05, 2018, 09:38:54 pm
content: Yep, that was me!

author: Simon Spiers
author_rank: Regular Contributor
timestamp: December 08, 2018, 03:46:31 pm
content: Ok it's working. The Shift register driving to comparators and fets had an open output resulting is a floating input and strange and confusing levels when fault finding. Ilso as the unit warmed up, 15 V rain noise became bad resulting in the input of the double fet having wildly swinging offsets that made it impossible for the unit to stay calibrated. This was mainly down to hi ESR values in the 15 volt regulators.