| Electronics > Repair |
| HP 3456A voltage offset |
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| stevopedia:
Since the offset is (as best I can tell) greatest in the low DC Volts ranges, I decided to try changing the high DC Volts switch Q109. This doesn't seem to have had much of an effect; I'll know better when the instrument is properly warmed up. I've got two PN4392s left... may be time to order more. Given that the main suspect here is gate leakage, it occurs to me that the diode-connected FETs Q106, Q108, and Q118 could equally be at fault here. I'll try probing them with some gentle hot air when I have the time. Speaking of, why did HP use diode-connected JFETs here? Is their leakage performance that much better than a simple silicon switching diode like 1N4148? Edit: I answered my own question: It is. If you want picoampere-level reverse leakage, it's a diode-connected JFET (including the PAD/JPAD series) or nothing. TIL. |
| Kleinstein:
The diode connected JFETs have pretty low leakage. They may have been cheaper than speical low leakage diode. Today there is the BAV199 as a cheap low leakage diode with usually bettter performance (less leakage, higher current rating) than a 2N4392. In some meters a JFET for the same type is also used to compensate for the gate leakage - in this case having the same type as the switches can help. It does not seem to be case here, as the diode connected FETs are used in pairs, with also a FET towards the negative side. For an intial test one could leave some of the FET switches out and this way be sure to not get a little leakage from multiple sources. As the excess input current is from the negative side it would be the diode connected FETs towards the negative side, that could be an issue. |
| Wallace Gasiewicz:
I think HP used this sort of FET Diode configuration in other HP gear that I have fixed. The FETs in the switching section are all the same type. Makes things easier in a lot of ways. In my 3456 repair : I used Thompson 2N4117 FETS as clamping diodes. I was able to get them on ebay.They have four leads, one for the case, which I left flying. |
| Kleinstein:
The 2N4117 (and 4118) are rather small JFETs and the maximum gate current is small and the series resistance is a bit on the high side. They may not provide enough protection. One could use these higher resistance FETs for the switches in some places. With anyway some 100 K from the protection another 3-5 K are not that bad. The HP meter may use JFETs selected for low leakage - the normal spec limit for the PN4392 don't look like sufficient (may depend on the manufacturer). |
| stevopedia:
There are a tiny handful of switch FETs in the 3456A that have generic JEDEC part numbers given in the "replaceable parts" table. The overwhelming majority of them have a custom HP part number, which I agree refers to some kind of special selection... but I haven't seen any record of what it is they're selecting for. Gate leakage is probably a good guess. Thanks for pointing out the BAV199--those look like nice little diodes! It's too bad you'd have to get the lead stretcher out (i.e. solder leads onto a SOT-23) to use them in this application. As an experiment I tried lifting the gates of Q106, Q108, and Q118. That didn't seem to have any effect, so I reconnected the gates again... and found I had modestly increased the offset somehow. Flux residue? Heating from two solder operations altering the components? No idea, but I'll try thoroughly cleaning the input switching area with isopropyl alcohol later. I tried readjusting the charge balancing circuit (R147 and R148) but this seemed to have almost no effect. During that last adjustment I found that with autozero and the analog filter enabled and an integration period of 100 cycles, the displayed offset changed from about -3.0 μV or whatever to about +35 μV. This only occurred with all three factors; changing any one of them brought the displayed offset back to sanity. I saw a bunch of high-frequency spikes on the input amplifier's output (TP303) that correlated perfectly in time with the ADC's slope changes, which were only present with the analog filter enabled. I think this might be because the TLE2071 I used in U307 is too fast/performant. I'll try swapping it with an OP07 just to see what happens. All that aside, I'll think aloud here for a moment. As has been established previously, gate leakage is the primary failure path we suspect. That implies we're looking for FETs that are 1.) off at least part of the time during a measurement, especially low-range DC Volts; 2.) have their gates pulled to -18 V when off; and 3.) are connected directly to one of the two main measurement nodes. By my reckoning, that makes my list of suspects the parts below, listed in no particular order: * Q109 (changed; no improvement) * Q101 (don't remember if checked or changed) * Q114 (changed; greatly reduced offset) * Q116, DC V low-range switch (doubtful; offset in high DC V ranges is similar to that of low ranges) * Q115 (changed; no improvement) * Q105, 4-wire ohms low switch (unchecked) * Q102 (changed; no improvement) * Q103 (changed; no improvement) * Q110, AC V switch (unchecked) * Q112, self-test #7 switch (unchecked) |
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