Seen a lot worse even on rentals. Pretty much the same here, other big $ projects going on right now so can't make an offer.
On Tue, Jan 5, 2016 at 5:28 PM, Tom Miller <tmiller11...@verizon.net> wrote: > That looks like a good one from what I can see. It would be nice to check > it out in person to see if the display has any dim digits. Looking at the > picture seems to show it has some dimming compared to the manual range > indicator. The price is pretty good also. Maybe they might negotiate down > slightly. > > It would run you about $600+ to send in for a Keysight calibration. > > Nice work on finding the problem with your 3457A. > > ----- Original Message ----- From: "Jerry Hancock" <je...@hanler.com> > To: "Discussion of precise voltage measurement" <volt-nuts@febo.com> > Sent: Tuesday, January 05, 2016 8:06 PM > Subject: Re: [volt-nuts] New member with 3457A problem > > > > Hello again, Just for the record, after spending another dozen or so hours >> on this problem, I found U216, “Precharge offset adj” to have a low >> resistance from pins 1,5,6 to pin 4, essentially one of the 4 transistors >> in the CA3096 is shorted (11.28 ohms) though not completely. I found this >> by scoping the signal coming into the input amplifier at pin 10 of U101, >> “Input Hybrid”, while putting the meter into an auto cal loop. This >> problem was difficult to track down as unlike the other Service Manuals, >> there are no waveforms available in the 3457a manual. I noticed that >> during the precharge cycle the waveform was taking a large negative dive >> instead of two relatively equal pulses, one that reads the signal and the >> other during precharge, I assume. This negative voltage from the precharge >> offset was then not being drained efficiently when the leads were open >> resulting in a negative to zero open voltage displayed on the meter >> compared to the typical ~+2.5V. >> >> So for the record, if you get a “0 Hardware error” followed by a 512 or >> 560 (512 + 32 + 16 errors signaled) in reply to the “auxerr?” command, take >> a look in the area of U216 and check the four transistors that are in the >> CA3096E. >> >> Lastly, as I commented on another site, after reading through the 3457a >> theory of operations, that there are a number factors that need to be >> calibrated out of a meter to ensure accuracy and repeatable measurements. >> In this case, obviously the input amplifier needs to have a stable offset >> voltage (zero). The precharge offset charges the approximate 10pf of stray >> capacitance out of the input amplifier to keep it from impacting the >> readings. In conclusion, it takes more than an accurate ADC, gain >> resistors, etc to make a good meter. I followed a number of threads where >> people were attempting to make their own 5.5 digit meter, I played around >> with some circuits myself, but nowhere did I see stray capacitance taken >> into account. >> >> Thank you for reading. By the way, there is what looks like a nice 3458a >> on craigslist in the SF Bay area. Initial price of $2,900 was dropped last >> night to $1,950. I wish I had some leftover cash from Christmas! >> >> Jerry >> >> On Jan 3, 2016, at 6:26 PM, Jerry Hancock <je...@hanler.com> wrote: >> >> I’ve been reading the list for a while, never had a reason to post until >>> now. >>> >>> I have an HP 3457a, actually two of them, both work great, accurate, >>> stable, etc., except one won’t autocal without an error 512 in auxerr - >>> ohms recharge offset. I’ve had the meters side by side for hours the past >>> few days and can’t seem to locate the problem. There is a difference in >>> the Ohms precharge offset in the failing meter. It looks like a factor mod >>> was performed to lower U216 below +15 by adding resistors to legs 13 and >>> 10. I know there were service notes sent out by HP in 1992 related to the >>> 512 error. the other thing of note is that occasionally when I power on the >>> meter, it will also get a 16 error, which is the offset. I ran diagnostic >>> 4 and calibrated the offset but occasionally the meter will not pass the >>> test no mater how the offset it adjusted. For those of you not familiar, >>> the diagnostic 4 puts the meter in a loop displaying HI or LO depending on >>> the adjustment until the repair tech (me in this case) adjusts the offset >>> correctly. At that point the >>> >> meter displays ‘Passed’. When the meter fails, usually when cold, the >> display oscillates between HI/LO like the ADC doesn’t have a connection and >> the input is floating. I tried swapping U101, the input hybrid, between >> the two units without luck. >> >>> >>> So I am wondering if anyone on the list has run into this problem in the >>> past, and if so, the solution would be appreciated. >>> >>> Thanks. >>> >>> By the way, I have a number of voltmeters, can’t seem to get enough, and >>> am pretty good with 3455A repairs having four of them. My black face 3455A >>> is one of my favorites. I also have 2 x 3456A, 2 x 3457A, 3 x 3478a, 2 x >>> 3476, a bunch of Keithely (191, 178, 199, 195, 196, etc) and others, mostly >>> older HP units like the 400. I am currently eyeing a 3458a. I guess I am >>> a volt-nut at heart. >>> _______________________________________________ >>> volt-nuts mailing list -- volt-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts >>> and follow the instructions there. >>> >> >> _______________________________________________ >> volt-nuts mailing list -- volt-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts >> and follow the instructions there. >> > > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. > _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.