Fluke 760A slowly but surely drifting upwards - Page 1
Forum section: electronics / testgear

author: kblue
author_rank: Regular Contributor
timestamp: March 30, 2025, 02:04:36 pm
content: Hello there!I've got another Fluke 760A "Calibrator" but with a fault, i'm not able to "pin point" the cause of it. If i, for example, set the 760A for lets say ... 10.0000 Volt DC output, set coarse, mid and fine adjust accordingly to have 10.0000 Volt DC shown at my 6 1/2 digit Multimeter, it starts drifting upwards. Even if i let it "warm up" for about 30min, 1h or even 2h it wont be "stable" and is always drifting upwards.Most of the contacts where dirty and i already tried to clean them. The behavior stays the same, but less "erratic". Before cleaning the contacts 10.0000 went to 10.0021 within one second. Now only the last digit is slowly but surely rising and rising and rising. The (service) manual calls for 30min warm-up time, which would suggest after 30min it should be "reasonable stable", yes? One unscientific chart for what i mean with the never ending rising in voltage is attached.Looking at the block diagram, only the source section is of interest, right? For looking into the "drift problem". So i checked some values given in the "Source Section Calibration".Oscillator BoardTP2 of the Oscillator Board wishes for -18V +/- 0.1 Volts and it is -18.002 after 30min. So this seems okay.Between Pin 6 or 8 of the same board and ground, the manual wants 400 Hz +/- 0.4 Hz and it is 400.000321 Hz after 30min. Seems okay too. (sometimes it "glitches to 401 Hz for a split second, not sure if this is normal or my test-setup)TP2 still the same board wants 1.8V +/- 0.2V and shows 1.8000 after 30min.+50V Power Supply BoardBetween pin 6 and common it should be +50V to +50.3V and it is 50.000VDC.Between pin 13 and common it should be -50V to -50.3V and it is -50.000VDC.Power Amplifier BoardI can't check this board, due to lack of components and equipment the service manual wishes for. Like 50k 20 Watts (!!!) resistor and a dedicated distortion analyzer.100V Indicator Board (Low Voltage Adjustment Calibration)With coarse at max, medium and fine adjust (5 turns) at center of control, the output should read between 3.6 and 3.7VDC and it is 3.654XXX drifting upwards as usual)A lot of these carbon resistors are already replaced with "vishay low tc" resistors. Also a lot but not all capacitors where replaced before i got the unit. Some special capacitors on the oscillator board and all the huge filter-caps are still the originals. The "fine adjustment pot" was also changed for a modern 10 turn precision pot before i got the unit.My gut feeling tells me, it might be still some dirty contacts with all the readout dials. Lots and lots of mechanical contacts and some where really dirty like ... black plaque dirty. The big jumps and erratic behavior already changed after some multiple hour long cleaning session with isoprop and very fine "brushes". As i am unable to dismantle the whole unit, as it seems it would take ages, mayhaps some dirty contacts are still messing things up. But i'm not sure if thats the case. It looks clean from what i can see. The carbon resistors at the dial switches are still the originals, i guess because the owner before had the same problem: its somewhat unreachable place.Best wishes!

author: kblue
author_rank: Regular Contributor
timestamp: April 11, 2025, 01:16:53 pm
content: Went for another intense cleaning session of all reachable switches and stuff (incl. the rheostat etc pp) with another solvent. The drift is nearly gone, not completely gone but by a margin better compared to the first cleaning sessions and worlds apart compared to "as the unit arrived" (read: without anything done). So i guess it has something to do with "dirt" at some places (lots of black stuff). Not everything is easy reachable.Guess in the long run, i need to desolder some stuff and take the whole "switch unit" apart for a dump in the (big ultrasonic) cleaner to get rid of most of the stuff what's left.Or i fill the pool with isoprop and dump the whole unit into it. JK.But at least, i can now dial in e.g. 10.0000 Volts and it takes way longer before it goes 10.0001 or 3. Before the second cleaning session it was more like 10.0000 -> 10.0005 -> 10.0012 -> 10.0016 ... in like 10 seconds.Best wishes.

author: kblue
author_rank: Regular Contributor
timestamp: April 12, 2025, 11:30:59 pm
content: Followed the "slowly rising / not as stable as it could be"-problem backwards and its already present in the "source section". Pin 5 (out of the PowerAmp PCB) the VAC is rising - not much, but it will never really settle (even after 1h runtime). The VAC (at 400 Hz) from the OSCILLATOR PCB, passing through the Switches (Coarse, Medium, Fine) seems very stable (or at least rising very, very, very ... very slowly. Way slower compared to the output of the PowerAmp PCB. The 50 Volt (+/-) Input of the PowerAmp PCB is stable after 30min runtime.So my guess would be, the instability and/or slowly rising problem that still exist, is somewhere on the PowerAmp PCB. I may be wrong, but it sure looks like.Something i also noted: If i connect my meters to either Pin 1 of the PowerAMP or Pin 5 of the same PCB, the rising and "little instability" is somehow ... even more slowed down. If i connect any meter to Pin 1 AND Pin 5 of the PCB, its again slower in rising and even less instability. Once i disconnect my meters from Pin 1 or 5 of the PowerAmp PCB its "jumping" a few mVolts (DC Output of the unit) and rising faster again. Like ... connecting some high ohms (from the multimeter) between the pins and chassis does "help". Not enough but still ... it's again bit bitter.Anyway, it would be nice if the unit is stable by its own with the digits i can dial an. It does not help, if i can set 10.0000 Volts DC and the last number is rising./edit: changed the 2nd picture / schematics ... because it might be confusing to have multiple transistors with the same designation ... the others are outside mounted on a heatsink / added the type of transistor extra.

author: kblue
author_rank: Regular Contributor
timestamp: April 13, 2025, 06:14:59 pm
content: Did a modification, just for testing purpose so far. The "root cause" of the drift is still to be found, but i made another tiny improvement. R12 + Q7 + CR2 (Oscillator PCB) are now isolated / encapsulated. By accident i noticed the output (like 10,0000 VDC) changed a LOT (like jumps to 10,00213) if any insignificant airflow is near these components. Due to all of the holes in the case, airflow around these components is inevitable. Made a little housing and now its again "a little bit more stable/less erratic" but still rising a bit. Even after 1h runtime it does not settle.

author: kblue
author_rank: Regular Contributor
timestamp: April 13, 2025, 08:46:43 pm
content: Discovered something again "by accident". I was about to measure the VAC Voltage from the source section (before it goes into the transformer) and the OUTPUT DC Voltage together.So i plugged VAC and VDC Meter both "minus" into the J2 (-) jack of the unit. So they share the same common. J2 and Chassis are already connected together externally with a bridge which was already present. So ... once i tune in the desired voltage, it jumps first and seems very, very stable. Neither VAC from Source nor VDC from the output changes. I have never seen this stable of a line with this instrument since i am tinkering with it.What to make out of it? Why is it sooooo stable now with these two meters connected? The only things which are shared between all the units (both meters and the 760A) are ... J2 (minus) which is connected to chassis and the power line (live, neutral and earth). The meter for VAC measurement is a HP 3455A and for DC its a Fluke 45.See the big attached picture for what is how connected. It was never ever that stable before and the stability is gone, once only the VDC meter (output of the 760A) is the only connected unit or only the VAC meter (connected between J2 output of 760A and Pin 5 (output of the poweramp board before it goes into the transformer)./edit: only the Fluke 45 connected to the output will change the output from 10.000 from start to 10.005 after a few minutes./edit2: switching the meters the other way around (Fluke 45 for VAC and 3455A in HighRes Mode for VDC) its unstable again and after few minutes it rises from 10.00000 to 10.00200 and so forth (or maybe the stability was a lucky draw, since i turned the unit of for the change)/edit3: changed back to 3455A for VAC (output poweramp-board and chassis/common) and Fluke 45 for VDC (output of the unit and chassis/common) ... its way more stable again - not perfect, but much better ... 10.000 stays 10.000 even after 10minutes

author: kblue
author_rank: Regular Contributor
timestamp: April 14, 2025, 04:50:06 am
content: The next day, turned the unit on and let it run for around 30min and it's okay ... not perfect, but okay. Set the output to 10,0000 VDC and it stayed 10,0000 for like 15min and switched to 10,0001 after that (still rising for sure and feels more and more like hours of runtime during tests do help, like ... all the components need to wake up after years of slumber. Just a reminder, most of the caps (except all the big filtercaps/blue spraque cans which there are a lot) are already changed for new parts as are most of the resistors (except the ones that are way to difficult to reach, these are still carbon comp. resistors).Something else i noticed, one filtercap/can which is labelled as some sort of 'oil filled filter cap' looked like it leaked a tiny bit? There was some oily film on top of it, not much and after cleaning the case the film did not return. No matter if the unit is running upside down, on either side etc pp. Mayhaps it was the same residue i found on other parts and from the old environment the unit was stored in. I'll keep this topic updated for any change or mod.