Datron 1061 DCV drifting to several 100V on display when I/P open circuit

Datron 1061 DCV drifting to several 100V on display when I/P open circuit
Posted by Ade911 on 25 Aug, 2023 16:21
Hi knowledge team,

I acquired recently two 1061 meters, the second powers up but on hold whilst i work on the first meter.

The first had dried popped caps on the power supply which have all been replaced +-15 and 5VDC. I also found after checking on BB M31 was faulty, which I replaced with a TLC81 device be it DIL and in a socket on extended legs and the backup battery was totally flat which has been replaced.

I started to try and calibrate / check voltages as the manual states but I've noticed that when the HI LO input sockets are not connected together the meter starts to show voltage on the display , this increase to several 100V. After reading allot on line I've re-seated all chips, cleaned with acetone all DC and I/P relays. The fault is still there , the voltage does refer to zero when the I/P is shorted again so its a drift issue.

Other issues which may be connected, [I don't have cal keys] so I shorted out the switch , the CAL display at the front doesn't illuminate but illuminates if I do a TEST.

Also after doing a TEST , I have ERROR 5,6,7 then pass.

Before trying to work out what may be wrong I was just wondering if any one else had experience this problems and what may be causing them.

Any support will be most welcome.

#1 Reply
Posted by Kleinstein on 25 Aug, 2023 17:25
It is normal that the votlage reading will drift when the input is open. However this only applies to the high Z modes, which are usually up to 10 V maybe 20 V with a few meters. So drifting up to some 100 V would indeed be strange, as these modes usually have a 10 M input divider connected. Drift in the 100 V/1000 V range may caused by bad relay contacts connecting the amplifier to the divider (or depending on the meters circuit making the ground connection on the low side of the divider). Dirty / oxidized relay or switch contacts are relatively common with old meters. Sometimes just operating them a few times can be a quick fix.

The frist test for the voltage is usually with a shorted input.
The speed of the drift with an open input (or a capacitor of some 10 nF) can give an idea on the input bias current.
AFAIK there are manuals available online (maybe only similar versions like 1062 or 1065) that should explain the error codes.

#2 Reply
Posted by Ade911 on 25 Aug, 2023 19:30
Thanks Kleinstein for the advice, interestingly if i inject a DC voltage from zero to 30 VDC [Max of an external power supply] the meter tracks the power supply to ~19VDC and then jumps to a ERROR OL , then if you reduce the input voltage at ~19VDC the meter says Im inputting 1974V !, at 15VDC I/P I see ~1500VDC on the readout, until zero VDC IP where the display shows several milli volts [Range is also set to Auto].

I wonder if I have the famous OPTO issue ?

#3 Reply
Posted by Kleinstein on 25 Aug, 2023 19:52
The error looks like a problem with the relay for the input divider (1:100). It looks like RL1 is never switching as intendent and thus 100 x the reading as long the the realy is still supposed to be on.
This could be the relay itself or the driver part (maybe an OK ?).
One could check if other relays in the AC part work as intendet.

#4 Reply
Posted by Ade911 on 27 Aug, 2023 16:31
Hi Kleinstein,

You were correct, i found that LD2 was held low ,unplugging the AC Bd resolved the problem, further investigation found M4 [AC Bd] to be faulty which after replacing from the other system then came to life and the relay started working correctly on the DC Bd.

I don't have the ability to calibrate the AC Bd, but i still get error 6 and 7, where does the calibration data come from in the circuit , maybe I can compare the CAL value against the existing circuit values. It also seems to be the same problem with the resistance bd, although i tried setting this up with my old hand held multimeter [which seemed to work on the DC bd] i can not pass test error 6.

#5 Reply
Posted by TheDefpom on 30 Aug, 2023 10:27
The meters have a 1999999 count, so going over 19.99v giving an error is correct.

To do a cal you need to set all the GPIB switches (can’t remember if all on or all off), also replace all the electrolytic caps on the analogue board (they are usually blue), they are ALWAYS bad.

I’ve done several videos on Datron repairs, you might want to watch them.

#6 Reply
Posted by Testtech on 30 Aug, 2023 23:23
Did you replace the 4 bootstrap supply capacitors? If the BS supply is defective, the whole front end will drift like crazy.

#8 Reply
Posted by Kleinstein on 01 Sep, 2023 07:38
When in a high impedance range (e.g. 2 V) it is normal to get a drifting, non zero reading for an open input. The meter has a not very stable input bias current in the 10s (maybe 100) of pA and with an input impedance in the 1-500 GOhm range (also not very stable and well defined) this may end up at 1.2 V or even out of the range.
The open circuit voltage is not relevant. Having it at only 1.2 V is more like a good sign, that the input bias is likely not very large (there is a trimmer at the input stage to adjust).

The more relevant voltage would be with a 10 M range resistor across the input, to measure the input bias current.

#10 Reply
Posted by TheDefpom on 01 Sep, 2023 19:40
It may not be the other boards which are the problem, each board communicate with the analogue board and it has opto isolators for each signal, they are known to go bad.