Fluke 895A DC differential voltmeter Repair - Page 1
Forum section: electronics / general

author: BertSP
author_rank: Contributor
timestamp: October 16, 2017, 09:04:25 pm
content: Real Newbie Here,I have acquired a unit and I would like some guidance in which direction to follow to try to get this unit into reasonable shape.It is rock solid below 10v, quite impressive. The problem is that as soon as I go to 10v and over, it is way off. In the order of 20%. In the 50v, I have to null at 60v. Since the lower voltages work well, which section of this unit should I be troubleshooting as I don't really want to upset the lower ranges.Hopefully it is something evident or discernible. I have not tried any calibration of the higher ranges yet.

author: dacman
author_rank: Frequent Contributor
timestamp: October 17, 2017, 12:30:48 am
content: If the 1 V range works, then about everything has been eliminated except for the range switching and associated range resistors. Does the 10 V range work with the MSD from 1 to 10? I'd clean the switches prior to troubleshooting any further. Be meticulous cleaning the switches, because the 895A can suffer from leakage.

author: BertSP
author_rank: Contributor
timestamp: October 17, 2017, 01:29:39 am
content: Dacman,Thanks for the reply. The unit works fine in the 1v Range and the 10v range.Its when I start using the 100 and 1000v ranges that things start falling apart. There is some type of consistency nulling the same voltage in the 100 and 1000 ranges. The reference used was a 10v and switching between ranges and decimals yielded low 8-9v. The peculiar behavior noticed was that the null meter kept creeping to the positive side. From the manual, the 1 and 10v range work off a 10v reference, and the 100 to 1000v work off the 1000v reference. From my limited understanding, the switches are dividers running off two different base voltage reference. So if the switches work with the 10v, they should work with the 1000v. So something must be wrong with the 1000v ref and the reading creep might be an indicator. This is the limit of my understanding. I will do some more sampling of Voltages using my 335A and see if there is a pattern for the 100/1000v ranges.

author: dacman
author_rank: Frequent Contributor
timestamp: October 17, 2017, 02:32:45 am
content: Ok, I suppose I should read the manual before I respond. It's starting to come to me.The 1100 V power supply is turned on once a high voltage range is selected (it isn't on when using the 1V or 10V ranges). Does it eventually settle? I'd check C401 through C404 and the parallel power resistors.

author: BertSP
author_rank: Contributor
timestamp: October 17, 2017, 06:47:51 pm
content: Here are some pictures, including the 1989 dated Zener and oven? Any one knows if these spragues are still available or a replacement?20uf -10/+100%, 500v, type 66D

author: dacman
author_rank: Frequent Contributor
timestamp: October 25, 2017, 01:16:51 am
content: I know this is belated but what I would do for a replacement capacitor is measure the distance between the mounting holes, and the max height and diameter the capacitor could be and then go research some newer style capacitors, such as these:http://www.newark.com/panasonic-electronic-components/eeu-ed2w220s/aluminum-electrolytic-capacitor/dp/96K9476http://www.newark.com/panasonic-electronic-components/eeu-ee2w220s/aluminum-electrolytic-capacitor/dp/39P9931Also, make sure the capacitor cases are not part of the circuit, else shorting wires might need to be added.

author: builder
author_rank: Newbie
timestamp: October 25, 2017, 01:47:54 am
content: Hi BertSP,I found no problems with the caps on my 895A but had to replace the HV transistors in the 1000V supply. They in a cascade arrangement, Q401-Q404. I purchased more modern higher rated ones and replaced the lot as it is not worth trying to test them at high voltages. This solved my problem and got the 100 and 1000V ranges working fine.Sorry not to be more explicit but I have moved house and mislaid the data.Hope this helps, builder

author: builder
author_rank: Newbie
timestamp: October 25, 2017, 05:19:28 am
content: Hi again from builder,Found my account of the mods to the 895A 1100V supply.I found 1450V at the output capacitor, feeds to 15K then Q401 and Q402 in series. Replaced these two with BUX85G. You may have an unstable component associated with this circuit.Hope this may help.