Low resistance values for Fluke 720A resistors in tank - Page 1
Forum section: electronics / general

author: perdrix
author_rank: Frequent Contributor
timestamp: April 10, 2022, 03:29:15 pm
content: I've got a 720A that is decidedly odd in that two of the resistors in the oil bath read LOW by a substantial amount (I forget exactly how much but very significant).Can anyone think of how that could happen? I "hacked" it by adding series resistors so I could get it even close to adjustment (yes I know that's BAD).How to fix (apart from send back to Fluke and pay most of the price of a new one for the repair ...)?Yes this is the same 720A as reported in this thread:https://www.eevblog.com/forum/metrology/how-to-check-how-bad-a-suspect-720a-is/msg1430974/#msg1430974

author: martinr33
author_rank: Frequent Contributor
timestamp: April 10, 2022, 04:52:20 pm
content: Without any numbers, it is difficult to diagnose the problem.I'd offer two sources of trouble:(a) drift in the resistors in the tank.(b) some form of abuse - overheated resistors. Fixing this one is an unknown, becaue of the importance of matched temperature characteristics.If (a), you can correct the resistors by adding parallel or series components.Check out Dr. Frank's thread here:https://www.eevblog.com/forum/metrology/fluke-720a-kelvin-varley-divider-repair-calibration-linearity-validation/msg3897077/#msg3897077The trim resistors do not have much influence on the stability of the end result, but I would still use the best you can find. Obviously, the bigger the trim, the more stable the trim resistor needs to be.Also, you can move the balance point of the bridge, by cranking the C deck resistors to their limit and then recentering the bridge with the S2 adjustement. This approach might bring the lowest values in the A deck into adjustment range. However, it is clearly subject to drift.The best approach is to rebalance the unit as in the above thread. Note also that Dr. Frank's unit has a few trimpots replaced with cermet, not wirewound, trimpots which did not appear to compromise the system.

author: MK
author_rank: Regular Contributor
timestamp: April 10, 2022, 05:50:19 pm
content: Could it be contamination in the oil? Or a resistor has been overheated enough to stretch and touch an adjacent turn.

author: perdrix
author_rank: Frequent Contributor
timestamp: April 10, 2022, 10:15:13 pm
content: Quote from: martinr33 on April 10, 2022, 04:52:20 pmR302 is 9.711k and R311 is 9.793k. Both should be 9.896k, so R302 is 185 ohms low (1.869%), and R311 is 103 ohms low (1.041%).D.

author: martinr33
author_rank: Frequent Contributor
timestamp: April 10, 2022, 10:57:31 pm
content: OK, that looks pretty awful..You are going to need to add series resistors to bring these two in line, close to the values that you calculated. They will need to be precision resistors to get the thermal characteristics you need, ideally 10ppm (for .01ppm precision) but you could go with less.You might end up with a combination of this series resistor for the bulk of the adjustment, then a parallel trim resistor for the adjustment circuit.The trim resistor can be of lower quality, and is going to be in the range of 15k.However, before you go any further, you should fully characterize the unit. You want to find out the range of every resistor at both ends of its adjustment pot - and as you do this, you will also end up cranking the pots to each end, which can help.Once you know how the pots lay out, you can develop your plan for adjustment resistors.One goal is to recenter the alignment bridge, which you do with the S1, S2 and S3 pots. By doing this, you can make it easier to keep the bridge in alignment.