Fluke 731 Repair, someone dropped a dab of solder on wirewound resistor.

[opa627bm]

Hi all,
I bought a Fluke 731A off ebay, and finally got power supply issue fixed and it started working again, however, the 10V output is never 10V, it is about 4~5 mV range off which I cannot adjust form the front trim pot.
After I open the back, I noticed a dab of solder on one of the wire wound resistor, I assume that is the reason why pervious owner give up on fixing it.
any idea how to get it working again ?

[Kleinstein]

The resistor wire is isolated and the isolation often is quite heat resistant. Even without the polymer isolation the NiCr wire is very hard to solder and is somewhat isolated by an oxide layer.  So chances are the solder drop is not contacting with wire at all. One could measure this with an ohm meter.
So I don't think the solder drop is part of the problem.  It is more that someone was alread in there and tried to fix the problem.

No Idea if it would be better to remove the solder blob, or keep it in place - both have there pros and cons.

If the problem is due to contact problem (e.g. a resistor in the trim part or the actual reference circuit) chances are it could reactor to temperature or mechanical disturbance.  A nasty type of fault is one that disappears (to come back after some time) after taking out the PCB.

[opa627bm]

So I got it off, the Value should be 11.424k but only reads 11.390k (which is way off from the 0.05% rating) I am trying to see if :
1. If it is easier to get one end of the wire off, I can take it off from the reel, recoat it and but it back together.
2. Find a VPG metal foil in exact value

This is a bummer because this chain is driving the voltage reference BJT base directly, form a divider 4.900K  -- 11.424k + Trimpot for the 10V adjustment...

[HighVoltage]

It looks like the damage did not come from the solder dab. It is some mechanical damage to a few wires. Maybe someone tried to remove another solder dab?

Its probably best to replace this resistor.

[alm]

This resistor is probably quite critical, though, so eventually you'll want to use a low tempco / low drift resistor (VHP). But for testing a lower spec resistor will do.

[Wallace Gasiewicz]

If you are going to try to remove the wire, note how it is connected. NiChrome does not solder well at all. It might be welded to a terminal.
You might want to try to straighten the slight damage to the resistor coils that are shown in your very good picture. Maybe one or two of them are shorted.

[bdunham7]

I would agree that a good place to start would be to get a small plastic dental pick and try to gently separate the crunched windings and perhaps put a dab of nail polish or similar product on there as insulation.  There are probably just a few windings shorted together.  If that doesn't work, you might add a small low-TC resistor in series to bring the resistance back to spec just for testing purposes (this wouldn't be stable long-term if the lower resistance is the result of damage) and if everything is good, start looking for a way to assemble a very-low-TC replacement.  I think the resistors in the chain are select TC-matched as well as very-low-TC, so this may be a struggle. 

[Kleinstein]

The original resistors are likely mainly slected for TC matching and may not even have a really low individual TC. So one would may have to replace both resistor.

Looking to repair the resistor is not such a bad idea. There are 2 spots visible that may have an inter winding contact. Fixing this may be all it takes. Getting back to the original resistance would be a good indecation that one got it fixed.

Keeping the wrong value and add a series resistor is a bit risky, as the unintentional contact to reduce the resistance is likely not reliable. I would consider a repaired resistor more reliable than the current state with likely some extra contacts.

[Edwin G. Pettis]

This resistor is very likely Manganin, esily verified by looking at the wire/lead joint, if it is soldered it is Manganin.  It is also very likely that the solder blob may have melted through the insulation of the wire which was a fairly low temperature type, it was often easy enough to 'burn' off the insulation with a soldering iron when making the wire/lead joint.  Once you have carefully removed the solder blob examine the wires underneath for any signs of damage.  It is possible to separate the wires if touching with a thin blade, the winding was also secured in place with an additional coating so once any touching wires are separated they should be stable.  I also noticed to the right of the solder blob what appears to be some possible damage to the wire turns, these need to be carefully examined for any metal to metal touching and spread apart as well.  The difference in measured resistance vs. measured resistance indicates that more than just two turned are involved.

The resistors in any given divider were usually matched for a particular TCR for the set to achieve a 'best' possible match, the TCR may or may not be specified on the resistors or in the parts list.  They were usually listed as a matched set and be replaced as such.  Replacing this possibly damaged resistor with a VHD resistor would not likely result in as good a TCR match as the original set, therefor I would not recommend such a replacement.

[alm]

Edwin probably has more experience with precision wire wound resistors than all the rest of the forum members together, so I'd absolutely listen to him.

The resistors in any given divider were usually matched for a particular TCR for the set to achieve a 'best' possible match, the TCR may or may not be specified on the resistors or in the parts list.  They were usually listed as a matched set and be replaced as such.  Replacing this possibly damaged resistor with a VHD resistor would not likely result in as good a TCR match as the original set, therefor I would not recommend such a replacement.

If the resistor is part of a divider and can not be repaired to satisfaction, then indeed you'd have to replace both resistors in the divider by precision wire wound or something from VPG. Ideally with a matched tempco.

[Wallace Gasiewicz]

Edwin:
How exact are the specs on Maganin. It it possible to use the length specs to make a very accurate resistor for example.
I would not think so but I thought I would ask.

[Edwin G. Pettis]

Off the shelf Manganin would very likely not have the similar TCR as the resistors in your instrument, stock Manganin has a TCR of ±10ppm/°C to ±20ppm/°C and the curve is hyperbolic around the 25°C reference point.  Those Manganin resistors went through a set of specific bakes and stress relief procedures to produce very low TCR....and note, to some degree it was hit or miss, in other words the resistors after processing were measured, their individual TCRs noted and then matched together for TCR that met the required specifications.

At one time winding by wire length was generally used to wind resistors, the ohms/foot was noted, the required resistance was divided by the ohms/foot number resulting in the approximate length of wire required for a given resistor value.  The resulting resistor still had to be 'calibrated' to the correct value/tolerance by removing wire from the bobbin until the value was satisfactory and then could be terminated.  Unfortunately it was found that winding by length measurement was difficult and in the case of the very fine gauges virtually impossible due to wire breakage during winding.  The practice is not that common anymore.

If I understand your question correctly, the general answer would be no unless the tolerance was rather sloppy, if you were referring to essentially what is called wind and terminate.

One other factor that needs to be considered in divider strings is the variance in power dissipation among the resistors (if they are of differing values), the easiest condition is where the divider resistors are all the same value then dissipation becomes somewhat less of a thorn.  If they are of different values then the power dissipation of each resistor must be considered to determine the effect of self heating on the resistors, this will cause an apparent difference in TCR even if the resistors have the same identical resistance and TCR.

Integrated resistor networks do reduce the dissipation difference to some degree but is rarely, if ever absolute among the resistors.  Probably the more important condition is that the temperature around the resistors remains as stable as possible thereby keeping the resistors at a stable operating point minimizing drift.

Manganin is one of the few resistor alloys that can be soldered, the prevalent Evanohm alloy requires welding to terminate.