Orbital Shaker Rheostat circuit

[biobort]

Hi all - I'm looking for a little bit of practical advice on an academic electronics project.

I'm working on rehabbing an older laboratory orbital shaker for my lab, and I am working on identifying the issue with its problematic operation. This is the particular product: http://www.bellcoglass.com/node/1677

A rheostat allows fine adjustment of the platform rotation speed of the shaker. At the higher end of the speed range the motor begins operating in a jerky manner. I opened the unit up and found that one of the resistors on the rheostat board appears to be fried (see pictures). I'm wondering if I can simply replace this resistor and expect this thing to work property, or if its destruction is a symptom of another failed component. My background is in biochemistry (phd) with some basic electronics experience, but this is a bit over my head so any help would be much appreciated.

 

[Asymptotic]

Try a resistor replacement - problem is, I'm not quite sure what value it is.

The first two bands (orange and white) represent 3 and 9. The next band is discolored from overheating, and may have been brown (1) or red (2).
On a four band resistor the 4th band represents tolerance, but I've never seen a tolerance of 'black' (which indicates a value of 0, or a multiplier of x1). On a 4 band resistor black could instead be used to indicate a temperature coefficient of 250 ppm/°K. In this case, if the third band had been brown it is a 390 ohm resistor.
It can also be read as a five band resistor without a 5th (tolerance) band. If the third band was brown, this color coding indicates a 391 ohm resistor, but I doubt the difference between a 390 and 391 ohm resistor is of practical importance in this circuit.

However, if the third band had been red (2) then it is a 3900 ohm resistor.

Can you measure it with an ohmmeter? Even if it has drifted it probably hasn't changed by an entire order of magnitude. My money is on 390 ohms.

Take a look at how resistor heating discolored the circuit board. A fair amount where the resistor body comes into contact with the board, about the same amount of discoloration at the 'outboard' lead, and a concentrated bit at the 'inboard' lead. Looking down from above, solder at the 'outboard' lead has a mottled, dark gray appearance caused by oxidation while the other lead (although it doesn't have the bright, silvery color of a good solder joint) is far less affected. This indicates the outboard solder connection was running hotter than the other. Since electrical current through each connection is identical this suggests the hotter connection has developed high resistance. This could have been during board manufacture (a 'cold' joint when initially soldered), or through solder fracture due to vibration.

Given this circuit board is mounted in a shaker I'm going with vibration-induced failure.

Normally, I'd suggest offsetting the replacement resistor from the circuit board by 5mm or so to improve cooling, and prevent resistor body heat from flowing into the board. In fact, this technique is often used with high wattage resistors during fabrication to prevent PC board degradation in the first place. However, the larger and heavier the device, the more mechanical inertia it has, and if the resistor isn't touching the board it will move around more due to vibration, and increase the likelihood of fracture in the solder pads. A compromise solution could be to install the resistor so the body isn't in contact with the board, and wedge a thin piece of high temperature silicone rubber underneath to damp the vibrations.

Take a close look at the circuit traces on the other side of the board. There is a good chance overheating caused one or more of them to raise up off the board. If so, it is a good idea to bridge them by soldering appropriately long pieces of thin wire (at least as large in diameter as the resistor leads) from the solder pad to a point a cm or so beyond where the traces lifted.

 

[dlgoff]

That resistor doesn't look fried to me. I would suspect a motor problem instead.

 

[dlgoff]

Take a close look at the circuit traces on the other side of the board. There is a good chance overheating caused one or more of them to raise up off the board.

There aren't any traces for this resistor as far as I can see. The leads go to wires as seen in this image.

 

[rbelli1]

The green component may also be an inducter. Is there any legend for that part?

BoB

 

[dlgoff]

On a four band resistor the 4th band represents tolerance, but I've never seen a tolerance of 'black' (which indicates a value of 0, or a multiplier of x1).

I think you're right that it's a four band resistor and the 4th band is it's tolerance. Here's a pic of my little color code cheat sheet. A black band indicates a 1% tolerance. I betting on 391Ω ± 1%. (edit: 390Ω ± 1%)

 

[Asymptotic]

I think you're right that it's a four band resistor and the 4th band is it's tolerance. Here's a pic of my little color code cheat sheet. A black band indicates a 1% tolerance. I betting on 391Ω ± 1%.

It bugs me a bit, through.
390Ω at 1% is 386.1Ω to 393.9Ω
391Ω at 1% is 387.1Ω to 394.9Ω

390Ω is a common value. A 391Ω resistor is definitely oddball, and it makes me wonder why a circuit designer would find it necessary to use such a thing.

@rbelli1 may have the right of it, but it looks more like a resistor than any axial inductor I've ever seen. Some of those have been squared off at the lead ends (in the manner of old-fashioned carbon comp resistors) in tan or black bodies with alphanumeric specs stenciled onto them. The ones that have used color-coding were too small for this, and looked very much like metal film resistors except their bodies had a more uniform diameter right up to the leads while resistors often have a slight bulge at the lead ends.

An ohmmeter measurement is called for. If resistance is on the close order of an ohm (versus hundreds of ohms) the inductor identification is likely.

 

[CWatters]

My first thought was the colours were the other way around but a four band never starts with Black (0).

Could the colours be Orange (3), White (9), Silver (x0.01) making it a 0.39 Ohm resistor? Motor current limit sense resistor?

 

[dlgoff]

390Ω is a common value. A 391Ω resistor is definitely oddball,

Oops. Yes. 390Ω not 391Ω. My bad.

 

[Asymptotic]

My first thought was the colours were the other way around but a four band never starts with Black (0).

Could the colours be Orange (3), White (9), Silver (x0.01) making it a 0.39 Ohm resistor? Motor current limit sense resistor?

Very possible - from the top view the 3rd band does appear more silvery than a thermally degraded brown or red. If so, my comments in post #7 about ohmmeter testing go into the trash bin.

Couldn't find a schematic (or even a manual) for a Bellco model 7744-02020, but the basic specs were 11 kg (25 lb) maximum load, 2.5 cm (1.0 inch) orbit, 10-350 RPM, and 115V +/-10% , 1ph, 60 Hz at 1.0 amps.

It is a DC drive (one of the quick-disconnect 1/4" tabs is marked 'MOTOR -'). 1.0 amps at 115V suggests motor power rating can't be more than about 1/8 HP, and a 0.39Ω, high precision resistor makes sense in an armature current sensing circuit for a motor of this size. If it is a current sensing resistor, a high resistance connection here fits with the 'jerkiness' symptom. High resistance >> high voltage drop >> abnormally high sensed armature current.

One way for a drive designer to handle current overload is to reduce armature voltage until sensed current drops to and below current limit setpoint. In an orbital shaker I'd expect loading is cyclic with one cycle per orbit. Particularly when operated in the high end of the speed range, the motor slows down when loaded more heavily (and sensed current feedback appears to exceed current limit), and speeds up through the relatively unloaded phase of the cycle yielding jerky operation.