One path was through a 470Ω ½W resistor which showed zero signs of heat stress and had the same value.


The other path(s) were the rabbit hole, where the trace beneath the darkest resistor was browned (replaced with adhesive copper tape to be sure), the network of BJT transistors, 4 Zener Diodes (More and More the target of my inquiry, but I'm unsure of the values of 3 of them.) A LOT of standard 1A diodes that test fine/no reverse leakage. Then add in two taps for power from the air-controlled signal input.

So, total is:
Inputs:
5 Potentiometers (2 controlled by foot/air pressure)
1 Single Stitch button (Double Pole, Triple Throw) That charges the cap and activates the motor at speed determined by 2 pots (1 user adjustable) for one stitch.
1 Reed Switch indicating Needle up

1 Output: Universal Motor that drives entire sewing machine gear train via 2 belts.

The SCR immediately next to the burnt resistors had failed open, along with the downstream motor control SCR.

I'm unsure What else happened at time of failure, as another tech had worked on the board before I got it. If I didn't have the photos from the last time this happened, I'd be completely guessing the values and components. This behavior is not much different from what I'm doing now, but I digress...

First picture is the board it connects to, and the air-diaphragm resistors/switch.

Second picture is component side, the spade connectors, from edge of board are: reedA reedB motorA motorB Light A Light B. The burnt resistors were around Th2, in the lower left-middle of the component side of the board, where you can see the CR-100 Sharpied in, along with initial guesses as to resistor values.

Third picture is reverse side of board, if you flip it vertically, and scale just a touch, you can trace the connections after a bit of spatial visualization. I sharpied in some diodes and caps to help align, but ended up aligning on pushbutton notch in board and notch in center where the air diaphragm goes through to visualize.

Last image is board soldered into housing and connected to motor, switch, light, etc.

Click Images for VERY FULL SIZE versions:



Housing and air control (cylinder in middle), power switches, power input:


Component side and top traces:


Copper side traces:


Assembled and in place to test with housing and board together:

 

I agree with the first one. Not sure how you get 825Ω for the second. I clearly see brown, black, green, red, which is 1MΩ 2%. How do you get 825Ω?

At the bottom of the resistor is a grey band, which is actually the first band. So it reads Grey, Red, Green, Black, Brown. The Grey, Red Green are the 825, the black means *10^0 (or "times 1") and the brown means 1%.

Welcome to the world of precision metal-film resistors.

Edit: The other possibility is 105K ohm at 0.05%, However I think that one is unlikely

 

@thatoneguy,

When I look at you written values on the board, I could be wrong, but think I see 820K. Should that not be just 820?

Also, it might be an idea to replace the resistors with a 1/2 watt units just make sure it was not a failure caused by the resistors succumbing to heat stress.

 

I just took my wife's Elna control apart, but it is very different. Older, simpler design with no pneumatics.

 

I just took my wife's Elna control apart, but it is very different. Older, simpler design with no pneumatics.

There are 5 boards based on this general layout for their "Air Series" of smooth control machines (The high end). They no longer support them, I'm trying my best to get a schematic through official channels.

This is the most complex of their boards, due to the single stitch button. On others it is simply blocked out and the board has maybe 1/3 of the components. The needle doesn't always stop at top of stroke, etc. After this board, they went to digital control, which I can work with a bit better until I get to the new ones that run a few steppers in addition to the main drive motor, and everything is TSQFP multi-layer. Relative to those, this one should be cake, since there are no ICs.

I'm guessing that one of the zeners (blue diodes) may be bad. Thus causing the charge on the cap to be 4x what it should be, and each 22V pulse-dump isn't discharging enough current. The problem is, I don't know what the actual values should be. I can make it "functional" by removing the cap, but then the needle doesn't always stop in the up position, which is a reason people laid out a lot more money for this model.

The other neat feature is the "auto-light", where the second little board comes into play. When there is draw from the motor, current is drawn through the coil, which is wrapped around the reed switch on the mini board, which in turn enables the light.

 

At the bottom of the resistor is a grey band, which is actually the first band. So it reads Grey, Red, Green, Black, Brown. The Grey, Red Green are the 825, the black means *10^0 (or "times 1") and the brown means 1%.

Welcome to the world of precision metal-film resistors.

Edit: The other possibility is 105K ohm at 0.05%, However I think that one is unlikely

Ah, I did not see the gray band. Thanks for clearing that up

 

@thatoneguy,

When I look at you written values on the board, I could be wrong, but think I see 820K. Should that not be just 820?

Yes, I wiped the K off with alcohol. I've haven't looked at it for a few weeks.