Just so we are all on the same page, and I could be wrong but if I follow the wiring in the picture I see that the AC controller is on the AC side of the circuit and outputs an AC (chopped at lower speed settings) this feeds the diode bridge. The bridge is just 4 diodes packaged together. The AC from the controller feeds the bridge, AC inputs, the DC output feeds the motor.
When measuring check to see if the meter has a setting for RMS if so set that on for readings from the AC contoller. I think this is the normal setting on most meters.
As for Dusty measurements it looks like the motor sees the 51.8 to 137.1 VDC so depending on load this sound like the area we need to be in. A little high for a 115V motor??? I would have thought they would have tried to keep the high end more like 115V but if its working so be it. It also could be that under load the real numbers go down.
So once again is this what everyone thinks this looks like?
Ed
First I must correct an error. The 137.1 should read 131.7 VDC.
The DC output to the motor swings from 51.8 vdc to 131.7 vdc. This is the same (approximately) on two different speed controls (my scroll saw uses the same module and I measured both).
With the dc motor unplugged (no load on the circuit board) the dc output sets at about 108 vdc and can not be varied.
PS These AC readings are RMS therefore the peak ac readings would then be 1.414 x the RMS.
"Making Sawdust Safely" Dusty
Sent from my Dell XPS using Firefox.
Looking at the photo's they have to be kidding. $300 for that?. I can buy a DVD player for $39. I do not know much about electronics but I would think you could go to the Jack Shack (Radio Shack) and buy the parts for less than $25.
I am a die hard Shopsmith fan. Don't blink when spending $$$ for their tools but this is almost an insult to our loyalty. They could at least sell the components for a repair not the whole unit.
johnm wrote:Based upon your measurements and picture 026, what I think SS did is put a light dimmer in series with the AC input to the bridge rectifier block (seems wierd, but I suppose it would work). Since the motor turns when DC is put to it (so it works), and the board is what smokes when power is applied, I'll put my money on the rectifier block being toasted. If it fails, it will probably put a short circuit or near short circuit across the AC (marked with a sine wave) terminals. You can test that by unsoldering it from the circuit and testing the AC terminals on the rectifier block with an ohm-meter. They should be very high resistance.
The board still may be toasted, but it wasn't the cause. You can try replacing the board with a garden-variety light dimmer (it's got 2 terminals just like the board) and see what happens.
Good hunting.
With the rectifier still in circuit:
and the ohm meter connected across the rectifier (- to - and + to +) the rectifier measures 3.6 meg ohms. Reversing the meter leads (- to + and + to -) the meter reading is infinite.
The potentiometer ranges from 178 ohms to 202 kohms.
"Making Sawdust Safely" Dusty
Sent from my Dell XPS using Firefox.
I know at one time Radio Shack sold a kit that you could make your own circuit board. If the jumpering fails you could go that route. And the Shack should have equivalent parts.
Also, have you talked to Shopsmith about this? They do like to take care of their customers and maybe you can get a replacement board for a lot less.
This is a bit off subject but maybe some know off hand if shopsmith still sells the switch only version of this? I have one of these on my old OPR, looks the same except it doesn't have the extra plug in or dial.
I tried a couple of key word searches but found nothing, and the parts list of the OPR doesn't show it except as an option you could buy... and no part number.
Ed
{Knight of the Shopsmith} [Hero's don't wear capes, they wear dog tags]
Just so we are all on the same page, and I could be wrong but if I follow the wiring in the picture I see that the AC controller is on the AC side of the circuit and outputs an AC (chopped at lower speed settings) this feeds the diode bridge. The bridge is just 4 diodes packaged together. The AC from the controller feeds the bridge, AC inputs, the DC output feeds the motor.
When measuring check to see if the meter has a setting for RMS if so set that on for readings from the AC contoller. I think this is the normal setting on most meters.
As for Dusty measurements it looks like the motor sees the 51.8 to 137.1 VDC so depending on load this sound like the area we need to be in. A little high for a 115V motor??? I would have thought they would have tried to keep the high end more like 115V but if its working so be it. It also could be that under load the real numbers go down.
So once again is this what everyone thinks this looks like?
Ed
I think Reible and Johnm both agree and I think they are right. It looks like a simple AC dimmer switch hooked to a bridge rectifier. I will guess the rectifer is toasted. I thought the output of the rectifer feed the dimmer but it appears to be the other way around. The voltages dusty is seeing is something is wrong if the motor is a 50 volt DC motor. I suspect the rectifier is so terrible that the motor is only seeing half the voltage dusty is measuring.
That is certainly not worth more than $25 and that is including a 100% profit.
Ed
I just found the receipt for my band saw stand (505992) which included the switch box. This number no longer crosses. I am unable to find a good number for the switch box.
"Making Sawdust Safely" Dusty
Sent from my Dell XPS using Firefox.
Can you read a part number on the voltage regulator (the part tucked down inside the metal heat sink).
I think that is the rogue part. If it was shorted it would momentarily draw maximum current in the path that is damaged (the path that ed identified with the red line).
Notice the solder pad where the black output wire connects. It is also discolored like it has been overheated.
It's the regulator. Dusty's final diagnosis.
I'll have to get one of my grand kids to see if they can read the part number on my regulator. Even with my new eyes and reading glasses I am unable to decipher the number.
"Making Sawdust Safely" Dusty
Sent from my Dell XPS using Firefox.
Two thing, one I have no idea if this would really work and I'm still out with a back problem so this will not be something I do but if as I expect a few of you have vast bins of storage (as I do) I'm sure I have an old rotary dimmer and a large enough bridge to put this circuit together... the hard part would be to have a DC motor to plug in... but if one were interest to see if this would work it would look something like this:
[ATTACH]2366[/ATTACH]
If you are not into working with AC votages don't start learning on this project, but if you are a hobbyist and know the dangers and can deal with it then have a go.
Keep in mind this is for educational purposes only. And most importain if you were to wreck something like the motor don't call me because I told it would work... it is simply an idea that one could test taking all responsiblies of damages that could occur... and no tell SWMBO it is my fault.
Second, Dusty the part I have looks just like the one here except for it doesn't have the dial or plug in for the DC motor. The ones on the stands from your URL are a much newer version so maybe what I have is just a piece of history now. I'll have to see if it has a part number on it somewhere.
Ed
Attachments
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{Knight of the Shopsmith} [Hero's don't wear capes, they wear dog tags]
