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1991 Central Machinery Lathe Speed Control Problem

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#1
Hello, All!

Thanks for having me on the forum! I've been learning loads reading through the posts. But I've got a bit of an odd problem, or rather an odd model with a usual problem.
I recently bought an old Central Machinery 7x10 mini lathe from a gentleman in my area who had only used it twice. It had been sitting in storage since 1992 when he last used it. I brought it home, plugged it in, and it would try to start and fail immediately. Every 4 tries it ran, but only at full speed. I figured that I'd tackle the speed control problem first, and replaced the plain B5K potentiometer that was in it with a new one. Soldered it in place, and now it starts up perfectly every time, but the speed is still stuck on full. I'm a but flummoxed, really! I tried to fix one problem, but ended up fixing the other one.

I've attached pictures of the lathe below. I'd really love to be able to fix this problem, with your help.
Thanks so much!
Willow
 

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BaronJ

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#2
Hi Willow,

Based upon your comments, I would suspect that the power devices (semiconductors) have gone short circuit.
You can check this with an ohmmeter ! But make sure that the machine is disconnected from the power supply.
 
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#3
Thank you for such a prompt reply! :D Now I have to figure out which bits are the semiconductors and how to fix them. Would I be replacing them?
 

BaronJ

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#4
Could do with a much bigger 10.jpg, to identify parts.
 
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#5
unnamed-11.jpg

Is this more helpful? I hope it comes fully sized in the post. Thanks so much!
 

BaronJ

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#6
Hi Willow,

Yes that is much better :encourage:
unnamed-11.jpg

The two devices ringed in red should be the semiconductors you need to examine. If you carefully bend them back towards the top of the picture
you should be able to read the numbers on them. With the numbers I should be able to identify them.

But please do this after you have disconnected the power. I would hate for you to get a belt...
 
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#7
Thanks so much for your help! I definitely made sure that the power was off so I didn't get zapped. :D The semiconductors are TYN 412 C041.
 

markba633csi

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#8
Looks like a very early SCR phase fired speed control- you might want to send it to Pete at:
www.olduhfguy.com
mark
 

BaronJ

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#9
Hi Willow,

I agree with Mark, it looks like a simple SCR controller. I have attached the PDF data sheet. This gives you the connections to the semiconductor devices. Testing these is easy but you do need a multimeter. These devices are not expensive and are available from Digikey and many other component suppliers.

If you look at the top right side of the first page, there you will see the details for the package and pin designations. You are checking for a short circuit between "A" and "K" , check with the multimeter on the Ohms or Diode range, make the check both way by swapping over the meter leads. It should read open circuit. If the reading is low then you will have to remove them to test.

Not knowing what your skills are makes it difficult to advise further. Since there are several circuit configurations that could be used for motor control.

HTH
 

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#10
Ok, thanks! I will check ports A and K. To be honest, my experience with this kind of thing is negligible. I'm a blacksmith/blacksmithing instructor who does small machining and welding jobs on the side. I'm doing this out of a love of making and repairing my own tools, and curiosity more than anything else. I've got a friend who has far more experience with circuit boards who has offered to help me with this one, and now that (thanks to you both!) I have an idea of where to start, we can give it a go.
 

BaronJ

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#11
Hi Willow,

If you get stuck, give me a shout !
 

markba633csi

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#12
Here is a likely pinout for the SCRs and a sketch of a simple test fixture you can make. You can use a cordless drill battery and a 12 volt automobile lamp. Unsolder the device from the board and connect as shown. The lamp should be off. Momentarily connect the gate lead to the anode and the light should come on and stay on till the power is removed if the device is good.
mark
SCRpinout1x.jpeg
SCRtestbed1a.jpeg

Test fixture for SCRs or diodes
 
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BaronJ

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#13
Hi Guys,

I had a test box for years using that circuit. An old battery charger transformer, 12 volt car side light bulb and a push button switch. Very handy.
I had forgotten about it till Mark posted.
 

markba633csi

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#14
Hi Baron- yes it's sometimes possible to check semis in circuit but to be absolutely, positively sure it's best to unsolder them and bench test.
I suspect however, that the OP's board has some other problem though- SCRs are pretty hard to kill...
M
ps looks like TYN412 is the part number- 12 amp, 400 volt devices- they do match the drawing I posted- If you need to buy replacements Littelfuse S6020L or S8020L would be a good substitute with more voltage and current capability
 
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BaronJ

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#15
Hello Mark,

Yes I agree with you completely. The test box I referred to had clip leads so could be used in or out of circuit. Many times I found that the device had gate failure rather than "A - K" shorts.

I did wonder if that circuit was the type that replaced two of the four diodes in a bridge, but couldn't see enough from the picture to determine that.

Almost any SCR with ratings equal to or greater in a TO220 package could be used.
 

markba633csi

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#16
Baron it looks to me that on this particular board they used a different bridge/scr arrangement than on for example the KB series controllers- I'm not sure how exactly- if they full wave rectified first then in theory they would need only one scr but apparently there are two. There may be additional diodes mounted on the back.
m
 
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