1991 Central Machinery Lathe Speed Control Problem

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
 
Last edited:
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.
 
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
 
Last edited:
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.
 
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
 
Back
Top