Help with a Dayton DC Speed Control

I have a Dayton DC Speed Control where the potentiometer is not working. It's almost like it's not there. The voltage is at 106 vdc no matter if the pot is fully opened or closed. The pot is good. Has anybody had any experience working on these and know what it may be. Thanks.

the pots do not fail very often, probbaly an scr. has it worked before? there is a field output and an armature output, one is fixed and the other variable.

Usually a shorted SCR will cause a blown fuse, as this will short one side of the diode/SCR bridge.
If it operates with the pot removed altogether then it is something turning the bridge on full after the pot.
Max.

In one form of speed control AC is bridge rectified to pulsating 120 CPS. Motor & SCR are in series, so a shorted SCR would just give full speed?

Bernard said:

In one form of speed control AC is bridge rectified to pulsating 120 CPS. Motor & SCR are in series, so a shorted SCR would just give full speed?

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Many of these are SCR/Diode bridge, if the SCR is shorted, no different from a bridge with a shorted diode, results in a blown fuse or similar symptom.
If it doesn't turn off when the other conducts this is the result.
Max.

I was thinking it may be the SRC's but wasn't sure. Thanks for the replies!

For it to be the SCR's causing continuous 106vdc, one would have to be shorted and the other open circuit.
Max.

I put in 4 brand new SCR thryristors and I'm still having the same problem.

Does it have tach feedback? If it has the option for tach feedback and you dont need it, it should be jumpered out. Otherwise floating voltages might appear to the drive as a tach signal, and the drive is trying to stop a freewheeling motor.

Tillman33 said:

I put in 4 brand new SCR thryristors and I'm still having the same problem.

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If it has 4 SCR's then it is a 4 quadrant drive.
Are you sure one pair was not straight rectifiers?
Max.

strantor said:

Does it have tach feedback? If it has the option for tach feedback and you dont need it, it should be jumpered out. Otherwise floating voltages might appear to the drive as a tach signal, and the drive is trying to stop a freewheeling motor.

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No it doesn't.

MaxHeadRoom said:

If it has 4 SCR's then it is a 4 quadrant drive.
Are you sure one pair was not straight rectifiers?
Max.

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They are the same type SCR's as was originally in there.

It could be in the firing circuit, some use an Opto for the SCR triggers, if this is shorted then the SCR's are always full on.
Max.

http://www.next.gr/automations/motor-control-circuits/speed-regulator-for-dc-motors-l12176.html

This isn't the actual schematic for the speed control, but it's the closest to what I could find. Where is the firing circuit?

C11, 4N28 is the opto coupler drive to the SCR's in that schematic.
Max.

there are two outputs in the dayton dc speed controls, one for the armature, and one for the field. one is variable and one is fixed, both dc.