12 V motor loss of power

[kellys_eye]

A PWM voltage supply may be good enough or this purpose, It gives a controlled average voltage but no speed control. It would normally have a big electrolytic input capacitor which is not necessary since the motor inertia will smooth the speed variation.

"no speed control"? - contrary to all the PWM speed controllers that currently exist?
"big electrolytic input capacitor"? - not specifically? and why would this be a problem?

Regardless.... personally I'd change the system to a stepper with LCD readout and programmability in a move towards CNC-ing the lathe!

 

[Bluejets]

Duke, Looks like the led is on the rectifier output. Guess we will know more if the op posts some sort of drawing.

 

[Donald Helm]

The SCR is probably a C106 or maybe an armoured cat fish! 4A. Sensitive gate, very common.
The transistors are npn, The 2N3904 is 200mA, The ZTX651 is 2A. Both common.
Could the 1R2004 be another 1N4002?

It looks as if the 30V AC is rectified with a bridge rectifier and if the motor speed and hence voltage is low, the SCR is triggered to raise it. The reference voltage will be controlled by the potentiometer.

Be careful of the mains voltage.
Measure the voltage across the outer terminals of the potentiometer.
Measure the voltage from the centre terminal of the potentiometer to the most negative terminal. Does it go up and down as you twiddle the pot?

Done a few more checks and to be honest I think the circuit board is fine. Transformer output is 15 v AC and supply to motor is 15 v DC and the motor revs go up and down when the pot knob is turned.
When I ordered a new motor , I searched the inernet and found RS Components had a 12 V DC motor identical in all dimensions and 60RPM . Just had another look at the new motor and it has www.mclennan.co.uk on it . i went to their web site and they have a lot a motors all to the same dimensions but different RPM etc and I think I may have purchased a wrong motor . As the old motor had no means of identification on it , I will somehow have to try and find this out.

 

[73's de Edd]

Sir Donald Helm . . . . .

I assume the electrollytic you refer to is the black capacitor which I replaced . It is 63V 1uF . Which is identical to the one that was originally on.

I hope that you still HAVE the old electrolytic, as I totally am in agreement with the 63VDC, but I really think that the capacitance marking just actually might have been 1M0. . . .which is being 1000 ufd.

Can you confirm that ?

Also can you measure the DC voltage that is being read at the BLACK paired wires that come out from the center contacts of your FORWARD--OFF-REVERSE rotary switch.
Get the voltage readings at minimum speed and also at maximum speed.

73's de Edd

 

[Minder]

I could see it being 1μf if it is part of the phase angle control into a diac etc.
M.