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Old 30th Sep 2021, 1:26 pm   #1
M3VUV51
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Default Singer 2502C pedal repair.

Hi all,i have a singer 2502c sewing machine,the issue is with the foot switch,it is either on or off,there is no ramp up of the power,its like flatout or nothing,so far i have tested the bt134 triac with a dmm,it seems fine,any ideas what the issue could be,would a duff cap or diac cause this?,i have measured the caps for s/c they are not shorted,not cheched capacitance yet or esr.attached is a pic of the pcb,have no idea of the history as just brought it for spares or repairs.
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Old 30th Sep 2021, 2:39 pm   #2
M3VUV51
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Default Re: singer 2502c pedal repair

would i possibly be correct in thinking the caps have gone low capacitance causing ing the diac to conduct faster hence switchind the triac to fully on faster hence no ramp up on the speed control? tia m3vuv.
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Old 30th Sep 2021, 9:15 pm   #3
rambo1152
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Default Re: singer 2502c pedal repair

Quote:
Originally Posted by M3VUV51 View Post
would i possibly be correct in thinking the caps have gone low capacitance causing ing the diac to conduct faster hence switchind the triac to fully on faster hence no ramp up on the speed control? tia m3vuv.
Possibly, but I would suspect the BT134 first.
Check for a short between MT1 & MT2

Looks like they've used a 500v type, replace it with a BT134-600 or 800
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Old 30th Sep 2021, 11:21 pm   #4
M3VUV51
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Default Re: Singer 2502C pedal repair.

Well,i though i would have a play,i turned the pot fully cw and now it ramps up,seems sort of odd.
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Old 1st Oct 2021, 7:31 am   #5
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Default Re: Singer 2502C pedal repair.

Hi, the bt134 tests fine,however turning the vr fully in one direction seems to nearly sort it,it now ramps up but is still rather fast reatching max rpm bit is now usable.
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Old 14th Oct 2021, 7:29 am   #6
M3VUV51
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Default Re: Singer 2502C pedal repair.

well pulled this apart again ,the pedal controlled a wiper on a carbon track on the pcb, i put a 300k resistor in series with it, the machine speed is nice and slow now with a slight push of the pedal, only thing I notice is the motor makes a growling noise at slow speed and gets hot after a couple of mins, the cooling fan is on the motor shaft so assume it's getting hot due to the low motor rpm, I've fitted a cooling fan from an old ps2 to the side of the motor housing, running from a smps fitted into the base of the machine, any idea as to the growling noise? i assume the motor is dc? thanks in advance.
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Old 18th Oct 2021, 6:33 am   #7
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Default Re: Singer 2502C pedal repair.

The motor is AC and you have a light dimmer circuit ib series with it.
The triac fires sooner or later on the AC cycles, later when the pot is higher resistance and sooner when the pot is lower resistance.
It will heat up when it receives a late phase power on, and as the speed is low, it will not cool properly;
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