Hello, folks. I need your help in repairing a faulty motherboard from a PFAFF 1471 sewing machine, circa 1986. The basic problem is no power. I tested the 120 volt supply lines all the way from the power cord through the housing connectors and into circuitboard. I got good continuity and voltage throughout and at least into the transformer. Hard for me to test further than that. I suspect one of three things:

1. The three 240 volt .630mA fuses are blown. They are measuring about 12M, 6M, and 6M ohms respectively. Aren't fuses supposed to have full continuity unless blown and thereby open? Or do thermal fuses behave differently? I realize the fuses could be blown but the cause for why this happened is more important and would be the bigger concern. I just need to understand how the thermal fuses work.

2. There are two identical components next to the sealed and soldered transformer. I suspect this to be voltage regulators and I see very crystally yellowish crud where it meets the circuitboard. In case I can't attach some pictures, it's flat black ceramic type with four prongs. It measures about 5/8" by 3/4" by 3/8"". The numbers on it are G1 KBL01 8615. I don't have a schematic and am trying to identify and obtain a replacement. I'll contact a sewing repair place and PFFAF soon to see if they can help me. Any ideas on where else to obtain this part or the entire motherboard? How can I troubleshoot this part?

3. The transformer is dead, in which case I'm dead in the water, too.

I'll try to upload some photos. I didn't have any luck in my last posting but I'll try...

 

 

 

Your fuses are definitely blown, the other objects are most likely bridge rectifiers.

 

Your board has significant environmental damage; it was in a flood or got peed on or the like. It will be difficult to make it reliable even if you can repair it since the corrosion creates low impedance current paths under components where you can't see them and can cause board layers to partially delaminate, breaking vias.

KBL01 is a 4A bridge rectifier.

 

Your fuses are definitely blown, the other objects are most likely bridge rectifiers.

Thanks! I didn't know they are call rectifiers. I had someone test the rectifiers and they were fine. I bought some new thermal fuses (actually soldered holders in so I can pop on and off the new ones easier). The sewing machine display started up! But it was much brighter than normal, and the motor started on its own. After about 30 seconds, no power again--motor stopped, no display. So I tested all the new thermal fuses and two were blown; one was still OK (show full continuity--I answered my own question).

I haven't had time but will troubleshoot soon...I plan to keep all harnesses off (see photos), pop in new fuses, then power it and see if the fuses blow again. If not, it tells me the mother board is fine and the problem is "up the line" where the harnesses go---towards the digital display circuit board/motor."

Any other thoughts given the above?

KJ6EAD: Thanks for the reply. No corrosion...I live in a desert. But your point is well taken re: ".....
since the corrosion creates low impedance current paths under components where you can't see them and can cause board layers to partially delaminate, breaking vias." So thanks for that. I hope it's not the case and what I'm suspecting. I'll post back within a week or so.

 

Amazing how they way over complicated a simple machine over the last 40 years.