oscillating spindle sander help?

The brushes will be under the black plastic cover, each held on by two phillips screws in your picture (below)

Visible is one wire from the PCB , going under the cover to the brush terminal. That is where you need to make contact with the probe.

You said the brushes 'look' fine - are they clearly visible - I can't see from your photos.

In essence we need to establish there is continuity from the mains input all the way through to the brushes.

Maybe my texts are too long and I lose you along the way..

Try this first and then we can return to try the measurements I already indicated, if needed.:

Leave the 'paperclips' in place. With the plug removed from the mains , set your meter to 'diode' measurement mode, as you did before.

Connect one probe to the plug live pin, and one to the neutral pin. With no open circuit you would see between 1.2v and 2volts or so.

That would mean the path all the way, through the cables, diode bridge, inductors AND motor brushes and windings is OK.

If you read open circuit, then there is ...an open circuit. NOW - turn the motor rotor a few turns by hand, and measure again. If you get a reading, then the brushes are not making good contact. If no reading then we need to get to the motor brush connections, under those plastic covers.

Joe

DID you read that i got -250v DC  when i measured between the two toroids as you suggested??

I get 0.7 V between the live and neutral plug pins.

Edited By mark smith 20 on 28/05/2020 15:26:03

Edited By mark smith 20 on 28/05/2020 15:41:18

Now who's not reading...!!! Sorry...I did not see that you got 250VDC at those two points! The comments went onto a new page and I missed that.

It is sort of good news, but with some confusion...the points across which you measure the 250V DC ( GOOD NEWS) should be the same point that the two motor brush wires are connected to.

Attached a cct as I understand of your PCB ( I have omitted the two black capacitors acros the input and output of the large inductor)

At each of the points marked A-A you should measure the '250V-DC'. If it is present at the two black wires on the toroids, then it must be present at the start of the wires ( on the PCB) going to the motor brushes.

Measure across the toroid inputs and then the toroid outputs to be sure. If you measure 70V at the actual motor wire ends on the pcb , then there is a break in the PCB track, or a wire solder joint there is bad.

Between the two actual motor carbon brushes measured removed from the motor im getting no DC but 106V AC

joseph its fixed i think I reflowed all the solder joints on the pcb and `vroom` it started , dont know if it was a bad solder joint or brushes badly seated , i suspect the pcb.

Thankyou for you help

Edited By mark smith 20 on 28/05/2020 16:51:58

I thought it had to be a bad joint on the PCB..Thats great! Happy Sanding.

Joe

Thanks.

I dont understand these bad/or dry joints though, how does it suddenly happen when it was working fine. I can understand vibration loosening things but not dry joints or whatever.

A dry joint is a soldered joint that has not properly flowed between the two parts. Typically you get the solder on the wire, but not the board, essentially the two are touching, not soldered. The joint seems firm as the resin flux acts as a glue.

Sometimes bad joints are duller where the rest are shiny or it's more of a blob than a pyramid.

Dave thanks for the explanation, i couldnt picture what was meant by dry joint.

Dry joints happen when the solder fails to "wet" the two parts of the joint. They are held mechanically in contact, but can fail electrically at any time. It seems to happen more with the new lead-free solder, than the old 60/40 lead -tin solder I used most of my working life. Such is progress.

Sometimes more that that - the joint may start out perfectly good, ie, a well made, properly flowed joint. But a lot depends of the actually mechanics of the parts soldered together. For example, a double sided through-hole plated connection of the PCB, with a snugly fitting wire in the hole, properly soldered will have a miniscule chance of failing under tension or vibration from the motor, etc.

However, that PCB is single sided it seems, and a single sided joint, with heavy wires soldered whilst inserted into a hole notably larger than the wire, relies on both electrical connection, and mechanical strength ONLY via the solder, a soft tin/lead combo ( yes, that one still has lots of lead..). all the stress is on the solder itself, and with vibration it fractures and the inter-joint becomes crystalline and either open circuit or high(er) resistance. A further cause is when the perfect joint is made during soldering, but the technician moves the wire , even a little, before the solder has properly solidified - the joint may look good, but internals it is already crystalline and waiting to fail.

That is why, on high reliability requirements for wire connections, the connections are crimped and not soldered..

In the High-Rel world, you will never find wires directly inserted into a hole and soldered - if not crimped, the hole will have a termination pin or pillar inserted and soldered, and the wire wrapped around the pillar and then soldered. Mechanical stability or strength should NEVER rely on the solder itself.

Joe