Upper body exercise bike power board bypass

Hello All, first post for me.

I have a used Scifit Pro 1000 upper body exercise bike that pooched the other day. It's old enough that there are no replacement electronics available, new or used. As well, I have been unable to find a schematic for that particular model.

Long story short, it's worked fine until the other day when I was using it and heard a pop and the electronics went dead. Because the resistance is provided by an electromagnetic brake, the bike has been rendered all but useless.

It is powered by an internal AC generator which provides power to the power board and the power board provides power to the display and the resistance brake. I assume that the control board controls the amount of voltage provided to the resistance brake to allow for varying levels of arm crank resistance. It was used when I bought it and did not come with the 12v power supply so I rigged up a spare one I had lying around and the display now works. (It worked fine without the external 12v power before it pooched the other day). However, there is no voltage going to the resistance brake so I'm pretty sure the power board is pooched.

I have carefully examined both the power board and the display board and there is no obvious evidence of a component blowing or being cooked. The AC generator does provide voltage to the power board when the arm crank is spun but again, no voltage is going to the brake when the arm crank is spun.

So, I was thinking if it was possible to supply a variable voltage to the brake, the unit would still be functional even though the control panel would be useless. I'm not sure if the brake is AC or DC or what voltage or amperage ranges the brake operates. The brake has two red wires that go to the power board.

Thoughts? Questions?

Things that make noise are electrolytic capacitors (especially when they see reversed voltage or over voltage), fuses, larger transistors or diodes if they rapidly see over current (less loud if the slowly heat up because the plastic is too hot and soft to shatter loudly in most cases).

Check the components I mentioned - remove one lead of the cap to measure capacitance.

Apologies, my inserting skills are limited. The only boards I've repaired are those with obvious blown components or fried connections.

Any of these jump out at you as reasonable suspects? See also previous pic.

Also, not sure how to measure capacitance. Would that be the diode test on my multimeter?

Can you add some higher resolution photos of the board topside, so all the components can be seen in detail?

What's the grey looking thing just left of the two white connectors at the top of the PCB?

Which connector goes where? I'm guessing the lower left one is from the generator and lower near centre is to the brake?

The first things to test are the six diodes just below the large blue capacitor - if they fail, there will be no power to the board.

I'll add some more pix in the next day or two. Not sure what the grey thing is.
Connectors:
1) on the previous pix, on the top, one goes to the 12v battery and the other to the 12v external power source. I have disconnected the battery as it is quite old and isn't putting out 12v and that it's now redundant due to the external 12v source.
2) Bottom of the pix with wire attached, connector goes to the electronic resistance brake
3) Lower left 3 pin connector goes to the generator

At least part of the board is getting power through the external 12v supply.

More pix

These are the best pix I can get with my phone.

DRJP said:

These are the best pix I can get with my phone.

Click to expand...

You'd be amazed how much better you can do if you set the phone so it's hanging over the edge of a small box and use the time-delayed shutter so the phone is perfectly still when the picture is taken.

Looks like the grey thing is a thermistor? That's right next to and connected to the Brake leads. Could I get lucky and this be the only thing pooched?

Also not sure how to test a thermistor.

Is it connected in series with the brake, or in parallel? The PCB underside photo is not clear enough to see the PCB tracks.

A thermistor should read as a resistor. If it's a VDR it would be connected across the brake coil to limit voltage spikes.

Does the large cap actually charge from the generator when you run it? That's presumably where the main power comes from, even if it has an auxiliary supply for other electronics?

DRJP said:

Looks like the grey thing is a thermistor? That's right next to and connected to the Brake leads. Could I get lucky and this be the only thing pooched?

Also not sure how to test a thermistor.

Click to expand...

Generally thermistors are visually damaged if they fail - bits broken off, or obvious cracks.

I've replaced many hundreds (if not thousands) of thermistors - deguass thermistors in colour TV's, or power-on surge limiters. I don't recall ever replacing one that wasn't visually faulty?.

But to test it, simply click a multimeter on ohms across it, it should read as a resistor - then with the meter still connected, warm it up with a hair drier - the resistance should increase or decrease, dependent on type.

rjenkinsgb said:

Is it connected in series with the brake, or in parallel? The PCB underside photo is not clear enough to see the PCB tracks.

A thermistor should read as a resistor. If it's a VDR it would be connected across the brake coil to limit voltage spikes.

Does the large cap actually charge from the generator when you run it? That's presumably where the main power comes from, even if it has an auxiliary supply for other electronics?

Click to expand...

Yes, the generator provides all the power for the unit when not attached to the 12v external power supply. In fact, when I bought it used a few years ago, when I got it home I though it would need an external power supply, but the generator powered everything. The 12v external supply does currently work to power the control board without the generator running, even though the brake is not working.

So I'm suspicious of the power circuit somewhere between the generator power input and the brake output, although I suppose that there's somewhere where the power is controlled to the brake output that's failed.

I'll hopefully start testing components later today.

What's the best way to check if the large cap is charging...can I test it while it's still on the board?

rjenkinsgb said:

Is it connected in series with the brake, or in parallel?

Click to expand...

It appears that the thermistor is connected in series between the large white resistor and the output to the brake.

DRJP said:

What's the best way to check if the large cap is charging...can I test it while it's still on the board?

Click to expand...

That's what I was hoping for, just see if it develops a reasonable voltage across the cap when the generator is spinning?

One of the first things I suggested was to check the diodes that rectify the generator output.