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Exercise arm bike power board repair

drjsp

Nov 22, 2023
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First off, I'm a complete novice when it comes to repairing circuit boards, checking parts, etc.. I took electronics in high school in the 70s...so...it's been a while.

In any case I have a Scifit Pro 1000 upper body exercise bike that went "pop!" and lost all the power a few weeks ago. I've isolated the problem to the power board. Power comes from the internal generator that spins when one starts pedaling, goes to the power board and out to the electronic brake, that provides variable effort resistance to the flywheel.

Voltage is being generated by the generator but there is no voltage exiting the board going to the brake.
There are no obviously damaged components; capacitors show no obvious damage nor are there any visible burned circuits or components.


It's an old model and replacement boards are unavailable and I have been unable to find a schematic online.
I do have pix of the board and have drawn a rudimentary schematic of part of the circuit. It's one of those boards where some of the connections are sandwiched between the layers and difficult to tell where all the components connect.

I've checked out a couple of the components but have been unable to isolate the bad component(s). Part of the issue is that I'm not sure how to test transistors and need basic, detailed help on how to do that. I can remove components from the board and resolder them back on with no issues.
If I can't fix the board, which I'm willing to keep trying to do, I thought if I could find a power source from which I could provide a variable voltage to the electronic brake, the bike would still be functional even though it wouldn't have the various programs that are in the display board. BTW, using an external 12v power source, the display board works fine but the problem is that the power board is not providing power to the brake despite being apparently told to do so by the control board.

So...where to start?
 

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drjsp

Nov 22, 2023
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My rudimentary schematic.
 

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Delta Prime

Jul 29, 2020
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Power comes from the internal generator that spins when one starts pedaling,
Are you sure? Don't you pedal with your feet?
Anyway I hope this helps.
 

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drjsp

Nov 22, 2023
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Are you sure? Don't you pedal with your feet?
Anyway I hope this helps.
I've got this and it's unfortunately not the schematic for my model. Might be a later or earlier modification. Mine is powered internally by a generator and has no fuses; the one in the service manual is 110v plug in.
Yeah, odd way of saying it, but they say "pedal" with your arms. Might depend on how flexible one is? :)
 

Bluejets

Oct 5, 2014
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Would be unusual to say the least for any pcb to have gone "pop" and then have zero visual effect of the "pop".
Did you do any test for power to the board, then the rectifier, or even the electrolytic cap?

A fuse as such would not necessarily be obvious to the uninitiated, especially on pcb's and again would be unusual not to be included.
Photos are too small to see detail.
 
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Delta Prime

Jul 29, 2020
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You going to have to determine what type of transistors they are.
In your case I would test out of circuit. As you stated previously no obvious damage to components.
Check the diodes they can fail short as well as open.
Did you take a look at the generator? Sometimes they include safety measures inside the housing you have to look real hard they're embedded in the center of the windings of the coil. If you heard a pop I'm thinking mechanical perhaps…
Is it possible that you're too strong now? And you broke it?
And welcome to maker Pro.
:)

 
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drjsp

Nov 22, 2023
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So there is power to the board from the generator but there's no power output to the brake. Additionally, there's a 12v battery input to the board which seems to power the display/control board, as well as an external 12v supply for the same circuit. The battery is ancient, so I removed it and hooked up the 12v. power supply, which powered up the display and control board, but still no voltage to the brake.

I did test remove and test two of the transistors, the IRFP 360 and the HFA 50PA60C Here's the results:

Multimeter in diode test mode

1) HFA50PA60C: Red lead on left terminal. Reads 437 with black on center terminal. No reading with black on right terminal.

Red lead on center terminal. No reading with black on right or left terminal
Red lead on left terminal. Reads 437 with black lead on center terminal. No reading with black on right terminal

2) IRFP360: RED lead on left terminal. No reading with Black lead on center or right terminal
Red lead on center terminal. No reading with black on left or right terminals
Red lead on right terminal. No reading with black on left terminal. Reads 594 with black on center terminal.


I put the + from the 9v battery to the G terminal and the - from the battery to the S terminal on the IFRP360.

I put the DMM on diode setting, put the red probe on the D and the black probe on the S and got 000 with the beeping sound. With the 9v leads removed it shows 000 and beeps. If I then ground the 3 leads together and put the DMM red to D and black to S it shows 1 with no beeping sound.

3) Also tested the large blue capacitor, rated 580 uF, measured on meter as 475 uF.

I'm not 100% as to the interpretation of these results, but I think the IFRP360 and HFA50PA60C are good, but I'm concerned that the capacitor may not be. I think that I also need to check out the IRF 744 transistor, yes?
 

kellys_eye

Jun 25, 2010
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Item (1) is a fast recovery diode (worthwhile downloading the datasheets of those devices just to see what's what). The diode readings you see on your meter are the forward voltage drops of the two diodes within that package and '437' is actually 0.437V and indicates GOOD diodes - nothing wrong there.

Item (2) is an N-channel MOSFET and sometimes difficult to test properly (unless it's gone 'short') as you can never tell if your meter has enough volts on the test leads to make it conduct. This particular device needs 2-4V on the gate pin to conduct. The 594 reading is, once again, actually 0.594V and the forward volt reading of the built-in protection diode across the source/drain connections - again, get the datasheet, it tells it all. The fact that the diode measures ok is good and that it doesn't show a short on any tests is also good but you can't properly test it without activating the gate circuit in its proscribed manner. Worth getting a $20 Chinese component tester as they can show all the spec you ever need and will serve you well in the future - just an idea......

Item (3) is fine. Electrolytic capacitors have a notoriously wide tolerance range (+10% to -25% in some cases) but that value is 'good'.

You can keep going, testing 'all and sundry' and you will eventually get lucky (possibly) but powering it up and doing some voltage checks whilst it's 'working' might cut a few corners.
 

drjsp

Nov 22, 2023
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Item (1) is a fast recovery diode (worthwhile downloading the datasheets of those devices just to see what's what). The diode readings you see on your meter are the forward voltage drops of the two diodes within that package and '437' is actually 0.437V and indicates GOOD diodes - nothing wrong there.

Item (2) is an N-channel MOSFET and sometimes difficult to test properly (unless it's gone 'short') as you can never tell if your meter has enough volts on the test leads to make it conduct. This particular device needs 2-4V on the gate pin to conduct. The 594 reading is, once again, actually 0.594V and the forward volt reading of the built-in protection diode across the source/drain connections - again, get the datasheet, it tells it all. The fact that the diode measures ok is good and that it doesn't show a short on any tests is also good but you can't properly test it without activating the gate circuit in its proscribed manner. Worth getting a $20 Chinese component tester as they can show all the spec you ever need and will serve you well in the future - just an idea......

Item (3) is fine. Electrolytic capacitors have a notoriously wide tolerance range (+10% to -25% in some cases) but that value is 'good'.

You can keep going, testing 'all and sundry' and you will eventually get lucky (possibly) but powering it up and doing some voltage checks whilst it's 'working' might cut a few corners.
Thanks. For a few bucks it might be worth it to replace item 2.

Any advice how to test the IRF 744?
 

Bluejets

Oct 5, 2014
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Google broken I take it..?
A 2 seconds search reveals a P channel mosfet...further, invest in a $20 component tester as referred above or....Google again...
 

drjsp

Nov 22, 2023
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Google broken I take it..?
A 2 seconds search reveals a P channel mosfet...further, invest in a $20 component tester as referred above or....Google again...
Google works fine, I have no idea what a P channel mosfet is. Remember, I'm a noob at this as when I took electronics it was when transistors were just becoming a thing. I thought it a better idea to get information from you guys, who know what you're doing, rather than try to figure it out myself. And...this being Thanksgiving...I really am thankful for the assistance.
 

Bluejets

Oct 5, 2014
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To quote many who have come before, what's the fun in being told everything on a basic level and reinventing the wheel.
Get off your lazy butt and find out some details for yourself, you just may learn something.
Gazillions of tutorials and info out there on the web.........In our day, we had to (at best) go to a library and try to find what we needed in 20 year old books.
 

drjsp

Nov 22, 2023
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Wow, Bluejets.
I'm not interested in learning all about electronic board repair, I simply want to try to fix my broken board.
Since you know so much about the internet, you'll note that there is contradictory information out there (which I've run across quite frequently in my internet searches prior to joining this forum) and having taken electronics pre-transistors (Maybe you missed the first sentence in my first post?), I don't know what I don't know. I've found no less than four different contradictory ways to test one of the transistors.

So, if you don't feel like helping, that's fine. I came to this forum to get some help in fixing my board after spending countless hours on the internet trying to figure it out for myself. I'm not sure why you would feel compelled to suggest that I'm lazy.
 
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Martaine2005

May 12, 2015
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This link is by far the easiest method.
Do a WWW for your mosfet and get the ‘gate, drain and source’ configuration first.


Martin
 

drjsp

Nov 22, 2023
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drjsp

Nov 22, 2023
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This link is by far the easiest method.
Do a WWW for your mosfet and get the ‘gate, drain and source’ configuration first.


Martin
Right...all the transistors that I could safely get off the board checked out OK. That was very helpful, thanks.
 

drjsp

Nov 22, 2023
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OK, this is odd, can someone explain?
I removed and tested the IRF 744 and it tested out fine. I also removed another large capacitor and checked it, and it checked out fine.
Then I replaced the components back on to the board:
2 Capacitors and 3 transistors
...and now I'm getting voltage to the brake. It *seems* to be increasing the voltage to the brake when I call for higher resistance on the display panel, but I'm not 100% sure of that. I'll have someone else see if they notice an increased resistance because mine might be wishful thinking.

So, two questions:
1) Bad connection that I inadvertently repaired when removing and reinstalling components?
2) Would it be reasonable to assume one of the components I removed for testing then reinstalled is faulty? I don't mind replacing all of them, the cost would be inexpensive.

Other thoughts?
 

kellys_eye

Jun 25, 2010
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1) Bad connection that I inadvertently repaired when removing and reinstalling components?
Quite possible. These effects are known as 'dry joints' and commonly caused by the component moving during the soldering process and before the solder has properly set. Other issues stem from the poor soldering of off-board connectors (sometimes added after all the main components have been flow soldered and done in a manual process). Even physical stress on the off board connector can work them loose.
 

Martaine2005

May 12, 2015
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Exactly as above.
A dry solder joint could have been the only issue.
On older Sony TVs, the only advice is to reflow every solder joint, they were notorious.

Martin
 
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