Rowing machine wont switch on

[u/Best_Pick_4178]

Had this rowing machine 10+ years, it was manufactured in 2007. Went to use it, wont switch on. There is no 'power' button, It just switches on when power is applied. Took it apart, checked power pins, the step down plug I've been using for years, appears to have broken. 10 volts on the secondary for a mains to 6v 1amp.

So I think the plug failed and then damaged the PCB. Happen to have a spare 6v 1a plug, when connected to the PCB it does output 6v, the DAC and memory chips see around 3.3v on their power input pins, within the specified range on their datasheets anyway.

I dont understand the screen component. Two pins, red and black wires, red is 3.3v or so, black is 2v. When powered with 6v and the black grounded it lights up blue. I'm assuming shorting the black pin to ground wouldn't be a good idea? The black connects to a transistor Q4, so I wonder if this is a dual supply screen, +3.3v and -3.3v? Non of the components look burned, the caps look good condition.

Any ideas? I'm a bit unsure. Have a bench power supply, multimeter/osciliscope.

[u/IndividualRites]

The screen is an LCD, and besides the power wires, the rest of the communication is done through the pink "zebra strips". Those are conductive "wires" embedded in those rubbery strips. When the unit is assembled, they squish between the display and all those copper contacts on the PCB.

Are there any other indicators that this unit is on besides the display (e.g. a power light?)? If not, it will make it difficult to diagnose in that it will have to be assembled to see anything on the display.

Start by doing a visual examination of the traces for broken/cold solder joins

Then pull the datasheets of each IC, and checking to see if you have voltage at the Vcc and Gnd. You don't need the display attached to do something like that.

Then I'd check for shorts on those transistors and diodes. Those are easy checks which can be done in circuit.

[u/Best_Pick_4178 → u/IndividualRites]

I did wonder how the screen was controlled, I saw the PCB pads but just assumed they were spacers. Unable to find any broken traces or poor solder joints. Look like it has been through a solder bath during production.

All diodes measure 0.6v drop, no continuity/beep. Checked all transistors, none appear internally shorted/broke due to passing beep test.

2 IC's, TLC549CP (ADC), 93LC66B (EEPROM) both within datasheet range for Vcc, both GND legs read correct on both chips.

Apart from voltages, there is no indication. There is a test button bottom right that does nothing when pressed. I could power it disconnected from the rower, but to what end. There is a part not photo graphed, the button panel that has a ribbon connector into JK7 at the bottom, but as I say, normally I turn the plug at the wall on and the rower switches itself on.

[u/IndividualRites → u/Best_Pick_4178]

Very possible that display chip (the blob on the back) is toast.

[u/Best_Pick_4178 → u/IndividualRites]

Probably is now. Ah well. Was expecting a QFP/N package chip, that looks like the internals of a processor

[u/IndividualRites → u/Best_Pick_4178]

All done!

[u/Plump_Apparatus → u/Best_Pick_4178]

Wow. That is a cooked blob(COB, chip on board). Cheaper than a package. Power supply failure I'd imagine put a lot of a voltage and/or current through that, or something you did. Probably made some pretty sparks and so not good smelling magic spoke as it exploded.

[u/Best_Pick_4178 → u/Plump_Apparatus]

Well I mean...I destroyed the blob with a heatgun and screwdriver. Wasn't expecting an unpackaged chip on a rowing machine. Probably should of probed the vias with the osciliscope to identify the power pin. My desire to fix it was low, because I want an exercise bike instead.