| Electronics > Beginners |
| Beginners Road To Repair |
| << < (4/11) > >> |
| BrianHG:
It sounds like his input power rectifier diodes are blown open, or, his AC power switch is open, or, the +12v regulator is shorted to GND, or the power input filter caps are blown to a short (this last one would be unusual without smoke and residue coming out of one of those 21 electrolytic caps). |
| don@cascadefellowship.org:
--- Quote from: Dubbie on June 26, 2018, 02:24:35 am ---Ok, so now there are a couple of possibilities, there could be a fault with the power regulators or their supply. OR there could be a short somewhere in the rest of the circuit that is pulling the voltage right down. If would be good if you could identify a way to disconnect the power supply from the rest of the board. That will allow you to see where the problem is. --- End quote --- Can I do that by unplugging the power supply? It's a barrel connector on the small pcb board. I've also just removed the large pcb board from the small board by unplugging the ribbon cables. Testing the regulators this way yields 3.4 volts on input and .08 volts on output of the 7812 and 17.9 volts on input and 5.3 volts on output. These seem identical to the readings I got when both pcb boards were connected. |
| don@cascadefellowship.org:
--- Quote from: StillTrying on June 26, 2018, 11:20:27 am --- --- Quote from: don@cascadefellowship.org on June 26, 2018, 02:06:10 am ---Ok. So both regulators seem to be input, ground, output (with pins down from left to right). While testing the 7812 with my DMM (black lead on ground) I'm getting 3.4 volts on input and .08 volts on output. On the 7912 I'm getting 16.9 volts on input and 5 volts on output. However, this pinout doesn't jive with the 7912 data sheet which shows ground, input, output (again pins down from left to right). I'm not sure which model the regulator without the printing is. --- End quote --- Some of your voltage readings seem like you might not have the DMM black on a 0V/gnd. I'd check if the 7812 and 7912 have a pin connected directly together, that will be 0V/gnd and then find which of the 3 pins on the nearby socket is 0V/gnd, and measure ALL voltages from there. --- End quote --- Ah! I think I may have been incorrect about the ground pin on the 7912. With pins facing down the 7912 is ground, input, output from left to right. I was testing that one incorrectly. Both grounds are tied directly together by a trace on the board. If I check both inputs and outputs using either ground I get the same results. Here are the readings from both regulators now tested correctly. 7812 Input 3.4 volts Output .08 volts 7912 Input -17.9 volts Output -11.9 volts |
| don@cascadefellowship.org:
--- Quote from: BrianHG on June 26, 2018, 11:37:08 am ---It sounds like his input power rectifier diodes are blown open, or, his AC power switch is open, or, the +12v regulator is shorted to GND, or the power input filter caps are blown to a short (this last one would be unusual without smoke and residue coming out of one of those 21 electrolytic caps). --- End quote --- I'll start learning how to check all of those components in that order and post my results. I can verify that all the components and the boards do not show and burn marks or blow outs. |
| don@cascadefellowship.org:
--- Quote from: don@cascadefellowship.org on June 26, 2018, 03:45:06 pm --- --- Quote from: BrianHG on June 26, 2018, 11:37:08 am ---It sounds like his input power rectifier diodes are blown open, or, his AC power switch is open, or, the +12v regulator is shorted to GND, or the power input filter caps are blown to a short (this last one would be unusual without smoke and residue coming out of one of those 21 electrolytic caps). --- End quote --- I'll start learning how to check all of those components in that order and post my results. I can verify that all the components and the boards do not show and burn marks or blow outs. --- End quote --- Checking the 3 input diodes with my DMM (with power off using the diode mode) yields between .56 volts and .59 volts for all three. Reversing the leads on each one showed no flow. |
| Navigation |
| Message Index |
| Next page |
| Previous page |