Trainer PAD-234A constantly lit up LEDs - need troubleshooting assistance

Discussion in 'General Electronics Chat' started by Beave2012, Oct 17, 2015.

  1. Beave2012

    Thread Starter New Member

    Oct 17, 2015
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    Late last year I recieved an Analog/Digital Trainer PAD-234A for a hobby. However with it, every time I power it up LEDs 5 & 4 light up with the power switch, and no other LEDs will light up when power is applied to them. I have tried troubleshooting but, I do not read any voltage off the connectors to those LEDs, and I am at a loss on what is happening.

    I am not borrowing the unit, so I can make repairs to it if I can figure it out. Any idea where to start looking?
     
  2. umphrey

    Member

    Dec 1, 2012
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    Power it on and check all of the power supply voltages against what you expect them to be by looking at the schematic. Verify that you can adjust the ones that you are supposed to be able to adjust. Power it off and check all resistances with a DMM and compare them to what you expect them to be by looking at the schematic.
     
  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Welcome to AAC
    Post some good quality pictures of the unit. PCB board both sides and the unit self
     
  4. dl324

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    Mar 30, 2015
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    Is this the Trainer you're talking about?
    pad234a.jpg
    And are these the LEDs in question?
    pad234a-leds.jpg
     
  5. Beave2012

    Thread Starter New Member

    Oct 17, 2015
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    20151018214752.jpg 20151018214810.jpg

    Exactly the same unit. So here are basic pictures. The 12, - 12, 5, +v, - v, and all hi Lo switches produce correct voltages. No voltages detected from those led holes.

    I am not greatly proficient with electronics yet, this was given to me as my way to learn. So i am not to your level of expertise. Thus why I'm asking for help
     
  6. dl324

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    The manual says that the 8 LEDs are buffered. Connect a wire from one of the switches to LED4 and toggle the switch. When the switch is down, that's 0V and the LED should turn off. Do the same for LED5. If the LEDs will toggle, the problem may be floating inputs on the buffers; don't know, haven't checked the schematics that were in the manual.
     
  7. Beave2012

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    Oct 17, 2015
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    Here are the underside pics, focusing on the led area. 20151018220123.jpg 20151018220154.jpg 20151018220232.jpg 20151018220259.jpg
     
  8. Beave2012

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    Oct 17, 2015
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    None of the leds respond to anything. When connected the toggle does nothing, when I connect it to other LEDs, those LEDs remain off as well. I'm thinking I need to learn what a buffered LED is... lol
     
  9. dl324

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    Below is the schematic for the LEDs. There's a resistor divider on the inputs; I don't know what the wire to the LED is doing, but without it the inputs would be at 0V with nothing connected.
    pad234_leds.jpg

    Someone was nice to you; that's a fairly expensive piece of equipment.
     
  10. dl324

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    Mar 30, 2015
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    Do you have a voltmeter? Could be the inverters for LED 4 and 5 are bad. Connect one of the switches to one of the LEDs. Measure the voltage at the input and output of the appropriate inverter. With 0V in, you should have about 5V out and vice versa. Luckily for you, there appear to be 4 unused inverters on the board. If the chips are in sockets, it would be simpler to replace the bad IC.

    Just electronics-speak for something that doesn't load the whatever is driving the LED.
     
  11. Beave2012

    Thread Starter New Member

    Oct 17, 2015
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    Someone was nice to you; that's a fairly expensive piece of equipment.[/QUOTE]

    I can verify I have 5v going into any of the other LEDs on the line, and they do not light up. I can also verify there is no voltage on inputs as well. However those 2 lights DO light up... Checking from below the actual LED is getting 5v in, and 2.4v after the led. And that is for ALL the other LEDs as well.... from checking the rear solder points anyways. All trace lines lead back to RN1, my guess is thats a buffered resistor or something?

    My father assembled it as part of his engineering degree. I thought he would have a clue where to start with troubleshooting, but he cant remember for the life of him about assembling it. Must have been a class he wasn't too interested in or something. Anyways, Christmas gift for me that I primarily use for the power supply and breadboard. I still have yet to figure out any of the other functions like clock, gen, freq, offset. etc.. However it is getting use at least.
     
  12. dl324

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    RN1 is just 5 isolated resistors in the same package.

    Start by measuring the input and output for the inverter driving LED1. With nothing connected to that LED, the input to the inverter should be 0V and the output about 5V. If the schematic for PAD-234 is the same as yours, the inverter you want to check is U3F; input pin 13 output pin 12. You should also measure the voltage at pin 14.
     
  13. Beave2012

    Thread Starter New Member

    Oct 17, 2015
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    If by inverter you mean the "pulse" ones, I just checked, they go from 5v to 0.24v when toggled, and the inverse on the other row. Same scenario on the other pulse switch. (learned something new about those switches today, thanks)
     
  14. dl324

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    Here's the manual I downloaded:
     
  15. dl324

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    I meant one of the "inverter" logic gates. Four of the LEDs are driven by U3 and four by U4.
     
  16. Beave2012

    Thread Starter New Member

    Oct 17, 2015
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    20151018230218.jpg All the bottom 3 pins out of u3 show virtually no voltage. (i assume that chip u3 was the inverter?)

    However I measured the input block on led0 and found 2.4v.
     
  17. Beave2012

    Thread Starter New Member

    Oct 17, 2015
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    Actually, if i put the negative lead on GROUND, and the positive lead on pin 14 of U3, it does jump around even getting into a full volt or more, i just cant tell how high. **Edit, jumped the gun, tried an analog voltmeter and it shows no fluxuation, the digital did for some reason.

    Pin 7 shows around 3v, pin 8 at 0v, pin 9 at 0, pin 10 at 3v, and pins 11,12,13,14 all at 0v
     
    Last edited: Oct 19, 2015
  18. dl324

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    U3 is a 74HC04 hex inverter. Pin 7 is ground and should be at 0V, pin 14 is Vcc and should be at 5V. The input and output for the inverter driving LED1 are pins 13 and 12, respectively. Since that LED is off, the input should be around 0V and the output should be around 5V.
     
  19. Beave2012

    Thread Starter New Member

    Oct 17, 2015
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    Alright, first I appologize for messing up the pinout, I got that wrong, pulled up the correct pinout and here are the results. PIN 7 is ground at 0V. Pin 14 is at 5v, Pin 13 is at 0V, and Pin 12 is at 3.5v. Does this mean my 74HC04 is bad?

    FYI Pin 1 at 0V, and Pin 2 at 2.5v, the spares seem to jump around irratically between 0-1V on one side and 0v on the other.
     
    Last edited: Oct 19, 2015
  20. dl324

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    Mar 30, 2015
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    The output seems a little low, but the LED is off so it may be okay. Connect one of the toggle switches (the ones with the paddles), apply 5V (switch up), and measure voltages on that inverter again.
     
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