- Joined May 20, 2021
Good day bossHave you checked if there is any DC supply to the electronics ? At a very wild guess I suspect that AC from the transformer comes in to the 2 pin connector at the bottom left of the board and is rectified by the 4 diodes just above it. I also suspect that the TO220 device is a voltage regulator to supply the electronics. I think checking that area would be a good place to start. Also you could check the three vertically mounted resistors that I think may be fusible resistors. If that does not lead you towards possible causes of the fault I think the next step is to trace out the schematic. (Reverse engineer it.) and we will then be able to make better suggestions.
"With the meter set to the 200 volts AC range measure the voltage between the two wires to the two pin connector in the bottom left hand corner of the board." gives me 19.7 ACIf the TO220 device is a transistor rather than a voltage regulator my guess is wrong. Your transistor test is not very helpful as you do not say which meter leads are connected to which transistor lead (Base, emitter or Collector.) and you don't say if it is a PNP or NPN device. (Or give the part number so we can find out from the datasheet.) Your first resistor test looks OK as the colour code indicates it to be a 10 ohm resistor. The second resistor test I am not sure about as I can't tell if the multiplier band is yellow (Indicating a 100 K ohm resistor) or gold (Indicating a 1 ohm resistor.)
The third one I think the resistor is 20 ohms (Red, black, black) but as you have your meter set to the diode / continuity range rather than the correct resistance range I don't know what to expect your meter to display for a 20 ohm resistor.
You should have first taken the voltage readings I requested before removing parts.
In post #7 I had wrongly guessed the level of your electronics knowledge and not given enough detail of exactly what to test. With the meter set to the 200 volts AC range measure the voltage between the two wires to the two pin connector in the bottom left hand corner of the board.
I think there are 4 diodes just below the large electrolytic capacitor in the bottom left of the board (Although I can only see 3. I think a 4th one is hidden by the capacitor.) I assume that thse are connected to form a bridge rectifier to rectify the AC input from the 2 pin connector. I also assume (As I can't see the etch side of the board. ) that the output of the bridge is connected to the large electrolytic capacitor. With your meter set to the 200 volt DC range measure the voltage across the pins of this capacitor. I expect a low DC voltage of probably less than 20 volts that will be regulated further on in the circuit down to 5 volts.
What is the part number of the of the top 14 pin DIL IC. (Your picture is not clear enough to read this.) Also what is the part number of the TO220 device that you say is a transistor ?
If the fault is not a simple power supply fault I think it is beyond my ability to lead you through a more complex fault.
I think the unit works by selecting taps on the transformer winding selected by the relays.
Alright thanksThose voltage readings see about right.
The TO220 device IS a 5 volt voltage regulator (Not a transistor.) You can't test it out of circuit with a meter.
Looking at the regulator from the side where the part number is visible with it's pins at the bottom the left hand pin is the input. The middle pin (And the tab.) are ground. The right hand pin is the output.
I can't find a datasheet for the 14 pin IC so I have no idea what it's function is.
Re check the resistor values using a suitable resistance range on your meter for the marked value of the resistor you are testing.
If you cant read the colour code on the resistors Google "resistor colour codes" to find out how.
With all components back in place.
With your meter set to the 200 volt DC range connect the black lead to the tab of the voltage regulator. With the red meter lead measure the voltage on the input pin. (I expect this to be about 25 volts DC.) Now measure the voltage on the right hand pin. this should be 5 volts. (You can set you meter to the 20 volt range for this test.)
If all the resistors test OK and the voltage readings are correct then I think I have gone as far as I can to help you. You will need to find a local electronics engineer to diagnose the problem.
Yes it hums when ever i pit it onI assume that the 19.7 volts AC to the 2 pin connector comes from a winding on the main transformer. (Unless there is a small one that supplies the logic.) When the unit was working was there the same hum from the transformer ? I think you said the unit was giving an output so this probably comes from a winding on the transformer. As there are 5 relays I assume that one relay selects the input voltage. Another will give the input + 10% (For example) another will give the input +20%, another will give the input - 10% and the last one will give input -20%. The transformer may be an auto transformer with 5 taps or it may be an isolating transformer with two outputs one of 10% of the input and the other of 20% of the input. the relays would then switch to either add or subtract 10% or 20% to the input voltage. You could measure the no load input current of this unit and compare it with that of a working unit of the same model.
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