| ben04srt |
I am confused on this one , it powers on and off while making a humming noise.
replaced power supply fets and thats as far as I got.
would shorted outputs cause this ?
Thanks
Ben |
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| Perry Babin |
If you're using a relatively small 12v power supply (or a battery charger) and there is excessive current draw, the hum may be 60 cycle hum (ripple from the 12v power supply).
With the transistors mounted tightly to the heatsink, measure the voltage across the emitter resistors in the audio section. You should read ~0.00 volts across them. If one resistor (or one group of resistors) reads higher than the rest, that will help you narrow down the channel that's causing the problem.
I'll assume that you checked all of the output transistors to make sure none were shorted. Check all possible combinations (legs 1-2, 1-3, 2-3). On SS amps, you must check every individual output transistor. Also check every emitter resistor to see if any are open. |
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| ben04srt |
| where are the emitter resistors |
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| Perry Babin |
The emitter resistors are the resistors between the large diodes in the audio section. This amp appears to have 1 ohm resistors in parallel with diodes. It's unlikely that any are open but you can check them anyway.
Check for voltage drop across the emitter resistors (as described above). |
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| ben04srt |
I am still new to repairing amps , but do you test the tip 102 and tip 107 the same or do you switch the leads on the dmm.
IF you test them the same way , than I think all 24 outputs are bad.
Thanks
Ben |
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| Perry Babin |
The darlingtons are a bit different than regular transistors but you should not read anything near zero ohms between any of the legs of the transistors. While in the board, having only 4 shorted transistors can make it appear that all are shorted.
That amp should use relatively high value emitter resistors (1 ohm I think). so you should be able to pick the shorted transistors out from the rest of the group. Find the ones that have the lowest resistance. The difference may be less than 2 ohms so you'll have to use ohms instead of diode check on your meter. |
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| ben04srt |
I am testing them off the board .
When I test them red lead on 1st leg black to 2nd or 3rd should the reading be .5 to .7 on diode check or am I doing it all wrong.
This amp was ran at 1 ohm mono for a few years an got so hot it unsoldered some parts from the board .
Ben |
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| Perry Babin |
| See attached file. Your readings will vary slightly. |
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| ben04srt |
I found 6 bad outputs and replaced them , but it still powers on for a few seconds then turns off then on and keeps shutting off.
Ben |
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| Perry Babin |
Check the drivers transistors on the driver board for the channel where you found defective outputs. If you're uncertain about the readings, compare the resistances from one board to the other.
As you can see, the amp has 4 rows of outputs. If any single output in any row has failed, ALL in the row must be replaced with matching transistors (being from the same batch will generally provide sufficiently matched parts). |
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| ben04srt |
Found out my multimeter was dying and giving the wrong readings. My power supply died also .
I did find more bad out puts though.
So I am going to replace all 24 just to be sure.
Ben |
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