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Old 22nd April 2009, 02:25 AM   #1
tbrooke is offline tbrooke  United States
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Default Total newbie troubleshoot

First chip Amp 250 VA transformer Peter Daniel LM3875

Checked DC on the PG- V- PG+ V+ on amp boards and it was about 36 volts except for one side of one board that was about 1 volt - must be poor connection on one but I am thinking 36 is too much. but a 25 volt transformer would be about 35 volts rectified.

The heat sinks are way too hot to touch, transformer seems to make a faint humming sound for a while then stops .

2 amp fuse doing fine

Not sure where to start with trouble shooting.

I did actually connect a speaker (very old) for a bit and got a very distorted noise for a bit. I've gone through the builders thread a few times and double checked things

Any suggestions

Tom
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Old 22nd April 2009, 06:29 AM   #2
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36 V is within the tolerances of mains voltage fluctuations and transformer regulation, so it is, what you can expect of a 25 V transformer. Too high for 4 Ohm speakers, but okay with 8 Ohm and adequate heatsinking.

Did you disconnect the transformer and check the voltage of the secondaries? If that is okay, check the PSU for
- shorts from soldering.
- diode orientation.
- capacitor orientation.
If everything is correct, check the output voltage of the PSU with the amp disconnected.

Then check the amp PCB for
- shorts from soldering.
- capacitor orientation.

Use a light bulb tester. And you can always ask Peter Daniel for help. He has a permanent thread on the Forum and an e-mail button on his homepage.

And post photos for improved troubleshooting.
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