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MP566 Tube microphone preamplifier / Re: MP566 fault troubleshooting help needed.

« on: September 15, 2025, 09:01:37 PM »

Thanks for the fast reply! I followed your instructions, and I can measure 12V between V- and TP8 and 225V between 0V and TP2 on both units. 
I will replace the U4 on both of them and let you know if they work fine now.
They both failed at the same time. I think it was a problem related to the 500 Rack PSU where  I had mounted them. One voltage rail was having problems.
I do not think they overheat. In your experience, what is the most common failure cause of the L272?

Cheers. 
Jorge

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MP566 Tube microphone preamplifier / MP566 fault troubleshooting help needed.

« on: September 12, 2025, 02:01:03 PM »

Hi, I have a pair of MP566 that I have used for some years, suddenly stopped working. I have followed the guides and read the forum for clues or maybe solutions. 
from the test guide
1. There is no short circuit on v+ or v- and GND
2. I get 0Vdc  when testing the High voltage between TP2 and 0V
3. On 1 unit the v+ is 15.9V and the V- is around+7V, on the second unit  the v+ is -6.5V and V- is-16V. 
4. The current consumption on the rails is 400mA on one and 30 mA on the other. 
5. I read in one of the threads to remove U1 U4 and the Tube   to check if the V+ rail and V- rail would get +16 V and -16V respectively.  I removed U1, U4 and the vacum tube on both units and  I do get +16V on V+ and -16V on V- . 
6. I am looking for the next steps to follow since on the other threads there was another issue. Regarding a reversed diode.

Thank you! Please let me know what other tests should I do. I am suspecting that either U1 or U4 are damaged, but how to test that?. The revision of the board I have is 1.5
BR Jorge