6.9v at Mic input with phantom power off?

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6.9v at Mic input with phantom power off?

Postby Diyer » Fri Oct 09, 2015 5:55 pm

I can't find the mistake. All cp board voltages are correct, and the preamp passes audio and sounds great...exactly like my other A12. But, I measure 47v with phantom power on, which is good, but when I switch it off, it slowly fades down, then slowly come back up to 6.9v. And my condenser Mic will accept this power, so I ultimately have know way of shutting it off. Any thoughts?
Diyer
 
Posts: 9
Joined: Sun Sep 27, 2015 2:09 pm

Re: 6.9v at Mic input with phantom power off?

Postby Diyer » Fri Oct 16, 2015 5:33 am

Fixed. I traced the issue to a possible leaking transistor on the phantom power circuit. I obviously can't swear to exactly where the problem was, but I was able to solve the problem by replacing Q1, the A06 transistor, Q2, the A56 transistor, and reflowing the solder on R35.
When I had the phantom power OFF, I measured about 47V at the collector leg of Q1, which is correct. But I also measured 7.4v at the emitter and base legs of Q1. I was expecting zero. After replacing Q1, (A06), I now read zero at the base and emitter legs, as well as pin 2 and 3 with phantom power off. YEA.
Diyer
 
Posts: 9
Joined: Sun Sep 27, 2015 2:09 pm


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