My fp 3 quit in 1 channel

[najo49]

My pre amp quit suddenly in one channel.the middle board and the right have all LEDs brightly working in. The left board the top LEDs are dim and the bottoms are out.the voltage into the middle board are 200 v on both sides. The voltages at 12 is 54 instead of 150.  And at 15 28 v instead of 75. I think the problem is in the shunt reg. board.as the out to the left board is where the low v. Shows up. Need suggestions . Thanks in advance. Jann olsen

 

[najo49]

Ps ihave swapped the tubes. All voltages at 7,9,10 are good. Remember this pre a has been working for years. J.

 

[Paul Birkeland]

What are the "I", "O", and "Kreg" voltages on each side of the center PC board?

 

[najo49]

Paul. On the SR board left side I is 205 v. O is 55 v. And k is 4.14V. On the right side I is 197V. O is 150V.and K is 100.1V.  J.

 

[najo49]

THat should be 10.0V in Kreg on the right side..

 

[Paul Birkeland]

On the center board, do you have MJE5731A's, or MJE350's?

On the far left PC board, can you pull the red wire to "I" on the "A" side, then recheck the center board?

 

[najo49]

Paul, I have me 350s. With the wire pulled I got, I 206. O is now 155 as it is suppose to be. K is still 4.1 v.  .Jm

 

[Paul Birkeland]

OK, so with no load, you have 155 volts at the plate, 16mA going through the 12AU7, and 4.1V on the cathode.  Unfortunately, this is way off the actual 12AU7 operating parameters.  (155V on the plate and 4.1V on the cathode means ~6mA of current is going through the tube, or there is some other issue)

The next step is to swap in a different 12AU7 and confirm that the problem persists.  Beyond this, I would buy a pair of MJE-5731A's and a sprung solder sucker, and replace the MJE-350's. (The 5731A's face the opposite direction)

-PB

 

[najo49]

Paul, you are talking about the mob 350s on the left board right? I assume the test of lifting the red wire from the left board meant the middle board was ok. I spent some time checking solder joints and resold erring some. When I tried it again the LEDs on the A. Top end glowed brightly and the voltage at term. 12 went up from 54 v to 150 just as it should be. But alas the volt,age to O went down to just a few volts. The b end still has all LEDs out.