Mainline final volt test fail

[Tbar]

So I have finished my Mainline 1.1 build but appear to have a final volt test issue.  All resistance tests and the early volt test passed without issue.  However, I am now getting wonky readings off the 6.3v pads.  Initially I was getting 8v on the + and 1.4v on the -.  I found a thread here with a similar issue that was resolved by clearing touching contacts on one of the socket pads; so I had a look those and found one spot where there could have possibly been cross contact (but not 100% clear there was - either way I adjusted it).  When I put things back together my problem had reversed!  Now I am getting upwards of 330v on the 6.3 pad.  Resistance check and all other volt checks still pass.  Pulled the tubes . . . same result. 

[Paul Birkeland]

Can you post some build photos? 

You should see about 50V on one of those terminals and 56V or so on the other.  If you are seeing 330V, then you may have your black meter probe in the wrong spot, there may be a missing ground wire, or some unusual soldering problem exists.  Definitely don't attempt to use this amp until this is resolved.

[Tbar]

I rechecked everything around the sockets, reinserted the tubes and now I am getting ~260v off both +/- of the 6.3v pads.  They were dead on 6.3v at the page 52 test. 

[Paul Birkeland]

What do you measure on the power supply board where the black wire leaves (the - terminal)?

[Tbar]

-275 reads 0. 
-6.3 reads 263

[Paul Birkeland]

Be sure the amp is fully discharged, then pop the wires out of the +275V pads (both red wires and the black wire), then leave those poking up into the air.  Recheck the voltages on the 6.3V pads, bearing in mind that you'll need to clamp your negative probe to that - terminal on the 275V pads on the power supply board.  Soldering in a small piece of scrap temporarily is a reasonable method to make this easier and safer.

[Tbar]

That puts me back up to +6.3v = 355v, -6.3v = 349v. 

[Paul Birkeland]

Can you unbolt the power supply board and show the bottom?  Can you let me know what meter you're using? 

[Paul Birkeland]

Also you could end up with this situation if the zener diode on page 35 is missing or possibly if it's installed backwards it could blow on you, then you'd end up with high voltage on those heater terminals.

It would be worth inspecting that zener diode for damage (if it's blown, it should be pretty obviously cracked and there may be a burn mark on the board).

If this is what has happened, there is a workaround that's pretty easy, so you don't really have to worry about sourcing replacement parts.

[Tbar]

No signs of scorching on the diode and the band appears to be the right direction.  Photos attached plus a picture of the multimeter in question. 

[Paul Birkeland]

If you pot the board back and get your meter negative clamp back on the - terminal of the 275V pads, what DC voltages to you measure on each side of the little zener diode back there?

[Tbar]

347v on the banded end.  0v on the un-banded end.

[Paul Birkeland]

Well that diode isn't diode-ing anymore. 

You can remove the 75K resistor from that board next to the zener diode, then replace the zener diode with a piece of wire, and your Mainline will work as intended.  You'll see 6.3V and 0V on the 6.3V pads instead of the 50/56V, but that won't have any influence over the operation of your amplifier.

-PB

[Tbar]

Ok I will give that a shot.  Is that likely to have just been a faulty diode or is there a risk I have something else bungled and fried it?

[Tbar]

Ok - did as instructed: now 280v on the 275 pads and 6.24 on the 6.3 pad.  I will give it a listen. 
Thanks for all your help.