Power on voltage reading failing at H1 and H2

[Mwest]

Hello,
I started this build back in 2005 and just finished up yesterday.  The kit is a Foreplay 3 with the sweetest whispers stepped attenuator kit.  All resistance checks measured out perfectly.  Voltage checks are not going well 

Power measurements at the transformer terminals are spot on as are most others.  The problem spots are:

H1 = 0 VDC
H2 = 6.5 VDC
10 = 484 VDC (Instead of 373 incidentally this is the exact same reading I pulled at 6)
12 = 0 VDC
13 = 0 VDC
21 = 0 VDC
22 = 0 VDC
25 = 0 VDC
31 = 0 VDC
32 = 0 VDC

I have recheck the wiring a few times at this point and have corrected two miss-wires.  I also reflowed the solder between transformer 4 & 5 and H4 & H5.  My readings at H4 & H5 are 0 VDC and 6.5 VDC respectively.

Would greatly appreciate any suggestions for what to check next.

Thanks,
Matt West

[Paul Birkeland]

The voltage between H1 and H2 should be about 6 volts (one probe on H1, the other on H2).

With your black meter probe at ground, there is a voltage reference that elevates the 6V rail. 

The last picture on page 31 shows how this works.  There are two resistors and the Z5U bypass cap, then a wire leaving the resistor junction and going to tube socket pin 9 on the left socket.  That is what you'll want to look at.  If the red wire going to A9 (I think that's the A socket) is grounded, you'd have issues with your "H" voltages.


As far as the rest of your issues, you have high voltage at your power supply, do you have it present at R7?

If you don't have an 0D3 plugged in, it won't get any further than this.

-PB

[Mwest]

Thanks Paul!

A9 has continuity to ground.  I am trying to trace the path and see where it is shorted but I am not seeing it yet.  Are there any tricks to find where a short to ground is occurring?  I am thinking maybe de-solder and check either side and then do the binary search thing with this technique.

R7 has about 482 VDC.

Very much appreciate your help with this.  This is my first electronic project so I am not well versed in circuit descriptions yet... indeed I had never soldered prior to this project.

Best regards,
Matt

[Paul Birkeland]

Hello Matt,

Can you shoot a picture of your FP-III that's the same as the one I referred to?

-PB

[Mwest]

Thanks again Paul,
I am uploading three pictures to try and cover the areas you described.  Also my initial miss wires were 1 & 2 on the power supply were swapped and the red wire was miss fed to 7 instead of 6.  Symptom of the miss wire was a blown fuse on the power inlet.

Best Regards,
Matt

[Mwest]

One more piece of information that I would like to volunteer... I pulled the tubes and pin A9 still has continuity to ground.

Thanks,
Matt

[Paul Birkeland]

You never did mention whether or not the 0D3 was in the socket when doing these tests.

Without it, the "H" voltages will appear as you have measured them.

[Mwest]

All power on and resistance tests were with the tubes plugged in to the sockets. 

The only test I have performed with the tubes removed was the A9 continuity to ground test.  I performed this test first with the tubes in place and then removed the tubes to see if the path to ground was possibly through the tube.  In both instances, with and without the tubes in place, A9 had continuity to ground.

Thanks,
Matt

[Paul Birkeland]

I would focus on why the twisted pair leaving the high voltage supply isn't providing voltage to the 8 pin socket.

-PB

[Mwest]

Thanks Paul,
Terminal 10 has power and terminal 5 is measuring at the expected 0 vdc.  Pin r7 has 482 vdc.  I am not certain why the interest into the twisted pair feeding the 8 pin socket. I resoldered everything related to the 8 pin socket and the twisted pair with no joy.

Cheers,
Matt

[Grainger49]

Matt,

The OD3 is your voltage regulator.  Paul is saying that the voltage from the transformer is not reaching the OD3 tube pin.  Without that the 12AU7 tubes won't work.

[Mwest]

Thanks guys,
I think I may have found the problem.  Would you please look very closely at the picture of the 8 pin socket.  There is an indent in the hole at the center of this socket.  Mine appears to be clocked 90 degrees counter clockwise to the picture in my manual.  My guess is that I am not sending power across the center tube, which I believe you are referring to as 0D3.

Much appreciate you taking the time to help a newbie with his first project.

Sincerely,
Matt West

[Paul Birkeland]

Yes, I didn't notice that at first, but you are correct.  This will indeed prevent power from reaching the rest of the circuit.

-PB

[Mwest]

Thanks guys,
I am much closer.  Rotating the socket and re-attaching everything to it helped.  A lot of my voltage readings are now corrected.  I also had to swap to the smaller of the two center tubes to get it to glow and give happy voltage readings.

Still no sound... but I need to collect a lot more details before I know why.

Thanks again for all of your help so far!

Sincerely,
Matt

[fullheadofnothing]

A bit confusing what you're saying here. The largest tube (0D3) is in the center octal socket, with the left and right 9-pin sockets being used for the 12AU7s. They should be the same size, or at least close enough in size to be thought of as the same. Can you confirm that you have one 0D3 tube and 2 12AU7s?