Extended Foreplay III Voltage and Resistance Check

[bmgrlfl]

Below are the voltage and resistance readings from my Extended Foreplay III.  The only readings outside of that stated in the manual are positions 12 and 21.  Listed below are all of my measured voltage and resistance values.  Any opinion on these two resistance readings at 12 and 21?  The preamp powers up and sounds fine, but I was just curious about these two resistance readings.  Thanks in advance.

Position - Specified Voltage - Measured Voltage - Specified Resistance - Measured Resistance

11 - 0.0V - 0.02V - 470K - 473K
12 -  150V - 149V - 83K - 60K
13 - -
14 - -
15 - 75V - 70V - Infinity - OL
16 - 0V - 0V
17 - 0V - 0V
18 - -
19 - 0V - 0V
20 - 0V - 0V
21 - 150V - 149V - 83K - 60K
22 - 26.5V - 26.4V - 30K - 30K
23 - -
24 - 0V - 0V - 0 ohm - 0.2 ohm
25 - -
26 - 0V -0V
27- 0V -0V
28 - -
29 - 0V -0V
30 - 0V -0V
31 - 75V - 66V - infinity - OL
32 - 150V - 150V - 150K - 150K
33 - -
34 - -
35 - 0V - 0V
36 - 0V -0V
37 - 0V -0V
38 - -
39 - 0V - 0V
40 - 0V -0V

 

[Paul Birkeland]

Your voltages are perfectly spot on, so the resistances are not such a big deal. 

12 and 21 are the regulated high voltage terminals for each channel.  With the extended upgrade, the path to ground gets convoluted, so resistance measurements may vary a bit depending on the meter.

-PB

 

[bmgrlfl]

Paul

My Extended FPIII is oscillating in both channels (scope trace hopefully attached to this post).  The oscillation took out the left channel of my power amp awhile back, so I started to investigate and tore apart the Foreplay and replaced the shunt reg board and C4S boards.  Boards have all new parts and also updated to MJ5731AG.  Boards were charred and black at B-I and B-O on both left and right C4S boards.  Shunt board was discolored too, but no charring.  Voltages all check out after the rebuild.  Resistance at 12 and 21 are the only thing with some deviation (around 60K instead of 83K). 

Any advice on where to start looking to troubleshoot this oscillation problem?  Also, swapped all the tubes, but no change in the oscillation. 

Thanks,
Brian

 

[Doc B]

You might double check the orientation and connections of all of the power supply filter caps.

 

[bmgrlfl]

Thank you Doc...double checked the power supply capacitors are all correctly oriented.  The caps are brand new Panasonic and were replaced when I re-did the PC boards.  Attached are a couple of photos of my build.  Power supply is separate and runs through a 3-ft long shielded umbilical.  There is also an earth ground wire in the umbilical that connects the two chassis's.  All power supply voltages are within tolerance.

 

[Doc B]

That's a pretty different configuration than the stock kit, which makes it rather difficult for us to troubleshoot. The oscillation could be layout related, from capacitance in the umbilical cable or some kind of ground loop. You might want to disconnect the supply from the preamp circuit and see if the supply oscillates by itself.

 

[bmgrlfl]

If I disconnect the power supply before the tube A and B C4S boards to measure for ripple/oscillation, would I load each regulated 150V B+ supply to draw 7.0 ma (i.e., C4S @ 3.5 ma x 2 from the schematic).  This would be a dummy load resistor of 21.4K Ohm (and 150V x 0.007A = 1.05W)?

I have re-checked the circuit and only the resistance measurement from ground to tabs 21 and 12 is out of spec.  I measure 58.8K ohm instead of 83K ohm as listed.  If I measure directly across the 150K ohm resistor between 21 and 22, I do get 83k ohm somehow.