Extended FP3 Voltage/LED Issues

[Xphile11984]

Hey Everyone, I'm getting some bad readings and was hoping for some help!  In addition, i've got no love on the A Socket B side LEDs. 

Save for below, everything reads normal:

32: 160K R (150K R)

12: 103v (150v)
15: 4v, (75v)
21: 103v (150v)
21: 17v (26.5v)
32: 174v (150v)

Here's what i've done thus far:
-Swapped/moved tubes
-Replaced LEDs on Socket A, B Side
-Replaced the MJE350 on A side shunt board
-Swapped output coupling caps
-And, of course, visually inspected the traces/joints/wiring/hardware.  I can't see anything wrong??

If someone has some testing advice or any other ideas, I'd love to hear them!  Anything would be appreciated.  Thanks!!

[Grainger49]

First, let me thank you for making your post so easy to follow.  Also for a list of what has been done and checked.  That will help us to help you.

Sadly, I don't have the information on the extended upgrade.  But I'll do my best with my knowledge of the old FP 2 and C⁴S boards for it.

First some help with the HLMP6000 LEDs.  Using you meter set to ohms you will probably get the led to light with the red meter lead on the anode and the black on the cathode (silver banded end).  I think I have that right.  I will double check on some I have downstairs.

If the resistance setting doesn't work, you might have a diode check position on your meter.  It will show ~1.6V forward drop when the red is on the anode and black on the cathode.  The cathode is always pointing toward ground even if the diode is receiving full B+ at the anode.  Still the cathode is not at but pointing toward ground through a whole bunch of stuff.

On terminal 32, 150k is within 10% of 160k, I wouldn't worry about that one.

Terminal 12 and 21 should be receiving the B+ straight from the 12AU7 (center) voltage regulator.  There is no longer a resistor between tube pins A1 and A6, right?  This, about +50% high, voltage kind of points to the voltage regulating circuit around tube tube R in the middle.  Verify component values and wire placement for the circuit board for the center tube.

Terminal 32's voltage may be caused by the same thing as terminal 12 and 21 above.  It appears to be fed from the regulated B+. 

That leaves terminal 15.

Terminal 15 is the cathode of the A channel cathode follower.  From there to ground is one of the C⁴S circuits.  Verify the components (values) and wiring. 

[Xphile11984]

Thanks for the reply Grainger!  Much appreciated!  Can you tell I used to work in tech support? 

I actually had to take a last minute business trip right after i posted (and only just returned.)  So proceeding through your checklist; I have a Fluke 179 with a diode setting.  Checking the LEDs, they return fine.  Interestingly i can get both to light on the A socket B side but only one to light on the B socket B side when testing (yet they work at run time.)

I definitely removed the resistor between A1 and A6 and everything else seemed to check out until i tested the 2.49K R at R3: 4 Ohms.  Wha?  Tracing that back a little further, sure enough, across terminals A and r of the LM431 shunt regulator, 4 Ohms. 

So then my next question, might this be the result of the issue (fried at boot) or could this be the issue itself?

Thanks for your help!

-Zachary

[Paul Birkeland]

Hello Zachary,

What voltage do you see on the K terminals on the center PC board?

Half the LED's lighting on one of the outer boards often means that the 2N2907 and 2N2222 transistors are swapped.

-PB

[Yoder]

I actually had to take a last minute business trip right after i posted (and only just returned.)  So proceeding through your checklist; I have a Fluke 179 with a diode setting.  Checking the LEDs, they return fine.  Interestingly i can get both to light on the A socket B side but only one to light on the B socket B side when testing (yet they work at run time.)

I had something similar happen with the Eros, and I found that I had installed one of the LED's backwards. May want to check the "bad" one for kicks and make sure the silver side is toward the straight edge.

[Xphile11984]

OK, i swapped out the LM431 and my resistances are back on target.. My voltages, however, haven't moved.. 

PB, I'm getting 3 VDC on the K of the A side of the center PCB, and the B side sees 6.6 VDC.  I swear the transistors are correct - i have checked and rechecked.

Yoder, my first assumption too, though i've verified them at this point.

AHHH!

Zachary

[Paul Birkeland]

Would you be able to measure the resistance of R4 on the center PC board?  It should measure 147k, but it's possible that you got some 174k resistors on accident. 

The cathode follower on socket A is not operating, which is consistent with the LED issue. 

It doesn't look like either regulator is quite doing its job, which would be good to nail down before going any further, then we can check on the LED's not lighting (I'd first start by double checking the black jumper wire).

I just repaired a Foreplay today with the same LED issue, and it was misplaced transistors. 

[Xphile11984]

R4: 147K on the A side, 80K on the B side.  I removed the B side's R4 and it IS measuring 147K out of circuit.  By black jumper wires i assume you're referring to those mounted to the PCB?  Yes, they're in place per the manual, continuity checks out?

Not sure if it matters, but i did just notice that the caps on the center PCB are actually marked differently?  Currently F224K X 250MPP1 rides the B side and F224K Y 250MPP1 rides the A side.  Writing faces outward on both - assuming there's no polarity.  X/Y, what's the difference?

[Paul Joppa]

The low cathode voltage (T15) suggests that the C4S is saturated; that's consistent with the lack of light from the LEDs (A socket, B side of the board). I would first suspect the LEDs are in backwards - particularly easy, since those should be reversed from the printed notations on the PC board. If this is true, it would cause excess current draw by the cathode follower, overwhelming the regulator and resulting in the low power supply voltage at T12.

The other problem seems to be a lack of regulation on the other channel. On the center board, this is the triode at socket pins 1-2-3, the A side shunt reg (431 chip, R3, and R4), and the B side C4S (R1, R2, LEDs, transistors). The regulated voltage is controlled by R4 and R3 on the A side of the board; you can check the voltage to ground at each end of the 147K R4 which should be 2.5v and 150v.

[Xphile11984]

Hallelujah!  Voltages are now spot on - LEDs are working! 

It would seem we were all correct?  it WAS the A socket B side LEDs, but they were NOT backwards, as i asserted.  Measuring the B side, i looked under the board and noticed (that after removing and replacing them a few times) the board was glazing over the joints - every time i had tested the LEDs i had taken readings from the top.. 

Apparently there was no continuity on the LED Cathode/I, nor was there continuity from the LED anode to R2 (top trace, but no continuity).  Double issue!  I pulled both, installed two new LEDs and everything immediately came back.

Thanks for the help, everyone!!  I think I'm good!  Now to test it out..

Zachary