Where's there smoke... (extended upgrade)

[Steve Reese]

Yesterday evening I finished installing the Extended parts into my Foreplay III. I did a visual check, and found a few mistakes on the A tube side. Did another once over, then put the tubes in and hooked up the power cable. Flipped the switch, and it started smoking on the B power supply side. It was on maybe 3 seconds before I turned it off. I had to look really hard on that power supply side before I found it to be the 220 UF 250V that's attached to 5U (striped) and 10U wasn't soldered completely on 5U. I resoldered, put the tubes back in, and put the cable on, then flipped the switch. No smoke, but diodes on B C4S light up on on the B side of the board. If I recall, the power shunt regulator board has no lights on the diodes. The A tube side C4S has all four diodes light up, although the A side of the board is much brighter than on the B side. I powered it off being afraid I would ruin more parts. Any suggestions will be greatly appreciated on where to look next.

[Doc B.]

Hi Steve,

Now that you have got it to where it won't smoke when you fire it up, go over your resistance and voltage measurements again, and note here any readings that are off from the manual.

[Steve Reese]

Thanks Dan I will. Most likely will be tomorrow evening before I can get any readings taken.

[Steve Reese]

I'm just guessing, but if I'm looking down at the preamp with the wires exposed, I'm referring to the tube on my right as A. Please correct me if I'm wrong.
Here are the readings I got last night:
Terminal                                Voltage                                            Resistance
    H1                                      5.83                                                  30.1
    H2                                      12.81                     
    H4                                        4
    H5                                        12
     1                                     (A) 88  (B) 91
     2                                     (A) 0    (B) 0
     3                                     (A) -0   (B) -0
     4                                     (A) -0   (B) -0
     5                                     (A) 0     (B) 0
     6                                     (A) 70   (B) 90
     7                                     (A) 187  (B) 221
     8                                     (A) -0    (B) -0
     9                                     (A) 199  (B) 223
   10                                      (A) 190  (B) 218

    11                                         0                                                    468K
    12                                        33                                                    82K
    13                                        -0
    14                                        -0
    15                                         4                                                      0
    16                                         0
    17                                         0
    18                                        -0
    19                                        -0
    20                                        -0

    21                                         32                                                   83K
    22                                         5                                                     30K
    23                                        -0
    24                                         0                                                      0 ohm
    25                                         0

    26                                         0
    27                                         0
    28                                        -0
    29                                        -0
    30                                        -0

    31                                         181                                                  Infinity
    32                                         187                                                    150K
    33                                         -0
    34                                         -0
    35                                         -0                                                       474K

    36                                         -0
    37                                         -0
    38                                         -0
    39                                          0
    40                                         -0

The PCB board B has no lights on section A, but B has lights.
Power Shunt Regulator PCB board section A has lights, but B only has it on the right diode.
The PCB board A has strong lights on A, but faint lights on B.

Thank you all who offer any advice.

[Doc B.]

The readings from 1 to 10 look about right, so it would seem that the power supply parts are working OK. What you have labeled as the A side looks like some current is being drawn downstream of the power supply, that is pulling the numbers below spec. I think we actually label the tube sockets the other way around in the FPIII manual, i.e. as you look at the underside A is the far left one and B the far right one. Looks like the shunt regulator board is not working, so that is where I would suggest looking first for any missed solder joints or mis-wires. It might also be worthwhile to check the transistors. I think I must have posted how to do that a few hundred times, but I don't recall a specific repository for the directions. You want to measure resistance of each pair combination of the three leads on each transistor. If any combo reads really low like 100 ohms or less the transistor is toast.

[Paul Joppa]

While Dan's away, I'll see if I can keep this moving...  :^)

First question, do all the tubes light up, that is, are their heaters getting power? Especially the center tube which is the regulator (socket "R").

If the tube is working, then it is not regulating but it is getting power. The regulated voltages are on T21 and T32, and both should be 150v - you are getting 32 and 187v respectively. Can you measure the voltages on R1, R2, R3, R6, R7, and R8? That will tell us something about how it is not regulating. (The 32v at T21 is the reason H1 and H2 are off.) You may need to unscrew the A and B socket PC boards and move them to the side to access the R socket pins - we'll do something with those sockets eventually; I just want to focus on the regulators right now.

[Steve Reese]

Thanks Paul, here are the readings I get on the R socket:
R1 - 177
R2 - 0
R3 - 2
R4 - 11
R5 - 33
R6 - 0
R7 - 0
R8 - 0

[Paul Joppa]

R4 and R5 should be soldered together - see the top of p.28 in the manual. These are the heater terminals, and this is the reason I asked of the tube was glowing. Presumably it is not, or at least only one half of it is glowing.

I see that R5 is at the T21 voltage, which should be connected to R6. I'd suggest carefully going over the connections to ALL the R-socket pins, but it is possible that these two errors could explain all the R-socket voltage anomalies.

[Steve Reese]

sorry... both A and B tubes are glowing, but the regulator doesn't show, but feels hot. I was in a hurry this morning before I had to leave on a beer run trip out of state.

[Paul Joppa]

... I was in a hurry this morning before I had to leave on a beer run trip out of state.

Ah! A man with priorities!! I'm a wine man myself, but I can respect that  :^)

No sweat; post again when you've checked out the suggested items.

[Steve Reese]

Okay, I removed the screws from the PCB board on the regulator tube, and in fact the readings on R4 & R5 are at 5. R6 is in fact attached to T21 and is reading of 33.

[Paul Joppa]

OK, now we have new data and can try a different theory.

R1-R2-R3 is one regulator triode (plate-grid-cathode), and I see that the cathode is at 2 volts. I'll assume that is 2.0 volts, which as about as small a voltage as the 431 regulator chip can produce. The plate voltage, at 177, is well over the 150v. It is hard to make conclusions until we know what the "B" tube is doing - I think you said it shows no LEDs on the "A" end, so there is something wrong there. We'll get to that after we finish learning what we can from the regulator situation.

R6-R7-R8 is the other triode. It has 33v on the plate, and zero voltage on the grid and cathode. This regulator feeds the "A" signal tube, whose PC board has all four LEDs lit, so it may well be working correctly. In that case, the zero volts at the cathode (R8) indicates a wiring error or a shorted (blown) 431 regulator. There should be a black wire from R8 to the "K" pad on the B end of the board - check the locations and the solder joints. Also inspect the PC board for solder bridges around the 431 chip.

Now getting back to the "B" signal tube, check the voltages on the tube pins 1-2-3-6-7-8. This will tell us a lot about whether the C4S loads are working correctly on that tube, and once we know that we can finish diagnosing the regulator on that side.

[Steve Reese]

Here are the readings for tube B:
1 - 9
 - 2 - 0
3 - 1
4 - 0
5 - 12
6 - 32
7 - 8
8 - 3
You didn't ask, but 9 - 5 to 6.

[Steve Reese]

Let me rephrase... the tube socket I'm referring to is on my left.... is that not Tube B? It's the one that the PCB board that only has lights on the B side of the board. What I'm calling A on my right side reads as follows
1 - 172
2 - 0
3 - 1
4 - 12
5 - 12
6 - 177
7 - 173
8 - 172
and 9 - 6.  This is the side that all of the lights on the PCB board that works.

[Paul Joppa]

OK, now I am confused! Please see Doc B's post in this thread where he repeats the tube identification from the manual, which is:

Looking at the chassis upside down, with the controls closest to you and the input/output jacks furthest away from you, the tube on the left is A, middle is R, right is B. (Also, all the PC boards have their "B" end closest to you, with the "A" end towards the jacks and power supplies.)

You can probably ignore anything I said about the signal tubes (A and B) after that post, since I was assuming you had corrected that mis-identification. I've been drawing conclusions from erroneous data after that post.

Bottom line, the regulators are not working correctly, and the "tube on the right" that has the high voltages and the lights that work, is also not functioning correctly. I can't tell about the other tube until its regulator is working and supplying voltage to it.

Right now I have a terrible cold and low-grade fever, and I'm sleeping about 15 hours a day. This has become too complex for me to deal with effectively. I'll take another shot at it in a couple days when I feel more competent. Meanwhile, would you summarize the tube voltages with the correct designations, and if your meter allows it measure the smaller voltages to 2 or 3 places? This is particularly important for the cathode voltages (pins 3 and .