[SOLVED] Need Help - Extended Foreplay III Upgrade

[Natural Sound]

Hi guys,

Its late and I'm going to call it quits in a few minutes but I wanted to get this out there before I do.

I have a problem post upgrade. It looks like something is pulling my Regulated B+ voltage down on both channels. I'm leaning towards some kind of C4S issue but I'd like a second (or more) opinion. The resistance readings checked out OK. I included several of the test point readings where the voltage is way off. Let me know if there are any other readings that would be helpful.

Preface - Pre-Amp was built stock and ran great for several years. No real upgrades except for Goldpoint Stepped Attenuators.
All components, part numbers, resistor values and jumpers on all three boards and associated wiring/solder connections have been checked at least 3X.
All LED's on SR board iluminated.
LED's on A side of C4S boards are very dim. (Both Channels)
LED's on B side of C4S boards are completely dark. (Both Channels)
Cathode LED's on tube sockets are very dim. (Both Channels)
6.25v on heater PS cap. All 3 tubes are glowing.
Tried several different tubes with similar results.

10A 213v (HV Power Supplies Functioning)
10B 213v (HV Power Supplies Functioning)
12   49.1v (Should be 150v)
21   49.2v (Should be 150v)
15   21.3v (Should be ~ 75v)
31   24v   (Should be ~ 75v)

22   9v    (Should be 22v)

Shunt Regulator A
I   212.3
O   50
K   4.91

Shunt Regulator B
I   212
O   46
K   4.84

A Channel (Right) C4S
A Side I   50
A Side O   29.7
B Side I   0
B side O   24.5

B Channel (Left) C4S
A Side I   45.6
A Side O   25.6
B Side I   0
B side O   20.84

Thanks,
Tom

[Paul Birkeland]

Would you be able to pop up a couple of photos?

This problem could be explained by having 47K resistors in the R4 position on the center PC board, but since none are included in the kit, it is likely something else.

[Natural Sound]

Paul,

I measured the resistance of the two R4 resistors in circuit.

The A side read 147k but the B side read 83k. So I lifted one side of R4 on the B side and measured it out of circuit. It measured correctly at 147k.

I'll try to take some pictures in a little bit. Is there anything specific you want me to shoot?

Tom

[Paul Birkeland]

Just the tops of the boards.

For most E-FP-III issues, we find that LED's are reversed or transistors are out of place.  In nearly all the repairs like this I have performed, it has just been a matter of rearranging the transistors.

-PB

[Natural Sound]

OK here are a few photos showing my lack of photography skills

[Paul Birkeland]

So, 50V out and 5V on Kreg on the center board indicates that the middle 12AU7 isn't being permitted to draw any current.

This is consistent with both sides, so I would quadruple check the transistor positions and orientations.  Something on both sides is sucking down a ton of extra current!

[Natural Sound]

So, 50V out and 5V on Kreg on the center board indicates that the middle 12AU7 isn't being permitted to draw any current.

This is consistent with both sides, so I would quadruple check the transistor positions and orientations.  Something on both sides is sucking down a ton of extra current!

OK, I quadruple checked  the transistor positions and orientations. Didn't find anything out of place. I made sure that I used a strong light and a magnifying glass.

Do you think that this is telling us something?

I measured the resistance of the two R4 resistors in circuit.

The A side read 147k but the B side read 83k. So I lifted one side of R4 on the B side and measured it out of circuit. It measured correctly at 147k.

[Doc B.]

Then you have to go to the next logical possibility for the cause of the problem. First would be a miswire. Next would be wrong resistors - either the wrong value or installed in the wrong pads.

[Natural Sound]

Then you have to go to the next logical possibility for the cause of the problem. First would be a miswire. Next would be wrong resistors - either the wrong value or installed in the wrong pads.

Thanks Doc,

Before asking for help I did the things you suggested time and time and time again. And upon your recommendation I did it twice more. When I built the boards I measured each of the resistors with my DVM. They were all within spec. I had one question that was resolved about 1/8watt resister supplied vs the 1/4w spec'd in the procedure. You can see from the photos that the parts are installed correctly into the boards. I've been over them and their wiring at least ten times now.

Its time to try something else. First thing to ask is if the procedure I'm working with has any errors that I'm not aware of. My manual Rev is 6-27-06. I've looked here http://www.bottlehead.com/smf/index.php/topic,3831.0.html and it doesn't resolve any of my problems.

The next thing I'd like to try (if it will work) is to remove the C4S boards and (re) install the 4 22.1k 3/4 watt resistors. Will the shunt regulator work like that without the C4S boards?

I'm starting to suspect a bad component or components. I'm super anal about by builds. During the assembly process I do a lot of parity checks. This is the first bottlehead kit/upgrade that didn't work the first time.

Is there any other isolation troubleshooting I can do? From what see and what PB confirms the problem is affecting both channels. Is there a way I can isolate SR board (verify is working) and then do the same with each of the C4S boards?

Thanks,
Tom

[Paul Birkeland]

I'd recommend the opposite.  You'll notice a red wire going to the "I" pad on each side of the center green board, and a red wire going to the "O" pad on each side of the center board. 

If you pop the I and O red wires out of each side, and temporarily solder each pair together, you'll essentially bypass the regulator entirely.

This will allow you to debug the outer pairs of boards and will be very easy to put back into place later.

-PB

[Doc B.]

Have you swapped tubes around to see if it affects anything?

[Natural Sound]

I'd recommend the opposite.  You'll notice a red wire going to the "I" pad on each side of the center green board, and a red wire going to the "O" pad on each side of the center board. 

If you pop the I and O red wires out of each side, and temporarily solder each pair together, you'll essentially bypass the regulator entirely.

This will allow you to debug the outer pairs of boards and will be very easy to put back into place later.

-PB

Thanks, I'll give that a try.

[Natural Sound]

Have you swapped tubes around to see if it affects anything?

Yes. I swapped them around and I swapped them out with another known good set. The voltages changed only about .5v or so.

[Natural Sound]

Update:

I bypassed the SR board like PB recommended. Now the A side (V1 unit 1) LED's are illuminated. However the B side (V1 unit 2) is still dark on both sides.

I Rechecked the connections that go from ground (19 and 39) and the cathode connections (15, 31 and then to pin 8 of the tube sockets) and nothing seems wrong there.

Can the MJE340 and 2N2222 be checked in circuit or must they be removed?

[Paul Birkeland]

How are the "O" voltages on the glowing board?

The center leg of the MJE350 and MJE340 is internally heatsinked, often these need a little extra time from the soldering iron.