Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Foreplay III => Topic started by: Natural Sound on October 10, 2013, 09:01:10 PM
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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
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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.
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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
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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
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OK here are a few photos showing my lack of photography skills
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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!
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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.
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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.
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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 (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
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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
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Have you swapped tubes around to see if it affects anything?
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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.
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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.
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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?
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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.
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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.
I'll check the voltages here in a little bit.
What do you mean when you say, "The center leg of the MJE350 and MJE340 is internally heatsinked, often these need a little extra time from the soldering iron."? Does it mean that I should leave the heat on that connection a little longer when soldering?
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Hmmm... Not good, the voltages on "O" pads are 48v on the (left) and 31v (right). The LED's do seem a bit brighter now.
BTW, the voltage starts off high ~ 230v and then drops when the tube is on for about 10 seconds or so.
NOTE: The shunt regulator is still bypassed at this point.
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I'd flip the outside pair of boards over and shoot some more pics.
Yes, my previous comment meant that when your soldering iron touches the center leg of an MJE350 or MJE340, the chip will attempt to dissipate some of the soldering iron heat, especially if you aren't using a station, so a little extra time on those joints will help.
-PB
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Thanks for the clarification. I do have a soldering station (HAKKO 936). The connections look solid but I'll re-flow them anyway.
In order to "flip the boards over" I had to remove them. Here are the pictures.
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On the top board, along the top edge I see two almost spherical solder joints. I think that the solder pad didn't get much heat.
Look at the solder joints on the vertically oriented transistors. Some look just right but a few are again spherical. It doesn't join the circuit trace and the component lead well.
I'd reheat the ones that look too round.
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On the top board, along the top edge I see two almost spherical solder joints. I think that the solder pad didn't get much heat.
Look at the solder joints on the vertically oriented transistors. Some look just right but a few are again spherical. It doesn't join the circuit trace and the component lead well.
I'd reheat the ones that look too round.
Thanks Grainger. I had a feeling that was going to come up. The photo is deceptive. If I were to turn the board on edge you'd see that the solder joint is more like an inverted V than it is spherical. If anything they have too much solder. The "vertical transistors" are actually the LED's. And again are not accurately represented in the photo. That said I'll hit those again with a little heat.
This shouldn't be that hard. Four resistors have been replaced with four constant current circuits. I'm pretty certain that I got some bad parts. It happens, I've seen lots of examples of this in my 35 years in the electronics industry.
At this point I have two options. I can eat the $350 for the upgrade kit and put the FPIII back to stock. Or I can buy new components, rebuild the boards and start over at a later date.
Neither of these options are desirable but I'm out of options.
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It's quite possible that the transistors are shot, but at a certain point it gets a little difficult to work out what has gone wrong by long distance. You can check the transistors for shorts between the leads, but be forewarned that a bad one doesn't always show up with that test. You can also send the preamp to us and we can make it work for you.
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OK Guys, don't give up on me yet.
I went with a troubleshooting method that I recommended earlier. I removed the C4S boards and tacked the 22.1k 3/4 watt resistors to the wires that were attached to the C4S boards. The idea being that if this worked I could rule out any wiring mistakes. I re-connected the Shunt Regulator board that had been bypassed. Fired it up and the 4 LED's on the SR board as well as the cathode LED on the tube sockets illuminated "normally." Measured the "O" voltages for the left and right supplies and both measured 149v. Good.
What I know.
1. The two HV power supplies and both sides of the shunt regulator board are functioning.
2. The C4s boards are not working but the wiring to them is good.
This prompted yet another inspection of the 2 C4S boards.
LED's installed correctly - Check
2N2907A transistor orientation and location Q1 on side A - Check
2N2222A transistor orientation and location Q1 on side B - Check
Measured 2ea 75k ohm resistors in position R2 - Check
Measured 2ea 237 ohm resistors in position R1 - Check
MJE350 transistor orientation and location Q2 on side A - Check
MJE340 transistor orientation and location Q2 on side B - Check
Black wire jumper connecting G to G - Check
Rechecked solder connections.
Measured for shorts across transistors - Check
Measured for shorts across LED's - Check
I'm very confident that I did everything as instructed. Whatever the problem is it exists on both boards which seems a little strange. It has to be component related, right? Am I missing anything?
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I'd be nearly certain at this point that your LED's are not oriented properly.
-PB
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I'd be nearly certain at this point that your LED's are not oriented properly.
-PB
I'll see if I can get them out without destroying them..
.
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Removing the LED's was easier than I thought. The leads are close enough to heat both pads at the same time. On the other side of the board I grabbed the plastic body of the LED with needle nose pliers and applied gentle pressure outward. Piece of cake.
While removing the LED's I paid close attention to the orientation. I was hoping to find a smoking gun but they were installed the way I remember when I built the boards. Darn.
So I went back to the instructions and read the section regarding the LED installation. Then I saw it. The B side LED's were supposed to be in reverse orientation from the A side on the C4S boards. I missed that part and installed them in the same direction (cathode (silver stripe) all in the square shaped solder pad) like on the SR board. BTW the instructions were crystal clear about this. It was just a bone head mistake on my part. I usually don't miss details like that. I make no excuses, it just happened. {smacks self in head}
Powered up and all the LED's illuminated. Moved on to checking voltages and everything was in spec. :) Installed into my main system rack and she is providing beautiful music to my Paramour II's.
In retrospect the dark LED's on the B side of the C4S boards should have been a dead giveaway. Oh well, live and learn.
Big thanks to all that helped especially PB! This was a humbling experience and I'm eating a big piece of humble pie.