Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Seduction => Topic started by: Alonzo on December 06, 2011, 05:07:39 PM
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I need some help figuring out a voltage issue on my build. I've built the Seduction with what seems the current upgrade path, C4S kit and .1 and 1.0 Obilgatto capacitors. Everything else is standard from the kit. The build went easy (no lost parts, no extra solder, no unexpected left over parts). The resistance checks seem OK, a little low but within spec. Voltages, not so much. My readings are low coming out of the CRCRC filter, T12 reads 101. (Also reading 101 at T34/44). T15 reads 126 (low but at the edge). The real screwie reading come from B tube, voltages are higher than expected. Here's the breakdown:
A1 62, B1 67
A2, B2 0
A3, B3 1.5
A4, B4 0
A5, B5 1.5
A6 62, B6 96
A7, B7 0
A8, B8 1.5
A9, B9 0
T25 153
T26, T36 0
T27 63, T37 98
T28, T38 0
T29 59, T39 93
T30 59, T40 91
All the LED's light, 2 on the C4S board are a little dimmer than the others but all are on. I did a visual inspection and I can't see any missed solder joints or different looking resistors. I've re-wet all my joints, swapped tubes, all with the same results. Anyone have any ideas on what to check next?
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Let's see. Everything looks OK except the B+ coming off of the B side of the C4S board.
Terminals 37/39/40 are all fed from that voltage.
The B channel incoming voltages are the only screwy reading I see. That means something on the C4S board is awry in the B channel. B1 and B6 are about 30V higher than designed. T37 and T39 follow each other, this is the problem coming off of the C4S board.
To get to the bottom of this there will have to be some digging into the board schematic, which we don't get.
A2/B2 (the grids) should be about zero just like the other two grids, A7/B7. They all should be zero. I don't know why the manual reads much higher voltages for A2/B2 than A1/B1 which are the plates. This is wrong, you are right.
I'm betting that you can get music, beautiful music, but you want to find out why the C4S voltage on the B side is 50% too high.
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A5 and B5 are the heater terminals and should read about 6 volts DC. You reported 1.5v but the fact that the LEDs glow indicate this is not correct.
The grid voltage as measured and in the manual are all zero, which is correct.
The most likely explanation is that the C4S feeding T37 is dumping too much current. Check the four small resistors (237 ohms) to be sure they are the right value, check the correct orientation of all eight transistors, and carefully inspect the back of the PC board for possible solder bridges between terminals that should not be connected.
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Also make sure the metal cans of the 2N2907s are not touching the PC board traces.
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Did you swap the tubes to see if the voltages follow?
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Thanks everyone for the help, I found the problem under the transistor connected to T37, the middle leg was bridged to the right leg by a glop of solder on the topside of the board. I would recommend everyone follow the advice of the instructions and get a good magnifying glass, took 2 minutes and I found the problem. Scraped the area with an exacto knife and now all the values are within 15% of nominal. Right now Melody Gardot is singing to me and you can't wipe my grin off.
I promised myself not to mess with it for a month, the chokes can wait while everything else breaks in...
Thanks again!
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It has been a long, long time since building the board for my Seduction but I always leave transistor leads long so they don't transfer heat into the junction inside. A couple of these look like the aliens that attacked in "War Of The Worlds" when left long but it helps keep them safe.