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grosen
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« on: January 29, 2012, 08:40:13 AM » |
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I’ve had a stock Seduction up and running for years and finally decided to the C4S over the holidays. Build went smoothly. Resistances checked out. Powered it up: LED’s all glow, start checking the voltages and everything looks perfect until … right at the very end of the voltage check, the LED’s all go dark.
I start checking everything again. The fuse is fine, and at first the resistances all seem ok . But then I notice something weird. The resistances at T6 and T9, which are usually high ohm fluctuations (as the captitor charges off the battery in the multimiter are) infinite — like an open circuit.
I get this elsewhere in the cirucuit too. On a whim, I reverse the polarity, attaching the red(+) lead on the multimerer to ground and using the black (-) lead as the probe, and now all of a sudden I get the usual high ohm fluctuation again.
Plug the unit in to check some voltages and things are odd from the start. For example:
Terminal Spec Measured
T6 6.5 9.3
T9 6.0 9.3
T12 135 165
T15 148 170
T27,37 70 165
T29, 39 70 165
T30, 40 70 165
One last bit of weirdness: When I go back to check the resistances after the voltage check, instead of getting an open circuit at T6 and T9, when I try to make the test … the display reading on the multimeter completely disappears. It comes back a second or two after I remove the probe from the circuit. But this happens several times. When I reverse the polarity, I get normal high ohm fluctuations at these points, and after that, when I check with normal polarity, I get the open circuit reading again at T6 and T9 …
I’m guessing that I fried a diode or maybe a capacitor in the power supply, but since I didn’t touch any of that in the C4S project, I’m mystified. Anyway, I don’t know how to troubleshoot this, so any help would be much appreciated.
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mchurch
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« Reply #1 on: January 29, 2012, 09:47:05 AM » |
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Hi;
I won't profess to be an expert here but one thing I noticed in my build was to make sure the right transistor is in the right location and that they are not down flush with the board as the metal can might short out the traces. Also verify that the wires connected to the board are intact. When I was stripping the wires I inadvertently pinched one of the wires and although it appeared well soldered it broke while I was performing the checks. The other thing to watch out for is overheating the transistors while soldering. This can lead to issues further on in time.
Someone else on this forum suggested taking readings across the legs of the transistors to see if any are open which would indicate the transistor is cooked. I am sure there will be others who can weigh in with more advice, these are just my initial thoughts.
If it was working well before the upgrade I am sure it is something that can be resolved.
Cheers
Mike
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Doc B.
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« Reply #2 on: January 29, 2012, 09:54:05 AM » |
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Actually a bad transistor usually reads shorted, not open. Did you perhaps short two traces together when you were taking measurements? That could create the sudden darkening of the LEDs as something got too much current and died.
I agree that checking all of the semiconductors is probably worthwhile. Anything that reads shorted across the pins is dead.
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Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.
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grosen
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« Reply #3 on: January 29, 2012, 10:45:50 AM » |
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Thanks for the amazingly quick responses. The transistors do not appear to be shorted out, and the wires to the board look ok. I'm not sure how to test the LEDs, but two of the LEDs on the board seem to be passing current in one direction but not the other, while the other two test as open in both directions. Could that be it?
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Doc B.
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« Reply #4 on: January 29, 2012, 11:10:33 AM » |
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One LED in each pair will sometimes light when you use the meter, and the other in that string won't. Is it one LED in each series pair that reads open, or are both LEDs in one string reading open?
As mentioned before, check to see if the metal cans of the 2N2907 transistors are touching the traces.
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Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.
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grosen
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« Reply #5 on: January 29, 2012, 11:25:14 AM » |
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Hmm... Now all of the LEDs are reading open. (Before it was just one in each string.) The transistor cans are not touching the traces.
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Doc B.
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« Reply #6 on: January 30, 2012, 02:59:50 PM » |
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Check each of your transistors for shorts by measuring resistance across each of the three possible combinations of two leads. Everything should be reading from about 1.5K ohms up to infinity. If anything reads low like 200 ohms or less that is a dead transistor.
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Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.
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Paul Joppa
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« Reply #7 on: January 30, 2012, 04:04:07 PM » |
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From the description, it seems the tubes are not lighting up. First check is, are they installed? :^) stranger things have happened - to me = ... If the tubes are missing, or the wiring to the heaters has become disconnected, then the heater power supply capacitor won't discharge, causing odd behavior when measuring resistance.
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Paul Joppa
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grosen
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« Reply #8 on: February 04, 2012, 01:43:56 PM » |
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YES! We're back in business. Paul was right. The wiring to the heaters had become disconnected -- don't know how. Anyway, all is well. FWIW, the C4S has done just what I hoped it would do. I'm running a Dynavector 10x5. It was a bit of a challenge for the stock seduction (compared to my Shure V15 IV), but it's just right with the C4S. Many thanks for the help. (Next stop, Eros.)
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