Hi Gang,
A friend was kind enough to give me a pair of populated boards, and it was gesture I greatly appreciated as I have been swooning over them for years. He made one "small" mistake which I thought was on purpose.. he left out the LED's completely. That made R29 smoke in both channels.
So, I replaced R29 on both boards and installed LEDs with the correct polarity. I brought the boards up and one of them still smoked R29. I found a short in Q11 and replaced it. This channel works properly.
The other channel didn't obviously cook any transistors. I just dropped in a fresh 100 ohm resistor at R29 and tested it out. This channel shows a dramatically lower output. The only oddity I can find is higher voltage at the Collector of Q11. I can't seem to figure out why.
I am going to be totally upfront and say I am a total rookie at this type of thing and usually can't successfully diagnose a circuit because I don't truly understand how it all works. I also can't count out an assembly error of some sort by my friend.
Any help is appreciated, I am dying to hear this thing!! I borrowed my friends and really liked it a lot.
Thanks in advance.
A friend was kind enough to give me a pair of populated boards, and it was gesture I greatly appreciated as I have been swooning over them for years. He made one "small" mistake which I thought was on purpose.. he left out the LED's completely. That made R29 smoke in both channels.
So, I replaced R29 on both boards and installed LEDs with the correct polarity. I brought the boards up and one of them still smoked R29. I found a short in Q11 and replaced it. This channel works properly.
The other channel didn't obviously cook any transistors. I just dropped in a fresh 100 ohm resistor at R29 and tested it out. This channel shows a dramatically lower output. The only oddity I can find is higher voltage at the Collector of Q11. I can't seem to figure out why.
I am going to be totally upfront and say I am a total rookie at this type of thing and usually can't successfully diagnose a circuit because I don't truly understand how it all works. I also can't count out an assembly error of some sort by my friend.
Any help is appreciated, I am dying to hear this thing!! I borrowed my friends and really liked it a lot.
Thanks in advance.
For each channel, the lone LED is part of two different current sources, and is essential.
If left open bad things will happen, so it's not just an optional pilot light.
There could be more damaged parts or assembly errors in the bad board, start looking.
Check all power supply voltages first.
If left open bad things will happen, so it's not just an optional pilot light.
There could be more damaged parts or assembly errors in the bad board, start looking.
Check all power supply voltages first.
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> R29 smoke
If this gets annoying, raise R29 to like 2.2k for further smoke-test. I can't predict all modes of smoke but this deters the most likely overcurrent path(*). Output drive is reduced, it will drive 20k load to several Volts but THD may approach a few %.... but that beats smoke. The wrong DC voltages observed should lead to any additional mis-wires or wrong-part troubles. When all seems well, fade-down R29 to 1k, 330, finally 100r as designed.
(*) Full 24V (wrong) across 2.2k makes a fat 1/4W in the 2.2k and no more in any other part. 0.262W, which a "1/4W" will stand for many minutes, long enough to "smell hot", your cue to shut-down.
If this gets annoying, raise R29 to like 2.2k for further smoke-test. I can't predict all modes of smoke but this deters the most likely overcurrent path(*). Output drive is reduced, it will drive 20k load to several Volts but THD may approach a few %.... but that beats smoke. The wrong DC voltages observed should lead to any additional mis-wires or wrong-part troubles. When all seems well, fade-down R29 to 1k, 330, finally 100r as designed.
(*) Full 24V (wrong) across 2.2k makes a fat 1/4W in the 2.2k and no more in any other part. 0.262W, which a "1/4W" will stand for many minutes, long enough to "smell hot", your cue to shut-down.
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PRR, the good news is it's no longer cooking parts. Just not working properly.
I'm going to dig in a bit further today. Any suggestions as to why voltage would be high at the collector on Q11?
I'm going to dig in a bit further today. Any suggestions as to why voltage would be high at the collector on Q11?