I was soldering in a bypass cap in my preamp when I accidentally dropped solder on another trace affectively connecting the negative and positive of one of the small 10uf 35V capacitors. Unfortunately I did not know this until I turned it on to listen. I replaced the cap in question and everything seemed right but the wattage power indicator (1st LED) on my amp showed that it was drawing power for the left channel even with nothing being played. So I thought I would replace the next cap in line in the circuit, which didn't solve the problem so I replaced the next cap and then things got worse. Since I was no longer getting a signal from the preamp now in both channels I decided to check the voltages.
Something interesting happened when I took the voltage readings. I started with the power supply and the positive pins read normal (+24/+15). When I measured the negative pins I got the following readings
Pin 6 = +19.5 (Should be -15)
Pin 7 = +10.4 (Should be -24)
Pin 11 = +10.2 (Should be -15)
Pin12 = +10.4 (Should be -24)
HOWEVER
After reading the Output and Phono Boards, which were all normal I went back and measured the negative pins again to double check myself and they were now normal too.
pin 6 = -15.3 (Should be -15)
pin 7 = -24.5 (Should be -24)
pin 11 = -15.1 (Should be -15)
pin12 = -24.4 (Should be -24)
In between each reading I would turn the preamp on and off to use Alligator Clips so as not to have a mishap with the probes. Is it possible that turning it on and off repeatedly somehow righted something that was amiss? Possible explanations? What is the next step? Do I need to trace a 20-hertz signal starting backwards from the output board? I don’t want to hook it up again until I know it is o.k.
Something interesting happened when I took the voltage readings. I started with the power supply and the positive pins read normal (+24/+15). When I measured the negative pins I got the following readings
Pin 6 = +19.5 (Should be -15)
Pin 7 = +10.4 (Should be -24)
Pin 11 = +10.2 (Should be -15)
Pin12 = +10.4 (Should be -24)
HOWEVER
After reading the Output and Phono Boards, which were all normal I went back and measured the negative pins again to double check myself and they were now normal too.
pin 6 = -15.3 (Should be -15)
pin 7 = -24.5 (Should be -24)
pin 11 = -15.1 (Should be -15)
pin12 = -24.4 (Should be -24)
In between each reading I would turn the preamp on and off to use Alligator Clips so as not to have a mishap with the probes. Is it possible that turning it on and off repeatedly somehow righted something that was amiss? Possible explanations? What is the next step? Do I need to trace a 20-hertz signal starting backwards from the output board? I don’t want to hook it up again until I know it is o.k.
To be honest. If there is 0V at the output and you have no oscilloscope. The best test is a power amp.
Shorting out a bypass cap will not harm the cap. It might harm the rectifiers or regulators driving it.
If everything has returned to normal, that might indicate that you left a little bit of solder in there that has now fallen loose.
A lot of -pre-amps use regulators that will protect themselves against short circuits.
Changing the caps was probably unneccesary.
Shorting out a bypass cap will not harm the cap. It might harm the rectifiers or regulators driving it.
If everything has returned to normal, that might indicate that you left a little bit of solder in there that has now fallen loose.
A lot of -pre-amps use regulators that will protect themselves against short circuits.
Changing the caps was probably unneccesary.
I believe that something in the positive rail is blown, because of all voltages in this pcb are negative values. Check for a resistor blown, a fuse (some green like resistors), a diode or a zener shorted out.
Good luck!!!
Good luck!!!
You have blown one of the rectifiers on that rail and now you have pulses of AC going into your rails.
No need to replace the capacitors after a short, they are fine, you can short them all day long! But don't leave that AC going too long into your boards!
No need to replace the capacitors after a short, they are fine, you can short them all day long! But don't leave that AC going too long into your boards!
To clarify, you are saying that if all of the voltages have returned to normal now that I could hook it up to my power amp and see how it sounds? I do not have an oscilloscope.
Hey Osvaldo,
Thanks for answering another one of my posts again!
To be clear, the voltages in the negative rail were positive but now have returned to normal. I took my measurements at the pins that supply each board and they were all normal on my second pass through of readings, but not the first. I will probably still look for something blown on the output board and power control board just to be safe before reconnecting it.
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If they are back to normal. I would check for DC on the output, that is normally a clear indicator of a PSU fault.
If output has no DC offset - give it a go.
If output has no DC offset - give it a go.
I measured the DC output last night and I think I know the answer to this question but I wanted to double check. Do I measure it across the 2 terminals, one probe on the left terminal and one probe on the right terminal or do I measure each channel separately, meaning one probe to ground and one to voltage at each terminal. When I measured the left and right channel separately I got zero DC. When I measured across the left and right channels there was about 1.5 millivolts or so but the meter kept fluctuating. I am guessing that measuring the left and right output terminals separately is the correct way?
Ultimately measure between output and ground of each channel.
But 1.5mV is as close to zero as most designs get.
It seems that the two amps have survived the solder blob.
But 1.5mV is as close to zero as most designs get.
It seems that the two amps have survived the solder blob.
Thanks again for your confirmation and explanation. I am going to fire it up tonight!
OK, my friend, if I can help you, better. Beware I´ll go vacations this friday, so I´ll be away from computer during this following 2 weeks. Aleluya!!!
Good for you! I refuse to check email on vacation too for my own sanity. I am going on vacation too from making any changes to my amp or preamp. I just finished upgrading all the caps in both and I need a mental breathier from the stress of worrying that I would mess them up (which I almost did) and waste all that money on designer parts. Now I am going to sit back, listen and enjoy. Then when I feel crazy enough and confident again I will tackle the resistors.
If you want to be really safe. Buy yourself a pair of cheepie RCA interconnects and put a 1uF cap in series with the I/P.
For a couple of dollars you might protect against catastrophic damage.
For a couple of dollars you might protect against catastrophic damage.
OK, no, I will use my free time to make some bricklaying in my own house and change the all day job.
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Is this to control the stray DC Voltage I am getting across the Left and right terminals? The left and right channel did read zero Millivolts independently. My measurement across the terminals could have been in error. However, can I simply connect the 1uf between the sockets inside the chassis? Is that 1uf per channel? So the positive lead of the cap to the positive wire supplying the voltage to the Right RCA socket soldering tab, and the negative to the negative? Repeat for left channel RCA socket?
or what is the purpose exactly?
Citation 17? Check the resistors on the output to the amp selector at the very left back (port for the plug for amp/speakers). There are resistors tied to ground there, one may be burnt and I mean toasted to no longer making any connection as in fired up. No testing needed just verify they are not blown (apart).
If all is well and your voltages check to what they should be (your 2nd time around test indicated so) I would plug in and put on some music (try some test speakers first). Maybe your DIM is flakey, due to not fresh batteries giving you hokey measurement the first time around.
If it was me... I'd get a fresh set of batteries in DIM and give a 3rd test, results same and 2nd test, then hook it up and play (again test speakers) and turn it up and enjoy.
Cheers,
Bob
If all is well and your voltages check to what they should be (your 2nd time around test indicated so) I would plug in and put on some music (try some test speakers first). Maybe your DIM is flakey, due to not fresh batteries giving you hokey measurement the first time around.
If it was me... I'd get a fresh set of batteries in DIM and give a 3rd test, results same and 2nd test, then hook it up and play (again test speakers) and turn it up and enjoy.
Cheers,
Bob
I got my DMI from China for $10 shipped 😀
I tested everything again tonight in the Citation 17 and it was all good. All Voltages normal, zero DC at the terminals and low and behold it played without a hitch. Still sounds great. I certainly can't explain what happened with he first set of readings but I am happy to have learned how to test the preamp. I think I am going to replace the resistors next to MF 😉 Thanks for your help guys!
I tested everything again tonight in the Citation 17 and it was all good. All Voltages normal, zero DC at the terminals and low and behold it played without a hitch. Still sounds great. I certainly can't explain what happened with he first set of readings but I am happy to have learned how to test the preamp. I think I am going to replace the resistors next to MF 😉 Thanks for your help guys!
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