Voltage Readings Following Soldering Error

[AmCan]

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.

[KatieandDad]

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.

[Osvaldo de Banfield]

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!!!

[SoNic_real_one]

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!

[AmCan]

Originally Posted by KatieandDad

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.

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.

[KatieandDad]

Sorry. I thought the - signs were just tabulation marks.

[AmCan]

Originally Posted by Osvaldo de Banfield

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!!!

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.

[KatieandDad]

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.

[AmCan]

Originally Posted by KatieandDad

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.

Awesome, thanks Mate! Thats the test I needed to know.

[AmCan]

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?