Hammer-time was popular in the day. I've got one out in the garage, it could be a smash hit...I'll see myself out now. Gotta go to the garage.Dremel time.I just use a dremel. Or a hammer.
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Has anyone opened up a wall wart?
- Thread starter 62vauxhall
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62vauxhall
Addicted Member
Time to bring this thread to a close.
I am tapping out.
These past 2 days I have spent much time pondering this thing. The wall warts do not seem to have been the issue. Rather, what ever is going on must be something internal within this AP-25 pre-amp.
Twice, I thought I had a Eureka! moment.
One was when I discovered that one leg of a recently installed capacitor was not soldered in place and another was an observation that caused me to question and change the polarity of the incoming DC power. That did eliminate the hum but the pre-amp did not function.
I could be wrong about the polarity swap but the outside barrel contacts of the RCA jacks should, I thought, go to the PC board ground. That meant that in it's state at that time the barrel contacts were connected to the +(pos) side of DC volts input. The swap I did now means the outside barrel contacts are connected to the -(neg) side of DC volts input.
But as mentioned, that did eliminate the hum but the preamp did not amplify the signal from a turntable.
I did do a DMM in diode mode check of the four 2SA950 transistors after I removed them and replaced with 2SA970's. All four 2SA950's passed. No fault found using a DMM in diode mode.
The hum I was hearing was faster than a 60Hz hum. Could be 120Hz? I do not have a frequency counter to check but what ever frequency it is, I cannot comprehend why it exists when the power source is confirmed as DC with no trace of AC present.
The incoming power immediately after the 7809 regulator I installed is 8.9V DC. Yet, at one of the RCA outputjacks, I measured about 0.39V AC. No AC voltage was detected at the other RCA output jack.
There is about 30mV or so of DC voltage across both RCA output jacks but I wasn't concerned by that low of a measurement.
Being confounded by this little black box, I figure it's time to give up and move on to something else like a 1949 RCA radio/phonograph combo.
I am tapping out.
These past 2 days I have spent much time pondering this thing. The wall warts do not seem to have been the issue. Rather, what ever is going on must be something internal within this AP-25 pre-amp.
Twice, I thought I had a Eureka! moment.
One was when I discovered that one leg of a recently installed capacitor was not soldered in place and another was an observation that caused me to question and change the polarity of the incoming DC power. That did eliminate the hum but the pre-amp did not function.
I could be wrong about the polarity swap but the outside barrel contacts of the RCA jacks should, I thought, go to the PC board ground. That meant that in it's state at that time the barrel contacts were connected to the +(pos) side of DC volts input. The swap I did now means the outside barrel contacts are connected to the -(neg) side of DC volts input.
But as mentioned, that did eliminate the hum but the preamp did not amplify the signal from a turntable.
I did do a DMM in diode mode check of the four 2SA950 transistors after I removed them and replaced with 2SA970's. All four 2SA950's passed. No fault found using a DMM in diode mode.
The hum I was hearing was faster than a 60Hz hum. Could be 120Hz? I do not have a frequency counter to check but what ever frequency it is, I cannot comprehend why it exists when the power source is confirmed as DC with no trace of AC present.
The incoming power immediately after the 7809 regulator I installed is 8.9V DC. Yet, at one of the RCA outputjacks, I measured about 0.39V AC. No AC voltage was detected at the other RCA output jack.
There is about 30mV or so of DC voltage across both RCA output jacks but I wasn't concerned by that low of a measurement.
Being confounded by this little black box, I figure it's time to give up and move on to something else like a 1949 RCA radio/phonograph combo.