Pin 4 is the problem. I'll see if I can determine what's driving it when I get back in the shop.
See if the voltage is always the same on pin 4 when it won't power up.
See if the voltage is always the same on pin 4 when it won't power up.
The last readings I posted the amp was not powering up.
Checked it today and amp is powering up. Voltage on pin 4 is at +0.190 and pulsing up and down by about 0.010 volts. Noticed as I continued to monitor it voltage slowly starts dropping and still pulsing as I described. After about 2 minutes it gets down to +0.130 still pulsing as described.
Checked it today and amp is powering up. Voltage on pin 4 is at +0.190 and pulsing up and down by about 0.010 volts. Noticed as I continued to monitor it voltage slowly starts dropping and still pulsing as I described. After about 2 minutes it gets down to +0.130 still pulsing as described.
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My thoughts are, there is a component in that circuit that while it is cold has internal connection but as it warms up and ever so slightly expands it looses that internal contention.
What's the resistance between the RCA shields and the main ground with nothing plugged into the RCA jacks?
What is the voltage on the RCA shields (black probe on ground) when the amp refuses to come on?
What is the voltage on the RCA shields (black probe on ground) when the amp refuses to come on?
Resistance between RCA shields and main ground: 119.5ohms
Voltage on RCA shields when amp will not power up: 0.00 volts
I did note there are areas of the transformer that are reaching 145 degrees Fahrenheit. Is this normal?
Voltage on RCA shields when amp will not power up: 0.00 volts
I did note there are areas of the transformer that are reaching 145 degrees Fahrenheit. Is this normal?
Is the transformer heating unevenly?
How long did it that to get that hot?
How hard were you running the amp?
How long did it that to get that hot?
How hard were you running the amp?
It is was one of the parts I replaced with one from my other board. The one that was on it appeared to have a spot on it where it had gotten hot. It also had a number of the components on the power side blown. I switched it back to the original one and it does the same thing. So in answering your question it applies to both transformers that I tried.
Yes, it is heating unevenly.
About 7 - 10 minutes.
Not hard at all. Gain on lowest setting. Volume on radio set low at 5 (volume goes to 30)
Yes, it is heating unevenly.
About 7 - 10 minutes.
Not hard at all. Gain on lowest setting. Volume on radio set low at 5 (volume goes to 30)
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I'll have to find the various bits of the amp I have to put it back together to run it to see how it heats up.
Hey Perry, I have an update for you. I was wrong about the transformer heating up unevenly. Apparently my Raytek IR thermometer isn't reading exactly where the laser is pointing, it is reading slighty under it. In paying attention to that, as I go around the transformer, I get a even reading.
The transformer is the only part I can find on the board that is heating up...
Also the amp is intermittently is powering up and down. May power up for 10 seconds and shut down, may power up for 15+ minutes before shutting down. When it does shut down, if I leave the power supply on, the amp eventually comes back on. That also varies, may only take a couple seconds to around a minute or so.
The transformer is the only part I can find on the board that is heating up...
Also the amp is intermittently is powering up and down. May power up for 10 seconds and shut down, may power up for 15+ minutes before shutting down. When it does shut down, if I leave the power supply on, the amp eventually comes back on. That also varies, may only take a couple seconds to around a minute or so.
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Do you think it is possible that the replacement caps I installed could be causing the problem?
I don't know why the caps would be causing a problem. Is pin 4 of the 494 still going high when it shuts down?
Do you have any loose/spare resistors, something in the range of 1k-10k?
I'd like to see how much it takes to keep pin 4 down if the resistor is connected between pin 4 and pin 7.
I'd like to see how much it takes to keep pin 4 down if the resistor is connected between pin 4 and pin 7.
I installed the 10k resistor between pins 4 and 7 of the 494. It produced a change it what the amp has been doing. After about 10 minutes of playing, the sound started cutting out. The difference is at this point, the amp never powered down or did the voltage rise on pin 4 as it had been doing.
For reference, what is the pin 4 voltage when it starts to cut out?
Have you tried using a heat gun to see if the fault was related to temperature?
And if so, which area of the board is sensitive to the temperature change?
Have you tried using a heat gun to see if the fault was related to temperature?
And if so, which area of the board is sensitive to the temperature change?
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