Hello.
I am new here. I was interested in electronics as a kid but never really developed the hobby. I have worked around electricity all of my life. I have decided now that I am going to pick it up and try to understand it.
Resistors, capacitors, diodes, I understand. I'm struggling getting transistors straight in my head to where I can look at a schematic in my head and simulate what I think should be happening.
My first project is to fix a car audio amp that my brother reversed polarity to 20 years ago. I tinkered with it a little back then, but never got anywhere with it.
Sherwood GA-1072BP
I have found a service manual/schematic that is listed for this unit, but is slightly different.
The differences I have found:
R558 - 10k in the unit, not the 22k listed
R568 - 5.1k in the unit, not the 100k listed
D520 - Does not exist. This line was fed from the AC line just beyond transformer T501. Instead, on this unit, it comes from the rectified line feeding the amp.
Between pin 2 and pin 13/14 of IC501 is what I believe are two thermistors in series. One mounted on each side of the case heat sinks, wired in series at about 170 ohm at room temp.
What I have discovered.
Highlighted components on the attached partial schematic have been tested
I put 14.4 volts into the unit as it lists on the schematic.
I measure 4.16v at pin 2 of the 494C instead of 4.3v listed.
I measure 3.75v at pin 1 of the 494C instead of 4.3v listed.
Output is at about 57v, instead of the listed 66v listed
I do have the 5v Vref at pin 13 of the 494C
I do have 10.6v at pin 12 of the 494C (10.5v listed)
Diode D514 was blown in half when I started and has been replaced with listed diode
Frequency of the oscillator is about 35khz
Pulse width of the two channels are the same.
The values of the highlighted components have been verified (probably some addition before I started highlighting. The pink highlighter was removed from circuit to check.)
I have a few questions:
1) Obviously, if someone can give me some direction, I would be excited to hear.
2) How does feedback work on 494C dual error amplifier work if they are tied together?
3) While powered on, if the error amps are working, should the inputs be the same? (Pin 1 = Pin 2 and Pin 15 = Pin 16) -
4) Am I barking up the wrong tree? as in, is the power supply close enough that the problem is probably in the amplifier section?
Thanks for any input