Bought this amp hooked it up and instantly went in and out of protection while making a ticking noise. I took it apart in my lab and realized one power supply fet was hot. The fet tested bad so bought 3 new Fets for power supply. Checked the UC3843B and found the following on the pins with Fets out:
pin 6 and had the same pulses on gate pad of mosfet
pin 4 sawtooth
pin 3 similar sawtooth but has 1.7 volts.
pin 8 5.5 volts
My thought is pin 3 should be below 1 volt for operation and followed the trace to Q4 transistor which tested bad. Have t replaced Q4 as I’m not sure this is the true problem. When power supply fets were in I had 60ish positive rail voltage at outputs, can’t remember exact number. Totally thrown off here with this amp.
pin 6 and had the same pulses on gate pad of mosfet
pin 4 sawtooth
pin 3 similar sawtooth but has 1.7 volts.
pin 8 5.5 volts
My thought is pin 3 should be below 1 volt for operation and followed the trace to Q4 transistor which tested bad. Have t replaced Q4 as I’m not sure this is the true problem. When power supply fets were in I had 60ish positive rail voltage at outputs, can’t remember exact number. Totally thrown off here with this amp.
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You are right about Pin 3 of the UC3843. I believe that the output or housekeeping supplies may be the reason why it is high.
You can check the DC voltages on Pins 1(Vdd) &2(CSD) of IC14 and IC15.
Check DC voltages around Q21 and ZD3,ZD4,ZD5.
Probe Pins 3 & 5 of IC11 for a sawtooth waveform.
All referenced to Power Ground. Post your results and screenshots.
Posting a pic will allow me to edit and point out testpoints etc. Also,I will be able to determine which of the Revs you have. Newer versions use a Microcontroller for the protection circuitry, older versions use an LM339.
You can check the DC voltages on Pins 1(Vdd) &2(CSD) of IC14 and IC15.
Check DC voltages around Q21 and ZD3,ZD4,ZD5.
Probe Pins 3 & 5 of IC11 for a sawtooth waveform.
All referenced to Power Ground. Post your results and screenshots.
Posting a pic will allow me to edit and point out testpoints etc. Also,I will be able to determine which of the Revs you have. Newer versions use a Microcontroller for the protection circuitry, older versions use an LM339.
Check D33 and D34-1N4148 if one is shorted replace and install any outputs or PS mosfets if necessary and power up amp as you have been.
If the amp is still in and out of protection, remove D18-1N4148 and recheck.
D33,D34 are tied to the outputs, back to protection,IC17-P7. Shorted D33 or D34 are common problems. D18 is tied to the NTC, and if this brings the amp out of protection there may be a problem with a resistor tied to the IC which is a common failure.
If the amp is still in and out of protection, remove D18-1N4148 and recheck.
D33,D34 are tied to the outputs, back to protection,IC17-P7. Shorted D33 or D34 are common problems. D18 is tied to the NTC, and if this brings the amp out of protection there may be a problem with a resistor tied to the IC which is a common failure.
Look for R71-383K ohm and R69-100K ohm. They create a voltage divider for over voltage (OVP) or under voltage protection (UVP), I don't recall which.
Relowing the solder to both resistors may do the trick, but I would recommend replacing both resistors, for a more reliable fix.
As I said this is a common problem, but it's possible something else, such as the IC17-LM339 has failed, but I'm fairly certain the problem is in the voltage divider.
Relowing the solder to both resistors may do the trick, but I would recommend replacing both resistors, for a more reliable fix.
As I said this is a common problem, but it's possible something else, such as the IC17-LM339 has failed, but I'm fairly certain the problem is in the voltage divider.
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