I’ve got a USA-200 that speaker wires accidentally touched and fried a few resistors. I replaced all resistors out of tolerance and tip36 & tip35 transistors. And also replaced a few MPSA06 transistors and MPSA56 transistors. Amp powers up drawing less than 1 amp of current but no sound. I checked the TL074 and sound is present on pins 1 & 7. The relay is engaging which should release the amp out of muting sound.
I don't think I have anything similar. Can you find audio on any of the discrete components in either channel?
This is the result of 40hz sine wave from one of the pins of the relay to the base of a MPSA06 & MPSA56. The sine wave looks perfect on the TL074 but looks like this after the relay.
What's the DC voltage measured directly across the speaker terminals of each of the two channels?
The amp may be in protect due to the DC offset.
What happens to the 2v of offset if you touch a load across the speaker terminals?
Do you have positive and negative rail voltage?
Do you have positive and negative regulated voltage on the power supply pins of the op-amps?
What happens to the 2v of offset if you touch a load across the speaker terminals?
Do you have positive and negative rail voltage?
Do you have positive and negative regulated voltage on the power supply pins of the op-amps?
Yes, I have positive and negative 15 volts on the op amp and positive and negative rail voltage. Once a load is applied to the speaker terminals the DC offset goes away.
Can you tell how the relay is being used?
Is the coil receiving and maintaining 12v?
Are the contacts reading 0 ohms (closed) when the coil has voltage?
Is the coil receiving and maintaining 12v?
Are the contacts reading 0 ohms (closed) when the coil has voltage?
Didn't you state that the signal was different after the relay, earlier, compared to the output of the TL074?
Yes, the first photo is the output pin 7 of the TL074. Second photo is coming from the relay to the MPS transistors.
What's the resistance between the common and NC relay contacts when there is no coil voltage?
And then the resistance between the common and the NO contacts when there is 24v of coil voltage?
And then the resistance between the common and the NO contacts when there is 24v of coil voltage?