I've removed Q301 and after reinstalling the amp with the same sub connected to Channel 1 it played briefly, and intermittently, before the signal stopped and it didn't play at all.
EDIT: for whatever it's worth, Q301 tested within spec in the ESR meter after being removed.
EDIT: for whatever it's worth, Q301 tested within spec in the ESR meter after being removed.
This is only a muting transistor. It's removal won't do any harm. See if you can find some other reason that the amplifier is no longer behaving like it did previously.
When you state that it played briefly, did it play as short of a time that it did before removing the transistor?
When you state that it played briefly, did it play as short of a time that it did before removing the transistor?
I just checked it again and realized I was mistaken: it's still playing a couple seconds on, a couple seconds off, but much quieter. With my phone volume maxed and the gain cranked way up it's just barely audible. My hearing isn't very good so I mistook it for silence at first.
Are you sure that you don't have a solder bridge on the pads for the muting transistor?
Are the other channels low?
Are the other channels low?
I connected the subwoofer to channel 2 and got a clean signal, a couple seconds on, a couple seconds off, as before. I guess that means I'm shorting the muting transistor on channel 1? I'll have to open the amp up and pull the board out again to be sure.
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If you leave the transistors clamped to the aluminum riser plates and you carefully monitor their temperature, you can operate the amp out of the heatsink, or at least just sitting in the heatsink, no need to take all of the time to completely disassemble and reassemble the amp every time you have to check something.
Good to know, thanks. Double-checked and there's no evident solder bridge or other short on the Q301 pads. No continuity.
Also, I went for a short drive today with the amp installed in my truck and powered, but not running, inputs disconnected, and by the time I got home it felt extremely warm, especially on the channel 1/2/3/4 power supply area.
Also, I went for a short drive today with the amp installed in my truck and powered, but not running, inputs disconnected, and by the time I got home it felt extremely warm, especially on the channel 1/2/3/4 power supply area.
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Operate all pots and switches through their entire range while listening to ch1 to see if the audio ever plays through at a normal level.
I was able to get a clean signal with my phone volume at medium and the gain turned up. Still quieter than would be suitable for highway driving. Turning the source volume all the way up caused a noisy, crackly signal. HP and LP filters are working as expected and the crossover adjustment seems fine.
Sorry for the delay in followup, thanks for all your help so far.
Sorry for the delay in followup, thanks for all your help so far.
After testing each channel, and without changing any volume or gain settings, channels 1, 2, 3 and 5 are all playing intermittently but at roughly the same volume. Channel 4 doesn't play at all.
Was ch4 playing previously?
Are you sure that the problem with ch4 isn't due to intermittent switch contacts?
Are you sure that the problem with ch4 isn't due to intermittent switch contacts?
I got an intermittent signal from it the last time I tried all channels. I verified that the pots and filter switch for channels 3 and 4 were working normally on channel 3 and they seem to be. I also cycled the filter switch with the subwoofer connected to channel 4 and got no output.
Does the DC voltage on the gates of the power supply FETs go to 0v when the audio cuts out?
What is the DC voltage on the gates of the power supply FETs when the audio is playing?
Don't let the probes slip.
If you're powering this from the battery of the vehicle, pull the two power fuses and use only a single 10 or 15 amp fuse.
What is the DC voltage on the gates of the power supply FETs when the audio is playing?
Don't let the probes slip.
If you're powering this from the battery of the vehicle, pull the two power fuses and use only a single 10 or 15 amp fuse.
Testing the main power supply (that drives channels 1 to 4), I get around +12v while music is playing and -3 to -2v when it cuts out. I get these values on the gate leg of all four MOSFETs in the power supply.
On the 494 driver IC for the PS for those channels, what's the voltage on all 16 pins? Copy and paste the following list and fill in the blanks. For the terminals that change when the amp is on/off, post 2 voltages for that terminal. Sharpen your probes to make them less likely to slip and short between terminals.
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:
Pin 15:
Pin 16:
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:
Pin 15:
Pin 16:
Here are the values I got. The voltages cycle through the values listed, holding at the first value for a second or two and then rising or falling as described, before returning to the third value.
There are two 494 ICs on this board so I tested both. I've attached a photo of the ICs on the board. My best guess is that the bottom one drives the subwoofer channel power supply and the top on the main power supply, but I'm sure you'd have a better sense than me.
Sub PS IC - Tested first
Pin 1: +2.5mV, spiking to +13mV and dropping back to +2.5mV
Pin 2: +5V, dropping to +1 to +3V and back to +5V
Pin 3: +85mV
Pin 4: +4.3V, dropping to +2.8V and climbing back to +4.3V
Pin 5: +1.6V, dropping to +0.7V and climbing back to +1.6V
Pin 6: +5V, dropping to +2.5V and climbing back to +5V
Pin 7: +2.3V, spiking to +4V, dropping to 0V, and climbing back to +2.3V
Pin 8: +12V, dropping to between +5 and +11V, and back to +12V
Pin 9: 0V, spiking to +190mV and dropping back to 0V
Pin 10: 0V, spiking to between +100 and +300mV and dropping back to 0V
Pin 11: +12V, dropping to between +4.9 and +11V, climbing back to +12V
Pin 12: +12V, dropping to between +5.8 and +11V, climbing back to +12V
Pin 13: +5V, dropping to +2.5V and climbing back to +5V
Pin 14: +5V, dropping to +1.8V and climbing back to +5V
Pin 15: +5V, dropping to +2.7V and climbing back to +5V
Pin 16: +2.5mV, spiking to +9mV and dropping back to +2.5mV
Main PS IC - Tested second
Pin 1: +3.5mV, spiking to +7mV, dropping to +3.5mV, dropping to 0V, climbing back to +3.5mV
Pin 2: +870mV dropping to +500mV and rising back to +870mV
Pin 3: +123mV dropping to +80mV and rising back to +123mV
Pin 4: +4.4V dropping to +2.7V and rising back to +4.4V
Pin 5: +1.6V dropping to +0.7V and rising back to +1.6V
Pin 6: +3.7V dropping to +1.0V and rising back to +3.7V
Pin 7: +3.5mV, spiking to +7mV, dropping to +3.5mV, dropping to 0V, climbing back to +3.5mV
Pin 8: +12V dropping to +7V, climbing back to +12V
Pin 9: +3V dropping to +0.3V, climbing back to +3V
Pin 10: +3V dropping to +0.3V, climbing back to +3V
Pin 11: +12V dropping to +6V, climbing back to +12V
Pin 12: +12V dropping to +6V, climbing back to +12V
Pin 13: +5V dropping to +2V, climbing back to +5V
Pin 14: +5V dropping to +2.7V, climbing back to +5V
Pin 15: +5V dropping to +1.9V, climbing back to +5V
Pin 16: +3.1mV, spiking to +9mV and dropping back to +3.1mV
There are two 494 ICs on this board so I tested both. I've attached a photo of the ICs on the board. My best guess is that the bottom one drives the subwoofer channel power supply and the top on the main power supply, but I'm sure you'd have a better sense than me.
Sub PS IC - Tested first
Pin 1: +2.5mV, spiking to +13mV and dropping back to +2.5mV
Pin 2: +5V, dropping to +1 to +3V and back to +5V
Pin 3: +85mV
Pin 4: +4.3V, dropping to +2.8V and climbing back to +4.3V
Pin 5: +1.6V, dropping to +0.7V and climbing back to +1.6V
Pin 6: +5V, dropping to +2.5V and climbing back to +5V
Pin 7: +2.3V, spiking to +4V, dropping to 0V, and climbing back to +2.3V
Pin 8: +12V, dropping to between +5 and +11V, and back to +12V
Pin 9: 0V, spiking to +190mV and dropping back to 0V
Pin 10: 0V, spiking to between +100 and +300mV and dropping back to 0V
Pin 11: +12V, dropping to between +4.9 and +11V, climbing back to +12V
Pin 12: +12V, dropping to between +5.8 and +11V, climbing back to +12V
Pin 13: +5V, dropping to +2.5V and climbing back to +5V
Pin 14: +5V, dropping to +1.8V and climbing back to +5V
Pin 15: +5V, dropping to +2.7V and climbing back to +5V
Pin 16: +2.5mV, spiking to +9mV and dropping back to +2.5mV
Main PS IC - Tested second
Pin 1: +3.5mV, spiking to +7mV, dropping to +3.5mV, dropping to 0V, climbing back to +3.5mV
Pin 2: +870mV dropping to +500mV and rising back to +870mV
Pin 3: +123mV dropping to +80mV and rising back to +123mV
Pin 4: +4.4V dropping to +2.7V and rising back to +4.4V
Pin 5: +1.6V dropping to +0.7V and rising back to +1.6V
Pin 6: +3.7V dropping to +1.0V and rising back to +3.7V
Pin 7: +3.5mV, spiking to +7mV, dropping to +3.5mV, dropping to 0V, climbing back to +3.5mV
Pin 8: +12V dropping to +7V, climbing back to +12V
Pin 9: +3V dropping to +0.3V, climbing back to +3V
Pin 10: +3V dropping to +0.3V, climbing back to +3V
Pin 11: +12V dropping to +6V, climbing back to +12V
Pin 12: +12V dropping to +6V, climbing back to +12V
Pin 13: +5V dropping to +2V, climbing back to +5V
Pin 14: +5V dropping to +2.7V, climbing back to +5V
Pin 15: +5V dropping to +1.9V, climbing back to +5V
Pin 16: +3.1mV, spiking to +9mV and dropping back to +3.1mV
Thanks for the detailed information.
Is the DC voltage across the main (primary) B+ and ground terminals dropping to about 6v when the amp tries to power up?
Are you still testing in the vehicle?
Is the DC voltage across the main (primary) B+ and ground terminals dropping to about 6v when the amp tries to power up?
Are you still testing in the vehicle?
With the amp powered but no input or output connected, the voltage across B+ and ground sits at +12V, dropping to between +8 and +8V and climbing back to +12V in the same way as described with the IC pins.
I'm now testing on my desk with a switched 12V 3A wall wart; it's not optimal but it's what I have available. It felt less risky than trying to balance the amp in a way where I could test the IC pins in the truck.
I'm now testing on my desk with a switched 12V 3A wall wart; it's not optimal but it's what I have available. It felt less risky than trying to balance the amp in a way where I could test the IC pins in the truck.
Wallwarts will typically fail the first time they are overloaded.
Do you have an old computer power supply?
In the truck, your amp could be starving for current due to a problem in the wiring or it could be pulling excess current and that was pulling the voltage down.
What fuse do you currently have in the amp?
Do you have an old computer power supply?
In the truck, your amp could be starving for current due to a problem in the wiring or it could be pulling excess current and that was pulling the voltage down.
What fuse do you currently have in the amp?