Gainclone - does not fire up.

[jazko]

Good evening,
I assembled my gainclone yesterday but it is not working. I'm using a 100 watt toroidal transformer 2x19 VAC. Both the PSU and the LM1875 PCBs were kits, and I am sure I did everything right (checked so many times). Right now I dont have a potentiometer(is it necessary or can do without)?

[AndrewT]

what is the voltage on the transformer secondaries with the rest of the amplifier completely disconnected?

[jazko]

2x19 volts no load - 26 volts rectified. I did not measure the voltage when did the test run, using 8ohm speakers. I suppose the amp can work without a pot.

[Greg Erskine]

Originally Posted by jazko

I am sure I did everything right (checked so many times). Right now I dont have a potentiometer(is it necessary or can do without)?

Hi jazko,

You haven't given us anything to enable us to help you. How about posting a picture or the wiring diagram. If you did everything right it should work (assuming the kits are OK).

The pot is only necessary as a volume control.

Did you measure the DC offset voltage before attaching any speakers?

regards

[jazko]

Greg,
DC offset is 0. Here are some photos. On the gainglone PCB's from left to right - Input (GND and +), voltage supply (- VDC, GND, + VDC), output (GND, +). On the power supply board - (blue - VDC, brown - GND, green - +VDC). On the other side AC - GND - AC from the toroid.

[Greg Erskine]

hi jazko,

DC offset is very unlikely to be zero. Are you using the mV range? I would expect around 10mV to 100mV.

Did you measure all the output voltages of the PSU before connecting the amps? Did you get 26-0-26 on both supplies?

It is best practice not to use green wire for anything but earth (or 0V). It will confuse us as we always expect green or green/yellow wire to be at earth potential.

Double check all wiring. Double check cap orientation.

Just try one amp at the time without the speakers connected. Measure DC offset. If less than 100mV, connect a cheap test speaker. Connect source and listen.

If you have already connect the power the wrong way the chips may have blown. Have your fuses blown at any stage?

regards

[THS]

Originally Posted by jazko

Greg,
DC offset is 0. Here are some photos. On the gainglone PCB's from left to right - Input (GND and +), voltage supply (- VDC, GND, + VDC), output (GND, +). On the power supply board - (blue - VDC, brown - GND, green - +VDC). On the other side AC - GND - AC from the toroid.

I'll recomend to double check the orientation off your psu caps.

[jazko]

Hello, everyone, I'm sorry about the wires, I will redo them afterwards. I will check the offset again, I was using the auto-range of my multimeter. Yes there is 26-0-26 on both channels and cap orientation is correct as well as wiring. No blown fuses, I'm using 1 amp fast blow.

[jazko]

Offset is 0.6mV for one of the channels and 1.1 mV for the other.Still don't know what is wrong Chips look fine, heatsinks are warm. Power supply 26-0-26 to both channels.

I will take it to uni this week and make some measurements with an oscilloscope.

[AndrewT]

heatsinks warm and output offset consistently low. Seems like there is no oscillation.
Measure signal level at the sig input terminals