Anthem Amp 2 debugging help

[ronenash]

Hi all,

I have an Anthem Amp 2 power amplifier for which I upgrade several caps. After the upgrade the resistors marked in red in the schematic are over heating. I suspect its a bias issue. Can anyone help me with instructions on how to set the bias for this amp. There is a bias potentiometer but I do not know what I need to measure when setting the bias.

Any other ideas why these resistors are over heating?

Thanks
Ronen

[gagnou]

Hi Ronen,

Nothing to worry about. Those resistors are essentially series connected between +-80v supply.
Current flowing through is 53mA, leading to 2.13W dissipation for each resistor.
The resistors are rated at 3 times this value so not really a problem.

Check the output, it should be close to 0 volts.

If you only replaced some capacitors then biasing of output transistors shouldn't have changed at all from the manufacturer's settings.

Regards,
George

[ronenash]

Hi George,

Apparently something is wrong since the resistors in one channel are blown. I disconnected power before the other channel was toasted. The resistors got so hot that I could not touch them.
I checked for the voltage at the output and it was 0.019v. The transistors did not get hot and I did get sound from the speakers although I cannot tell if it was distorted or not since I used small test drivers.
Maybe its not the bias but something else. Any other ideas?

Regards,
Ronen

[Jan Dupont]

Nothing to do with the bias I think....
Which caps did you upgrade ?

[ronenash]

Hi Jan,

I replace C7, C8, C13, C16 with Wima MPK10 capacitors. I also installed 0.1uf Wima MPK on each of the power supply filter capacitors.

I appreciate your assistance,
Ronen

[gagnou]

I take it that you mean Wima MKP10?
If so there's no 330pF value in that series for C13 and C16.

Replacing those caps with wrong values could lead to oscillation and heating of the resistors.

Regards,
George

[ronenash]

Yes George, the 0.1uf is indeed MKP10 and the 330pf is FKP2. I can try to return the original 330pf capacitors. Do you think the 0.1uf (C7, C8) can also be a potential problem?

Again, thanks for your assistance.
Ronen

[gagnou]

I see no reason to change c7 and c8 to something better than cheap ceramic or polyester caps.
Wimas MKP10 are overkill for the position in my opinion.
Q14 and Q13 serve the secondary function to protect the output stage in case of short-circuit or abnormally low loads.

FKP2, 330pF should be fine, although the original NPO ceramics should be just as fine.

Were all four of the resistors overheating or just two of them?

[ronenash]

As far as I remember all four resistors were heating.
One more thing I just remembered, I also replace the 0.1uf (C14) in the Zobel network.

I don't have the amplifier infront of me right now but I will check again tomorrow.

I will try to return the original 330pf capacitors this weekend to see if it makes a difference.

[gagnou]

Hi Ronen,

A polypropylene cap is considered perfect for the zobel network.

Was the zobel resistor also overheating?

Regards,
George