One Heat Sink Hot - One Not?

[skippy_ps]

I've got a MAC 4100 with the left side heat sink pretty hot (assuming this is the right channel) and the right side heat sink only slightly above body temp. The left sink doesn't pass the "5 second finger on test." Hot! This after receiver on for about 30 mins at not really loud volume level - maybe about 3 - 10 watt peaks on front panel indicators. Balance knob centered, sound from speakers is balanced.

DC offset is about 5 mv right channel and 19 mv left channel. Receiver draws .5 amps at idle, volume all the way down. Don't see any pins to check bias voltage.

Any ideas?

Murray

 

[EchoWars]

Obviously the bias is grossly misadjusted...if it's that hot, the life of the transistors will be shortened. Get it adjusted.

 

[dewickt]

You need to adjust the bias, it's the only pot on the driver board.
Operate at 120V with no input signal, AC current should be approx. .3A.
To adjust - turn pots full CCW
rotate pot CW to the point where the line input power or current begins to increase then back off slightly to the point where the line input just reaches it's lowest value.
Now do the other channel.
The 4100 should idle with the AC power at about 40W or current at .3A if you can not get it close to this amount you have a component failure in one of the outputs or drivers.

 

[skippy_ps]

OK Now

Bias pot for the hot 'sink got turned down about 1/4 turn. Sheesh! Turned it down all the way and then up 'til amp meter started hopping around at about .31 amps. Adjusted right 'sink too, down about 1/8 th turn.

Seems ok now - both 'sinks at about the same temp (a tad cooler than body temp.)

Thanks EW and Terry. :thmbsp:

Murray

(meter shows .33 amp now)

 

[fatrhino]

what kind of meter is that?

 

[skippy_ps]

Kill A Watt

Shows voltage, amps, watts, voltamps, hertz, power factor (pf) and kilowatt hours - has a clock built in to see how much juice has been consumed over time. Watts are adjusted for power factor - a complicated subject that I have only a limited understanding of.

Sells for about $30. Got mine at J&R about a year ago. mfg is p3international.com. Put www. in front of that.

Murray

 

[dewickt]

That looks like my next buy for the bench, I like it.