I noticed another potential issue with my 1a today...
When the amp is powered on, the right channel measures 0.00-ohms across the output terminals, which I think is about normal. However, the left channel measures about 49-ohms, which seems wrong. Has something in a feedback loop perhaps become disconnected? I reflowed many of the solder joints on this channel, but apparently still have an issue with it. It sounds fine, though, despite the strange measurement.
Any tips here? I seem to have a never-ending stream of trouble with this amp...
When the amp is powered on, the right channel measures 0.00-ohms across the output terminals, which I think is about normal. However, the left channel measures about 49-ohms, which seems wrong. Has something in a feedback loop perhaps become disconnected? I reflowed many of the solder joints on this channel, but apparently still have an issue with it. It sounds fine, though, despite the strange measurement.
Any tips here? I seem to have a never-ending stream of trouble with this amp...
I noticed another potential issue with my 1a today...
When the amp is powered on, the right channel measures 0.00-ohms across the output terminals, which I think is about normal. However, the left channel measures about 49-ohms, which seems wrong. Has something in a feedback loop perhaps become disconnected? I reflowed many of the solder joints on this channel, but apparently still have an issue with it. It sounds fine, though, despite the strange measurement.
Any tips here? I seem to have a never-ending stream of trouble with this amp...
*** Note: Mr Pass posted while I was typing. Of course he is right and I assumed you were taking the reading after the amplifier was disconnected and powered off ***
You took that measurement with the amplifier off and disconnected? Yet you were able to use it?
0.00 Ohms across right channel out? Not good. This means the right channel is subjected to driving across a dead short. There is a fault and it will be very risky to power up the amplifier until the fault is corrected. Could you have created a solder bridge at some point when reflowing the solder joints? Just a thought.
I would print out the schematic that Zen Mod provided and use that to narrow your search. The beauty of a stereo amplifier, though, is you have two identical circuits to compare each other with. We know we have a problem with the right channel. You can start taking resistance measurements to ground at likely test points to see if you have any clues as to where the fault might be. At some point you will have to lift one or two connections of the component you need to check. Assuming there is no physical short accross any of the circuits then I would suspect a semiconductor junction has broken down or a capacitor is shorted.
Capacitors are easy enough to check. You should expect to see it go from low resistance to open as it charges. Small value caps do this quickly. Perhaps faster than the meter can show. Large value caps will do this over a period of time as it charges to the maximum voltage the meter provides. This will not tell you everything but the main concern for now is that there is no damage to the dialectric that is supposed to insulate the plates from each other. It is possible it could be arcing at higher voltages (carbon trace) but this is a very small possibility and will not account for the 0.00 Ohm reading.
Most meters have a diode check nowdays and can be used to check the basic integrity of a semiconductor junction. It is usually necessary to check the device out of circuit to get an accurate indication. Treat a transistors as though it was two diodes wired with either their anodes or cathodes being common. You should see it conduct accross the junction with the probes connected in one direction (a reading of 500 to 700 Ohms is typical of a silicon device) and read open the other way. You will need to look at the junction between base to collector and base to emitter to see if it acts like a diode. Last check is between Collector to emitter which should read open. I have seen a transistors read normal until this last check and show a direct connection at this point.
That should be enough to get started. Once you find the problem then you will have an idea as to where to go from there.
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The ohm measurements were made with the amp on. With the amp off, both channels start reading in the hundreds of k-ohms and the meter moves upward into the M-ohms.
As for DC offset, the right channel sits at about 25mV, the left at about 10mV.
I was just wondering if output resistance was a measurable thing, which prompted this test... I guess the difference in measured resistance is nothing to worry about, then?
As for DC offset, the right channel sits at about 25mV, the left at about 10mV.
I was just wondering if output resistance was a measurable thing, which prompted this test... I guess the difference in measured resistance is nothing to worry about, then?
Ah... never mind. An Ohm meter cannot be used on an energized circuit and give you a meaningful reading.
Output resistance with your Forte has little meaning, depending on the question being asked. The output transistors, to my mind, act as a signal variable resistors between the power supply rails and the load. So the output resistance will vary with drive signal.
Unless I have missed it there does not appear to be an offset adjustment in the schematic. I think the combination of R8 and C2 is supposed to be a filter to provide DC compensation for the the channel. And I believe R15 is the bias adjustment that can be measured at the test points 14 a~e and 15 a~e. But you will have to have a reference to tell you what should be read at the test points.
Are you trying to eliminate the hum problem that you mentioned before?
The really bad hum problem has been fixed, for now. I reflowed the solder joints, which fixed the right channel DC/hum issue.
The remaining hum is very minimal and is normal, I suppose, for this type of amp.
I guess I'll call the amp 'fixed' for now...
Might be the wise thing to do. There is always a risk of accidentally damaging the amplifier by continuing to fool with it. You can always try swapping the inputs and then swap the outputs to see if the minor hum follows the switch or not. You will want to power down the Forte when you do this. But if you have music again then you have accomplished what you set out to do.
Have a happy!
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