Schreck,
A variety of things can cause the symptoms you describe. Do you have access to a Variac? Do you have a good digital VOM? What experience do you have sleuthing problems?
The first 4 small signal transistors can cause problems, especially the first 2 as they get banged on by excessively strong input signals. The pre-driver and driver transistors are highly biased and carry substantial current and can be prone to failure.
I once "repaired" a DH-220 with a bad channel by resoldering the joints that connect C1 to the circuit. Over the years solder joints can become corroded and intermittent.
With a Variac I would slowly ramp up the voltage while tapping on the board's components with something like a plastic toothbrush handle while listening for problems that might be caused by a loose connection or broken device.
Another strategy is to apply heat and cold, selectively, to different board components to try and identify one that may cause more or less problems depending on temperature.
Perhaps someone else has a suggestion or two. But, you need to better identify which part/s or solder joints are causing the symptoms before you begin throwing parts at them to solve a problem.
Also, wiring may have bad solder joints that can cause problems.
The manual has a table of voltages expected at various points on the circuit board. You could do some voltage measuring referenced to ground. Be careful as the power supply filter caps carry a lethal amount of charge!
A variety of things can cause the symptoms you describe. Do you have access to a Variac? Do you have a good digital VOM? What experience do you have sleuthing problems?
The first 4 small signal transistors can cause problems, especially the first 2 as they get banged on by excessively strong input signals. The pre-driver and driver transistors are highly biased and carry substantial current and can be prone to failure.
I once "repaired" a DH-220 with a bad channel by resoldering the joints that connect C1 to the circuit. Over the years solder joints can become corroded and intermittent.
With a Variac I would slowly ramp up the voltage while tapping on the board's components with something like a plastic toothbrush handle while listening for problems that might be caused by a loose connection or broken device.
Another strategy is to apply heat and cold, selectively, to different board components to try and identify one that may cause more or less problems depending on temperature.
Perhaps someone else has a suggestion or two. But, you need to better identify which part/s or solder joints are causing the symptoms before you begin throwing parts at them to solve a problem.
Also, wiring may have bad solder joints that can cause problems.
The manual has a table of voltages expected at various points on the circuit board. You could do some voltage measuring referenced to ground. Be careful as the power supply filter caps carry a lethal amount of charge!
First I would remove the 3 small self-tapping screws so you can fold the PCB out to get at its back side.
Then, I would reflow all the solder joints to make sure the joints are still OK.
The MOSFET output devices are very rugged and almost never fail, but it was smart to swap them around as a test.
It is very difficult to do PCB level work on an amplifier without a Variac. Do you know about the light bulb in series with incoming AC as a way to reduce voltage? Anything to control the voltage would be helpful.
Then, I would reflow all the solder joints to make sure the joints are still OK.
The MOSFET output devices are very rugged and almost never fail, but it was smart to swap them around as a test.
It is very difficult to do PCB level work on an amplifier without a Variac. Do you know about the light bulb in series with incoming AC as a way to reduce voltage? Anything to control the voltage would be helpful.
Well . . . I guess the next thing to try is to selectively heat and cool various components to determine if any of them change their popping sounds.
There is a product, sometimes called "Cool-It" which is a refrigerant in a compressed gas can that can direct a very cold spray on various parts. You could do this while monitoring for pops and crackles.
Also, you could use a heat gun or hair dryer to direct hot air at various parts to determine if heating them makes a difference.
If these two tests produce no results, and because your tapping on parts and reflowing solder joints don't seem to make a difference, I guess you are to the point of having to begin to throw parts at the circuit.
If you had a signal generator and oscilloscope you might better determine at which part of the circuit the noise begins, which could point you to which part/s are faulty. Otherwise, you have to use a guesswork strategy of which parts to replace.
The first 4 transistors are often a problem but they should be matched to some degree on hFe, so just buying some parts may not be the answer. The DH-220 does have a DC nulling circuit, but if the parts you choose differ too much on their hFe they could produce DC offsets too large to be corrected. So, if you could pre-select some parts and get them matched at least somewhat you could install them and see if they make a difference.
The other parts should be matched PNP/NPN to a degree -- if possible. Replacing them one at a time is an attempt to get rid of the problem. Also, you could monitor the various voltage check points and attempt to see if their voltages jump around when you hear the noises.
Obviously, I am grasping at straws here. Good luck. Others may have better suggestions on what to do.
There is a product, sometimes called "Cool-It" which is a refrigerant in a compressed gas can that can direct a very cold spray on various parts. You could do this while monitoring for pops and crackles.
Also, you could use a heat gun or hair dryer to direct hot air at various parts to determine if heating them makes a difference.
If these two tests produce no results, and because your tapping on parts and reflowing solder joints don't seem to make a difference, I guess you are to the point of having to begin to throw parts at the circuit.
If you had a signal generator and oscilloscope you might better determine at which part of the circuit the noise begins, which could point you to which part/s are faulty. Otherwise, you have to use a guesswork strategy of which parts to replace.
The first 4 transistors are often a problem but they should be matched to some degree on hFe, so just buying some parts may not be the answer. The DH-220 does have a DC nulling circuit, but if the parts you choose differ too much on their hFe they could produce DC offsets too large to be corrected. So, if you could pre-select some parts and get them matched at least somewhat you could install them and see if they make a difference.
The other parts should be matched PNP/NPN to a degree -- if possible. Replacing them one at a time is an attempt to get rid of the problem. Also, you could monitor the various voltage check points and attempt to see if their voltages jump around when you hear the noises.
Obviously, I am grasping at straws here. Good luck. Others may have better suggestions on what to do.
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