Hafler DH-220 problems
I posted some replys on old threads for Hafler amps, but seems no one is looking at them, so I decided to start a new one. I need some basic help on 2 old hafler dh-220s I picked up on eBay that are exhibiting different problems. To avoid confusion, I just want to discuss one amp. If I can resolve it, then I will move to the other.
So the problem is with the dc offset in the right channel. When I got the amp, before firing it up, I took some good advice and checked the bias and offset in both channels. The left channel I easily dialed the offset in to .27A (my meter doesn't give me the third digit, so 270mA was as close as I could get to spec of 275mA), and the offset, measured at the speaker outputs, I got to 0V, with fluctuation up and down through 2.3mV or so, without going negative.
1. Is this fluctuation normal, or do I have a problem here?
On the right channel, I also got the bias offset easily to 270mA, but the dc offset would not go below 203mV with the pot at full ccw stop. I sprayed and worked with electro contact cleaner, and the pot will definitely adjust upwards to about 700mV, just not down to zero. I have checked the resistors around both sides of the pot (without removing from the board, and without the amp on), and they seem to read very close to the values that are banded on them - definitely within the percent value band.
So, based on threads on this forum and others, I suspect I have transistor issues either feeding the offset pot, or others earlier in the power input circuit. Looking at the hafler manual, it gives a schedule of values on each leg of each transistor on the pc-19c board, with instructions that they are:
"measured with 120 volt line, no signal. with respect to the ground bus between the two capacitors in the power supply."
Since I have very limited experience with basic amp architecture, but think I can repair this amp with a little guidance, my question is:
2. Does this mean I should power on the amp, with NO input or output connected, and connect the neg lead of my MM to the wire connecting the two big 10,000mF filter caps (which has multiple neg wires soldered to it), and use the pos lead of the MM to touch each leg (emitter, collector, base) of the transistor to read the voltages?
If I do not get values close to those stated in the manual, I will hopefully identify the problem, replace the offending pair of transistors with a new, BALANCED pair of transistors, and hopefully be able to bring the dc offset in line so I can get this amp going.
Any and all responses will be greatly appreciated.