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|27th January 2013, 06:50 PM||#11|
Join Date: Jan 2013
Another DH-220 Offset problem
Thanks for this great post. I'm a total newbee to electronics, but am trying to repair 2 DH-220's I purchased from different sources on eBay. My intent is to bridge each one to run as mono amps, but before bridging, I checked the bias and DC offset. On one amp, I was easily able to set the Bias on both channels to .27 A (my meter won't give me the third digit, so 270 mA is as close as I can get to spec of 275 mA). I was also able to set the Offset on the LEFT channel (measured at the output 5-way speaker posts with the amp ON) to 0 V, with the voltage fluctuating up and down through +2.8 mV, but not going negative.
1. Is this fluctuation normal?
On the RIGHT channel, I was only able to get down to 206 mV with the P1 pot at the full CCW stop. I have cleaned P1 with LPS Electro Contact Cleaner to no affect, and have checked the resistance of surrounding R4 & R5 and R6 & R7, and all are VERY close to their banded values.
I have not checked the transistors since I’m not sure exactly how to proceed. I read in the Hafler DH-220 Manual to test each leg (collector, base, emitter) “…with 120 volt line, no signal, with respect to the ground buss between the two capacitors in the power supply.”
2. I assume this means with the amp ON, and my tester’s black lead should be connected to the ground wire connecting the two big 10k mfd filter caps (C403, C404) while I touch the red lead to each of the transistor legs I am testing?
3. Also, the only pictogram of the PC-19C board I can find is in the afore-mentioned manual downloaded from the hafler website, but it has a digital glitch across the top part of the board, nulling some of the labels. Can you please check the attached pdf with my assumptions and let me know if they are right?
4. Finally, from your above quoted post, I gather that proper diagnosing of the transistors on these amps is to start at the beginning of the circuit and work your way through, as errant values at the beginning affect the values of later components (makes sense). From what I can tell, this would be pretty much working from the top of the board toward the bottom, from Q1 through to Q13. And, that most of the transistors work in match pairs (Q3 & Q4; Q5 & Q6; Q7 & Q8; Q10 & Q11) and should be replaced in matched pairs (this from reading on the forums and looking at the voltage chart at the bottom of page 12 of the manual). Is this correct?
Any responses would be GREATLY appreciated. Once I get this amp in shape, I will post problems with a right channel buzz in the other amp. Thanks!
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