DH-220 Clone (SMART TA-242) WAY too high bias current issue? - Page 2 - diyAudio
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Old 27th January 2013, 06:50 PM   #11
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Default Another DH-220 Offset problem

Quote:
Originally Posted by Dick West View Post
Beady, I am taking the liberty to put this conversation up here on the thread you started so that others may see/learn from your experiences, plus perhaps provide more expert advice than I can give. Also, attached is the measured table of values you just made and sent me in a private message. Others can examine your values and also compare them to the values printed in the Hafler D-220 manual. Here are my thoughts on your situation:

1. Beg, borrow or steal a Variac to use while you are trouble-shooting and repairing your amp.

2. Understand that the topology of this Hafler amp is dual complementary. The Pos side of the circuit is an almost mirror image of the Neg side of the circuit, and that measured values from similar and opposite parts of the circuit will be almost the same, but with reversed polarity.

3. Understand that a problem in the beginning of the circuit will distort voltages further down the line, so the place to start for repair is at the beginning of the circuit. The exception may be Q9 which may require replacement, or its present behavior may only be symptomatic of problems earlier in the circuit.

4. The first 4 small signal transistors (the two pairs Q3 and 4 -- Q5 and Q6) are the source of most problems with these amps. P1 can be adjusted to attempt to null out problems with DC offset, which you have. But, I believe your analysis is correct in that Q1 may be "toast." However, you have to measure voltages around Q1 and Q2, as P1 is adjusted, to determine if they can be made to be more mirror imaged (identical but of opposite polarity). Once that part of the circuit is repaired, you can determine if more work further down the circuit is required because the voltages in the remaining circuit are very dependent on the behavior of the Q1 and Q2 parts of the circuit. Ultimately, you should be able to use (P1) to adjust the collector voltages of Q1 and Q2 to be very similar.

5. Because this is a mirror image (complementary) circuit, the hFe of their complements should be very similar. The Q3,4 pair must have an identical (+/-) 10% match. Ditto the Q5,6 pair. And these corresponding pairs should be as similar as possible in their hfe. To get the close hfe matching for these 2 pairs of devices might require you to purchase 10-20 of each polarity to get the matched pairs required. There are on-line sources for the purchase of matched pairs if you do some investigating. This matching was very critical for the DH-200 which did not have the P1, Q1, Q2 circuitry for DC offset nulling. It is still important for the DH-220, but ultimate matching requirements are not quite as stringent.

6. Once the DC offset problem is corrected down to the base voltages of the Q7, Q10 pairs (with corresponding opposite polarities), re-measure the rest of the circuit and post your results here.

7. The output MOSFETs are very rugged and rarely cause a problem, unless massively abused. Although the driver pairs (q7,8 and Q10,11) sometimes cause problems their troubleshooting is simpler than the first part of the circuit.

8. Precise voltage matchings can be tricky and repair difficult to get them exactly as in the Hafler table of voltages because the feedback circuit can correct (or mask), to a degree, for some voltage irregularities. To what percentage I don't know, but if you get the final output DC offset down to below 30 mV your should be OK.

Now, let the flames begin as those more expert than me may have some corrective statements. However, that is the way we all learn. But, get a Variac for your repair!

Good luck. I will watch for further postings at this thread.
Dick,

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!

mark
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File Type: pdf PC-19C_board.pdf (489.7 KB, 19 views)
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