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Old 3rd April 2012, 08:17 PM   #1041
tvrgeek is offline tvrgeek  United States
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Location: Md
Glad to here your OnSemi parts were good. I tossed mine and bought Fairchilds. I was able to match 2 pair of each out of a dozen within 1%.

I too have my 120 back together just last night. Subjectively far improved.
New main caps, Hexfred rectifier, moved the VAS loading compensation to MC, selected correct gate stoppers for the Exicons, removed all other compensation except the feedback bypass which was necessary to tame the outputs. The slew is now symmetrical and about twice what it was, noise floor about 8 dB lower and line harmonics down by 12 dB. If I do another round, it will be a JFET IPS ccs and TMC with further additional C after the rail separation resistors. I may just leave well enough alone. It is quite a bit better. Bias is DOWN to 110. Now, my 220..... Starts out with a symmetrical topology. The MC boards were single ended IPS. I kind of agree with the textbook, it is the signal that needs to be symmetrical, not how you do it.
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Old 1st May 2012, 04:58 AM   #1042
rss388 is offline rss388  United States
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Sorry to hijack the thread. I have a pair of DH220 boards and the output MOSFETs that I removed from a pair of amps that I converted to monoblocks. They are fully stock but functional. Anyone interested please send me an offer.
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Old 17th May 2012, 02:25 AM   #1043
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Location: Athens, Georgia, US
Stormrider: I refer you to your post #1032
Quote:
Originally Posted by Stormrider View Post
... This amp had MPS8099/8599 for the diff pairs,
Are those MPS8099/8599 transistors still good? Are they matched?

I have an amp w/DC offset issues that uses those transistors. 2 or 3 each per channel in what I believe is a double-diff amp.

In the link below, see post #32 for a parts list; post #33 for a drawing .

will pay for Belles 1 Series amp info & Schematic

thanks in advance, Tony

Last edited by gp4Jesus; 17th May 2012 at 02:34 AM.
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Old 19th May 2012, 01:56 AM   #1044
mmerig is offline mmerig  United States
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Default Hafler DH220 front-end voltage puzzler

Here's a strange situation:

After a voltage surge (lightning), the right channel of my Hafler DH220 had muted sound, as if the speaker fuse blew, but it was not that. The troubleshooting has been difficult (but interesting) for me, but I bet those that know this circuit would have some ideas and may find this puzzler interesting.

The amp was not on during the surge, and is plugged into a switched outlet. The surge probably worked its way through the earth lead.

Further complicating matters, I slipped with a probe and shorted two leads on Q1 (ccs for b+), and this caused some smoke on the pc board.

So, after that, I started from scratch, checking all components and replacing all of the transistors to be sure. Here is what I found:

Very low bias current (about 20mA), no surprise there. The MOSFETS checked out okay (checked gate-to-source voltage versus current). The power supply is okay.

1. Resistor R30 was burned, while R31 and R32 were low (36 and 40 ohms, instead of 47). I replaced all of these. All other resisters checked out okay, as well as the trimpots.

3. Checked all of the diodes, including the zeners, which I pulled to check the voltage. All okay.

4. Removed and checked all transisters on the board, and Q5 was broken (probably related to my shorting Q1). One lead on Q6 broke as I pulled it so I could not test it. All other transistors tested okay on beta and leakage. I ended up replacing them all gradually as I worked my way through the circuit. While I was at it, I matched every pair, even those not in the differential pairs.

5. After setting the DC offset (now < 5 mV), all of the voltages at the transistor leads were okay until I tested the collectors at Q8 and Q11. The collector voltage was +10 for Q8, and +7.5 for Q11. All the voltages at Q9 are off too (e,b,c = 7.5, 8.1, 10; should be -1.3, -0.6, 1.5)

The collector voltages at Q8 and Q11 should be about 1.5 and -1.4 volts, and as I understand it, the collector voltages here can increase, up to the 10 volt zeners, if there is a high load on the speakers and the feedback circuit tries to correct for it.

The other odd thing is at turn-on, the bias current is about 150 mV but drops exponentially to 20 mV (rapid, then gradually), as if there is some sort of load or leakage.

The voltages remain the same with the mosfets removed. I don't have a scope to look for ringing, but with the fets removed I think this is unlikely.

There is no signal on the input, and all the signal grounds are good, and no shorts anywhere in the wiring to the MOSFETS, speaker connectors, or the inputs. Checked for solder bridges.

5. Checked all of the electrolytic caps,and most of the film caps (only some bypass film caps skipped). All okay. Replaced the rail caps (100uf) caps and the feed-back cap (470uf) to be sure, as these are harder for me to test.

What could be causing the high voltages at Q8 and Q11 and the points beyond where expected (e.g., base at Q12 and Q13)? Why is there low bias current and voltage?

The gate voltages at the MOSFETS fluctuates close to zero (<15mV), and the source voltage is -25 for the N- and P-devices. Drain voltage is okay (+66 or - 66v). To be sure, I swapped in some other MOSFETS and problem persisted.

The hfe for the differential pairs is 95 to 105. Is this high enough and close enough?

Thanks in advance for any suggestions.


Perhaps this problem will improve our insight on the circuit, sort of like learning about the brain by studying damaged ones.

By the way, I put a new surge protector at the outlet, I get a lot of lightning here.
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Old 19th May 2012, 09:11 PM   #1045
tvrgeek is offline tvrgeek  United States
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Fixing feedback circuits is fun isn't it. It does not sound like it, but if you have high freq stability problems, the outputs will get really hot fast. With both sides bad, check around Q9.
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Old 20th May 2012, 12:15 AM   #1046
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The manual contains a table of voltages expected at various parts in the circuit. Was it of any help in your troubleshooting?


Quote:
Originally Posted by mmerig View Post
Here's a strange situation:

After a voltage surge (lightning), the right channel of my Hafler DH220 had muted sound, as if the speaker fuse blew, but it was not that. The troubleshooting has been difficult (but interesting) for me, but I bet those that know this circuit would have some ideas and may find this puzzler interesting.

The amp was not on during the surge, and is plugged into a switched outlet. The surge probably worked its way through the earth lead.

Further complicating matters, I slipped with a probe and shorted two leads on Q1 (ccs for b+), and this caused some smoke on the pc board.

So, after that, I started from scratch, checking all components and replacing all of the transistors to be sure. Here is what I found:

Very low bias current (about 20mA), no surprise there. The MOSFETS checked out okay (checked gate-to-source voltage versus current). The power supply is okay.

1. Resistor R30 was burned, while R31 and R32 were low (36 and 40 ohms, instead of 47). I replaced all of these. All other resisters checked out okay, as well as the trimpots.

3. Checked all of the diodes, including the zeners, which I pulled to check the voltage. All okay.

4. Removed and checked all transisters on the board, and Q5 was broken (probably related to my shorting Q1). One lead on Q6 broke as I pulled it so I could not test it. All other transistors tested okay on beta and leakage. I ended up replacing them all gradually as I worked my way through the circuit. While I was at it, I matched every pair, even those not in the differential pairs.

5. After setting the DC offset (now < 5 mV), all of the voltages at the transistor leads were okay until I tested the collectors at Q8 and Q11. The collector voltage was +10 for Q8, and +7.5 for Q11. All the voltages at Q9 are off too (e,b,c = 7.5, 8.1, 10; should be -1.3, -0.6, 1.5)

The collector voltages at Q8 and Q11 should be about 1.5 and -1.4 volts, and as I understand it, the collector voltages here can increase, up to the 10 volt zeners, if there is a high load on the speakers and the feedback circuit tries to correct for it.

The other odd thing is at turn-on, the bias current is about 150 mV but drops exponentially to 20 mV (rapid, then gradually), as if there is some sort of load or leakage.

The voltages remain the same with the mosfets removed. I don't have a scope to look for ringing, but with the fets removed I think this is unlikely.

There is no signal on the input, and all the signal grounds are good, and no shorts anywhere in the wiring to the MOSFETS, speaker connectors, or the inputs. Checked for solder bridges.

5. Checked all of the electrolytic caps,and most of the film caps (only some bypass film caps skipped). All okay. Replaced the rail caps (100uf) caps and the feed-back cap (470uf) to be sure, as these are harder for me to test.

What could be causing the high voltages at Q8 and Q11 and the points beyond where expected (e.g., base at Q12 and Q13)? Why is there low bias current and voltage?

The gate voltages at the MOSFETS fluctuates close to zero (<15mV), and the source voltage is -25 for the N- and P-devices. Drain voltage is okay (+66 or - 66v). To be sure, I swapped in some other MOSFETS and problem persisted.

The hfe for the differential pairs is 95 to 105. Is this high enough and close enough?

Thanks in advance for any suggestions.


Perhaps this problem will improve our insight on the circuit, sort of like learning about the brain by studying damaged ones.

By the way, I put a new surge protector at the outlet, I get a lot of lightning here.
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Old 20th May 2012, 03:23 PM   #1047
mmerig is offline mmerig  United States
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Thanks to tvrgeek and Dick West.

Would the feedback circuit be operating at all with no signal and the MOSFETS out of the circuit?

The outputs, if you mean the MOSFETS, don't get hot at all, as there is neglible bias current to them. The heat-sinked rivers on the board don't get very hot either -- I'd say they are at normal temperture (about 40 to 45 C).

And yes, I referred to the voltage chart in the manual, that's how I knew the voltages were okay up to Q7 and Q10. I also used the left channel voltages for reference too; some of these are slightly different than what is in the chart. As mentioned in the OP, the big deviations on the bad board are at Q8 and Q10 collectors and everything that follows those voltages.
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Old 20th May 2012, 10:38 PM   #1048
tvrgeek is offline tvrgeek  United States
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With the outputs out, you have no feedback and it will do very strange, and maybe bad things to the VAS.
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Old 21st May 2012, 04:41 AM   #1049
mmerig is offline mmerig  United States
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Thanks again, tvrgeek.

Normally I would hesitate to remove the MOSFETS during testing, but what motivated me to remove them was a letter to Erno Borbely, published in the "Power Amp Projects" book (Audio Amateur Magazine, 1996). Borbely worked for Hafler and likley designed the DH200

The letter writer's amp, with a similar circuit as the DH200, was having ringing problems, and when the MOSFETS were removed during testing, the ringing stopped. In Erno's reply, there was no warning about removing MOSFETS. His advice centered around adjusting the capacitance around the MOSFETS.

Double-checking the caps near the MOSFETS would be worthwhile, but the point-to-point wiring has not changed, and the FETS are fine, so my guess is the problem is in the PC19 VAS board, but I am stumped as to what it is. Other posters mention the tendency for this amp to ring if improperly modified, but I had not changed anything on it in years.
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Old 21st May 2012, 11:08 PM   #1050
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I really wish you would come up with a solution to your PCB problem. I, too, have a DH-220 PCB that has, essentially, identical symptoms. I have no drive to the gates and the MOSFETs do not get warm but I have triple checked all voltages and everything seems to be in order, including MOSFET swaps to ascertain that they are functioning.

So . . . . I set this board aside 2 years ago intending to pick it up again and try a new approach. I am monitoring closely your trials and hope your eventual success might inspire me to give another go at my PCB. I hate to throw parts at something that otherwise seems to check out OK, but it seems I am down to having to try this. The final drivers (Q12 and Q13) will be the first pair on the replacement list (fingers crossed)and , as needed, will work back from there.
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