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Old 22nd May 2012, 03:47 AM   #1051
eyoung is offline eyoung  United States
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Hola Y'all,

these amps are bad for bias generator transistors (np2222? ithink) to go bad and poor solder joints ...look for faint dark hairline circle around lead on back side of board. Replace all the electrolytics. cold outputs is a sign of under bias.

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Old 22nd May 2012, 10:08 AM   #1052
tvrgeek is offline tvrgeek  United States
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Many of the transistors cannot take full rail voltages and the currents when you have a failure. Changing one part at a time is not always useful. I would recommend pulling them out and measuring their gain on the bench. With the transistors out, you can check all the resistors.
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Old 22nd May 2012, 05:45 PM   #1053
mmerig is offline mmerig  United States
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Thanks again tvrgeek and eyoung,

I know my original post was long, but in it I explain that I tested the transistors (on a Heathkit IT-18, hfe and leakage) and ended up replacing them anyway, in pairs, except for Q9 which was replaced but is not paired with anything.

The voltage problem I am having has persisted after replacement, and I have re-checked several transistors, and the problem still remains. I also checked or replaced all of the 'litic caps, resisters and diodes, pulling the ones that needed it. As tvrgeek says, some of the volatges are a bit high for the device tolerances, so I could pull the transistors and test yet again.

One thing I have not tried is using high-beta transistors in the differential pairs (now they are about 100). I have some 2n5401 and 2n5550 in the 400 to 500 range. Erno Borbly recommended using diff. pairs in the 180 to 450 range range for his early version of the VAS similar to the Hafler. But his design did not have a trimmer for dc offset, and the design depended on reasonable matching and high hfe to keep the off-set low. I doubt this will make a difference, but will try anyway.

As Dick West mentions, problems can be hard to track down, and these boards seem fragile to me. I have had failed bias currents before, and Q9 is usually not the problem. I have never had one where the voltages check out but the MOSFTS have no bias current though -- that seems even more mysterious than what I have.
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Old 3rd December 2012, 10:46 PM   #1054
zcables is offline zcables  United States
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Working my way through this thread as I'll be getting a factory pair of DH200s tomorrow from a friend which I'll be setting up for vertical bi-amping. Parts are still in good condition, but simply due to age, I'll be changing out caps. My first mod will be changing both amps to 35A bridge rectifier ahead of installing new filter caps.

Has anyone tried installing a power supply board such as the below linked? It looks like it has two different halves, so it could feed both channels of the amp, with twice the factory capacitance in a tidy package.

4 10000uF 100V High Quality Power Supply Board for Power Amp DIY | eBay

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Old 3rd December 2012, 11:13 PM   #1055
zcables is offline zcables  United States
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... I understand this board would not simply be a plug and play for the old filter caps...
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Old 4th December 2012, 12:49 AM   #1056
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I have a DH200 sitting in front of me, and that board will only just barely fit mounted to the bottom of chassis. It might fit better mounted vertically.

You will probably need to add a softstart circuit. There is already a pretty large inrush spike with these amps with just the stock +/-10mF. You will kill the power switches, and stress the transformer and bridge rectifier(s) without something to limit the inrush current.

EDIT: Also, beware. There's a pretty good chance those caps are fake.
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Old 4th December 2012, 12:55 AM   #1057
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For instance, real Cornell Dubilier caps 10mF/100V from Mouser or Digikey are between $10 and $50 each. They would have to be buying thousands of these caps to make any money on that board.
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Old 4th December 2012, 04:42 AM   #1058
rfj5106 is offline rfj5106  United States
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Default E-bay power supply board

I have purchased similiar ps boards with capacitors for amps I have built and they have worked well. A soft start circuit (as mentioned by Stormrider) is probably a VERY good idea as is a relay system to prevent turn on/off thumps.
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Old 4th December 2012, 05:09 PM   #1059
mmerig is offline mmerig  United States
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As with other recent posters, I am a little suspicious on the quality of the caps, as the board is so cheap. Also, it seems like the rectifiers on the board would be redundant with the bridge rectifier you plan on using. Why not just get decent 10k or (larger capacity if they fit) caps and use them as direct replacements for what is in the DH200? This would leave you the option of putting in a larger transformer in the future.

Then of course there are the driver boards that this thread has discussed in
depth.

A better switch and in-rush limiter are also a good ideas.

I have 40k uf of ps caps in my DH220 with a higher-rated switch, and have had no problems with the power supply (without an in-rush limiter) for many years. The last problem I had, with a blown-out driver board, happened when the amp was not even on (cause = lightning strike).

Good luck, and please let us know how the sound improves with veritical bi-amping.
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Old 4th December 2012, 05:15 PM   #1060
zcables is offline zcables  United States
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Any link to a high current switch that would work in the DH200? I'm not against getting large filter caps (twice bypassed) and calling it a day coupled with the upgraded bridge rectifier.
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