Rebuilding a Hafler DH200 Part 2 - diyAudio
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Rebuilding a Hafler DH200 Part 2

Posted 16th April 2013 at 10:57 PM by Stormrider

Where was I? Right, back to the workshop and cue the music.

Sorry in advance for my photos. They get the job done, but some of them look pretty bad now. I'd retake them but the amp has long since been returned to it's owner.

Once both channels were running happily with their new parts on the benchtop power supply I turned my limited attention to the chassis. The phenolic insulators sandwiching the RCA jacks in place were starting to crumble so, some shiny new jacks were fitted courtesy of ApexJr.com for 99 cents. The input wiring was done with 22AWG shielded wire.

Click the image to open in full size.

The pro Hafler models like the P125 and the P230 have a solid aluminum bracket for the ground connections between the main power supply caps. The DH200 and DH220 models only got some tinned wire which isn't very convenient or nice looking. I fashioned up a new bracket with some scrap aluminum and tin snips.

Click the image to open in full size.

Not pretty, but better than what was there originally. The amp was rewired using 18AWG, PTFE insulated wire.

Click the image to open in full size.

I tested the 10,000uF/75Vdc main caps with a B&K LCR meter and found they were still in good working order. These caps seem to last so I don't replace them unless it's needed.

Once the wiring was done I did a soft power up test with a lightbulb limiter and a 40W bulb, here's mine:

Click the image to open in full size.

The lightbulb flashed and then quickly dimmed down, signifying a happy amplifier. When powering an amp up for the first time after a major repair I usually have multiple DMM's measuring various voltages and currents. In this case I had two measuring the rail voltages, and one measuring the current draw on the positive rail at the fuse holder. I only powered up one channel to start with, leaving the two rail fuses out of the other channel. I had a fourth DMM measuring the output DC offset. Everything looked good with both channels, so I disconnected the lightbulb limiter, plugging the amp straight into the 120Vac mains.

I set the bias on both channels to 275mA per the directions in the Hafler manual. I took a final measurement of the output DC offset; with both channels being under 10mVdc, the work matching the input differential pair transistors paid off.

Next I hooked up my 6.7 ohm dummy load (three 50W, 20 ohm Arcol resistors on a heatsink), an oscope, and a Leader Lag-120B signal generator. Sine and square waves looked good, and I verified the channels were matched for gain (this amp had a gain of 19).

All done. I replaced the amps top cover, and hooked it up to my DIY speakers. Not much else I can say besides that the amp sounded great, and I surmise that it will give many more years of service.
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  1. Old Comment

    DonA

    Ummmmm.... I know aluminum looks good, but I would caution all who embark on these sorts of DIY projects to never, never use aluminum as active, conductive pathways of any kind, period. This is true for low level signals as well as the grounding in this project.

    Why?

    Because aluminum is notoriously difficult to use in attaining stable, low resistance electrical connections, especially over long periods of time. This is true when used in connections to different metals than itself, and even to itself. It can be done, but requires special preparation and hardware which I won't go into here. Go and read the Electrical Safety Code which applies to your area and you'll see there are all kinds of restrictions in its use. Google the subject and note that there have been many house fires attributable to the problems inherent in using aluminum wire. Talk to an electrician. Suffice it to say that for the kind of projects we do here, using aluminum bus bars and wire is far more trouble than it's worth and is an invitation for problems. Copper is my conductor of choice but if you are independently rich, you can use silver or gold... it's just $$$.

    I'm not saying don't using an aluminum chassis for shielding... That's fine but use something like lock washers that bite into both the metal and the solderable terminal where you connect to it. And don't forget all the other proper grounding and wiring practices which I won't go into here. I just hope I have saved someone some grief down the road with this bit of info.
    permalink
    Posted 22nd May 2014 at 02:45 AM by DonA DonA is offline
  2. Old Comment
    Stormrider's Avatar

    DonA

    Quote:
    Originally Posted by DonA View Comment
    Ummmmm.... I know aluminum looks good, but I would caution all who embark on these sorts of DIY projects to never, never use aluminum as active, conductive pathways of any kind, period. This is true for low level signals as well as the grounding in this project.

    Why?

    Because aluminum is notoriously difficult to use in attaining stable, low resistance electrical connections, especially over long periods of time. This is true when used in connections to different metals than itself, and even to itself. It can be done, but requires special preparation and hardware which I won't go into here. Go and read the Electrical Safety Code which applies to your area and you'll see there are all kinds of restrictions in its use. Google the subject and note that there have been many house fires attributable to the problems inherent in using aluminum wire. Talk to an electrician. Suffice it to say that for the kind of projects we do here, using aluminum bus bars and wire is far more trouble than it's worth and is an invitation for problems. Copper is my conductor of choice but if you are independently rich, you can use silver or gold... it's just $$$.

    I'm not saying don't using an aluminum chassis for shielding... That's fine but use something like lock washers that bite into both the metal and the solderable terminal where you connect to it. And don't forget all the other proper grounding and wiring practices which I won't go into here. I just hope I have saved someone some grief down the road with this bit of info.
    Thanks for your input. I agree, aluminum is not the best material to use for the ground buss however, it is not a fire hazard. There are many amps that use aluminum ground bars including Haflers. Notice there are internal star lock washers in use here. Also note that the terminals on most old screw type caps, including those used by Hafler, are aluminum.
    permalink
    Posted 22nd May 2014 at 12:36 PM by Stormrider Stormrider is offline
 
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