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23rd July 2012, 02:15
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24th July 2012, 07:38
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Hafler P125 (DH120) One Channel Not Working.
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diyAudio Member
Join Date: Dec 2009
Location: North
Carolinia/Piedmont
The first problem is the output transistors are no longer available. I switched to Exicons, but it
took a LOT of work to get it stable. I changed the compensation design totally. After months of
work and a lot of advice from folks here, I have a very nice amp.
BTW, The 120 circuit is nothing like the DH 200.
Yes, the main caps will be bad by now. So will ALL of the electrolytics. The low power transistors
are available, that is good. If you have a bad output, you are basically screwed. BUT, if the fuses
were good, it may not be a bad output. ( unless someone replaced the fuse. ) Remember, there
are both fuses on the rear and inside.
This amp has a nasty habit where if any transistor is bad, it takes out most of them in that side.
It also can really confuse you by taking out a couple of resistors. Be aware the inputs are, or
should be, matched. It also has a habit of frying the lands around the bias string resistor. THat
big one that gets hot. Too hot. I went with a higher wattage further from the board.
The wires to the outputs are tines, so they tend to break. Your problem could be a simple as a
broken wire.
Don't know if the later versions had the mono bridiging and dyna quad. I removed all of that.
If you are not an experienced tech who just has never seen the HAfler before, you are over your
head.
diyAudio Member
Join Date: Jul 2012
fas42, you the man...
I followed as you suggested with the poking stick and found 2 resisitors that turned the sound off
when I pressed them. This also happens on the good channel.
It also looks like the resistor (5100 ohm) got overheated at some point because it looks brown at
the connection point.
Here's the board showing the problem resistors, r28 is a 10 ohm, and r10 is a 5100 ohm 2w
metal film.
I'll see if I can find replacements for these and see what happens.
dinky,
1/29/2020
Hafler P125 (DH120) One Channel Not Working. - Page 2 - diyAudio
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24th July 2012, 10:11
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24th July 2012, 11:33
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diyAudio Member
Join Date: Apr 2004
Location: Norwich, UK
Those are both resistors in the VAS stage, so if they are not working it will cause loss of sound.
Fit a modern 0.6W metal film resistor for R28, and fit R10 so that it stands off from the PCB by
~10mm to allow airflow. R10 probably runs hot and it has cooked R28. Do this on both channels.
Banned
Join Date: Jun 2012
Location: NSW, Australia
Good to hear!
A weakness in one channel can easily mean the same thing in the other, as you found, and as I
found. The gods of chance are usually not so cruel as to have both go bad at the same time ...
Frank
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