Hi,
I've been given a Hafler P125 power amp with on channel not working. The prior ouner opened and checked the fusers and looked around but couldn't find anything. He gave it to me and I want to see if I can fix it.
Where should I start? and what should I be checking? I downloaded the DH-120 manual which looks to be the same as the P125 and it contains schematics and a voltage chart...
Plus any info on this amp would be appreciated.
dinky,
I've been given a Hafler P125 power amp with on channel not working. The prior ouner opened and checked the fusers and looked around but couldn't find anything. He gave it to me and I want to see if I can fix it.
Where should I start? and what should I be checking? I downloaded the DH-120 manual which looks to be the same as the P125 and it contains schematics and a voltage chart...
Plus any info on this amp would be appreciated.
dinky,
bad news... if you don't know what you should be checking it is highly unlikely that you can actually get it working again, unless you get very very lucky. Usually more than one thing fries and lets the magic smoke out...
The voltage chart is not going to help much, because if it is fried, but not totally cooked, running it at full voltage will fry whatever can or will fry...
do some resistance checks if you are sure there is no residual PS voltages around. check the fuses with the ohmeter to start.
if you don't find the problem there, best wait for some other advice, and/or bring it to someone with the gear and experience to do this sort of repair... unless ur guy doesn't mind you having "on the job training" that is.
_-_-bear
The voltage chart is not going to help much, because if it is fried, but not totally cooked, running it at full voltage will fry whatever can or will fry...
do some resistance checks if you are sure there is no residual PS voltages around. check the fuses with the ohmeter to start.
if you don't find the problem there, best wait for some other advice, and/or bring it to someone with the gear and experience to do this sort of repair... unless ur guy doesn't mind you having "on the job training" that is.
_-_-bear
check out the ginormous DH200 thread , start at the beginning...you will need a basic test bench which starts with a good soldering iron, a variac, and a light bulb test cable(use search function for multiple construction ideas)...study up on safety as there are Lethal voltages inside most power amps....
start reading...
Regards, Elwood
start reading...
Regards, Elwood
In one sense you're in excellent shape, if you're comfortable with using a multimeter, because you have a working channel next to the non working specimen. There is quite a good chance that the DC levels at precisely corresponding points in the 2 copies of the channel circuitry will vary, probably quite dramatically, which should give you excellent clues. If you measure the DC voltage at a circuit point in the good channel, check the same point in the bad, starting with the feed from the voltage rails, then going to the input and working towards the output, you may find something.
I have a Perreaux which died completely in one channel many years ago, nothing obvious whatsoever. No schematic or material to refer to; I used this technique steadily until I got to a simple 1/2 watt resistor: yes, it had gone open circuit. Absolutely nothing to show a problem, no heat stress signs or discolouring at all. Replaced it, we were cooking again! But, there must have been a design weakness, or bad batch of resistors used: the exact same resistor in the other channel went bad a few months later!! The amp went on for years after without missing a beat ....
Warning: unless you're 100% comfortable with trying this technique, do not proceed!!!
Frank
I have a Perreaux which died completely in one channel many years ago, nothing obvious whatsoever. No schematic or material to refer to; I used this technique steadily until I got to a simple 1/2 watt resistor: yes, it had gone open circuit. Absolutely nothing to show a problem, no heat stress signs or discolouring at all. Replaced it, we were cooking again! But, there must have been a design weakness, or bad batch of resistors used: the exact same resistor in the other channel went bad a few months later!! The amp went on for years after without missing a beat ....
Warning: unless you're 100% comfortable with trying this technique, do not proceed!!!
Frank
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Thanks for your suggestions...
Well, I don't think the left channel is blown.
That channel has hiss but at a lower volume. Then there are times where it's working fine...Then it's got crackle and lower volume. So based on this I don't thnk it's blown.
I measured the voltage on those big caps and they are both the same one side is -54.3volts, the other +54.3.
I turned the level knobs, flipped the stereo/mono back and forth, swapped inputs between left and right, but I didn't find anything wrong as far as contacts...
I can tell you that it's a clean amp when it's loud...62 watts stereo.
Is it worth trying to fix?
dinky
Well, I don't think the left channel is blown.
That channel has hiss but at a lower volume. Then there are times where it's working fine...Then it's got crackle and lower volume. So based on this I don't thnk it's blown.
I measured the voltage on those big caps and they are both the same one side is -54.3volts, the other +54.3.
I turned the level knobs, flipped the stereo/mono back and forth, swapped inputs between left and right, but I didn't find anything wrong as far as contacts...
I can tell you that it's a clean amp when it's loud...62 watts stereo.
Is it worth trying to fix?
dinky
Certainly worth trying ... with what you're saying now, sounds like it's a solder joint gone bad, typically on the circuit board. Or a connection within a component, less likely. What I'd suggest doing now, is run it at a low volume with signal coming through, and with a long, slim wooden stick, a stirring spoon from the kitchen drawer held at the wrong end is ideal, gently tap at the leads of the components where they enter the board. Or the components themselves ... gently does it! With a bit of luck you should get a strong reaction at some point, and then all that might be needed is a touch with a hot soldering iron to restore the connection.
Good luck!
Frank
likely to be a bad transistor, small one. that is a symptom where you get it to work, then a pop then noise... freeze spray, one transistor at a time usually makes it come back on. then you know which one to replace!
It is also possible that you have a broken solder joint on a pad. so you can "touch up" all the solder connections on the bottom of the board if you wish.
_-_-bear
It is also possible that you have a broken solder joint on a pad. so you can "touch up" all the solder connections on the bottom of the board if you wish.
_-_-bear
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.
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.
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,
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,
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