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Old 23rd April 2013, 09:58 PM   #1
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Default Need help fixing Hafler P505

I bought Hafler P505 recently (almost identical to DH500 & P500). Supposedly, one of the volume pots was scratchy and just needed to be cleaned. After messing with it a bit, I find it's in need of much more than some contact cleaner. I plugged it into an 8ohm guitar cab with 1000hz on the input and the left channel sounded great, but the right channel was incredibly scratchy when I turned the volume knob. It seemed okay when I was not turning the knob, so I tried hooking up my B&Ws. As soon as I turned the volume on the right channel it made a horrible diminishing static sound and the amp's overload protection kicked in. I immediately turned the amp off and disconnected my B&Ws (which thankfully were not harmed).

When I measure the DC voltage at the speaker terminals, both channels measure about -7mv with the volume pot turned all the way down. If I turn up the volume pot on the left channel the dc voltage stays the same, but if I turn up the volume pot on the right channel the dc voltage begins jumping around randomly (22mv to 200mv to 1v to 70mv to -15mv, and so forth). I assumed it was just a leaky cap, so I replaced the electrolytics on the driver board. I measured the outputs again and still got erratic dc voltages on the right channel. After that I pulled the driver board out and checked all the diodes, resistors, capacitors, and transistors with my multimeter. None of them appear to be shorted and all the resistors and capacitors seem to be in spec. I measured the power rails and they are measuring +/- 94v for each channel (which I assume is ok. schematic says +/-90v).

I tried swapping the volume pots from one channel to the other, and the problem stayed in the right channel. So, the issue does not appear to be related to the volume pot.

I'm kind of a noob at the whole troubleshooting thing, so I could really use some help from those of you with more experience. What do I need to check next?
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Old 24th April 2013, 04:47 PM   #2
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Here's a link to the manual (with schematic) for the DH-500. From what I understand, the driver boards are the same on the P505.

http://www.hafler.com/techsupport/pd...00_amp_man.pdf

Should I use my scope to compare the sine wave from my oscillator with the sine wave at the output, and other points within the circuit? If so, which points in the circuit do I need to check?
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Old 25th April 2013, 11:47 AM   #3
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You are not getting much help. See this thread about old amp repair vintage amplifier repair/upgrade manual
For test I load amps with 8 to 10 ohm resistors with more wattage capability than the amp. I'm using 5 ohm 225 watt resistors in series now, which allows me to put a 4 ohm car radio speaker in series with a back to back 3300 uf cap (minus to minus) across one resistor, so I can listen to the output as well as letting the DC dissipate in the resistor. When my amp whanged into DC, it would pop and then sound funny. Turns out my DC cause was a bad solder joint on the input of an op amp, probably there since it left the factory in 1994.
You can probably fix this probably with a DVM since it is a DC problem. Be sure to attach the meter negative to the speaker negative as you test, as 94 v across your heart could stop it. Use one hand at a time, and wear safety glasses in case a semiconductor blows its top. A high wattage light bulb or room heater element in series with the AC input can cut the energy to shorts and limit the size of any explosions. I used a room heater with a bad tip over switch. Be sure to keep any AC wiring inside a grounded box with a fuse, you don't want a hot wire to come out from under a screw and flop around in the open.
I would start at the base drive to the output and look for out of balance DC voltages when DC is on the output, working back to whereever it comes from. Like I said, mine sounded funny when putting out DC, so I didn't need two meters. Could be a bad solder joint, bad component or a poor connection of the wiper on a bias potentiometer. Look for Vb-e on transistors not at 0.6 v, voltage on connections between resistors and transistors being different at two ends of a continuous run, voltages across blocking capacitors being zero (internal short) etc. Like I said, my DC cause was at the input op amp, you may have to trace back all the way through the amp, or you might get lucky. Have fun.
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Last edited by indianajo; 25th April 2013 at 11:51 AM.
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