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Old 24th August 2010, 09:27 AM   #1
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Question Need help with Hafler DH-500...I'm a newb.

Hi everyone, I'm new to this forum and come in hopes getting some help.

I just found a Hafler DH-500, which was used for PA duties in it's past. The owner hadn't turned it on in years, but recalled a 60 Hz hum on output. I brought it home, plugged it in without speakers connected at first and it would turn on and stay on just fine. Then I proceeded to hook up a pair of 8 ohm Klipsch and upon turn on, I heard a different click and the amp turned off. I checked all fuses and found them intact...only to find that it had blown the wall socket fuse. So I had to go flip the switch outside, then fired the amp back up without speakers again and it worked.

Any idea what's going on? Why would it do that only when speakers are hooked up? Also, how do I check DC offset on this amp?

I hooked up my multimeter probes to the speaker binding posts and measured around 450-520 mV (~0.5 V). Does that work to measure it that way, and are those okay numbers? What is optimal DC offset for this amp?

I would like to get this thing operational so I can monitor for hum and fix it if possible. Then I will tackle some upgrades on it. Thanks for any help!

Cory
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Old 24th August 2010, 10:46 AM   #2
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DC offset is measured as you have done at the speaker outputs. 0.5 volts won't damage anything but is far too high and needs investigating. There's a speaker output relay... if that was tripped then you could get incorrect reading as output floating. Could that be the click you heard ?

You need to measure the output line internally in the amp with no load, which is on fuse F401 (for right channel). I would say it should be under 60 mv.
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Old 24th August 2010, 10:53 AM   #3
Mooly is offline Mooly  United Kingdom
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You'll be under moderation for the first 5 posts or so.

Do you have the manual ?
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Old 24th August 2010, 04:26 PM   #4
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There's only one relay inside, so it was probably a DC offset issue tripping the protection circuit.

Edit: the click could have been one or both thermal switches opening to protect the amp, or the fan speed switches turning up the fan. Did you notice the power switch lamp blinking?

Be sure that you are measuring the DC offset with the inputs shorted. Floating inputs or a preamp with DC on it's output will affect the output. You can measure at the fuses as Mooly suggested or at the protection relay. DC on the input could have caused your problem. Check the preamp for DC on its output.

If you blew a mains breaker, there is something seriously wrong with the amp. The normal line fuse on the amp is a 15A slow blow, so it isn't terribly surprising that you tripped a breaker without blowing it. But the amp should draw around 200W at idle, not in excess of 1800. Thank the protection circuit for tripping without destroying your speakers.

Put a light bulb in series with the amp as a current limiter so that you don't blow the breaker again. If the bulb lights, the amp has a problem.

You should pull the rail fuses to test one channel at a time. Hopefully it's only one channel with a problem. Start with no fuses in the rails to verify that the power supply works properly, shut down and bleed the main filter caps with a 10K 3W or better resistor before installing one channel's rail fuses. test the channel, pull its fuses once the main filter caps bleed off, check the other channel.

Bias should be ~450 mA total per channel (measure by putting an ammeter in place of a rail fuse, start with it set on its highest current range.) Offset and bias are adjustable using the two open frame trim-pots. If the wiper of P2 loses contact the amp will bias too high. If it doesn't adjust back down to normal you may be able to clean the pot, if not replace it.

Ampslab has the output devices if you need them, but $$$.

Here's a link to the manual if you need it http://www.hafler.com/techsupport/pd...00_amp_man.pdf

Last edited by BobEllis; 24th August 2010 at 04:38 PM.
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Old 24th August 2010, 11:31 PM   #5
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How do I actually test the voltage at fuse F401? I'm completely new to testing things like this. The most I have done is set the bias on my old tube amps. Is there a risk of shock at the fuse? Sorry if this sounds like a dumb question.

Is there a way to measure the DC from the PC-9 driver board like the manual says? Even this confused me a little. How would I test from points 7 and 8? Do I just ground to the chassis? Thanks for the help.

I'm on another hi-fi forum that is helping me a little to, so bear with me.

Cory
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Old 24th August 2010, 11:51 PM   #6
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Also, which fuse is F401 if I'm looking at the amp from the front panel?
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Old 25th August 2010, 02:16 PM   #7
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There is always risk of shock working on an open amplifier. Be careful. Notice that there are several exposed terminals where you could come in contact with mains voltage.

Main board Point 7 is the board ground - reference most test voltages to it. Note that it connects directly to the black binding post for that channel, which makes it really easy to reach.

main board Point 8 is the same as one end of F401 is the same as the point I mentioned at the relay. Any of which is where you would connect the positive terminal of your voltmeter. Use the one you feel most comfortable clipping a test lead to. You could also connect to point 7 or 8 on the PC-9 board. If there is an amplifier fault you could have 90V here.

F401 is either fuse on the rear panel for the appropriate channel.

Last edited by BobEllis; 25th August 2010 at 02:18 PM.
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Old 25th August 2010, 02:46 PM   #8
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The DH500 circuit has potentiometers for bias and offset. The bias pot is the one close to the middle of the board, don't get into diddling it to correct for offset problems, or you'll quickly have bigger ones. The pot functions are printed in copper on the top side of the board. It would be reasonable to wash the pots with a little contact cleaner spray before moving them, and then running them back and forth a little (two or three times) with the power off, before attempting adjustment. Note the original position so you don't send the adjustments to Mars before tuning them.

If you can measure a lot of ripple on the main filters with your multimeter in AC mode, you may as well stop and replace those first, unless the amplifier has a bias problem that is overloading the supply even at idle. It would be handy to measure line current upon power-up at this point.

Last edited by Andrew Eckhardt; 25th August 2010 at 02:51 PM.
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Old 25th August 2010, 08:56 PM   #9
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Ok, so I think I made some progress...thanks for the help!

After an hour warm up, I put the black(-) lead of the meter in a hole on the chassis for ground, and touched the red(+) lead to points 7 and 8 on the PC-9 board which gave me a reading of: Right channel: ~ 7 mV
Left channel: ~ -21 mV


Another thing on the 60 Hz hum...I set the meter to read AC Hz and put the test leads in the speaker binding posts. I got a reading of 59.99 Hz on the right channel, but when I touch the amp, it jumps to ~60.05 or so. Left channel gives a reading of 0.00, but when I touch the amp, it also jumps to
~60.05 Hz. Any way I can troubleshoot a ground connection problem on this amp?

FYI, it have the PC-10 A boards in my amp.

Cory
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Old 25th August 2010, 11:30 PM   #10
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Good news that the amp seems to be operational.

Are you getting the hum with the inputs grounded? The signal ground connects to the chassis at a screw between the input jacks. Lift that and check for continuity to the chassis ground. You shouldn't have any continuity.

You can also substitute a 50-100R resistor for the jumpers from the RCA jacks to the chassis ground to provide the signal ground some isolation from the chassis ground. As a last resort, clip one of the jumper/resistors. This is the method used in the Leach amps.
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