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Old 5th April 2007, 12:49 AM   #1
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Default DH 220 trouble

Hello all,

I am a rank amateur here. My DH 220 Hafler amp started making strange noises. It makes spontaneous popping and cracking noises on the right channel only. These happen whether or not the preamp is plugged in.

What causes this? Can an amateur repair it? Is it worth repairing? Who can repair it?

Thanks!

JMG
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Old 5th April 2007, 02:10 AM   #2
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Do these noises seem to correlate with the temperature of the amp?

You could try reflowing all the solder joints on the back side of the PCB.

You could replace the 3 electrolytic capacitors on the PCB of the noisy channel.

You could alternate heat and cold applications to various parts of the PCB to see if they correlate with the noise. This might help to identify a possibly noisy component, like a transistor or resistor.

You could try tapping various parts of the PCB with a plastic tooth brush handle to determine if noise correlates with tapping of certain parts.

These are some amateur "brute force" tricks to trouble shoot. Be careful with the lid removed and when power is applied to the amp. The large filter caps can pack a lethal charge if touched. I usually keep one hand in my pocket when working around live voltage.
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Old 5th April 2007, 04:05 PM   #3
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Dick,

Thanks for your reply. There is no correlation with temperature that I can tell. The noise starts within a few seconds after powering up the unit. It started for the first time just a few days ago.

Prior to that, I had had a few instances of the right channel not playing at all when the amp was hot. I came to the conclusion that it had to do with the amp being inside a cabinet and building up some heat when left on and unused. Cutting it off a few minutes would seem to cure it.
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Old 6th April 2007, 01:59 AM   #4
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I believe you were overheating your amp. Heat is the enemy of solid state circuits and shortens the life of electrolytic capacitors.

Have you read the manual for your amp. It must have several inches of free space on all sides so it can ventilate, and never use inside a closed cabinet.

Well, if the noise does not seem correlated to temperature then try some of the other things mentioned.
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Old 6th April 2007, 03:56 AM   #5
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Are the capacitors the likely culprits? Is there a way for an amateur to test them?

Thanks,

JMG
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Old 6th April 2007, 07:27 AM   #6
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Quote:
Originally posted by jmgreenlee
Are the capacitors the likely culprits? Is there a way for an amateur to test them?

Thanks,

JMG

considering how many caps you have in both channel,it's easiest to change them all........after all this years they will die soon,if not already
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Old 6th April 2007, 02:26 PM   #7
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jmgreenlee,

You have to remove the caps from the circuit to test them. Take the advice of Zen Mod and replace all of them.

There are 3 caps on each channel circuit card. Replace all of them with new caps that have the same ratings in uF and temp, i.e. 100 uF at 85 C and 100 WVDC.

Some guys use expensive botique caps but the jury is still out on how much difference they make. Get some low ESR caps from Panasonic (or similar). Your amp is 25+ years old and most caps have begun to die.

The power supply caps probably should also be replaced but as the popping sounds are only in one channel (not both) they are probably not the villains right now. Once the popping is cured consider replacing the power supply filter caps.

While you are in there to replace the 3 caps on each channel's circuit card reflow the solder joints on the back sides of the PCBs to make sure each component is still securely (physically and chemically ) attached to its trace.

FWIW I recently "repaired" a DH-220 that had an intermittent sound in one channel. All I did was retouch up the solder joint of C1 to its connections on the back side of the PCB.
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Old 6th April 2007, 02:29 PM   #8
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Note that the 3rd cap in each channel is non-polarized. Check its uF value carefully and observe its voltage rating. Check the parts list in the manual for these replacement values.
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Old 6th April 2007, 03:06 PM   #9
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....mmmmmmm sounds like the differential stage. Dual diffs. Good matching is required Q1, Q2, Q5, and Q6. (4 pcs) High Hfe and matched is good. They also need pretty high BVceo ~200V

...or feedback shunt cap C-5 470/6.3V (NP!!!!)

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

Good Luck
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Old 6th April 2007, 11:28 PM   #10
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jmgreenlee,

Well by now you are getting a lot of advice, some of which might be conflicting.

I refer you back to my first post in this thread that listed some trouble shooting tricks. The dual-diff input section has 4 transistors all of which must be rather closely matched on hFe. Their replacements would be a difficult and expensive proposition. Before you begin throwing parts at a faulty circuit you need to do some sleuthing, some troubleshooting, to better make hypotheses about possible problem devices.

Often a faulty transistor that is "popping" will alter its behavior when it is heated or cooled or tapped with a plastic toothbrush handle. Try this on your transistors on the bad channel. Ditto for the resistors on that channel. Also, reflow the solder joints on that channel. And, tap on various parts to determine if they make the popping worse or better when struck.

All of these strategies just might point you to the bad device/s that make noise. Then, you could begin parts replacement with better assurance of success. How are your soldering skills? Do you have a meter or VOM that can measure the hFe of transistors?

As for replacing the caps, this is just good preventive maintenance and should be done regardless.

Lots of luck.

Dick
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