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Old 24th July 2005, 10:29 PM   #1
AR2 is offline AR2  United States
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Default Help with Hafler 9505

Hello,
I got from friend Hafler 9505 which has wierd noise on one channel. It starts with pop and than it follows with crackling noise which is followed with pop again and than it is quiet. That goes on and off every 15-20 seconds. Does anyone possibly has a guess what this could be?
Thank you
AR2
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Old 24th July 2005, 11:51 PM   #2
anatech is offline anatech  Canada
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Hi AR2,
Either someone is blowing bubbles with chewing gum or you had better get some more information.

A schematic would help some of us (me for one), and information whether the noise was heat related, motion related (tap it) or age related. I have no idea how old the unit is. Have you swapped the preamp leads to see if the fault is even in the amp?

As it is, my complete guess would be in the front end. I have seen older transistors doing this in the diff pair.

-Chris
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Old 25th July 2005, 06:00 AM   #3
AR2 is offline AR2  United States
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Hi Anatech,
I do notg know exactly how old is unit, my guess is 5 - 10 years maximum. This is current amp from Hafler. I do not have schematic for this unit, but I found one for previous model 9500.
Here is the link.
http://k-amps.8m.com/cgi-bin/i/Power...afler_915c.jpg

This problem is happening any time - it is not heat related. It acts when the amp is just turned on, as well as when it warms up.

Certainly, problem is in the amp, as I mentioned it belongs to my friend. With my amp in the chain there are no problems.

Thank you
AR2
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Old 25th July 2005, 01:25 PM   #4
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Default Re: Help with Hafler 9505

Quote:
Originally posted by AR2
Hello,
I got from friend Hafler 9505 which has wierd noise on one channel. It starts with pop and than it follows with crackling noise which is followed with pop again and than it is quiet. That goes on and off every 15-20 seconds. Does anyone possibly has a guess what this could be?
Pops are usually related with capacitors and crackling to pots, particularly if they stabilize after getting warm.

Check if the bias pot is a multiturn one. If it is not it might need changing. If the circuit is the same as 9500, I would add a capacitor in parallel with P1.

Suspect capacitors for pops are C1 and C2, with a necessary checking on C7 and C9. If the small values are not film types, preferably polystyrene or polypropylene, change them anyway.

If the problem is on the active parts the search migh be more difficult. Update the passive parts first and see if it gets solved. Be careful when you install the new pot: try to set it at same value P1 was or follow general bias set proceeding.

Carlos
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Old 25th July 2005, 09:56 PM   #5
anatech is offline anatech  Canada
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Hi carlmart,
I can not agree with you. You stand a very good chance of making things worse by just changing things out. Multiturn controls are at least as likely to be noisy as single turn also. Pops and crackling may occur with semiconductors.

AR2,
Most intermittents will respond to tapping or temperature changes (cold spray / heat gun). Could be a bad solder joint too. Lets find the fault first.

-Chris
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Old 26th July 2005, 12:39 AM   #6
Davet is offline Davet  United States
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Default Problem with Hafler 9505

I suspect it is most likely a cold solder joint. I have purchased a number of Hafler DH-200 amps of late and I have had one where the transistors just fell off the PCB.

I would start by taping the boards with a disposable speaker hooked up; just in case.

Based on my experience, it wouldn't hurt to reheat all the solder joints on the PCBs.
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Old 26th July 2005, 01:01 AM   #7
anatech is offline anatech  Canada
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Use flux with the reheat. Try not to add too much solder. Clean the flux off when you are done.
-Chris
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Old 26th July 2005, 04:35 AM   #8
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Default Re: Help with Hafler 9505

Quote:
Originally posted by AR2
Hello,
I got from friend Hafler 9505 which has wierd noise on one channel. [...] Does anyone possibly has a guess what this could be?
Thank you
AR2
My 9300 did that a few years ago; it turned out to be a leaky transistor. I also agree with anatech:

Thou shalt not futz with the bias pot.


Cheers,
Francois.
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Old 26th July 2005, 05:11 AM   #9
AR2 is offline AR2  United States
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Thank you very much for all advices. I still do not have schematic for 9505 model. I openned amp and checked if anything looks suspicios. Interestigly enough circuit board looks burnt arond transistors (on the 9500 schematic) Q8,Q9,Q10,Q11. I do not know if it is old damage and someone worked on it already, or it is new and is reason for the problem.
Didn't have time to do any more checking on it, I just observed but I will try to do some more poking next weekend.
Thank you all
AR2
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Old 26th July 2005, 09:12 PM   #10
anatech is offline anatech  Canada
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Hi AR2,
What you see is just age I think. Resolder the connections in those areas, but first tap around to see if you can make it fail. Start tapping in the overheated area. Use a plastic rod or something like it.

You may want to replace the transistors and possibly resistors in the overheated area. Electrolytics are on the menu too. But not until you solve the fault.

-Chris
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