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Old 14th August 2007, 11:17 AM   #1
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Default Lesson learned - fried my Hafler - what went wrong?

Well, I'm pretty sure one channel of my beloved Hafler XL280 is dead - and I killed it.
I know I probably cannot repair it at this point, but I am very curious as to what some of you might think about the symptoms.
In a nutshell, this is what I did:
I was refurbishing my old XL280 with new caps and I wanted to check bias and voltages before putting it back in service. Fired her up with a Variac and a 60W lightbulb in series.
The first channel came up fine and I eventually got bias set perfectly at full AC.

When I fired up the other channel, I noticed immediately that it was drawing more current than the other side from the glow of the lightbulb. I tried making some bias adjustments at 60 volts and less, but my ammeter bounced around like crazy and I wasn't able to reduce the bias enough to rev her up to full AC, even with the lightbulb in series. At one point, the DMM beeped that it had exceeded the 400mA limit, but it was only a split second and I recall it started reading around 200mA immediately after beeping.
Bias should be about 300mA at full AC.

I thought my bias pot might be dirty / bad, so I shut down, let the current drop and then turned the pot from min to max a few times to loosen any dirt etc. I turned the pot down to minimum again and turned the amp back on.
That was when it got scary and depressing.

The amp was barely drawing current.

The bias is totally unaffected by the bias pot, but I tested across the pot and got exactly 0 to 1000 ohms from stop to stop. I don't know how or if to test the wiper, so I haven't done that.

What would fry from a momentary bias peak that would leave the board with a stable bias current that is 10% what it should be? Why doesn't the bias pot do anything anymore?

I would really like to know what I did even if I can't fix it... closure, you know. This amp was an old friend.
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Old 14th August 2007, 11:35 AM   #2
SY is offline SY  United States
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Actually, you don't really know how high the current went or whether that channel oscillated its way to the grave- voltmeters are MUCH slower than the time it takes to transmute silicon into carbon. My guess is that you've got some blown transistors and (if you can get the output MOSFETs), the amp is repairable, though not without some expense.

You DID check all the fuses, right?
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Old 14th August 2007, 05:51 PM   #3
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Yes, I checked all the fuses. To check bias you pull one of the fuses and place the ammeter across the clip.

I've gotta tell you, I REALLY tried not to fry this thing. I read everything I could find on this website about powering her back up. I actually bought a second hand Variac and made a current-limiting extension cord with a lightbulb across the hot line. All my fiddles with the bias trimpot were at 60volts AC or less through a 60W bulb.
Nothing ever got hot, and I am still wondering what went wrong.

If the bias adjustment can fry the amp that easily at 60volts, why do they make the range of the trimpot so high? Was/is my trimpot bad - it seems to test fine but does nothing.

Are there any atypical situations where the bias on a driver board just sticks at 35mA instead of 285?
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Old 14th August 2007, 07:14 PM   #4
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Is the fuse in your meter still intact?
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Old 14th August 2007, 07:59 PM   #5
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To megajocke:
My friend, do you know who you are?
You - are - my - HERO!!!!
Oh man oh man oh man I am so excited I think I'm gonna plotz!

After reading your oh so simple suggestion I removed the fuse panel on my multimeter with shaking hands,
... and, you know what, the 400mA fuse blew! Hallelujah, I have rarely deserved such mercy I can't believe I'm gonna get another shot at this thing - I will keep you posted.

Seriously, I might have never thought to check the meter because after all, I'm one of those imbeciles that takes on projects he should probably leave to the pros.

Way to go DIYaudio. I see people praising this site in their posts all the time, now I truly know why.

If I get this amp up and running again, I will be making as large a donation to this site as I can afford....
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Old 15th August 2007, 02:39 AM   #6
anatech is offline anatech  Canada
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Hi HaflerFreak,
May I make a suggestion?

When setting bias currents, many procedures include using a series ammeter in a supply line. I do things a little differently. I made up some resistors with fuse caps on them. 1W metal oxide jobs. I also have some 5 W metal plate units I use for big amps. They are 0.1 ohm and I do have a couple 1 ohm models. I then stick them in the fuse holders and measure the voltage across them.

No leads carrying all the B+ hanging around and the ability to measure channel to channel at will. Neat eh?

-Chris
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