I blew up all my amps :irked: (Help needed, hafler adcom)

[Septimus]

Hi everyone, haven't been here in forever but my amp just acted up and I thought of this forum.

Due to a intermittent short that developed in my DIY braided cat-5 speaker cable I fried 3 amps. 1 of them over a year ago that I always thought was weak and kind of always expected to fail so I never even checked if there could have been a problem. It was a YBA Complete integrated amp that I sold online, broken, for about 200.

The second one was a nice adcom GFA 535II that went into thermal protect and then never worked right after that. I thought maybe the speaker had a problem and I checked it with my multimeter and one speaker had a very high value, while the other had a very low value for resistance. :dunce: Assuming this was the problem I hooked up some other speakers and ordered a new amp on ebay. The adcom sits in the basement, waiting to get fixed or sold.

The third one was this nice "Sound Valves" amp, a rebranded Hafler SE240. This unit was NOS, and not 10 minutes in I hear a pop and see a little smoke. It doesn't stop playing at this time, it just had a little smoke and I quickly turned it off (2-3 seconds at the most) . Now I'm really frustrated and check the speakers again. My cheap-*** multimeter that had been sitting in a humid garage for 2 years and I should have known better led me astray. The speakers were fine and then I found the short in the speaker cables.

I called up a repair shop I found online that was supposed to be reputable. They said the hafler is irreparably damaged. The unit never even stopped playing music, both channels. It seems to me that only one component failed in the right channel that would need to be replaced. I popped the top on the amp and couldn't find anything burnt looking or damaged in any way.

Long story short...

I blew the right channel by shorting it out. I want to figure out if the hafler is something I can repair myself (and later on, I want to look at the adcom too...) . If it is only replacing one or two items and checking a few values on the board with a (NEW!) multimeter then I would be very interested in doing that.

I snapped a few pics and posted them online.

http://picasaweb.google.com/Bryant.CW/FriedHafler

Maybe someone will see something out of
order, or could use them as a diagram to help me identify likely suspects. They can be downloaded and are fairly big, so there is reasonable detail there.

In either case, thanks in advance for any help!

[gain]

Originally posted by Septimus

...

It doesn't stop playing at this time, it just had a little smoke and I quickly turned it off (2-3 seconds at the most) .

...

I blew the right channel by shorting it out.

this doesn't make sense if the channel got shorted you would certainly not hear any sound while it was shorted, at least not from that channel. does the amp still play? if one channel is dead and the other still works, and you are reasonably certain the cause of failure was shorted speaker cable, then the fist place i would look is the output stage of the dead channel ... especially between the collectors and emitters.

[Septimus]

Well, it was the right channel on all three amps so the fault was in the system somewhere. Both the adcom and the hafler got very hot on the right channel when they failed. The only thing common to all three failures was the speaker cable, and testing it with the multi meter shows a short when its bent a certain way. Could it be that the short was very small and only allowed a little current through it?

On the integrated amp it played while smoking, I turned it on again and it didn't work. On the adcom it said thermal protect , turning it off and allowing it to cool I tried it again. The right channel made a zipping sound when I turned it on and was immediately in thermal protect again. I haven't tried the hafler, I didn't want to damage it.

[gain]

well, there are guys on this forum with MUCH more knowledge and experience debugging amps than i have, so you may want to get their opinions first, but here is what i would do ...

take the amp and wire a light bulb (say 60W, 120V) and wire it in series with the mains hot wire going to the amp. leave speakers disconnected or apply an 8 ohm dummy load at the speaker terminals. short the inputs. apply power. the bulb will act as a current dependent resistor, lighting brighter and dropping more voltage as more current flows, (hopefully) preventing damage in the event there is a short inside the amp. if all is well, bulb should glow bight when you first power up, then quickly settle to a dim glow or off.

if bulb stays bright, begin looking for dead shorts (with power removed!), starting with the transformer itself, working through the power supply (esp the rectifier diodes, snubbing caps and reservoir caps) and eventually to the OPS. disconnect wires where you can (eg between the PS and amp board) and observe if the short goes away. you must find the cause of the short before you proceed any further.

if the amp will power up normally (bulb glowing dim) and you can get sound out of one channel, begin by testing the OPS of the dead channel. look for cooked components, esp resistors and transistors. check the diode junctions of all the output devices with the multimeter set on diode test. better still, if you have the time and patience, check every piece of silicon on the entire channel.

doing this procedure should at least point you in the direction of the problem. oh yeah, and don't forget to come back and tell us what you found when you get done testing!

regards and good luck.

[unclejed613]

don't try to get sound out of the amp with the bulb in series with the amp, as the extra current draw will cause the bulb to drop more voltage.

[Septimus]

Originally posted by gain
... you must find the cause of the short before you proceed any further...

The short was in the speaker cable, unless you mean that the damage would have caused a short in the amp which will now need going after.

To clarify, the hafler never stopped playing on both channels, I just turned it off after the popping and the little bit of smoke. Can I check it by just hooking it up again, or would turning it back on risk further damage?

*45 minutes pass...*

Well, I didn't wait to check it out and here's what I got:

I plugged in the amp and turned it on with nothing hooked up to the input or output and it didn't explode.

I hooked up my multimeter and measure the voltage between the +DCV and -DCV fuses on both channels; they were the same.

I measure the output of each channel from the speaker terminals with nothing hooked up to the inputs. On startup the left channel (the good one) read about -2vdc, and the right channel (the questionable one) read about +3vdc. After being powered up for a moment however, they both go to zero. I unplug the amp and let it sit for about 20 min while I eat some dinner and then I come back to it. I turned it back on and plugged in one of my speakers to the right channel and it played as normal. I then tried the left channel and it also played as normal.

Its been playing for about 10 minutes now at a low level, and the right channel is not running hot.

I'd like to think that all was well, but since I saw the smoke I can't imagine that its all ok.

What steps should I take now?

[Septimus]

UncleJed:


You are in Denver?

I'd love to take it somewhere local and have it fixed.

Let me know.

[gain]

awesome, great news about both channels still working!

in that event, the smoke you saw most likely came from some component that is obviously not directly related to processing the signal. check stuff like rail bypass caps, smoothing/snubbing caps, zobel network if applicable, etc. also check the other functions of the receiver to make sure they are still working (PA, tuner, RIAA, tone controls, etc) because perhaps the smoke might have come from one of those boards instead of the amp.

[Septimus]

Its just a 2 channel amp.

As for those things you suggested checking, if I had any idea what they were, I'd get right on it!


I'm definitely a total novice when it comes to this.

[Stormrider]

Just so you know schematics/service manuals for the Haflers are available at hafler.com

I took a look at the photos and nothing really jumped out at me. Look at all of the capacitors, especially the electrolytics and see if they are bulged or leaking. If any are you need to replace them and clean the board.

Make sure the dc offset for both channels is below 50mv. To do this connect an 8ohm resistor across he output terminals and put your meter in parallel with the resistor. When you first power up the amp it'll jump to a few volts and then level off.