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Dac Ah Repair.

post #1 of 16
4/14/08 at 12:57pm
Thread Starter 
New here. Looks like a great resource.
Recently purchased a Pacific Valve modded Dac Ah. Found that one channel is outputting a very low level. Emailed Pacific about the problem and they advised that it is probably the Op amps and that they can be replaced diy. I am handy but have no experience with electronics.
Can anyone point me to an explanation or step by step instructions for doing this? How difficult is it?
Thanks for any guidance.
Steve
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post #2 of 16
4/14/08 at 1:05pm
depends if the opamps are socketed

afaik they're not unless pacfic valve installed sockets for you

when you remove the top lid of the DAC-AH, the opamps are 8 pin integrated circuits on the top right of the PCB just below the RCA jacks

personally if it arrived faulty i'd insist on a replacement from pacific valve
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post #3 of 16
4/14/08 at 1:23pm
Thread Starter 
Thanks for the reply. Must not have been clear about my purchase. This was not purchased from Pacific. Was purchased used and the seller is offering a refund. Just thinking I might use it to learn a bit.
This is what Pacific says about their upgrade:
DAC AH-M1 takes this architecture one step
further by upgrading with the OPA27AM Op Amp
for greater detail and clarity. In addition, we install
an op amp socket for future upgrades.
So it appears that there is a socket. Does this mean it can simply be unplugged and replaced? Is there a source showing pictures perhaps?
Thanks again,
Steve
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post #4 of 16
4/14/08 at 1:31pm
Sounds like you have a socketed op amp and you just need to pop the old one out and place the new one in.
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post #5 of 16
4/14/08 at 5:13pm
Quote:
Originally Posted by Scoly View Post
Emailed Pacific about the problem and they advised that it is probably the Op amps and that they can be replaced diy.
It is very unlikely that an opamp simply stopped working on its own. Even if the opamp is the problem, it is likely an indication that something else is wrong with the circuit. I'd return it and get one that works.
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post #6 of 16
4/14/08 at 5:39pm
Thread Starter 
Appreciate all of you for taking the time to respond. Sounds like returning is the best of my choices.
Steve
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post #7 of 16
4/15/08 at 4:30pm
Thread Starter 
After replies here I decided to try swapping the opamps to see what would happen. Befor the swap the left channel was outputting at a much higher sound level than the right. After the swap it was the right channel outputting at a higher level.
My assessment is that one of the opamps is in fact bad and that a replacement will fix the problem.
Where do all of you purchase parts such as the OPA627am at a reasonable price?
Thanks
Steve
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post #8 of 16
4/15/08 at 6:08pm
I was about to suggest that you did that - good work!

Texas Instruments have a very flexible samples policy - have a look at the OPA627 page on their site and see if there's a free samples link. There usually is. They'll probably send them to you for nothing. If you have a friend who works for an electronics company then they'll definitely send them to them FOC.
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post #9 of 16
4/15/08 at 6:10pm
Ah, they're only available as paid for samples, unsurprising now that I've checked the price of them. I can get them here in the UK from Farnell, I don't know where you are but digi-key is usually a good bet in the US.
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post #10 of 16
4/15/08 at 6:29pm
Thread Starter 
Thanks Jambo. Did find a source though he is moving and has to sort thru boxes to find them.
Have been reading a lot here about modding the dac ah and keep coming across the "passive modification" being mentioned. Is there a link to what this is and instructions on how?
Thanks again,
Steve
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post #11 of 16
6/23/10 at 5:30pm

My DAC-AH's output is at very low volume and there's a LOT of noise, plus when you flick the input selector to the empty socket (I use coax and nothing's plugged into the optical input) I get a really distorted signal, like what you'd get if you were trying to catch a certain freq transmission on communications equipment (think of some alien flicks and they're picking up alien transmissions on short waves, etc). I was advised it's most likely the digital receiver chip, which I think was heat-damaged considering the mods done to this unit before I got it had heatsinks on the DAC chips but the I didn't realize the full extent of the heat generation until I opened it up. All the sinks are really hot, I should have used it with the cover off. Problem is I cant find the schematics and I can't read the markings on it.

 

Would anyone know the details on that chip? Part no., manufacturer, etc?

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post #12 of 16
6/28/10 at 8:37pm

You can reference the circuit diagram here.

 

http://www.audiodesignguide.com/Headphone_opamp/theHeadphone_opamp.html

 

It's best to leave the top cover open even with the heatsink installed as it still get quite hot and the heat sink ppb added more heat inside the chasis.

 

Good luck.

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post #13 of 16
6/29/10 at 3:21am

Thanks for the link. Now the DAC-AH is making some popping noises. I hope I get this fixed pronto, the Icon Mobile sounds similar but it's just not enough, especially now that my car's processor is giving me erratic L-R gain biasing.

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post #14 of 16
8/9/10 at 2:40pm

OK, update. Got an advice via email, based on the schematics and the mods on it, that most likely the TDA1543's went bad, but he was not 100% on it. I took the plunge anyway, and...turns out there's still something else wrong with it. Now it outputs absolutely no sound, or just a few minutes ago, it was spitting out a humming sound. Any theories as to what might be wrong with it? Like the caps? Or this chip smack between the power caps? (I'm no engineer BTW, so I'm not quite sure what that chip is for)

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post #15 of 16
8/13/10 at 4:14am



You need to study the schematic and trouble shoot with a DMM,  if you don't know how to do this I highly recommend you send it back.

 

 

Quote:
Originally Posted by ProtegeManiac View Post

OK, update. Got an advice via email, based on the schematics and the mods on it, that most likely the TDA1543's went bad, but he was not 100% on it. I took the plunge anyway, and...turns out there's still something else wrong with it. Now it outputs absolutely no sound, or just a few minutes ago, it was spitting out a humming sound. Any theories as to what might be wrong with it? Like the caps? Or this chip smack between the power caps? (I'm no engineer BTW, so I'm not quite sure what that chip is for)

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