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Circuit error in Lite DAC-AH: Easy fix - Page 8

post #106 of 209
Thread Starter 
Quote:
Originally Posted by _Rand_
Stupid question, are there any capacitor values that would "work" that I might be able to pull off an old soundcard or something?

Nothing permanent, just something that would work well enough to make sure I haven't done any real serious damage. Or work until I get a proper replacement even.

I don't think its likely I did any major damage of course, especially given the state of the capacitor I screwed up, but I suppose damaging one of the TDA1543s isn't totally out of the question, but I don't think I could do that just by having a soldering iron in proximity for a few seconds...

I have a ton of mostly useless old computer hardware, much of it from the P2 and P3 era. They are of course, covered in caps of varying values.

The only high voltage caps I spotted though were some 4.7uf 50v ones.

Just remove the damaged caps. They are just local power supply decoupling caps to reduce power supply noise. If the noise you were hearing goes away then you'll know for sure that the caps are bad. It would still be a good idea to replace them but maybe not mandatory if it sounds OK.

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post #107 of 209
Quote:
Originally Posted by eladioc
where did you guys get your opa627's? Any place have them for cheap?
Eddie Wu at DIYkits in HongKong sells them for $6 a piece.
http://www.diykits.com.hk/

Screaming deal when you consider the equivalent part goes for $18 at digikey, which itself is an excellent reasonably priced source for parts in the US.

Yesterday I bought ten from Eddie. With the $120 I saved on OPA627s alone, I bought a Lite DAC AH.
post #108 of 209
Quote:
Originally Posted by Kim Hardee
Just remove the damaged caps. They are just local power supply decoupling caps to reduce power supply noise. If the noise you were hearing goes away then you'll know for sure that the caps are bad. It would still be a good idea to replace them but maybe not mandatory if it sounds OK.
Well that helps, would I have to jump them with a peice of wire or just leave it?

Edit: It seems I would have to jump both caps I removed, but I'm too lazy. At any rate I don't get static with them gone, just the squeal I used to get with no cable attached.

Whether or not this means its just the caps or not I don't know.
post #109 of 209
Thread Starter 
Quote:
Originally Posted by _Rand_
Well that helps, would I have to jump them with a peice of wire or just leave it?

Edit: It seems I would have to jump both caps I removed, but I'm too lazy. At any rate I don't get static with them gone, just the squeal I used to get with no cable attached.

Whether or not this means its just the caps or not I don't know.
Definitely DO NOT jump them. That would short out the power supply. Now you can replace them whenever you want to.
post #110 of 209
Quote:
Originally Posted by DDF
Eddie Wu at DIYkits in HongKong sells them for $6 a piece.
http://www.diykits.com.hk/

Screaming deal when you consider the equivalent part goes for $18 at digikey, which itself is an excellent reasonably priced source for parts in the US.

Yesterday I bought ten from Eddie. With the $120 I saved on OPA627s alone, I bought a Lite DAC AH.
$6? $80 HK converted to US $ is $10.28... how did you get $6?
post #111 of 209
So I have enough capacitors on the way to replace C5,6,7,8,12,13,14,15 with .1uf 63v Polypropylene Film caps.

Assuming getting those in doesn't work, where should I look next? A messed up TDA1543? Im pretty determined to get this working again, as well as (re)learning some electronics stuff.

I still get static with the two caps removed, I forgot to hook up the other end of the cable before (I'm a moron, what can I say.) So I'm not terribly sure just what i did to this thing if the noise should go away with them unhooked.

As I understand it the stuff on the right hand side of the board, past the capacitors and whatnot isn't even necessary for the passive mod so would someone mind linking me to info about how to do that?

What I don't understand is the voltage thing.

No matter how much I turn the trim pot pins 5 and 7 on the TD1543s always measures about 3.88-3.9v. Im talking like 20 revolutions here, I'm thinking the trim pot is busted actually, but I don't know for sure if thats normal or not.
post #112 of 209
Quote:
Originally Posted by eladioc
$6? $80 HK converted to US $ is $10.28... how did you get $6?
Impeccable fashion sense?

Pretty straight forward to deal with, I wrote and asked for a quote for 10 OPA627s and a Dac-ah, that's it. Shipped it one business day later. I plan to mod this:
- OPA627s
- cap changes
- ensure the voltage is right
- heat sinks
- if needed, modify the top with perforated metal insert, so it can ventilate.

BTW, shipping was $40, no idea what it would be for just the OPAs.
post #113 of 209
Thread Starter 
Quote:
Originally Posted by _Rand_

No matter how much I turn the trim pot pins 5 and 7 on the TD1543s always measures about 3.88-3.9v. Im talking like 20 revolutions here, I'm thinking the trim pot is busted actually, but I don't know for sure if thats normal or not.
You shouldn't have to turn the trimpot more than 1/2 turn. It sounds like maybe you somehow shorted Vref to something else, maybe one of the DAC outputs. The Vref trace is very close to the one of the DAC output traces. Check where pins 5,6,7 are soldered on the TDA1543 next to the resistors and damaged caps to see if any of the pins shorted to other pins when you soldered the resistors. Measure the voltage on pin 7 (Vref) of the DACs. It should be about 2.1V. It shouldn't change much as you turn the trimpot, but if it's below 1.9V or above 2.2V, then Vref is probably shorted to something.

The only components on the right side (opamp side) of the board that are needed for the passive bypass are the resistors R35,R35 and the caps you are replacing. I don't think you should attempt the passive bypass until you fix the Vref (trimpot) problem, though.
post #114 of 209
I get 2.15v between pins 4 and 7, is are those the ones I'm supposed to be measuring?

Also I can't see any places where traces are shorted, everything looks like it should be connected as is, following what traces I can see anyways.

Playing around with my continuity tester it appears one pin of the trim pot is connected to pin 7 of the TDA1543s and the other two pins are connected to pin 4 of the TDA1543s, this continues across all pins.

Assuming there is something wrong with the trim pot that isn't just connected to the missing caps what is a suitable replacement? Perhaps I accidentally broke something internally trying to free the screw from the white glue they use to secure it.
post #115 of 209
Thread Starter 
I agree -- it sounds like something might be wrong with your trimpot. I guess you could take it off the board and measure the resistance as you turn the screw to see if it is defective. You can probably find a replacement at Digikey.com. I believe it's a 1K ohm pot.
post #116 of 209
Thread Starter 
Take a look at Digikey part number 490-2278-ND. It might work for a trimpot replacement.

Can you describe the noise you're hearing?
post #117 of 209
The part you listed seems to be discontinued.

I found this though: 3296W-102-ND from the picture it looks VERY similar. Might even be the same part actually but branded with a different name. The stock one appears to say BAOSH 3296. Thats not in stock though. 3299-201-ND is however and appears to be identical. 3296W-102LF-ND also appears to be OK.

Im fairly certain its a bad pot now though, the specs for the 3296 says its a 25 turn pot. Mine NEVER stops turning. I must have done 100 turns before I gave up .

Wish I had decided on that before I placed and order for the caps though.

As for the noise its static, and its also much lower volume than is normal. I need to turn my amp up to about 3/4 for it to be the volume I normally have at about 1/4-1/3.

The static is especially bad on low frequencys.

EDIT: If im going to order a trimpot I dont want to waste $14 on getting a $3 item shipped so I'm going to need to buy $25 worth of stuff in order to not get digikeys handling + international fees.

I can fill most of that up with replacing the major caps on the board but that still leaves me about $6-7 short. Is there a fairly inexpensive opamp (In the $3-5 range) that would be worth replacing the stock ones with perhaps?
post #118 of 209
Thread Starter 
Sorry that the part was discontinued. I was looking at a 2004 catalog.

I can't recommend any opamps. Maybe someone else can.

So, your volume was reduced significantly after you soldered the new resistors in parallel with R35,R36? That shouldn't have happened. Can you measure the resistance between pins 5 and 7 on the TDA1543s with the power off? It should measure about 340 ohms.

Also, is the "static" you're hearing only on loud passages?
post #119 of 209
I get 18.3k between pins 5 and 7 when the meter is set to 200k. Thats pretty far off 340 ohm.

Do keep in mind there is nothing at C6 or C8 at the moment, and while I was taking those out I put 150ohm resistors at R35 and R36. A little off the 163 ohm ideal, but I had 150 ohm resistors handy.

As for the static, its constant as long as there is audio playing. There is also a bit of a squeaky noise when it first starts up forgot about that bit earlier..
post #120 of 209
Thread Starter 
I'm so sorry. I meant to ask you to measure the resistance between pins 6 and 8. I was thinking of the wrong pinout. You should get about 300 ohms since you changed the resistors to 150 ohms.
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