Rex's 0404 repair chronicles...

jbeckva

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#1
Figured I'd share with the group. :mrgreen:

Awhile back Rex had a channel go out on his EMU 0404 sound card. Took a bit to get here, but I now have it to troubleshoot.

I haven't even powered it up yet, and can definitely see a problem. The output resistance on the good channel is around 50 ohms (guess that's "good" or to be expected), but it's not even 4 ohms on the bad channel.

Seems there are two tiny transistors of some sort near the output connector. One's reading "good", the other "bad". Question is... what kind of transistors are these (gotta find one methinks). All I see is "6KI" (or maybe "8KI".. ) written on them. Anyone (sam) heard of these little SMT beasties?

Quick to tshoot, but I hate waitin' for parts.. :study:
 

Skywavebe

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#2
Hi Jerry,
It is really hard to say without technical documentation which is going to be hard to get I bet. I would think that transistors near the output would be muting transistors with one terminal grounded. One of these is shorted probably. If you are used to working with SMD type devices, you could lift the one that seems to be attached to the bad channel side. The number is likely a house of manufacturer number or maybe a date code. These things made in China as most stuff is, are not meant to be fixed- although some of us will try.
Even if you were to send it back under warranty it would go in the can and they would just send you a new one. There is no budget to pay a well qualified Technician with all the tools needed to fix it when they could easily give 100 of the boards away for what it would cost to set this up. If you can get the channel to work with that transistor lifted and gain seems to be correct, then with the danger of some pops or noise on that channel when the computer is turned on, you could always work around that by turning on speakers after the computer was on. Also off or down previous to turn off. Chances are you could put a typical transistor on the board depending on the signal fed to it and the type of transistor is not that particular with regards to gain- Hfe or other specs as all it does is act like a switch. A little more info really have to be obtained to be sure but that is my input on the thing. I was handing my boss, Director of Engineering an Antex card some years back that was brand new (they are close to $600 a copy) and a surface mount capacitor fell off of it. So don't feel like you got a cheap board- even the expensive ones made in China fail just as easy- their quality is low across the board. And I used to complain about Japanese quality. HA!
 

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#3
Thanks Sam,

Yeah that pretty much confirms my suspicion about the purpose of them. Collector's (or source.. it *could* be a FET) right at the output line and emitter (or drain) is direct to ground. I'm going to lift it and see if that does it.

What were those common muting transistors on teacs? The ones with the very fast transition times?

(psssst.. with 10 years of component level t-shooting, you bet I snuck over to the 2M shop on many an occasion, including learning SMT.. :rabbit: )
 

jbeckva

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#4
Yep, that did it. I lifted both - I didn't like the fact that the other one was only showing 50 ohms when compared to another like soundcard I have measuring 90K. Plugged it up, installed the drivers, and sound's coming out of both channels.

Interesting predicament. When powering down the PC I did have a noticeable spike on the line, so I'd recommend not having it as a selected input if doing either power up or down on the PC. Other than that - it's working fine.

So hey Rex - we can leave the transistors out, or I'll keep looking for them. Whatever you want me to do... :thumbright:
 
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#5
jbeckva said:
Yep, that did it. I lifted both - I didn't like the fact that the other one was only showing 50 ohms when compared to another like soundcard I have measuring 90K. Plugged it up, installed the drivers, and sound's coming out of both channels.

Interesting predicament. When powering down the PC I did have a noticeable spike on the line, so I'd recommend not having it as a selected input if doing either power up or down on the PC. Other than that - it's working fine.

So hey Rex - we can leave the transistors out, or I'll keep looking for them. Whatever you want me to do... :thumbright:

Right on! Thanks!


I'd like to have them there if possible but could just have ya send them to me when ya find them. I'm sure I could solder them in place.

What do you think?
 

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#6
Rex Everything said:
Right on! Thanks!


I'd like to have them there if possible but could just have ya send them to me when ya find them. I'm sure I could solder them in place.

What do you think?
You sure? They are tiny little buggers that you'll need to have a scope and a steady hand to install. Let me go get the camera and take a pic first.
 
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#7
jbeckva said:
Rex Everything said:
Right on! Thanks!


I'd like to have them there if possible but could just have ya send them to me when ya find them. I'm sure I could solder them in place.

What do you think?
You sure? They are tiny little buggers that you'll need to have a scope and a steady hand to install. Let me go get the camera and take a pic first.

Scope, just to install? I do have one and should have a place to set it up soon.
 

jbeckva

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#8
Haven't seen any replies to my inquiries on what a good P/N is for 'em, but heck... you probably won't even miss them. In fact, in some circles removing those is a performance "tweak" as they're right in the critical signal path.

I'll box 'er up tonight and ship 'er out tomorrow.
 
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