[dhp]

I'm posting on behalf of a friend (I'm helping him build his amp..) I have his PPA board so I can troubleshoot it here.. the problem is this.
Yesterday his PPA powered up fine, no problems at all and he wired up panel components.. Took the PPA home, today worked on it again... realized he forgot to adjust the buffer bias.
While adjusting buffer bias he shorts output resistor to one of the legs on Q33 (output transistor).. We dont notice and scratch our heads wondering why meter reads OL. I touch the top of the TO-92 and it's HOT.. burn-like warmth and I start to smell solder so we turn it off and desolder the TO-92s from the left channel..

Thinking that we could adjust the bias on the other two, we power up the amp again, this time TO-92s arnt hot, but the R-23 burns a hole through itself and we quickly shut off the PPA and call it a day.. my question is this

Why would the R-23 burn? It's rated for 1/4 watt at 2.2 ohms so that's 1/4=I^2*R. so I^2 = 1/8 amp. so that resistor would have to pass about .35 amps through when the Q-23 can only output 50 mA... whats going on here?

Oh and we get to replace the entirety of ground/left/right buffers.. just to be safe =p

 

[en480c4]

I shorted out an entire channel adjusting the bias on my PPA v2.
Check Page 2 of the following thread:
http://www6.head-fi.org/forums/showt...ht=PPA+smoking
A friend took pity on me and rebuilt the channel and fixed the problem.
Good luck!

 

[ATAT]

The TLEs dont power the buffers right? (IIRC tangent's schems show only HV+ and HV- powering the buffers) So replacing the entirety of To-92s should work.. anyone confirm?

 

[n_maher]

Quote:

Originally Posted by DieInAFire I'm posting on behalf of a friend (I'm helping him build his amp..) I have his PPA board so I can troubleshoot it here..

Sure you are

. Don't be afraid to join team smoking resistors - all the cool kids are doing it. With your user name you're a natural.

Quote:

Originally Posted by DieInAFire Oh and we get to replace the entirety of ground/left/right buffers.. just to be safe =p

I wouldn't, not until each of those torched their own resistors. It's not fun replacing all of the components, trust me. Just replace the parts that I list in the thread that en480c4 linked to on the affected channel and that should do it.

Good luck, I hope you've got a desoldering pump.

Nate

 

[Clutz]

Quote:

Originally Posted by n_maher Sure you are . Don't be afraid to join team smoking resistors - all the cool kids are doing it. With your user name you're a natural. I wouldn't, not until each of those torched their own resistors. It's not fun replacing all of the components, trust me. Just replace the parts that I list in the thread that en480c4 linked to on the affected channel and that should do it. Good luck, I hope you've got a desoldering pump. Nate

Replacing all of the transistors is definitely not fun. I've done it before. In fact, saying it's not fun makes it sound like it's way more fun than it actually is.

Cheers,
Clutz

 

[ericj]

Just gonna go ahead and recommend that y'all familiarize yourselves with the solution to the "oops my probe slipped" problem:

http://www.allelectronics.com/cgi-bi...-BR&type=store

J-hook probes.

Use 'em.

Even better, design boards that have through-hole plated test points away from the components that can accept the little loops the hooks are designed to clip onto, or at the very least a plated hole to jam the end of the probe into.

 

[n_maher]

Quote:

Originally Posted by ericj Even better, design boards that have through-hole plated test points away from the components that can accept the little loops the hooks are designed to clip onto, or at the very least a plated hole to jam the end of the probe into.

I challenge you to find the "spare" room on the PPA board for test points. Have a look , there isn't any.

Nate

 

[dhp]

Quote:

Originally Posted by n_maher I challenge you to find the "spare" room on the PPA board for test points. Have a look , there isn't any. Nate

yep

On the other hand, i can safely say I have smelt the smell of Vishay/Dale Smoke. (Alliteration

)

 

[tangent]

Quote:

Originally Posted by n_maher I challenge you to find the "spare" room on the PPA board for test points.

Indeed. In fact, I wanted to put them in from the start, but we never found the space for it.

 

[ATAT]

tangent.. how about adding this extra peice of advice .. I've found this helpful (after I watched DieInAFire fry both signal channels, and fry R23 on left ... gawd those RN55s smell _awful_). there are two holes in the output pad .. (that arnt connected to resitors). Wire up a wire to the unused hole (ie not connected to output.) use an alligator clip to that one.. now you only have to worry about one side rather than both sides.. win!

 

[n_maher]

Quote:

Originally Posted by ATAT tangent.. how about adding this extra peice of advice ..

I don't mean to sound like a wiseguy but I think part of the assumption is that if you're willing to take on a project like the PPA you're smart enough to figure this out own your own. Also, you can measure from the bottom of the board where there's less likelyhood of slipping and turning the PPA into a signal flare.

For the record, the transistor replacement only takes about an hour to do if you've got a decent desoldering pump.

Nate

 

[dhp]

Quote:

Originally Posted by n_maher I don't mean to sound like a wiseguy but I think part of the assumption is that if you're willing to take on a project like the PPA you're smart enough to figure this out own your own. Also, you can measure from the bottom of the board where there's less likelyhood of slipping and turning the PPA into a signal flare. For the record, the transistor replacement only takes about an hour to do if you've got a decent desoldering pump. Nate

yeah, took about 5 minutes to clear up the transistors for another transistor implant. And I did measure from the bottom of the board. My hand slipped anyways. It was tragic.