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mbates14 7th January 2010 03:06 AM

Couple of tough-dog amps
Working on a couple of tough-dogs.

One of them is an audiobaun A6004T. Keeps burning outputs, but it might be user error... It had shorted outputs QA11, QA12, both a 2SC4467 and 2SA1694.

Also had both drivers shorted, and top blown off of QA10 i think it was? then RA40, RA41 and RA42 open and burnt to a crisp. figured everything else out via the other 3 channels. With outputs removed, other 3 channels work fine. no problems.

So i replaced all those parts, and checked everything else for opens, shorts, etc etc. the differential and VAS stages all test fine. its odd... But i do not have these exact output transistors, all i have are the TIP35C, TIP36C pairs, as most amps use them.

So i used those. The channel sounds fine within the first 2 or so seconds on powerup, then it starts to cut in and out between protect for a few seconds before the output transistors short completely, and hold it into protect. Replacing the outputs, same exact thing happens. It might be me, not being able to sub out the 2SA/2sC outputs for the TIP set....

Thats the first amp.

Moved second amp to new thread.

Perry Babin 7th January 2010 03:13 AM

The biasing is wrong for the 35/36. With the outputs clamped, monitor the DC voltage across the emitter resistors in the channel with the 35/36. Does the voltage begin very near 0.000v DC and slowly increase?

Start a new thread for the ppi.

mbates14 7th January 2010 03:18 AM

i wouldnt know becuase it only lasts about 8 seconds before the outputs go bye bye. emitter resistors get hotter than a firecracker as well towards the end. When the amp is first on though, it sounds ok, but i cant turn it up becuase if i turn it up, the sound starts kicking the amp into protect before eventually it kicks itself in and out of protect on its own at idle, before outputs short.

Perry Babin 7th January 2010 03:26 AM

Is the emitter of QA8 connected to the base of QA10?

If so..
1. short legs 1 and 2 of QA8

2. connect your multimeter (set to DC volts) across one of the emitter resistors in that channel

3. clamp all transistors

4.insert a 10 amp fuse in the B+ line and power the amp up.

If the DC voltage across the emitter resistor is more than 0.000v for more than ~1 second, shut down the amp and post the results.

mbates14 7th January 2010 03:31 AM

I think so, but i dont have the amp in front of me.. its at the shop ill have to check everything out tomorrow. But at the rate im loosing outputs, cant afford to keep blowing them. lol.

Perry Babin 7th January 2010 03:34 AM

If you clamp the outputs tightly to the heatsink and use a 10 amp fuse in the B+ line, there's virtually no way to blow any outputs. Clamp ALL transistors so you don't blow any others.

mbates14 7th January 2010 03:41 AM

Also, is this with a load on the output? or no speaker/load connected.

Perry Babin 7th January 2010 03:54 AM

Nothing connected to the speaker terminals.

mbates14 8th January 2010 12:52 AM

Ok, i put the right outputs inside there, $11 bucks later. its still pulsating between protect and power.... Also on powerup, both emitter resistors shoot up to about .258V on poweron, then back to 0v, then .258, kinda in an oscillating fashoned.

I still dont trust using the 10amp fuse either. also i dont want to short out any pins for the simple fact that i cant get to much. its really compact in there.

Perry Babin 8th January 2010 01:03 AM

What do you mean by the 'right transistors'? Do you mean exact replacements?

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