Advent 300 - help diagnosing a short

[special_20]

Hoping maybe some of the wonderful techs around here can help me out. I have an Advent 300 receiver. I performed a full recap (all electrolytic caps replaced as well as tantalum caps) per the David Reaton website. After recap, amp was brought up on dim bulb tester. Amp was working fine. DC voltages were adjusted back to factory.

Amp was in regular use for a few weeks until amp suddenly stops working. No sound from speakers. Open it up and find blown fuse. Replaced fuse, turned on. The protection relay went click. Reset the protection relay via red button, and the 3 Amp fuse blew. This time unit starts humming through speakers.

Did a quick visual inspection and I'm seeing no obvious burned/broken components, popped caps or even any hot components or discolorations on the board. I pulled output transistors block and tested all 7 transistors on the heat sink. Both right output transistors tested bad. Replaced outputs devices on both channels with 2n6490/2n6487. Also replaced emitter resistors as they were testing high. Power supply caps still test good on ESR meter. Reflowed a few suspect junctions from the factory.

Put in a new fuse. Dialed variable resistors all the way CW assuming I'd need to reset DC voltages Upon fire up, relay is still kicking off after 2-3 seconds. Fuse isn't blowing. Not reading any DC voltages on emitter resistors during brief interval of fire up. Any other suggestions about where to head next in troubleshooting a short here?

 

[special_20]

Also, pulled 3A fuse. With fuse out, breaker isn't tripping. Double checked power supply voltages. Per manual, line voltages should be 22vdc; I'm reading 24.5 vdc. I have upped the capacitor values from 3000 uf to 4700 per recommendations on other audiokarma threads.

 

[NAD80]

Checked the SM. 24Vdc +/- 10% (21.6Vdc-26.4Vdc) at F400. Did you check all the transistors in the right channel amp?

 

[special_20]

Ok, maybe I've got a slightly different manual on power supply voltages. Seems like my tranny, caps, and rectifier must still be working nominally.

I checked all 6 transistors (and the voltage regulator) mounted to the heat sink Q312/313 were both testing fine. Q315 and 327 both tested bad. Did not dig further back into circuit as this board is tricky to find component legs on. Not the cleanest design... Any in particular of concern or should I try to pull them all sequentially going back?

 

[NAD80]

I would check the rest of the transistors in the right channel. At least they give the pin out of each device / parts list to check parts. A lot of times its more than just the output pairs that are bad. The SM from hifi engine doesn't allow higher magnification of the schematic of the part numbers.

 

[stereoguy70]

Sounds like you had some weak transistors in there that after the recap, they saw the voltages they were supposed to and didn't care for it. I would definitely check the rest of the transistors in the power amp channel that had the issue. Also, triple check your caps you replaced to make sure the polarity is correct on them.

 

[CT Jim]

Amp was in regular use for a few weeks until amp suddenly stops working.

If a cap was in backwards wouldn't this statement not be possible? But, maybe the amp was bumped and either created a bridge between two points, or somewhere a solder trace broke?

 

[vaathi]

I am in the same boat. I was running this Advent as amp for my computer speakers ( canton MX) for couple of months now. I try to switch off the unit when I am done with it but my kids are not judicious about switching it off when it is not in use. Anyway today when I switched it on, there was a short burst of static and dreaded burning of electronics smell and no more sound.

 

[special_20]

had a chance to pull the transistors and diodes. One of the pre-drivers was shorted. Replacement on order. Hopefully this does the trick.

 

[Ken770]

I’m having a similar problem. -22vdc fuse blows. Did you find the fault?

Ken S