AVR Center Channel Failure

[highdesert56]

Hi guys,
I have an issue I was hoping somebody could give me a hand with. 

I purchased a new center channel speaker and watched a movie at a somewhat moderate volume.  About 20 seconds after turning it down when the credits were rolling I heard a sound like a headphone jack being plugged in while powered up and lost the center channel.  It's there, just really low volume and highly distorted.

After determining the speaker is not the issue, I found that there is a trace on the amplifier circuit board that burnt like the element of a fuse.

I've included a screen shot of the schematic with a purple X showing where the fault occurred.

I know enough to be dangerous but not in a good way and would greatly appreciate any guidance as to what to test/replace/do next.

[james_s]

Is that the speaker output the burned trace is feeding? Is there no fuse?

I would start by repairing the burned trace, then check the output transistors. Could something have shorted the output causing the trace to act as a fuse? You *might* get lucky and have no other damage but you'll have to start testing parts.

[highdesert56]

Is that the speaker output the burned trace is feeding?

W1518A goes directly to the + binding post through some filtering so I think so.

Is there no fuse?

The only one I've seen is on the power supply and I haven't come across any others so I assume it's virtually fused somewhere.
I found the fault by noting there was no -65v on the middle leg of Q1079P.  There is no continuity between any of the legs on the power transistors for the center channel and I'm not sure how to test them further.

Could something have shorted the output causing the trace to act as a fuse?

I checked the speaker wire at both ends after the failure and they weren't touching and I've checked the amplifier board components to see if a foreign object shorted anything but couldn't see anything obvious.
I do appreciate your help so thank you.

[Armadillo]

Measure the ohms across the Red line collector to the Dotted Red line collector to find if there is a short circuit.

Other than the transistors, the 2 capacitors in series also look likely culprits.

[highdesert56]

Well, I found it, but I'm not sure I understand what's going on.  I suspect an insulator was compromised between the transistor and heatsink.

I found that pin C on Q1079P was shorted to the chassis.  After I removed the board from the heatsink there was charring between the transistor, heatsink and screw.

What causes this type of failure?  I'm going to order a transistor.  Is there anything else I should replace along with it?

Thanks again guys, I really appreciate it.

[Armadillo]

Wow, good find.

Aside from the mica sheet or insulation sheet, there is the screw plastic sleeve that you must insert in so that the screw or the body do not contact the heatsink.

If its new set, then must be poor manufacturer quality.

Not usual, unusual, I would say.

[highdesert56]

That was going to be one of my next queries:

What insulator sheets do I get?  These transistors seem to be a one off deal and I don't know whether a TO3P or T247 sheet will be big enough.

None of the transistor screws had any type of insulator on them; just a large flat washer.

This thing was still under warranty but I didn't feel like lugging it 50 miles to the authorized repair center... That, and I'm a glutton for punishment.

[Armadillo]

The set should look like this, you can see the screw with the insulator sleeve.

https://www.ebay.com/itm/5Pcs-TO220-Transistor-Triac-Thyristor-Insulator-Insulation-Protection-mica/132408053186?hash=item1ed4228dc2:rk:5:pf:0

But I prefer mica insulator sheet. Last infinito, so it seems.

Search ebay, buy the correct package Transistor Size type.

[highdesert56]

I ended up ordering a TO218 mica insulator kit and Yamaha transistors from amazon.

My thinking is that I'll use a short section of small diameter heat shrink to go over the threads of the screw between it and the transistor in the event that the ones in the kit won't work.

Thanks to everyone who posted and looked and wish me luck that I don't start a fire when I flip the switch. 
I'll use the 60w incandescent-in-series trick in an attempt to avoid that.

This forum is great!

P.S.
Has anyone seen a Yamaha or any amp for that matter without insulators on the power transistor screws?

[Armadillo]

I need to elaborate;

The sleeve is also a cap where the screw can press-in or press-on without touching the heatsink. Please see picture.

[highdesert56]

Yes, the kit I bought has those included, but the screws that Yamaha uses are of a different type.

They're more of a sheet metal screw than machine screw and I'm not sure the hole in a TO218 is the same size as the STD05P 5 pin transistor Yamaha uses on this model.

I was thinking out loud about the heat shrink as a method to improvise an insulator between the screw and transistor.

Thanks for clarifying though; I need all I can get. 

[Chris56000]

Hi!

Sorry to be a pain in the proverbial, but where did you get a manual with the coloured signal and power supply lines – I can only find copied ones with the coloured lines all thick black lines – did you get it electronically or on paper?

Any chance of a link?

Chris Williams

[highdesert56]

I bought it for 13 bucks from these guys: www.service-manual.net

I may have been able to find it elsewhere but it's a high resolution PDF with links and very thorough.

I'm $50 into a repair that could've been warrantied but I'm enjoying myself and I hate other people working on my stuff.

[Chris56000]

Hi!

Thanks for that – I don't object to paying for a maker's original high–res PDF with colour hightlighting in – I object to paying that (or more!!) to get a Chinese copy of a Chinese copy that's 1 bit b&w about 10 dpi and looks like spiders have been dipped in toner and left to scuttle all over the page!

Chris Williams

[highdesert56]

Spiders, haha!   Good visual.  Too many opportunities to be ripped off out there.

Did you end up finding what you needed?  Are you doing some AVR repair?