Denon Receiver

[Jimi]

Hello,

I'm trying to repair a Denon AVR-789 home theater receiver. It's about 3 years old and not under warranty. When it's plugged in and powered up, the green power light flashes, then turns red and flashes. I opened it up to take a look for anything obvious and it looks like a power transformer issue. The resin on the top of the transformer looks like it has melted and cracked, and dripped all over the inner workings of the transformer. I haven't taken it out to get a closer look yet. I'm wondering if this is possible to fix, or if the transformer needs to be replaced. I'll take a picture later if you think it would help. Any thoughts or ideas would be appreciated. Thanks!

 

[davelectronic]

Hi there Jimi.
Pictures do help, it might not be the transformer, some tooling lines get messy, hence possible varnish over spill, it wont have affected the unit from new.

But having said that its possible the transformers at fault, but usually there the least likely component to fail, i would be more interested in the power board, but photos do often help, often some one out here will spot the fault. No guarantees but worth a look, not a bad manufacturer.

 

[Jimi]

First picture is what we think the problem is.

 

[davelectronic]

Ok if you have one, and our comfortable with mains potential circuits, and only a small part, the primary side of the transformer is dangerous.

Move in with a multi meter set to a high voltage AC range on the PCB or where the secondary winding is still AC, look for power, typically 30 volts and up to a possible 60 volts or so, if you have a uniform output on the secondary AC its unlikely to be the transformer, the varnish looks like over spill, age and heat would crack it.

Does it smell like electrical burning, the typical, thats burnt out smell, this is a give away for burnt out winding insulation, dont confuse this with the ordinary under the lid electrical smell.

The power board should be connected to the secondary windings, look here for power / voltage, look for dissformed capacitors, burnt resistors etc.

Thats a start, but dont do it if your totally unfamiliar with mains voltage, and secondary moderately high voltage.

See what you find from that, this is how i would start looking, other members might do it differently.

 

[davelectronic]

I cant make it out clearly but there is an electrolytic capacitor down one side of the transformer near two leads a black and blue fair thickness leads, it looks at horizontal to the transformer. Have a look at this capacitor, i see what looks like over heating on one side, the side i can see, another brighter picture would help.
PS, it might not be, its hard to tell, better lit picture would help.

 

[Jimi]

 

[davelectronic]

Ok good pic's no it not burnt the capacitor i thought was, it looks ok, can you move in with a multi meter, set on a high AC range to test the transformers secondary for power.

If you go forward from here your working on a live circuit, it might pay to look about for damaged capacitors typical signs are blotted tops where they vent if over loaded, or burnt or damaged resistors, i would also use a continuity test on the fuses, power of for a while, or remove fuses for testing, dont remove all at once, or you might replace in the wrong order, but dont work on it powered up if your not totally comfortable.

 

[Jimi]

Haha, thanks. I don't have a multimeter here tonight, but I'll try that when I get a hold of it. The capacitors appear to be in good shape and nothing looks burnt. It doesn't smell burnt either. I can see 6 fuses in plain view and none of them are obviously blown. I'll post again when I get a hold of my multimeter. Thanks for your help.