onkyo A-8200 popping fuses..

[marko]

just bought an onkyo A-8200 as i thought it would be ideal for a little home project but to my dismay it's faulty soon as i turn it on it pops a 2.5a 250v fuse located just after the transformer, i'm not that clued up on home audio repairs so any thoughts/ideas would be welcome!

 

[oldmanaudio]

You may have your hands full. First you must try and isolate the short. Most likely a output transistor has failed and being the main part of the power supply is on the amp board you just can't unplug it. A variac would help but I would guess you don't have one. You may want find a repair shop near you.

 

[marko]

the power supply is not on the board and is seperate so could be unpluged/desoldered i guess easy enough?

 

[oldmanaudio]

the rectifier, filter caps and regulators all live on the main amp board.

 

[marko]

so if an output has gone what's stopping me just replacing it? apart from a potential 240volt

 

[oldmanaudio]

When something shorts there can be many causes and other transistors may be taken out or a drive transistor may have failed first then caused other failures. If you just replace them it may fix it but the possiblity the new tranisitors can be taken out again.

 

[oldmanaudio]

Make sure you unplug it when working on it.

 

[marko]

Make sure you unplug it when working on it.

i thought just the caps would give a shock? there are 2 big 50v ones on the main board..

just checked the outputs and they seam to check out ok on the MM.

 

[oldmanaudio]

What do you mean they check out fine? How did you come to that conclusion?

 

[marko]

What do you mean they check out fine? How did you come to that conclusion?

i know it's not 100% conclusive but i tested all the 4 outputs for shorts and they ohmed out fine. i usually work on car amps so i'm trying to apply what knowledge i have from there. i'm keen to learn about home amps and open to all critisismn and hints/tips

 

[oldmanaudio]

So you removed the output transistors?

 

[marko]

no, just did a quick check in situ just to get a quick idea of what's going on. if it makes a differance i'll remove them all together..

 

[oldmanaudio]

It can but if you don't have any dead shorts across any of the terminals E C B lets try something different. Can you unplug the transformer from the main board or is it soldered? just want to check if trans is shorted this is rare but should not be over looked.

 

[oldmanaudio]

also you are in the U.K. supply voltage is 240volts right, is this a U.S. amp 120volts?

 

[marko]

it's a uk spec one so 240v (fixed, no 120 option), there is 3 wires soldered from transformer to board.

 

[oldmanaudio]

makes it harder can you remove them from the circuit board tape them so don't touch each other and see if the fuse blows? The good thing here is the main power switch s901 at the bottom left of the schematic is manual not controlled by logic aka a remote. other wise removing the secondary windings would shut down the amp totally. We are looking for a shorted transformer which is not very common but should be checked. Kiss is an acronym for keep it simple stupid. I am keeping it simple to walk you through it. Getting late over there?

 

[marko]

yeah, it's late i will try the above tomorrow after work

 

[oldmanaudio]

ok

 

[marko]

just desoldered the 2x red and 1x black wire from the transformer and it's still popping fuses!! not sure how to test a transformer but i guess it's faulty!

Mark.

 

[oldmanaudio]

Marko,
One last thing the other side of the trans the yellow and white wires connect to a small circuit board. Disconnect connector 931 and see if the fuse still blows. This will isolate the trans completely.
Harry