Noob attempting to fix a blown NAD T-760 AV receiver

[Agent24]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

According to the schematic pin 5 of U109 is supposed to connect to analog ground, so that jumper was probably installed in factory to fix a PCB error.

[Gatchaman]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

Okay cheers. :-). Still looks bloody crude for a in-house job though.

[Gatchaman]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

Went back to the site where the transistor exploded and tested a few more things around the area. Looks like a load of resistors in the K ohm range have lost resistance and dropped down to silly levels. Going through the components near the transistor but this time I'll look at a larger area and build a proper parts list. CPC here I come..... again :-)

Used the new soldering iron today and I'm such a tit for not upgrading years ago. Wow is all I've got to say.

[Agent24]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

Went back to the site where the transistor exploded and tested a few more things around the area. Looks like a load of resistors in the K ohm range have lost resistance and dropped down to silly levels. Going through the components near the transistor but this time I'll look at a larger area and build a proper parts list. CPC here I come..... again :-)

Resistors don't typically drop in resistance. Their resistance can increase or they can burn out and go open-circuit but due to the physical construction it's practically impossible for a resistor value to drop.

Likely you have misread the value (some of the colour bands can look really similar in bad light) or you are testing them in-circuit - make sure you at least desolder one lead or remove the resistor completely before testing. If you don't, other components in parallel with the one you are testing will cause the reading to be wrong. Also make sure your multimeter has a good battery - a low or flat one can seriously throw some readings off.

Check the schematic and find which parts are actually connected to that transistor, don't just test things near it because they may not actually be electrically connected to it.

Used the new soldering iron today and I'm such a tit for not upgrading years ago. Wow is all I've got to say.

Good to hear!

[Gatchaman]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

Cheers and will do!

[Agent24]

Re: Noob attempting to fix a blown NAD T-760 AV receiver

From the idea here: http://www.repairfaq.org/REPAIR/F_au...#AUDIOFAQA_013

You should be able to narrow down whatever is tripping the protect circuit to a specific channel.

See pages 29-32 for front and rear amp schematics. Speaker outputs on the right are switched on and off with relays, M501 and M502 on the front board, and M801 on the rear board. Check the inputs to these as stated for DC.

Also note on page 11 there are 5 test points (one for each channel) which are supposed to be set to 2mV for an alignment procedure - but in the case of a fault they may end up being wrong as a symptom. I would check all these for discrepancies against the spec and against each other, it may also help find where the fault is coming from.

Remember too if you compare other voltages at the same points at each channel and find that one is different from the others, that may well be your problem area