(alternate title: patience, grasshopper...)

Recently I complained about the underwhelming replacement receiver, which my readers (yes, all ten of you!) might actually remember.

The issue wasn't just that it was doing poorly in stereo mode, but rather that it didn't drive the main/front speakers worth a damn. Next to no sound even with the volume at 11.

Well, there's a reason for that Sony box not doing its job - and I found it and fixed it. Poring over the service manual (yes, there is one and I found in less than an hour) after work today I thought Hmm, mostly digital, zero adjustments, few discrete components. As it's a deader I might as well open it up and look for obvious problems and/or gut it.

(For the youngsters out there: "service manuals" are magic tomes of arcane knowledge, hardly heard of on this side of y2k. In the hands of a wiza^Wstubborn old phart these can provide great enlightenment.)

Alas, no obvious leakers of the magic smoke presented themselves. So, after staring at the circuit schematics in the service manual a lot more (yes, I have no life), I decided that checking the power amp transistors and the driver IC would be Good Things To Try, given that the manual specified test voltages for them.

This meant a total disassembly of the box as you couldn't get to any of the test points from above. A while (and hundreds of screws) later I had it all in pieces. The transistors proved to be ok. The driver IC wasn't testable with everything disconnected.

A bit of online research revealed that these driver ICs (NEC µPC2581V) do run very hot, are not unheard of dying themselves and easily fry components in the vicinity. Sony didn't put any heatsinks on them in this model and a number of electrolytic caps were basically touching the IC cases - and there's your problem.

These caps weren't testable in circuit, so I started desoldering. Three out of the four that I replaced were cooked, one still had some marginal capacitance left.

Lots of screws later, a final function test: the thing works! Evenly firing on all channels. Hooray for being stubborn!

(Never mind that this exercise took me a good five hours this evening, and that in the meantime I had given up on the Sony and replaced it with a NAD T741...)

[ published on Wed 23.07.2014 00:00 | filed in brainfarts | ]
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