Old stereo receiver repair - Sankyo SRC-2020 - right channel - no output

[Skimask]

Friend of mine comes over today with this old school stereo receiver with a blown right channel.  Barely any output at full volume, mostly all severe distortion, crackling, etc.  Left channel is fine.
I normally wouldn't mess with but he says it was his Dad's, used it in the garage for years and years while they both worked on cars, etc.etc.etc.  Ok.  Fine.  Friend of mine.  I'll take a look-see.

-Thou shalt check all voltages.
--And thy voltages checked well.

-Thou shalt look for rotten solder joints.
--And said solder joints AND wire wrap joints look well and fine.  Or at least as fine as a 20+ year old piece of dusty stereo can be.

So, the output amp is an ECG1331.  Can't find any real decent data on it, but it crosses over to a NTE1331, and a suitable sub's are STK461, STK463, STK465.  Same type of thing with different wattage and voltage ratings I assume.

Found a couple datasheets for the STK463 with some internal diagrams, and a generic power amp schematic for it...
http://pdf.datasheetcatalog.com/datasheets/1150/499377_DS.pdf
and
http://pdf.datasheetcatalog.com/datasheets/37/499379_DS.pdf

Here's my voltage readings with it on, warmed up, operating, balance centered, low volume, music out the left side, nothing out the right side:  (Interesting differences highlighted)

Left Channel - OK
1- Left Ch In  -.07VDC
2- Left Ch Fdbk -.075VDC
3-GND
4-Left Channel BIAS -1.3VDC (.3VAC component)
5- -30VDC
6- Left Ch Fdbk 2 - 0VDC (.3VAC component)
7- Left Ch Output - .25VAC
8- +28VDC

Right Channel - Nothing (pinout mirrored image of left channel)
16- Right Ch In -0VDC
15- Right Ch Fdbk -.55VDC
14- GND
13- Right Channel BIAS +14.9VDC
12- -29.75VDC
11- Right Ch Fdbk2 +.68VDC
10- Rt Ch Output +.68VDC
9- +29.25VDC

From looking at the internals of the amp (http://pdf.datasheetcatalog.com/datasheets/1150/499377_DS.pdf),
I shouldn't have +14.9VDC on pin13, as it's tied to the base of what looks like a Darlington pair, but that section doesn't look like it's the actual driver.  Nonetheless, I think that voltage on pin 13 (right channel) should be ~1.3V like pin 4 (left channel) since it runs up to the base of that transistor pair.

Replacement part off of ebay will run me about $17.  Cheap enough.
Judging from the condition of the speaker wire the receiver was attached to, it's entirely possible that the right channel wires shorted together and took out the amp...but I'm not going to jump to conclusions just yet.

Any thoughts from the masses...

[BMack]

Clean the pots.

[Skimask]

Not the pots, not this time anyways.
Not even a broken pot with a rotten wiper or a rotten switch.

Addendum to post #1:
I checked the pots involved, volume, balance, speaker selection switches, etc.  Resistances changed, switch contacts moved, and so on.

[Skimask]

Since pin 10 is at low voltage, but pin 13 is at high voltage, it is definitely sure that the NPN output darlington or base resistor is dead.

Using the part designations from the left channel, you're talking about TR11 or R7?  (again, using the left side as a reference, but transposing the parts to the right)

[mzacharias]

The STK 463, 465, etc had lots of problems. Noise problems were very common. Beware of replacements. New Chinese copies rarely work well.

If you can find a "New Old Stock" on eBay for example that would be best.

[Skimask]

Ya, I ran into those situations ~6 or so years ago when rear projection TVs were going out all the time...before LCDs really hit the market at decent prices.
I paid my dues ordering cheap STKxxx parts.  It only took me twice to learn my lesson.  Most of the time the end user wanted me to at least "try" the cheap parts.  Ya, they came back for more.
More money in my pocket.

[BrookeAdams]

Just for fun I took a quick look, I dont know how similar the supporting circuitry is to the schematic but those can be verified anything inside the IC is a moot point.

[Skimask]

The supporting circuitry is close enough to the datasheet, almost generic in nature, R/C filter, D/C block, etc.  With the ECG1331 removed, ohms & caps (meter leads going both ways) checks between the pieces/parts on the working left and dead right channel show the pieces parts to be close enough.  Wish I still had a working Huntron 2000 for this purpose alone.  Come to think of it, why haven't I checked the ECG1331 itself?  Derp moment...  I'll do that tomorrow.  If the internals match the diagram, and match you shorted base-collector suggestion, it should have 2 diode drops between 11-13 and 1 diode drop between 12-13...if the base-collector is shorted.  Nonetheless, it should be practically identical between the left and right channel.  New part will be here on Monday-ish.

[Skimask]

And there ya go.
Pulled the old amp, put the meter in diode check, compared it with the new amp.  No comparison.  A short here, no short there.  One diode drop here, two diode drops there.  And the list goes on.
Replaced amp.
Ops Checked Good.
That friend of mine is semi-ecstatic.  I got his ol' man's stereo up and running again.

[ghuttanus]

Where did the op purchase a replacement ECG1331?

I stumbled on this thread, while searching for experiences debugging/replacing STK465 modules.  My experience was similar to the op’s – high voltage on pin 13. A Toshiba SA-725 receiver with one bad channel (no output).  The receiver had a Sanyo STK463 amplifier module.  Voltages on all 16 STK463 pins were as expected (per the service manual), except pin 13 measured +22v instead of -1.25v.  I ordered a replacement STK465 from a Chinese seller on Amazon (Alberta Low Prices Shoip), which took a few weeks to arrive, but fixed the problem after installation.  Many people recommend avoiding cheap STKXXX replacements, but Sanyo doesn’t manufacture them anymore, and I didn’t see any other options.  In any case, the problem appears to be fixed (reliability is another question).

[Skimask]

Don't remember where I got it.  More than likely from an ebay source.  That's where I've gotten most of my STKxxxx type chips from.  I've only been burned twice when I took a chance at paying $10 for an otherwise $50 piece.