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Sony STR-2800 Two Blown Fuses

Measure across the pot (with the power off) and set it to the lowest resistance. That way you know it's set properly for miniumum bias.
 
Q707 & Q708 are fine. Their bias pot (RT701) is also fine and measures roughly the same as RT751. Turns out all the output transistors were in fact insulated from the heatsink. The continuity I was getting seems to be through the collector leads to chassis, which the heatsink is connected to. Is this normal?

Just so I understand correctly, what you're suggesting is setting the two bias pots to minimal resistance and then trying to bias, using the test points, per the repair manual (no input, connecting the DMM on DC to each channel's test points and then carefully raising the resistance until reaching 50mV for each channel)?

If both channels were basically the same during the last test run (only difference being between Q756 and Q706), including misaligned diodes, why is it that only the right channel was affected? Shouldn't we be seeing a somewhat symmetrical damage?

Binkman - I'll be looking into building a current limiter. Thanks.
 
What I want you to do is to pre-adjust the bias pots to minimum resistance before powering it up, as a precaution, so we know it won't have full bias on it when you turn it on.
 
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yes above or you will fry stuff for sure. The dr and I are adamant about this step..

Please confirm CCW=0mv, It's a 'peeve' of mine in that all of the sony sm's "I've" read to not indicate CW or CCW like other mfg. models do. I only discovered this adjusting an str something I don't have anymore and just a tweak.. wtf.. backwards? So I tried another and was opposite, too.
thks.
 
Okay, what I'm getting when checking for continuity between chassis/heatsink and all the output transistors' collectors seems like the behavior of a capacitor. For a few seconds, there's a complete short circuit and then the resistance gets higher and the circuit opens. Then, when I reverse the probes, it happens again for a few seconds until the resistance rises and the circuit opens again. Is this normal?

I've set both pots (off the board) CCW to their lowest resistance - about 4kΩ.

Here's a merged image of the schematic for easier viewing - http://i.imgur.com/LgSIwpc.jpg (press on the image for higher resolution).
 
What you are seeing is a capacitor, it's the main filter caps. It's normal.
The bias pot should go down to close to 0 Ohms. 4K is too hight. the pots are bad, replace them. This may be the problem.
 
What voltage and power rating should I be looking for? The Sony manual only specifies resistance.

While we're on the subject of voltage and power - what bulb should be used in the current limiter?
My mains voltage is 220V and the Sony's power consumption, as stated in the manual, is 170W for the European model. If I'm not mistaken, that's a current draw of 0.7A.
 
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Regarding the pots, just buy whatever sealed pots (Bournes are good) Have the same resistance and the lead will fit.
 
dbt bulb. 100w seems accepted both 120v and 220v. bulb goes bright and hopefully dims steady, mainly to check for ps and potential shorts.
 
Very well. I'll try to find suitable pots and assemble a current limiter. Fortunately, I stowed a batch of 100W incandescent bulbs when they were more readily available. Thanks again.
 
Just an update - Current limiter is ready to use, but I had to order new MJE15032/33 transistors and Bourns 3386G 20kΩ trim pots (instead of the original 22kΩ), so it will take about two weeks until the next power-up.
I've tested all the transistors on the power amp board on both channels and they are all fine.
 
I finally received the pots. The 100W bulb in the current limiter didn't light up and when connected directly to the wall, nothing smoked. DC bias adjustment in the manual calls for 50mV on each side, but with the 20kΩ pots, it only goes up to 22.4mV on the right channel and 25mV max on the left. Still haven't tried to drive speakers but headphones work. Is the bias reading a problem? Is there anything else that I should test out before connecting everything back?
 
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