Pioneer Sx-1280 Idle Current Question

[Bobs34969]

Hello everyone! I am happy to be a part of such an awesome and knowledgeable forum! I'm having an issue with my 1280 that I hope someone can help point me in the right direction. I purchased this 1280 from a yard sale a few weeks back. The previous owner told me he bought it broken, and hoped to repair it one day but never got around to it. He said it would not power up. I purchased it and took it home. After some diagnosis, I noticed it was all original and needed some love. I fully re-capped the soft start board, replaced the soft start relay as the contacts inside were quite dirty, and replaced the 15,000 UF caps with 22,000 UF Chemi-con caps. I also removed the heatsinks and found that 5 of the 8 output transistor were blown. With the shorted transistors isolated it powered up, and both relays closed! With some help from this awesome forum, I did the modern transistor modification with the 1302 and 3281 transistors.The voltages on the AWM-157 board check out according to another thread I read. My readings are:
Pin 8 = +72V
Pins 11, 12, 13 = +32V
Pins 20, 21, 22 = -32V
Pin 25 = -72V
Pins 16, 17, 18 = +13.25V
I know the soft start and protection board are working well because with all 8 transistors isolated, both relays close and stay closed. When I installed the new transistors (Which I'm 99.9% I did it correctly, Ha!) With both Pre-amp boards plugged in, the protection relay trips. with just the right channel plugged in, it stays closed. and with the left channel only plugged in it trips the protection relay. My issue is with the left channel completely isolated, I keep blowing a PNP (1302) and NPN (3281) transistor on the right channel when the unit is powered up for more than 1 minute. My Fluke shows me its a direct short from C-E. Luckily on my mouser order I ordered more than enough transistors for this very reason!! I can get the DC offset to a reasonable level, but the as soon as I throw the switch, the idle current is at 100mV and climbing. The highest I saw it get to was 400mV + and I shut it down. I am assuming this is the reason why I am blowing the transistors? What causes the idle current to be so high and continue to climb? I cant seem to find a reason on the schematic and I need help from the gurus!! The weird thing is that it only blows 2 transistors. Not all 4 on the right channel. one PNP and one NPN. The other two test fine. I am looking forward to hearing from you guys, and happy to be here! Can't wait to crank the volume knob on this thing once it's working!

 

[mbz]

First step is to build a dim bulb tester, a current limiting device, numerous plans on AK, it's basically an incandescent bulb
connected in series to the active AC. Did you check the driver transistors for the blown outputs? Someone is asking the outputs
to work hard. Pioneer gurus will chime I'm.

 

[Bobs34969]

Thanks for your reply mbz! I will check out the drivers this afternoon. Could they be the reason why my idle current is so high, and continues climb on power up?

 

[rcs16]

The op stage bias generator is made up of the constant current source, the pot, stv diodes. If the circuit becomes open or effective resistance goes up, then op stage bias runs away. This why one uses the dim bulb to limit it as a quick check. Check the above components and connections

 

[zebulon1]

I think the best approach is to check every component on that amp board.
The STV’s have a specific forward voltage. Check that.
Check every resistor including the emitters.
Check every diode.
Replace all the driver transistors even if they test good.
Check the pre stage transistors. Replace if needed.
By not using a DBT you will stress all the silicon components and the ones that survive will fail the next power up or later.
Build that DBT. Learn it’s use before you continue.

Here’s some tips:
Use the other amp as a guide.
Use the scat and compare values. If one is off from the good amp and scat, lift one leg and recheck.
Ceramic and film caps are considered good.
E caps replace if you can’t test them.

This will not take you very long, stay organized. Take notes.
The 1280 is a wonderful receiver. I’m glad you stopped and joined this forum.