BD139 Odd problem- NAD 3140

[pdlemen]

I have an old NAD receiver that uses these Phillips brand BD139's both in the +25 /+45 volt rails and in the amp section as the temperature sensing Q631 and Q632. I could not adjust the bias in one channel and after troubleshooting found that if I switch Q631 with Q632,the problem followed what I presumed to be the bad transistor. I then stole one of the regulators (Q801) , subbing it out with an equivalent and installed it in the bad channel. No joy...the problem of the bias not adjustable remained. I tried the OnSemi brand BD139 and still nothing. So far the only transistor that allows the correct circuit operation is one of the original Phillips. I should note that the one that "works" has a different date code than the other three so someone has been in this trying to resolve the issue in the past. I've read a few posts about how the On's are junk and need a suitable substitute for Q631/632. Any help/thoughts would be great. Another note, all four Phillips transistors read good on the diode scale of the Fluke meter and I am going to test for beta on all four tomorrow. The manual for the unit is using the same p/n for all four.

 

[pdlemen]

Testing the beta showed that the "working" transistor at 91 while everything else was way below or way above 100. This circuit became very finicky after doing Ceurvels'
mod for stabilizing the bias circuit in order to attain a more comfortable pot rotation. Having many 2SC2958's in stock, I found a couple with similar hfe's. While not case 126's, a couple holes and screws on either side to hold them to the heatsink remedied the situation. I wonder how many other people have done that mod and ended up at the same place. How many 3140's hit the dumpster out of frustration. If you hated the 80's as much as I did, not enough of them.