DC Offset Adjusted!!
This thread has drifted a bit, but In case someone finds it in a search for how to adjust DC offset in a Sony STR-7055, I have now successfully adjusted the DC offset in this receiver. Here are the details:
Fortunately I was correct in my identification of the differential pairs: Q702 / Q 703 for the left channel and Q752 / Q753 on the right channel. They are all 2SA705 silicon PNP transistors.
(I'm a complete novice at this, but I studied the schematic and read all I could find on the subject.)
I ordered 20 2SA705s, and sorted them by hFE, looking for pairs that measured as closely as possible, then replaced Q702 / Q703 and Q752 / Q753 with matched pairs.
Initial turn on, on DBT looked normal, so I plugged the receiver in, turned it on and warmed it up, checked and set the gain, then checked DC offset.
Before replacing transistors, the right channel read 105 mV. It now reads -15 mV.
Before replacing transistors, the left channel read 95 mV. It now reads 4 mV.
The differential pair removed from the right channel had hFE readings of 166 and 199. They were replaced with a pair reading 331 and 332.
The pair removed from the left channel had hFE readings of 129 and 190. They were replaced with a pair reading 261 and 262.
The 2SA705s that came out of the receiver all had paint markings on them (dots.) I wonder whether those might be factory sorting marks.
Thank you again to everyone for your help in this thread. There are still a couple more things to resolve with this receiver, but for now it appears that DC offset is no longer an issue.
