UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Help needed with

PJL · 19th Mar 2023, 10:02 pm

IC6 Pin 11 looks very wrong! It is supposed to be the inverse of IC24 pin 13 (TP15), can you check the voltage on TP15.

The switching is very complex, Sz is biased to '0' by R34 (10K) BUT via S2/1R it can connect to RN when R27 (100K) connects it to SPECIAL CHANNEL SELECT pulled up by Q2 when Sc is 0. At best the potential divider created here seems likely to impact noise rejection.

What it does highlight is how critical it is to perform each test with the same settings as they interact in surprising ways.

19th Mar 2023, 10:02 pm	#262
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Help needed with - Racal Dana 9081 IC6 Pin 11 looks very wrong! It is supposed to be the inverse of IC24 pin 13 (TP15), can you check the voltage on TP15. The switching is very complex, Sz is biased to '0' by R34 (10K) BUT via S2/1R it can connect to RN when R27 (100K) connects it to SPECIAL CHANNEL SELECT pulled up by Q2 when Sc is 0. At best the potential divider created here seems likely to impact noise rejection. What it does highlight is how critical it is to perform each test with the same settings as they interact in surprising ways.