I’m trying to repair a Beocord 3500. I’ve succesfully repair the mechanical part as now the tape is running fine (play, ff and rev). But I’m stuck with no audio output.
I’ve checked with my oscilloscope while playing a tape. I can see audio L/R going out of the tape heads to the P.B amplifier L/R (DIAGRAM A AF and control). I have audio signal on pin 7 and 14 (input) of the Dolby decoder chip IC1 (LM1131) but nothing out of it (pin 9 and 12). DIAGRAM B
Also the digital vumeter is full lit all the time when I press play.
I’ve swapped the LM1131 with a brand new one with no more luck.
I’ve ran out of idea on this. Any help will be much appreciated
So , I will answer myself as it can be usefull for anyone having the same issue.
I found that the 8vREF signal was around 17mV and not 8V. This 8VREF is used on 2 opamp, on the lm3915 of the vu meter and …as a voltage reference on the LM1131 dolby chip. From the schematic, i suspected a faulty capacitor (C417 on dolby PCB) and… I was right. It was short cut so i swapped it with a brand new one and everything works fine now!