Just in case somebody else has the same problem: I fixed my Denon PRA-1500 input selector instability problem, and here is a short how-to:
Symptoms: after turning on the PRA-1500 the input selector switch (button operated) lost its memory, making it necessary to press the desired selection setting again and again. The problem went away after some time while the preamp was warming up.
Diagnosis: the preamp is built around a Toshiba TC9152P analog function switch. The TC9152P features an 'ALL_OFF' input with positive logic, which is connected in the schematic to a monostable operated by VDD. Any glitch on VDD higher than 0.8V will trigger a pulse on the ALL_OFF input, which causes the TC9152P to go into a 'no select' mode.
Solution: replacement of electrolytic capacitors in the power supply section did not solve the VDD glitch problem. Finally I found that the 18V linear regulator IC14 (NJM78M18FA) produced the glitches. A very expensive and sturdy looking TO220 part, but probaly a bad design. I replaced it with a 2A L78S18CV equivalent from ST Micro. And I did replace the -18V counterpart NJM79M18FA with an ON semi equivalent for good measure.
Symptoms: after turning on the PRA-1500 the input selector switch (button operated) lost its memory, making it necessary to press the desired selection setting again and again. The problem went away after some time while the preamp was warming up.
Diagnosis: the preamp is built around a Toshiba TC9152P analog function switch. The TC9152P features an 'ALL_OFF' input with positive logic, which is connected in the schematic to a monostable operated by VDD. Any glitch on VDD higher than 0.8V will trigger a pulse on the ALL_OFF input, which causes the TC9152P to go into a 'no select' mode.
Solution: replacement of electrolytic capacitors in the power supply section did not solve the VDD glitch problem. Finally I found that the 18V linear regulator IC14 (NJM78M18FA) produced the glitches. A very expensive and sturdy looking TO220 part, but probaly a bad design. I replaced it with a 2A L78S18CV equivalent from ST Micro. And I did replace the -18V counterpart NJM79M18FA with an ON semi equivalent for good measure.
And I did replace the -18V counterpart NJM79M18FA with an ON semi equivalent for good measure.
What means ON equivalent ?
I have the same problem with my pra-1500
What means ON equivalent ?
I have the same problem with my pra-1500