Denon AVR 1909 repair (-15V rail problems)

[kristupas]

Hello,

Got a problem with my AVR1909. Problem began with unit going into protection mode after ~5-10 min. Checked power supply voltages and found that -15V is pretty low (~ -14), drops to ~ -13 and and around that moment protection mode kicks in.
Changed regulator (KIA7915P ->L7915CV, insulated central pin from radiator of course), got 0V on 15V rail. With this temporary setup (photo, only -15V regulator) and disconnected -15V pin from input board (tested that pin only when disconnected, i don't get it short to ground) i get constant -15V (looks like PSU is working fine?).



Any clues where to look next?  Thank a lot everyone!

[kripton2035]

all the power supplies can get a protect mode if they don't have the correct values. also any shorted output transistor.
you must verify with the schematic. the 7815 is a good start it often fails.

[kristupas]

Output transistors seem fine to me. 7815 also seems fine (gives constant +15V). I see problem in -15V rail. When i disconnect -15V pin from input board (the one with R2A15218FP IC) to the regulator, i get normal -15V (measured output from regulator), with it connected i get ~0V.
Maybe anybody have experience with that input board? Could R2A15218FP IC be a problem? Any other components?

[mzacharias]

Do replace the + and - 15V regulators. Too much work to get to them only to have it's mate fail. The originals were pretty cheap to start with.

You should be fine if the tab of the -15 reg is properly isolated, although personally I would prefer the plastic package.

[kristupas]

On this particular model they are very easy to access, mounted on a small daughterboard No need to unscrew power board at all. I will replace +15V one just in case (had them both ordered together), but do any other components tend to fail along -15V rail on these receivers?

[shakalnokturn]

Bad solders on interconnect?

[mzacharias]

On this particular model they are very easy to access, mounted on a small daughterboard No need to unscrew power board at all. I will replace +15V one just in case (had them both ordered together), but do any other components tend to fail along -15V rail on these receivers?

Pretty much just the regulators. The ones down low failed the most in my experience, so be aware of that.

[kristupas]

I replaced both -15V and +15V regulators, -15V (7915) output capacitor and input zener diode (33V). When i disconnect -15V rail output i get nice -15V from the regulator, with both -15V rails connected i get 0V. +15V rail is fine. Maybe someone has any ideas where to look next? Thanks!

[kripton2035]

unsoldier every item (first the silicium ones) from the -15v rail, with the help of the schematic
or use a milliohm meter (or a precise esr meter) to find the short

[kristupas]

Looks like found one shorted op-amp along the -15V rail, but still have short in this part of protection circuit (picture). Disconnecting R188 makes it go away and amp turns on fine.
Replaced C179, ceramic SMD cap C180 not shorted, all resistors and Q143 seem fine for me. Red points (come from input/output board) measure ~ +3.6V each, does that trigger protection circuit? Thanks a lot for any ideas

[kripton2035]

build yourself a shorty device : http://kripton2035.free.fr/Continuity%20Meters/continuity-short.html
it's really easy to setup on a veroboard, and it is an invaluable tool to find shorts on a pcb.
you can get at around a centimeter of the short using the tone frequency difference.