anderslober
Yikes...not again!
Hi forum. Working on a c356bee amp. Its a bit sick….needed a lot. Amp powers on now on normal power, but the -26v rail is 50vdc. Please see picture of scat. Lm337n fail? Thanks.
NAD c356bee amplifier voltages isc.
- Thread starter anderslober
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I assume you mean -50Vdc
Flow is from right to left, candidate is Q42 voltage regulator. You will see the -55V coming in further to the right
anderslober
Yikes...not again!
Q42/2sa1220 tested good…q44 tested good too
anderslober
Yikes...not again!
R46/51ohm was crispy…..I didnt have any 51 ohm, so I put in 47ohm…..bad idea?…or..?
anderslober
Yikes...not again!
I changed q42 to brand new ksc1220….sill same problem…
anderslober
Yikes...not again!
Dbt power. I can turn ON amp and relays click…light turns blue….after a few seconds….light turns red(protection). I have 5.5vdc on the left speaker output….I can just manage to measure it before protection. Right is good…0.008v……
If I dont turn on spkrA…light remains blue.
Bias has been set at 6mv…both channels.
If I dont turn on spkrA…light remains blue.
Bias has been set at 6mv…both channels.
anderslober
Yikes...not again!
Dbt power Just seen the rail voltages -51/+39…..the + is to low…..power supply time…I disconnected the red/white rail connectors from ps. Measured at the pins with full power -55.1/+49.7……+rail still to low…what could cause that on ps? The ps has back-mounted smd components…….me no liiike……
One of the big caps does seem slightly popped on top….
One of the big caps does seem slightly popped on top….
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The -26V rail is set by Q42base
If Q44 is off then there will be no/very little current flow in the loop R42, R44, R48 so Q42b will still be about -50Vdc.
Also, that -26V rail tracks the +26V rail via R414, R412, R410, if any have failed then Q44 will be off.
Suggest the following dc voltage measurements, DBT not connected
- one side of R414 (+26Vdc?)
- other side of R414 (-1Vdc?)
- one side of R48 (-53Vdc faulty)
- other side of R48(-53Vdc, faulty)
- resistance check mentioned resistors, in circuit is fine, need to confirm none are open
Regarding the +26V rail
Voltage is set by Q41base so if Q43 is off then the +26V rail will be abnormally high.
Look at faulty Q43 or failed zener IC301 or failed resistor R49, R411, R497, R496
anderslober
Yikes...not again!
Wow..thanks…..great explanation. Will do. I did measure every component/voltage in the area concerned…only the pesky +50vdc. The voltage isnt actually used fir anything, unless the mDC is installed(not in this unit).
