Audio Systems: Denon Receiver DRA-295 wont power on, rectifier diodes, standby switch

Question
QUESTION: hi,
 My DRA-295 Denon A/V receiver died today during lunch: it's not getting
any power at all. i notice two problems:

 1) the On/Standby switch does not latch in the "On/Standby" position, so i'm
sure it's broken. but when i hold it in, to close the switch, even then the unit
does not power up. i verified with an ohmmeter that holding the switch in
closes the contacts.

 2) when i hold the On/Standby switch closed, i do not hear the usual
characteristic sound of the relay closing. i'm shocked that both components
would break simultaneously.

 FYI, i checked both fuses and they're good. Measured 120VAC at the power
cable as it enters the chassis and it's good there. there's no power on the
fuses. but now i notice that the small transformer right next to where the
120VAC enters the circuit board, is hot and smells of burning plastic. the
much, much larger main power transformer is many inches away. right next
to the small, hot transformer is the relay that i must have grown used to
hearing every time i turn the receiver off and on. i'm guessing that
transformer has open-circuited. any thoughts, or knowledge of what that
power circuit looks like?




regards,
joey.



ANSWER: The small transformer (for standby power) could be defective or more likely the rectifiers have shorted down causing an extra load on it to make it heat up.

That is what I would look into.  If you can find the rectifier diodes for the small supply, pull them out and see if the transformer stops heating up.  Then you will know they need replaced.  Also, look for a short on the board somewhere nearby the secondary side of the small transformer.

Good Luck,
C



---------- FOLLOW-UP ----------

QUESTION: hi C,
 thanks for your input. i did make some progress in the diagnosis. there are
two transformers: a large one and a small one. the small one is labeled "Sub
Trans" on the PCB, and is fed directly by the 120VAC. the primary coil is
open-circuited. the secondary coil is 18.5 ohms. i guess it's broken.
 the large transformer's primary coil is connected in a simple series circuit as
follows: 120VAC input, RLY 104 (relay), two fuses in parallel (8A and 6.3A),
and the large transformer's primary coil. i think the Sub Trans's secondary
coil is part of the power-on circuit, including the On/Standby button, RLY
104, and other things. So i'm thinking that the open circuit on the Sub Trans
is causing the turn-on circuit to fail.  If i try to bypass this turn-on circuit by
manually closing RLY 104, i hear a loud buzz and no LED's come on. i
thought maybe i could bypass that circuit, as a test, but maybe there are
other relays that need to be closed. i see three other relays, two are on the
speaker outputs.
 so:

 1) i might try to replace just the Sub Trans to fix it.
 2) what could be causing that buzz when i close RLY 104?
 3) the On/Standby button is just a spring-loaded doorbell-type button: it
has no "on" position and "off position": if you press it in, it just pops right
back out to its original position. i took it apart to see if it was broken, but it
wasn't. it looks like all it can do is send a pulse to another circuit. are you
familiar with this type of power-on circuitry? it seems overly complicated to
turn on a stereo.

 thanks for your thoughts.

regards,
joey.

Answer
Yes, it is complicated circuitry.  You are doing fine.

The sub transformer may be the problem, but just replacing it will usually not solve the problem as something caused it to go.  Rarely does a transformer fail on its own - not impossible, just un-likely.

And, you will probably have to repair the button.  Make sure it is the right switching system if you Rube-Goldberg it.  It may be a logical switch that tells a processor inside what to do rather than actually switch the power.  You can follow the wiring to check that out.  As you observed, it may be just sending a logical symbol to the processor.

Does this set not have remote control with ON/OFF features? If so, that is why it needs the remote power supply.  There must be a small amount of power whenever the receiver is "plugged into the wall" so as to have the IR receiver ready for a signal from the remote at any time.  The power management system operates from this IR receiver and the main power switch does the same thing as the remote control codes do.   

If you get stuck, call Jim McGuiness at Denon.  Use their main number and just go through their annunciator system to reach Mr. McGuiness; he is familiar with this model and can give you some basics from recall.

Also, this model, I think, has the room to room control options with add-on boxes for multi-room applications.  That may have something to do with the complexity of the IR receiver, also.

Thus, the solution to your problem may be a very simple matter inside that controller; OR it may be a very complex one........   

Best wishes,
C

PS: Denon is at:

DENON ELECTRONICS (USA), INC.
19 CHAPIN ROAD, P.O. BOX 867, PINE BROOK, NJ 07058-9777 USA
TEL: 973-396-0810 www.denon.com