Denon DRA-435R shuts off the speakers output

V

ve ze

Active Member
Hello everybody.
I am trying to find a fix for my 1991 Denon receiver DRA-435R. It has a very pleasing sound and has all the features that I need. I got it from a thrift shop and after listening to it for some time -- I'd like to keep it. Though I have others too :))
But it has a problem. It shuts off the speakers at random time. It may happen at different intervals of time, starting from 15 mins or it may go silent after several hours of working. When it shuts off the speakers, there is still sound on the headphones. The unit’s speakers protective circuit is obviously responsible for that.
Here’s what I did so far to diagnose this.
- First, I have a feeling the relay seems to work as expected. When turned on, there is a click. indicating that the power is being turned on. In 2-3 secs the speakers protection relay engages and the sound is heard. Then in some unpredictable time it clicks and disconnects the speakers. This can be seen, as I took off the outside cover on the relay. When the relay is engaged there is current of 21.5-22V DC on the coil. When it is disengaged – there is no current there.
- I adjusted the idling current according to the service manual.
- I also checked the voltage on the speaker terminals in the back. Here is smth that looks strange to me. The AC voltage on the right and left terminals is different. And it differs significantly, almost doubles. When the right channel shows ~20mV, the left would be on 50mV or so. Of course, the voltage does not stay on the same reading, but it fluctuates proportionally. May that be the reason that the protective circuit shuts off the connection to the speakers? Now, what may be the reason for that difference in the signal strength? The balance knob is set in the middle.
Fully understand the unproductiveness of troubleshooting remotely, but I am running out of ideas.
Could anybody share your thoughts on this?
Thanks for your help.
 
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Are you playing a stereo audio mix through the receiver, or are you feeding something like a sine wave to both channels? If it’s a stereo audio mix, there’s no relationship implied between the left and right channels, as they are a stereo mix. So, you would not necessarily expect the voltages to be the same. If it was a test signal of the same amplitude into both channels, of course you would. That being said, the receiver likely has leveling potentiometers for both channels, but you need a clean test signal for that.
 
ve ze said:
- I also checked the voltage on the speaker terminals in the back. Here is smth that looks strange to me. The AC voltage on the right and left terminals is different. And it differs significantly, almost doubles. When the right channel shows ~20mV, the left would be on 50mV or so
Click to expand...
Measure for DC voltage at the output.... AC voltage would be and AC signal voltage. DC voltage will trip the protect circuit. AC signal won't.
 
Thomas Rieck said:
Are you playing a stereo audio mix through the receiver, or are you feeding something like a sine wave to both channels? If it’s a stereo audio mix, there’s no relationship implied between the left and right channels, as they are a stereo mix. So, you would not necessarily expect the voltages to be the same. If it was a test signal of the same amplitude into both channels, of course you would. That being said, the receiver likely has leveling potentiometers for both channels, but you need a clean test signal for that.
Click to expand...
I will get the test signal tomorrow and check with it.
I agree, the stereo mix is not the same on both channels, but ... a) I use mono FM and b) the levels are different in strength by 100% ( 20 and 50 mV ) seems a little too much to me but I may be wrong. Will test tomorrow with the test signal and post.
 
Most likely you have and issue in the protect circuit or the power supply for the protect circuit.
 
Don't need a signal gen.. Right now. Just a multimeter...
 
There are 3 main things that will trip the protect circuit
1. high DC voltage present on ,at least, one of the amplifiers outputs.
2. Protect circuit is damaged
3. Power supply for the protect circuit is missing.
 
For right now .
1. Volume control at min.
2. No load on the amplifiers ( no speakers )
3. No input selected.
 
avionic said:
There are 3 main things that will trip the protect circuit
1. high DC voltage present on ,at least, one of the amplifiers output.
2. Protect circuit is damaged
3. Power supply for the protect circuit is missing.
Click to expand...
Checked for DC on both channels. There is the reading on both. Same pattern: the right is 1/2 lower than the left. Strange, I thought that there should not be DC on the speakers connection.
When the connection is broken by the relay, the right channel shows dead 0. The left has some voltage on it 1 or 2 mV and then it gradually goes down. That may take 10 or 20 secs.
 
ve ze said:
I thought that there should not be DC on the speakers connection.
Click to expand...
That is correct... Thats why DC voltage will trip the protect circuit(relay).And disconnect the speakers.. But you need roughly 500mv or more to trip the circuit.
 
ve ze said:
When the connection is broken by the relay, the right channel shows dead 0. The left has some voltage on it 1 or 2 mV and then it gradually goes down. That may take 10 or 20 secs.
Click to expand...
Key is to measure DC offset before the relay contacts at the amplifier output..Can't measure the DC offset at the speakers while it is in protect mode.
 
ve ze said:
What can generate that surge that trips the circuit?
Click to expand...
Possible bad solder connection in the power supply / protect circuit. Not really a " surge" per say. More like a broken circuit .
 
Check for cracks in the solder around TR407 & TR408 .. Testing you here.:biggrin:

Or just find them and shoot some good clear close-up photos of them and the surrounding areas. So I can look them over.

Know how to solder and de-solder ? If not..Learn..:D
 
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avionic said:
Check for cracks in the solder around TR407 & TR408 .. Testing you here.:biggrin:

Or just find them and shoot some good clear close-up photos of them and the surrounding areas. So I can look them over.

Know how to solder and de-solder ? If not..Learn..:D
Click to expand...
Found the TRs on the board and looked around them. Did not notice any cracks or loose solder there. Lightly touched them to press in a bit. Will monitor them now. You want to see the photo from the bottom side of the board?
Yes, I can solder. :)
 
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