Denon PMA-350II Amplifier repair advice needed.

[jimboH]

Hi there I have an old Denon PMA-350II than needs some love.

I'm not an electrical guru so please excuse my ignorance.

The unit works well for a while then the sound cuts out (power?) then the relay's can be heard clicking and the sound returns. This happens over and over again. I had initially thought that perhaps it was the relays that were faulty but now I have a hunch it may be the transformer/power section?

Any advice would be greatly appreciated.

It's probably not worth fixing $ wise? but I'd be keen to try just for the sense of satisfaction ;-)

Thanks in advance.

J

 

[Nigel Goodwin]

The relays are generally speaker protection relays, and a faulty relay usually wouldn't 'drop out', the normal failure mode would just be dirty contacts. It's also almost certainly not the power supply, I would imagine it's something triggering the protection, quite possibly a poor joint on one of the larger components - even on one of the relay coil pins (I've seen that in a number of surround sound amps).

 

[jimboH]

Hi Nigel. thank you for taking the time to reply. When I turn the unit on it takes about 6 seconds for the relays to click on. the big black circle at the top the photo (i assume is the transformer/power supply?) makes a buzzing/hum sound then slowly quietens and after it's finished it powers up.

the only bit in the board that looks dodgy is the little pink (resister?) at the bottom. it has some kind of blue stain around it. looks almost like oxidisation.

 

[Nigel Goodwin]

Hi Nigel. thank you for taking the time to reply. When I turn the unit on it takes about 6 seconds for the relays to click on. the big black circle at the top the photo (i assume is the transformer/power supply?) makes a buzzing/hum sound then slowly quietens and after it's finished it powers up.

the only bit in the board that looks dodgy is the little pink (resister?) at the bottom. it has some kind of blue stain around it. looks almost like oxidisation.

It's a capacitor, and (in the picture) looks like glue round it?.

There are two little three pin sockets to either side of that capacitor, I'm presuming those are for measuring quiescent current?, if you check the voltage on those they should be near zero volts - if they are higher (positive or negative) of afew hundres millivolts, it will trigger the speaker protection.

 

[jimboH]

Hi Nigel. Assuming there is a voltage reading at one of the two three-pin connectors does that infer the issue (bad connection etc) is "between" these three-pin connectors?

it's a capacitor, and (in the picture) looks like glue round it?.

>>To be honest, it looks more like a liquid spill, but to my knowledge, there have been no spills.

 

[Nigel Goodwin]

Hi Nigel. Assuming there is a voltage reading at one of the two three-pin connectors does that infer the issue (bad connection etc) is "between" these three-pin connectors?

No, I'm presuming the three pins connect across the emitter resistors for setting quiescent current, so should be at zero volts, anything that upsets the DC conditions of the amp would cause the voltage to be high or low. The point is to see if the amp is faulty, or the protection circuits.

>>To be honest, it looks more like a liquid spill, but to my knowledge, there have been no spills.

I can only go from the picture, and it looks like aged glue.

 

[jimboH]

Hi Nigel. just ran a test. the quiescent pins slowly increase in voltage over time. I spent 5 minutes hovering over the amp till my back got sore.

I'm reading max 10.00mv on the first quiescent pins and max 9.5mv on the last set of pins. during my test, it did not trip/reset. ill endevour to run another test and wait till it trips to see the max mV when it trips.

Also the cap with the glue around it has a fluctuating voltage from + to - back and forth?

 

[unclejed613]

actually, the bluish stuff on the board doesn't look to me like glue, it looks like water damage.... denon uses a white colored glue, and it doesn't look anything like that. i magnified the picture so i could see it better. that looks like one of the protection circuits. if you clean up that area and resolder everything, as well as replace any components that have corrosion on their leads, it is likely to fix your problem