DNM Amplifier Repair India

[Hemz]

Hi Friends,

I have now taken a new repair project of two DNM amplifier. This is a good amplifier as far as 20 Watt RMS per channel and 80 watts for sub is concerned.

Now the problem is one of them has no sound. Display works fine.

The problem began with the subwoofer sounding bad ( continues flapping, if i am not getting the correct word, please forgive) then one day all the channels stopped working.

With some initial trouble shoting i find that the sound signal from the small channels are reaching the relay but the relay is not turning on to connect the speakers.

i have a idea that there will be some kind of comparator ckt which controls the replay and is now protecting the speakers from being damaged.

Please assist, as i have two of them.

The amp is good so i intend to repair it myself.

Any help, any ideas is appreciated.

Thanks,
Hemant.

 

[Max Headroom]

The brown stuff on the board is contact glue that has decomposed.
In this state it can be conductive and corrosive, causing circuit misoperation.
You need to carefully remove this 'dead' glue...you may have to remove some caps to do so.
The amp might work after this.

Dan.

 

[Hemz]

thanks,

i did clean all the glue that was visible but no change in the state. am i suppose to get 17 or 18 Volts at the output, even when the unit is turned off.

 

[Mooly]

DC voltage at the output is not normal when an amp is turned off.

If the amp is off then the speaker relays must be 'open' so where is your voltage coming from.

Try putting a 10k resistor across each output and recheck the voltage is now zero. It could be stray pickup you are measuring.

You should check the DC voltage at the output of each amplifier before the relay (amp on).

 

[Hemz]

Hi,

The DC output is at the collector of the output transistor C5198 And -ve 1.3v at collector of A1941, from the sub output pair transistor, in switched off condition.

further, the -ve output eventually fades away..but not the +ve collector voltage.

 

[Panelhead]

Brown material

If that is glue that has run across the circuitry there sure is a lot of it. Look under the board. I think this is from a leaky capacitor or two.
Look for bulges and soft cases. The large filter caps may be in rough shape. I think the circuitry is acting up and that is why the output relay is open.
How hot did the amps run when operational?

 

[Mooly]

Checking residual voltages when the amp is off isn't really a valid test state.

You need to check that all the DC rails are correct when the amp is ON.

Then check the DC offset at each channels output.

All that brown stuff looks like glue to me. We used to see a lot of that stuff in commercial equipment and its horrible stuff and yes, it can cause issues with high impedance circuitry.

 

[Hemz]

All the power related capacitors were removed and cleaned the pcb as far as possible with little left over of glue.

this part is done. the small regulator capacitors were also cleaned, like 7812, 7912 and 7805.

the input to the lm1785 and dc rails..fixed.

now tracing the relay control ckt.

i will keep posted as i proceed further.

Thanks everyone for their sugesstions and advice ...will definetly be able to solve this one..

thanks again...apriciate it.

 

[Hemz]

c5198..collector voltage 48v
2sa1941..collector voltage 47.9v
LM1875..+vcc 22.7v
-vcc22.6v

i think the supply rails have been improved.

 

[Hemz]

and further only the 7812 is bit hot..
also can we check dc voltage at out of 1875..with dc voltage setting on DMM?

 

[Mooly]

The output pin of the LM1875 is probably meant to be zero volts if all is well. I say 'probably' because without seeing a circuit I wouldn't 100% commit.

 

[Hemz]

Hi,

One of the local suggestions was to clean the PCB with IP solution, so all the power capacitors were removed and the area was thoroughly cleaned, Guess what!!!.. Its working. so the cheat element in my case was the GLUE ( can't even imagine ).

The five channels have been working fine now but the problem it started from, the distorted output is still there.
now i think it needs a scope to check the culprit. i do have a DIY PC scope but though not that accurate..lets see.

 

[Mooly]

That's great. The glue is evil stuff, in fact you might even be able to get a voltage reading from it if you stick the probe into it hard enough.

 

[pued]

Which circuit is used?

 

[Hemz]

Guys..back to square 1..again not working..
there is something..that i need to further look for..this seems to be intermitent problem

 

[Mooly]

Having applied heat could be one clue, as could something physical (you must have at least mauled the boards around a little bit removing the caps).

Poor soldering on those devices on the small heatsinks could be prime suspects, as could poor soldering on those metal oxide resistors... anything that runs hot.

 

[Hemz]

hi,

This one was finally completed, both of them.

The problem being an OP AMP 4558, it was used to change the frequency response of the sub.

The input resistor, pot, was out and open i guess provided infinity gain.

Changed it with near term value and that's all. It works like charm.

Thank you all for your valuable inputs, my amp was thoroughly cleaned in the process.

Will come back soon, with onkyo TXL55...soon.

Regards,
Hemant.

 

[Mooly]

Thanks for updating the thread Pleased to hear you got it sorted in the end.