Denon POA-2400 repair

bazarimajom

Member
Recently I got my hands on this amp however when I turn it on the led 4/4 blinks while the others stay lit, with DBT. No input or speakers are connected. The service manual says "DC offset voltage for Right channel exceeds set level". So I checked some voltages on the board KU-9143-3 and found a deviation at IC502 (TLP521-1) which is an optocoupler. At legs 1-2 it should have 72V which is correct but at legs 3-4 there is excessive voltage 43V where it should be 389mV at leg 4 and -1.67v at leg 3. On the other side the voltages are very close to spec (+a slighty lower because of the DBT with 100W bulb). My question is: Is it enough if I change this part only, and what is a suitable replacement part for this? The closest I found was this but I'm not sure if it will be ok. Any suggestions are welcome!
 
How did the voltages look in the output stage? I'd bet your trouble is coming from a shorted transistor(s) in the output stage.
 
I didn't even want to get involved in this, as this amp is very sophisticated, and way beyond the skill of the amateur. However, when people who don't know these amps start chiming in, I have no choice.
How did the voltages look in the output stage? I'd bet your trouble is coming from a shorted transistor(s) in the output stage.
I've fixed a couple of those amps, and it probably isn't the opto-coupler. Check the vicinity of the opto-coupler (and indeed the entire amp board) for cracked solder joints. Some of those small signal transistors are operated right on the edge of SAO, they run hot and cause cracked solder joints. They also get a little sketchy after being operated like that, and should be replaced.
 
I didn't even want to get involved in this, as this amp is very sophisticated, and way beyond the skill of the amateur. However, when people who don't know these amps start chiming in, I have no choice.

I've fixed a couple of those amps, and it probably isn't the opto-coupler. Check the vicinity of the opto-coupler (and indeed the entire amp board) for cracked solder joints. Some of those small signal transistors are operated right on the edge of SAO, they run hot and cause cracked solder joints. They also get a little sketchy after being operated like that, and should be replaced.
And yes, I have fixed these amps.
 
Shorted output transistors tend to cause the main fuse to vaporize. Problems farther back tend to cause the output to swing a bunch of DC onto the output.
 
Thanks for the info Ray and tarior!

I'll check the small signal transistors. For preparation I might just order them in advance, thus I'm looking for a suitable replacement part for the following types:

2SC3334 - 2SC3467?
2SA1321 - ?
2SA968 - ?
2SA988 - ?

For the rest I can order the exact types as in the SM, but if you can recommend better for them I'd appreciate it!

KU-9143 board transistors:

2SK184C
2SC2705 - OK
2SA1145 - OK
2SA1360 - OK
2SC3334
2SC1815 - OK
2SC1841 - OK
2SC3334
2SA1321
2SC2238 - OK
2SA968
2SC3334
2SA988
 
I tested the all transistors (based on this) out of circuit except the 2 FETs (TR502, 504), replaced the optocoupler. Still the voltages are not ok. The solder rings of one 2SA1360 are pretty much destroyed and also the traces were lifted by a previous failure or someone else's soldering "skills" (I'd bet on the latter). I'll upload a photo of if later today. Can I use short jumper wires from the transistor leads to bridge the destroyed trace parts? I'll check the diodes, zeners next.

TR506, 508: 2SC2705 ok
TR510: 2SA1145 ok
TR512, 514: 2SA1360 ok
TR516: 2SC3334 ok
TR518: 2SC1815 ok
TR522: 2SC1841 ok
TR524: 2SC3334 ok
TR526: 2SA1321 ok
TR528: 2SC2238 ok
TR530: 2SA968 ok
TR540: 2SC3334 ok
TR542: 2SA988 ok
 
The output transistors are not connected. I checked the diodes and they also all seem to be fine (I did test them in circuit, which were suspicious were desoldered). The destroyed traces were for one the 2SA1360 were bridged. After putting the diodes back I did a test power on with DBT, and it powered up fine, the right channel DC offset voltage indicator did not blink meaning it was ok. Powered down and powered on again, the blinking came back. Now I'm stuck as I don't have the skill to trace the broken component and I'm having troubles reading the schematic. Do you have any suggestions how should I proceed ,where should I start to check the voltages and in which direction should I follow (if this makes sense)? Sorry for the noob questions but I have no techs available in my vicinity.
 
I don't have a schematic in front of me, but I do know that the 2SC2705/A1145 are biased way hot in that amp, and will become intermittent. I would recommend replacing any transistor there that has been run hot enough to crack up the board, even if it checks OK with a DMM.
 
Thank you tarior, I'll do that! I was checking the resistors as a last resort and meanwhile I checked the diodes again. D548 conducts in both directions so it should the troublemaker!
 
The circuit of the POA 2400 is very similar if not identical to that of my POA 6600a. Just yesterday the final went to protection. The problem was that part of the temperature sensor trace had come off . The causes are different and they do not believe it is an offset problem unless the trimmers have failed. Check them and if they work do not alter the values. Also check the electrolytic capacitors on the board.
 
Unfortunately no progress. I asked a friend of mine who is a test engineer (electronics engineer) to take a look at it at work at his spare time, but he didn't yet have the time for it.
 
I have 2400 also that has a DC offset on Ch 2. Similar above I found 65VDC on the opto-isolator pin 4. After removing the input CCA from the back of the amp I found the offset went away. After cking the ground on the board I found it missing which led me to the ground strap on the bottom of the KU-9143-3. I found there was no connection between the two ground wires and the ring terminal. Replacing the connector with a crimp and solder connection resolved the missing ground. On power up I found TR512, TR518 shorted. This of course resulted in a few resistors overheating. I have decided to replace all the front-end transistors however I am having trouble finding a replacement for the 2SK184C (TR501-504). Anyone found a replacement?
 
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