razpones
Member
Hello everyone
This is only my second post here, why am doing this is, I am trying to learn but for the most part am not sure of what am doing most of the time, so i go by instinct. This time it is about a receiver that a friend bought out of Craigslist, a post Covid buy that was really hasty and the unit was turn on but not tested for sound. He paid $40 and went home to discover that it didn't produced sound. He gave it to me, as a thanks for fixing his main unit (for which the JVC was to be a replacement), a Harman Kardon HK440 Vxi that needed a good cleaning of the pots.
The unit turned on and displayed the radio and it settled on a station, so I started to troubleshoot by testing if the preamp was working, I did it by connecting a tape deck to the tape record output from the receiver.
Listening on the tape deck headphone jack, I heard the cd with distortion on both sides and on FM and AM the volume was so low I could only hear a whisper with the full monitor volume up. So I checked the board and found a couple of caps that were open, changed them for some pull outs i produced from an old tv and also re soldered most of the board, as I believe there was bad connections from cold solder. After that, I got full sound on both sides of the headphones on all inputs, FM and AM could be heard at full volume and clear.
So I went to check on the power amp board, and after a long time testing and checking with a magnifier, I finally found a burned resistor (R710 on the board, 82 Ohms 5%), this lead me to discovering a transistor on that circuit (Q702 on the board, 2SC2389 Transistor) and a ceramic cap (C708 on the board, 27pf 50v), finding both to be open.
Now in that same circuit there is one of the two ICs in the unit (IC701, njm4560dd by Dainichi), this IC is measuring on pin 7, 2.58 V, where it is supposed to measure 0.8 V per the service manual (included section).
My question, why is that IC measuring wrong after I took the faulty parts on the circuit, all i can think is that the IC comes before the parts that went bad and it is the culprit, or that something that is feeding the IC has a component that is bad on the rail?, I don't want to buy all the parts I found bad and connect them just to have the short out as I turn on the unit. Do i have to replace the IC? it is not expensive, but again I fear that something that comes before the IC is also faulty and would mess all the parts replaced.
I'm including the part that is relevant on the service manual, maybe someone with more experience can point out my mistakes? Like I mentioned am a rookie as far as electronics, so I don't know how the current flows on this circuits and why this IC is measuring that out of spec.
One of the things I noticed is that only one of the two relays click when I power the unit, the one that clicks does right as you push the power button, and I assume the other one would click a few seconds after that, turning the power amp on. So that is part of the problem I assume.
Obviously fixing this is not a critical thing, as I got the unit for free (or a little work) and I don't care much for it, other than am trying to learn and am curious and would like to get it to work again. I appreciate all and any help. Thanks.
This is only my second post here, why am doing this is, I am trying to learn but for the most part am not sure of what am doing most of the time, so i go by instinct. This time it is about a receiver that a friend bought out of Craigslist, a post Covid buy that was really hasty and the unit was turn on but not tested for sound. He paid $40 and went home to discover that it didn't produced sound. He gave it to me, as a thanks for fixing his main unit (for which the JVC was to be a replacement), a Harman Kardon HK440 Vxi that needed a good cleaning of the pots.
The unit turned on and displayed the radio and it settled on a station, so I started to troubleshoot by testing if the preamp was working, I did it by connecting a tape deck to the tape record output from the receiver.
Listening on the tape deck headphone jack, I heard the cd with distortion on both sides and on FM and AM the volume was so low I could only hear a whisper with the full monitor volume up. So I checked the board and found a couple of caps that were open, changed them for some pull outs i produced from an old tv and also re soldered most of the board, as I believe there was bad connections from cold solder. After that, I got full sound on both sides of the headphones on all inputs, FM and AM could be heard at full volume and clear.
So I went to check on the power amp board, and after a long time testing and checking with a magnifier, I finally found a burned resistor (R710 on the board, 82 Ohms 5%), this lead me to discovering a transistor on that circuit (Q702 on the board, 2SC2389 Transistor) and a ceramic cap (C708 on the board, 27pf 50v), finding both to be open.
Now in that same circuit there is one of the two ICs in the unit (IC701, njm4560dd by Dainichi), this IC is measuring on pin 7, 2.58 V, where it is supposed to measure 0.8 V per the service manual (included section).
My question, why is that IC measuring wrong after I took the faulty parts on the circuit, all i can think is that the IC comes before the parts that went bad and it is the culprit, or that something that is feeding the IC has a component that is bad on the rail?, I don't want to buy all the parts I found bad and connect them just to have the short out as I turn on the unit. Do i have to replace the IC? it is not expensive, but again I fear that something that comes before the IC is also faulty and would mess all the parts replaced.
I'm including the part that is relevant on the service manual, maybe someone with more experience can point out my mistakes? Like I mentioned am a rookie as far as electronics, so I don't know how the current flows on this circuits and why this IC is measuring that out of spec.
One of the things I noticed is that only one of the two relays click when I power the unit, the one that clicks does right as you push the power button, and I assume the other one would click a few seconds after that, turning the power amp on. So that is part of the problem I assume.
Obviously fixing this is not a critical thing, as I got the unit for free (or a little work) and I don't care much for it, other than am trying to learn and am curious and would like to get it to work again. I appreciate all and any help. Thanks.
