Lasonic TRC-920
Moderator
I get like that some times, maybe set it aside for a while and then come back to it.
Or do like me, beg Norm to fix it
Or do like me, beg Norm to fix it
Fixing a legend.....
- Thread starter blu_fuz
- Start date
BoomboxLover48
Boomus Fidelis
I'm pretty sure it will end up in Norm's hands to fix it.Lasonic TRC-920 said:I get like that some times, maybe set it aside for a while and then come back to it.
Or do like me, beg Norm to fix it
BoomboxLover48
Boomus Fidelis
I strongly believe amp ICs are the issue! I bet! 
Careful not to overheat the leads.
I want to see Joe all excited when it is working!
You should post videos of it!
And the countdown starts ....
Careful not to overheat the leads.
I want to see Joe all excited when it is working!
You should post videos of it!
And the countdown starts ....
baddboybill
Boomus Fidelis
I replaced both amp chips and loaded them up with new paste...... still broken :-(
It's probably something small like a bad transistor
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It's probably something small like a bad transistor
Sent from my iPad using Tapatalk
Northerner
Boomus Fidelis
BoomboxLover48
Boomus Fidelis
baddboybill said:It's probably something small like a bad transistor
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...sure Bill! You really make me laugh ...
BoomboxLover48
Boomus Fidelis
mmmmm....
the whole unit is very complicated to begin with.... lots of built in resistors and PCB traces on both sides .... it might need help from the Magician 'Norman the Great'!
I think it is waiting for his magic touch.
Just love to see it all working well.
the whole unit is very complicated to begin with.... lots of built in resistors and PCB traces on both sides .... it might need help from the Magician 'Norman the Great'!
I think it is waiting for his magic touch.
Just love to see it all working well.
baddboybill
Boomus Fidelis
...sure Bill! You really make me laugh ...Well Royce I had a unit that powered up but didn't have sound. Guy I bought it from thought it to be preamp or something to do with it. Well he was partially right as it was just a transistor that needed to be replaced. All I'm saying is it could be as simple as that. I've replaced more transistors and diodes causing issues than caps and resistors. So thinking about it maybe just as simple as a diode causing it as well.
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BoomboxLover48
Boomus Fidelis
Take it easy Bill!baddboybill said:Well Royce I had a unit that powered up but didn't have sound. Guy I bought it from thought it to be preamp or something to do with it. Well he was partially right as it was just a transistor that needed to be replaced. All I'm saying is it could be as simple as that. I've replaced more transistors and diodes causing issues than caps and resistors. So thinking about it maybe just as simple as a diode causing it as well.
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I was just joking! I know this is pretty complicated to fix. I've seen zener diodes and transistors causing complete malfunction in many units. I agree! We can list a lot of components that can cause the issue. Could be even a broken ground lead.
'Humming' is a nasty problem to fix. I'm sure Joe will figure it out sooner or later! Good luck Joe!
Ghettoboom767
Member (SA)
Honestly,I'm sure it may very well be more than that but humm sounded like a bad ground to me but I praise Joe for his perseverance on this!! Norms knowledge is amazing,so wish you guys get this Cadillac of Panasonics going.
Tracing circuits and replacing components is one of the toughest things there is to do in a Boombox.
Good luck Joe!!
Tracing circuits and replacing components is one of the toughest things there is to do in a Boombox.
Good luck Joe!!
BoomboxLover48
Boomus Fidelis
Now....The one and only choice....
I had a British friend who knew it all...great guy ...my age group... but he passed away... miss him everyday!
i am still recovering from that shock! ..so hard to believe he is no more...
He was my idol...the master of electronics...knew it all from A to Z..miss him so badly!
He would have pin pointed the problem in a heart beat!
I had a British friend who knew it all...great guy ...my age group... but he passed away... miss him everyday!
i am still recovering from that shock! ..so hard to believe he is no more...
He was my idol...the master of electronics...knew it all from A to Z..miss him so badly!
He would have pin pointed the problem in a heart beat!
Some of you might have been following this thread so I thought I would update it to let you all know the current status. Recap: Joe has this RX-5350 with a hum/oscillation that could not be resolved. He got to the point where he felt he was stuck and needed more help to get this fixed once and for sall, so we worked something out where he would mail it to me and I would work on it as time permitted.
Some of you might be aware that my life has been really unsettled the past couple years. It still is, hence the reduced number of posts and lack of projects to share. In any event, when the boombox arrived, I did a quick test to verify everything according to my understanding. I immediately noticed that the noise could be changed and intensity varied by operating the mixing control knob. This was never reported or noticed by Joe.
***********
Lesson #1. Thoroughly check and test everything, even if seemingly unrelated. It took me about 2-3 minutes of checking to discover this, long before 1 screw was ever removed. The finding is important as you are about to see.
***********
The boombox then went into my closet for safekeeping since I was not ready to work on it. However, it was clear to me that I would focus on the mixing circuitry since it could certainly be related. Here the boombox sat until recently when Joe began dropping hints about the 5350. Stuff like "I miss my 5350, wink wink", etc.
Anyhow, a couple days ago, I pulled this out, disassembled it and looked it over. I then referred to the circuit diagram to look at the mixing circuit (for those following along, you can refer to the circuit diagram portion to follow).
I then tested the appurtenant transistors in circuit and testing showed they tested fine powered down. I then began testing the components. Found one resistor (R410 47R) which could not be seen since it was apparently a "printed" resistor. However, I do know, by looking at the circuit diagram, that the emitter (E) of Q403 is tethered to ground through R410. Therefore, testing resistance between (E)Q403 to Ground should read 47ohms or less. The reason resistor resistance tested in circuit must be the same or less than the value of the resistor itself is that you are reading either the resistor value itself, or the resistor value + any parallel circuitry resistance. Students of electronics will understand how parallel resistance will affect a circuit and if you wish to know, then just google since I don't want to complicate this thread any more than necessary. I read 168 ohms. Something is amiss.
The printed resistor is on the component side of the board and the circuit crosses over through these 2 vias (tunnels that connect traces on different sides or layers of a PC board together).
As you can see, no resistor is visible, but there is corossion where the resistor should be. That resistor is located immediately behind the mixing control pot.
So we are going to fix this by cutting that portion of the bad circuit to isolate it. It might be tempting to just drill through the vias and install a discrete resistor through those holes. Don't. Also, it is possible to calculate an appropriately value resistor to make that circuit 47 ohms total with the failing resistor in parallel with the new one. Once again, don't. That resistor is failing and probably will continue to change in value. Also, there could be stray capacitance introduced that could affect the circuit and maybe cause or allow oscillation to continue. The solution was to simply cut that trace with an exacto knife, make the cut wide enough to ensure no electrical connectivity. See cut.
Now after testing with meter to ensure ohms is infinity between (E)Q403 to ground, we are going to bridge those two with a new 47R 1/4W resistor, embedded in a clear heat shrink tubing. Clear is optional but rather than having future serviceman think it's just a jumper, a clear tubing allows them to see the resistor and value so there's no misunderstanding what was done.
After reassembly, hum was gone and boombox is now quiet. In fact, it sounds dang good, no doubt helped immensely by the recapping job. I am calling this issue, FIXED. For those of you who are just now seeing this thread, and you have A LOT of time, you can go back and read all 13 previous pages. Also note RULE #1 above. Had we observed and noted the relationship of the issue with the mixing circuit to begin with, we probably would have had this fixed long ago without getting it shipped at all.
One last note, while it may seem by following along with this post that this issue was a simple diagnosis and easy fix, I will say and I think Joe will concur that when you see the entire circuit diagram which is 100 times the size of this little snippet, things no longer seem so clear and simple.
Some of you might be aware that my life has been really unsettled the past couple years. It still is, hence the reduced number of posts and lack of projects to share. In any event, when the boombox arrived, I did a quick test to verify everything according to my understanding. I immediately noticed that the noise could be changed and intensity varied by operating the mixing control knob. This was never reported or noticed by Joe.
***********
Lesson #1. Thoroughly check and test everything, even if seemingly unrelated. It took me about 2-3 minutes of checking to discover this, long before 1 screw was ever removed. The finding is important as you are about to see.
***********
The boombox then went into my closet for safekeeping since I was not ready to work on it. However, it was clear to me that I would focus on the mixing circuitry since it could certainly be related. Here the boombox sat until recently when Joe began dropping hints about the 5350. Stuff like "I miss my 5350, wink wink", etc.
Anyhow, a couple days ago, I pulled this out, disassembled it and looked it over. I then referred to the circuit diagram to look at the mixing circuit (for those following along, you can refer to the circuit diagram portion to follow).
I then tested the appurtenant transistors in circuit and testing showed they tested fine powered down. I then began testing the components. Found one resistor (R410 47R) which could not be seen since it was apparently a "printed" resistor. However, I do know, by looking at the circuit diagram, that the emitter (E) of Q403 is tethered to ground through R410. Therefore, testing resistance between (E)Q403 to Ground should read 47ohms or less. The reason resistor resistance tested in circuit must be the same or less than the value of the resistor itself is that you are reading either the resistor value itself, or the resistor value + any parallel circuitry resistance. Students of electronics will understand how parallel resistance will affect a circuit and if you wish to know, then just google since I don't want to complicate this thread any more than necessary. I read 168 ohms. Something is amiss.
The printed resistor is on the component side of the board and the circuit crosses over through these 2 vias (tunnels that connect traces on different sides or layers of a PC board together).
As you can see, no resistor is visible, but there is corossion where the resistor should be. That resistor is located immediately behind the mixing control pot.
So we are going to fix this by cutting that portion of the bad circuit to isolate it. It might be tempting to just drill through the vias and install a discrete resistor through those holes. Don't. Also, it is possible to calculate an appropriately value resistor to make that circuit 47 ohms total with the failing resistor in parallel with the new one. Once again, don't. That resistor is failing and probably will continue to change in value. Also, there could be stray capacitance introduced that could affect the circuit and maybe cause or allow oscillation to continue. The solution was to simply cut that trace with an exacto knife, make the cut wide enough to ensure no electrical connectivity. See cut.
Now after testing with meter to ensure ohms is infinity between (E)Q403 to ground, we are going to bridge those two with a new 47R 1/4W resistor, embedded in a clear heat shrink tubing. Clear is optional but rather than having future serviceman think it's just a jumper, a clear tubing allows them to see the resistor and value so there's no misunderstanding what was done.
After reassembly, hum was gone and boombox is now quiet. In fact, it sounds dang good, no doubt helped immensely by the recapping job. I am calling this issue, FIXED. For those of you who are just now seeing this thread, and you have A LOT of time, you can go back and read all 13 previous pages. Also note RULE #1 above. Had we observed and noted the relationship of the issue with the mixing circuit to begin with, we probably would have had this fixed long ago without getting it shipped at all.
One last note, while it may seem by following along with this post that this issue was a simple diagnosis and easy fix, I will say and I think Joe will concur that when you see the entire circuit diagram which is 100 times the size of this little snippet, things no longer seem so clear and simple.
Bingo, bango, BONGO!
As Norm said, I never noted that the frequency would change with the mixing knob. Honestly, I never knew that it did but even if I knew, I probably would not have remembered to mention it. I'll go back and check my videos because I don't know if I noticed it.
After all this, I have to say, I am so proud of the work and effort I put towards this and I didn't make it much or any worse off than it was! Without your help Norm, this box wouldn't have been here long because I would have sold it off for parts which would have been sad. Thank you for sticking with this and helping all that you have. I appreciate every bit of it.
Honestly, I'm happy it was something easy! Wooohooo
As Norm said, I never noted that the frequency would change with the mixing knob. Honestly, I never knew that it did but even if I knew, I probably would not have remembered to mention it. I'll go back and check my videos because I don't know if I noticed it.
After all this, I have to say, I am so proud of the work and effort I put towards this and I didn't make it much or any worse off than it was! Without your help Norm, this box wouldn't have been here long because I would have sold it off for parts which would have been sad. Thank you for sticking with this and helping all that you have. I appreciate every bit of it.
Honestly, I'm happy it was something easy! Wooohooo
Cpl-Chronic
Member (SA)
Man, what a nightmare for me trying to find a PRINTED resistor, & fix it. Norm KILLS it again....skilllz Mad skillz.....
not to mention the photos & editing involved in the thread itself....holy crap!!
Cpl
not to mention the photos & editing involved in the thread itself....holy crap!!
Cpl