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[SOLVED] Hitachi 8290E Line out levels on Line In !?


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#1 Dancorp

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Posted 15 September 2018 - 01:40 PM

I propose this evening to you to look into a rather original problem concerning a Hitachi TRK-8290E
As you know,,  I update boomboxes with the addition of Bluetooth and a personal lighting system that reacts to the music.
 
 
To propose all this, it is obvious that the station must be equipped with a line input for Bluetooth, but 
also a line output. It is this line output that will be analyzed by my arduino to display the light effects correctly.
 
This is the completely weird problem I have with this boxboom:
When the box is in the Line In/Phono position, the lineout level is very low.
 
These are some elements that may or may not help in the reflection:
- In Radio position, the line output is good (expected levels, my lighting reacts correctly)
- The integrated  VuMeter shows that the line input level is good.
- For the same volume, the sound levels are equivalent between the different sources.
- The service manual shows that the line output is pumped to the function selector.
 

http://37.187.140.11..._SM_HITACHI.pdf

 

(Follow the DIN connector at the top of page 9)

 
Does anyone have an explanation, I don't understand what's going on. :(
Thank you in advance for your insights 
 
Dan
 
Translated with www.DeepL.com, because it's late here in marseille (France), i'm tired :)

 



#2 Superduper

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Posted 15 September 2018 - 10:33 PM

Not sure I am understanding your question.  For phono, you are using RCA jacks right?  And for line-in, you are using Din pins 1/4 with switch set to Line, right?  Because if you use Din as Line-input but your switch is set to phono, the signal will be grounded.

 

For noise reduction, the line/phono switch grounds the unused mode.  For example, if you set mode to phono, the switch will ground line, but if you set mode to line, the switch will ground the phono signal.  Phono signal is through RCA and Line-in signal is through DIN.



#3 Dancorp

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Posted 16 September 2018 - 07:49 AM

Thank you for that explanation. 

But I'm in the right position!
When listening, there is no problem with the line input. It is only a level problem for the lineout on the din.
For the radio, the level is normal, and for the Line In/ Phono, the lineout  level is very low !


#4 Superduper

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Posted 16 September 2018 - 08:56 PM

Attached File  hitachi_block.png   247.22K   5 downloads

 

I want to show you something:

 

Take a look at block diagram near right bottom.  I circled a switch.  See position P/L?  When in this position, it looks like it connects to +B3.  Not sure how much voltage is there but it's a power source.

 

Now follow that to the left and then up....  see VR401 & VR402?  They are varistors.  I'm not sure what is their function here but if they pass .7v or more to the base of Q410, then that transistor will turn on and allow current to pass from C to E (ground).  The C also happens to connect to the LINE-OUT of Din.  But since that switch only supplies current to the transistor in P/L mode, that means that line-out will not be affected in any other mode by that transistor except when switch is in P/L position.  

 

Does that make sense to you?  Ok, I'm not sure what is the intended reason for that circuit is, or if it is actually passing .7v or more, but if it is, it potentially shunts the line-out signal to ground.  Could this be the way it was designed?  I don't know.  Are those varistors working properly and as intended?  I don't know.

 

So what happens if you remove either VR401 or VR402?  Then that makes Q410 irrelevant and no longer effective.  If you want to confirm if removal of either or both of these makes Line-out active again, you can, easy enough to do.  BTW, the block diagram is probably only showing you one channel (L or R).  That means there is probably an identical mirror circuit for the other channel.



#5 Dancorp

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Posted 17 September 2018 - 04:07 AM

Thank you for this very interesting analysis!

I'm trying to understand the purpose of the varistor and the transistor it drives.
Why not just ground the lineout on the switch itself, on the L/P position?

Anyway, I'll locate Q410L/R (or VRs) and shunt it, we'll see what happens!



#6 Dancorp

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Posted 20 September 2018 - 03:27 PM

Good News Superduper :)

Thanks to your idee, the lineout is now fine on each position.

As expected, the solution was to cut one of the VRs. :)



#7 Superduper

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Posted 21 September 2018 - 12:16 AM

:clap:  :thumbsup:  :w00t:



#8 hopey

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Posted 26 September 2018 - 03:55 PM

Is this for copy protection or because the phono is low input that the shunt will eliminate interference?