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Old 25th June 2007, 11:29 AM   #1
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Default low audio on left channel jvc vn-300

Just realised to tick the 'have searched' box before I could post...

hi guys, hope someone can help me here...

Have an old jvc vn-300 solid state amp that really pumped out quite a nice sound, but now for some reason the left channel is putting out about 1/3 of the volume of the right channel. If I adjust the L/R balance to 2.3rds towards left and crank up the volume, both sound good, but no where near full volume ( only on R channel).

Now I have done some preliminary tests ( I have a scope - albeit an ancient phosphor dual channel ).

I have attached a link to the full size schematic ( amp came with the original instruction book which included the circuit).
Quick test around the DC Voltage points (not all of them, but randomly) in the diagram shows they are correct.
using a wave generator on a computer the output of the L channel is almost exactly 1/3 the full wave of the Right channel.
The only thing that is differing is the DC offset at points 801/802 where the signal is the same amplitute on both, but 801(L) has 0v dc offset and 802(R) has +0.15v offset and the signal amplitute of 0.1v p-p on both.
measuring the voltages at the transistors x1-x1 reveal correct DC voltages as in the schematic. I swapped the input path at 801/802 (wired) to see if it was a problem before the power stage, but made no difference, and though one of the elna 1000u caps was stuffed (as it was a bit wobbly) I replaced it and didn't make any difference - so swapped it back, they are hughe Elna caps.
pulled out a few random resistors I thought might look like trouble, but with no fault, or variation found.

Any help would be appreciated, see attached Full Schematic, if it's not detailed enough I can email through the hires scanned image.

you can click on image to take you to picassaweb where you can either download, or view full size.

Also I can really tell if the volume pot is log or linear (it's an old metal alps 100kdual), can you from circuit? as it's noisy and I will be replacing it.

Click the image to open in full size.
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Old 28th June 2007, 11:43 AM   #2
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wooot!... fixed it.

Seems a blown cap in the preamp stage was causing the problem.
c219, 100u25v, looks like it was already replaced before as it was a 220u25v, so I threw in a 220u35v.

Keep up with the good work, this site rocks man!...

picked up some 16mm marine ply for cheap price :-), now just gotta work on what enclosures to build.
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Old 3rd October 2009, 01:54 PM   #3
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Hello I seek this diagram for a long time, can one L have by email please
Thanks
Alain
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Old 5th April 2010, 04:36 PM   #4
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Hi all! i was given an old JVC VN-300 amp, it only has audio output on one channel, in mono mode i have output on both loudspeakers...(have'nt chequed the tape out yet)...does anyone have a clean schematic for this amp? I've found one on Picassa but resolution is way too low....judging from the one channel left alive, the sound is definitly to my liking, making some much more expensive things sound "muddled" and unexciting...thanks in advance!
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Old 16th September 2010, 12:26 PM   #5
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Quote:
Originally Posted by alaindelagironde View Post
Hello I seek this diagram for a long time, can one L have by email please
Thanks
Alain

Hi all! Looking for thisdiagram too for a long time,could send one to my email please? Thanks in forward
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Old 22nd February 2011, 07:56 AM   #6
Moksha is offline Moksha  Australia
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The JVC VN-300 is a really fantastic amp... i "downgraded" from a Yamaha natural sound amp a few months back for the VN-300 (which i bought at a market for 20$) - the sound like one of the previous posts stated is not muddied, very transparent and clear... i have since added a tube preamp which has warmed up the sound cosiderably... which i think is the only drawback to the amp.
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Old 19th April 2013, 11:03 AM   #7
alayn91 is offline alayn91  France
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Hello,

Is it still possible to have the full sized schematic ??

Thank in advance.
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Old 10th February 2014, 02:14 PM   #8
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Hi guys, got vn-300 for 15 years or so, worked like a charm. Love it.

Anyway, couple of moths ago I've started to notice some noise on speakers, static ambient noise and when volume is adjusted, creaking sound.
So I figure that volume "dynamic resistor" (or adjustable resistance resistor) rocker (not sure how it called in EN) worn out.
It has a code: 4iC-Qvz1201-005-100kΩBX (and very nice "MIJ")
Anyone can suggest proper replacement part? (I would like to use this amp for another 15 years or so)

I've opened it up and saw that I need a schematics first before I begin

Quote:
Originally Posted by alayn91 View Post
Hello,

Is it still possible to have the full sized schematic ??

Thank in advance.
Luckily got original manual that came with it (somewhere in the house) I'll post it as soon as I find it.
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Old 10th February 2014, 02:15 PM   #9
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Join Date: Feb 2014
Hi guys, got vn-300 for 15 years or so, worked like a charm. Love it.

Anyway, couple of moths ago I've started to notice some noise on speakers, static ambient noise and when volume is adjusted, creaking sound.
So I figure that volume "dynamic resistor" (or adjustable resistance resistor) rocker (not sure how it called in EN) worn out.
It has a code: 4iC-Qvz1201-005-100kΩBX (and very nice "MIJ")
Anyone can suggest proper replacement part? (I would like to use this amp for another 15 years or so)

I've opened it up and saw that I need a schematics first before I begin

Quote:
Originally Posted by alayn91 View Post
Hello,

Is it still possible to have the full sized schematic ??

Thank in advance.
Got original manual that came with it (somewhere in the house) I'll post it as soon as I find it.
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