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My JVC RC-838JW ** Project Twisty Treat


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#1 Cpl-Chronic

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Posted 11 May 2016 - 03:52 PM

Hey all....I managed to warngle a near minty looking 838 with the dreaded dead left channel.  Took it to my vintage repair guy & he changed out the amp IC so now both channels work in Biphonic mode but the left channel is still dead in mono & stereo modes....GRR!!!

 

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Even the back badges look brand new & the speakers perform like new...

 

Well, I put it aside & managed to get a fully working parts box, beat up to heck & back, with good tape deck.  The sliders are pretty stiff, probably cleaned by a noob with WD40 but both channels work.  I'm waiting on some DeOxit Fader lube to properly clean them & lubricate. So I'm in the process of fine tuning the tuner section & testing but here's a short video of the tape deck in operation...

 

Next will be transferring the guts of the working unit to the minty cabinet.  In the meantime, here's a video of it on the test bench.  It isn't together yet but you can hear how good it sounds...

 

 

 



#2 blu_fuz

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Posted 11 May 2016 - 03:56 PM

Where is the twisty treat?!?!?!?!



#3 Cpl-Chronic

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Posted 11 May 2016 - 09:23 PM

Where is the twisty treat?!?!?!?!

 

Glad you asked Blu! :-D

 

Here's the thing....

 

The 838 is a great blaster....amazing sound for its size, one of the first of that 80's style back in the late 70's, legendary JVC build quality, a killer manual deck & rugged good looks...

 

I was really surprised at the use of solder joints to electically connect the control board to the amp board at a 90 degree angle...big mistake..That's where the 'Twisty Treat' comes in....

 

If you are suffering from a 'blown channel' syndrome & no sound comes out of 1 channel, especially the Left, then check the solder joints, connecting the control board with the amp board...Demo vid soon to follow...

 

The Volume slider is on the corner of the control board, & positioned 90 degrees from the corner of the amp board while all the connections between the 2 boards are hard soldered points prone to cracking etc...When the 2 boards get twisted towards OR away from each other, by a fall, incorrect installation, etc. the solder joints will crack at the edge where it connects to the trace.....also if any of the sliders takes a hit during a fall, knock, or twist, it's very easy to crack the tracks inside the slider, opening a channel completely =  no sound...volume, balance are both prone to this issue....

 

So, while I was fitting the working chassis into the minty cabinet, the amp board got torqued a bit & once I got it correctly seated & tested, the left channel was gone again....just like the 1st chassis...GRRR!!!  Took a quick look & realized I had a broken solder joint at the L volume lead, invisible to the eye until you flexed the connection slightly & could see the joint lift away from the trace...BAM!!!

 

.fixed it with a wire jumper, more flexible than a solid joint...presto, back to working normally, correctly & beautifully...

You can see the volume slider above & the wire jumper I used to re-connect the left channel of the volume to the amp board's connections

nb6dly.jpg

 

video coming soon....

:-D



#4 MyOhMy

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Posted 11 May 2016 - 10:12 PM

^ ^ ^ I never ceased to be amazed at the skills and knowledge of the many forum members here.  :bow: Your description of the design flaw, the potential for problems, the means to identify the problems, how to rectify them (with a little hack thrown in!) was put in such a way I managed to follow everything with a clear mental image.  Even I (yes, ME!) understood the whole process. :yes:  Thank you for the clarity needed by a mere mortal such as myself. :rock:



#5 baddboybill

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Posted 11 May 2016 - 10:27 PM

Very interesting 🤔. Thanks for sharing 😎👍


Bad Boy Bill

#6 Northerner

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Posted 12 May 2016 - 01:16 AM

Great fix...useful to know. I love this box, one of my absolute favourites :-)

#7 SLO

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Posted 12 May 2016 - 01:32 AM

^ ^ ^ I never ceased to be amazed at the skills and knowledge of the many forum members here.  :bow: Your description of the design flaw, the potential for problems, the means to identify the problems, how to rectify them (with a little hack thrown in!) was put in such a way I managed to follow everything with a clear mental image.  Even I (yes, ME!) understood the whole process. :yes:  Thank you for the clarity needed by a mere mortal such as myself. :rock:

I gotta agree with M.O.M.  even I was able to get a visual in my head of the design flaw  :w00t: I love my 838 so this info is key! Good lookin out Cpl  :bow:



#8 blu_fuz

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Posted 12 May 2016 - 07:28 AM

AAAAaaaaaah. That all makes sense to me!



#9 Cpl-Chronic

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Posted 12 May 2016 - 09:38 PM

All back together & fully working!! :yes:

 

 

 

BAM!!!



#10 Lasonic TRC-920

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Posted 13 May 2016 - 08:11 AM

Very Interesting. The single channel issue with the 838 is SO COMMON, and in the past has been diagnosed simply as a BAD AMP CHIP.

 

I have this issue with my 838. I'm going to have to have me a look!

 

Thanks for the heads up!



#11 Cpl-Chronic

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Posted 15 May 2016 - 09:13 PM

Very Interesting. The single channel issue with the 838 is SO COMMON, and in the past has been diagnosed simply as a BAD AMP CHIP

 

Well, I knew I had just tweaked something on the amp/controller boards since it was working, not 10 minutes before, on both channels until I tried to install the 2nd donor chassis into the minty cabinet.

 

The solder joints joining the 2 boards together seemed the most likely & sure enough, it was...

 

If this helps anyone fix theirs please give feedback on the thread if you can.  It would be interesting to see if a lot of misdiagnosed problems were a bad joint along that connection...

 

Thanx,

Cpl



#12 Cpl-Chronic

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Posted 16 May 2016 - 01:00 AM

Been tinkering with the original chassis from the near mint unit & discovered a cracked track in the balance slider for the left channel, rendering the circuit open...using a jumper & testing brought the left channel to life but a bit stronger than the right...

 

see where the blue wire bypasses the control for the left channel...

 

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Now I have to try & fix the balance, Bass & Treble sliders.  All three have issues on one channel , oddly both Bass & Treble are open on the right channel....The problems are definitely in the sliders all 3 have open circuits, no matter the position of the pegs...



#13 crazygamer

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Posted 16 May 2016 - 01:07 PM

That´s a cool looking box, i like it, i hope to have one too, they are great !  :thumbsup:



#14 Lasonic TRC-920

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Posted 16 May 2016 - 07:39 PM

Been tinkering with the original chassis from the near mint unit & discovered a cracked track in the balance slider for the left channel, rendering the circuit open...using a jumper & testing brought the left channel to life but a bit stronger than the right...

 

see where the blue wire bypasses the control for the left channel...

 

2wmm3v7.jpg

 

Now I have to try & fix the balance, Bass & Treble sliders.  All three have issues on one channel , oddly both Bass & Treble are open on the right channel....The problems are definitely in the sliders all 3 have open circuits, no matter the position of the pegs...

 

Very interesting. I'll be watching to see where this all goes. I really want to get my 838 up and running again



#15 Cpl-Chronic

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Posted 16 May 2016 - 10:14 PM

It's Bi-PHoniC!!!

 

 

Cpl



#16 MyOhMy

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Posted 17 May 2016 - 03:25 PM

:clap:  :clap: :clap:  :clap:  :clap:  

 

Well done! :thumbsup:



#17 Northerner

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Posted 18 May 2016 - 12:45 AM

Nice repair!! I will remember this fix. Luckily mine is 100%. Glad you got it sorted mate :-)

#18 crazygamer

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Posted 18 May 2016 - 01:59 AM

Absolutely gorgeous radio, very well done. I have a chance to get one which is complete but i was said that "it doesn´t work at all", are they easy to work on or pain and misery to work on ? Everything´s soldered on i guess, no socket connections. 



#19 Cpl-Chronic

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Posted 18 May 2016 - 06:15 AM

I'm still working on the first chassis from the original 838, with the blue jumper across the balance fader...We'll see how it goes once my supplies are restocked.  I ordered some conductive glue for fixing the fader cracks(Balance, Bass, Treble) & a couple of solder removers..I figure, if I can get the faders working again I'll have 2 working 838's... :-D

 

Stay tuned...mods are on the way... :w00t:

 

Cpl



#20 riker1068

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Posted 18 May 2016 - 01:53 PM

:cool:



#21 Cpl-Chronic

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Posted 18 May 2016 - 06:04 PM

OK, All finished up with a VU LED mod to make it my own....

 

 

Cpl



#22 Northerner

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Posted 19 May 2016 - 01:11 AM

Love it! Bit more subtle than your usual bling-fest and I love it...the fact that the vu's are in the middle make it look just right. Very nice indeed...top work :-)

#23 jimmyjimmy19702010

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Posted 19 May 2016 - 01:36 AM

Very cool mod! That has to be the cleanest example of that model I've ever seen - simply stunning.

What a beast!

Nicely done,

James... :-)

#24 Cpl-Chronic

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Posted 19 May 2016 - 07:30 PM

Thanx guys!! This is the first 838 I bought originally for a horrific price, even tho the left side was dead.  It was so minty. it looked like it came out of a time capsule..

 

Ok, back to the first chassis....

 

Finallly got my solder remover in the mail from China & couldn't find any in my home city of Windsor, Ontario.  So China wins again.

 

Anyways, finally was able to remove the balance slider & open it up.  No broken tracks but one of the finger pairs for the Left channel fell out of the casing when I pulled the tabs up & removed the track board...

 

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I'm guessing that's my problem...ya think?? 

 

Time to get out the tweezers & jb weld & fix this thing & retest.  If that is a go then I'll tackle the Bass & Treble faders next...I have a good chance since this thing sat with almost 0 hours of use & no oxidation.  Everything is fresh as fark!!

 

May still have a 2nd fully working 838 when I'm done...*fingers crossed*

 

Cpl



#25 SLO

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Posted 19 May 2016 - 11:33 PM

That LED mod is dope! Not too much, and not too little, jus perfect for this box, Cpl strikes again  :-D



#26 Northerner

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Posted 19 May 2016 - 11:37 PM

Awesome, likely a much cleaner fix than trying to fix the track

#27 trippy1313

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Posted 19 May 2016 - 11:58 PM

Looks great, as usual. This box has so much going on already, the subtle but solid LED mod works great, I really like it.

#28 Cpl-Chronic

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Posted 23 May 2016 - 12:39 PM

well, attempt failed but i may have a line on new sliders...we'll have to wait & see...



#29 Gnarlyman

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Posted 07 June 2018 - 01:46 PM

I'm glad to see your fix for this problem here, my Fader has the same problem with the left channel cutting out completely. I'm gonna check all this stuff to try and fix it. Mine also cuts out the left channel in Mono and stereo modes, works on both channels in Expand and BiPhonic though.



#30 vermouth

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Posted 15 August 2018 - 07:25 PM

Looks really nice. Thanks for the tips. I just picked up an 838 for restoration and am reading up on them.