My JVC RC-838JW ** Project Twisty Treat

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!!!




















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...

https://www.youtube.com/watch?v=Ja8MIwwglww

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

blu_fuz said:

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

Click to expand...

Glad you asked Blu!

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


video coming soon....

^ ^ ^ I never ceased to be amazed at the skills and knowledge of the many forum members here. 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. Thank you for the clarity needed by a mere mortal such as myself.

Very interesting 🤔. Thanks for sharing 😎👍


Bad Boy Bill

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

MyOhMy said:

^ ^ ^ I never ceased to be amazed at the skills and knowledge of the many forum members here. 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. Thank you for the clarity needed by a mere mortal such as myself.

Click to expand...

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

AAAAaaaaaah. That all makes sense to me!

All back together & fully working!!


https://www.youtube.com/watch?v=LmvImk26tM0

BAM!!!

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!

Lasonic TRC-920 said:

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

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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

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...



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...

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

Cpl-Chronic said:

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...

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...

Click to expand...

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

It's Bi-PHoniC!!!

https://www.youtube.com/watch?v=MNjkPZMToO0

Cpl

Well done!

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

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.

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...

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

Cpl