JVC / Victor RC-838 balance issues

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Lasonic TRC-920

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Was working on my Japanese Victor RC-838 trying to figure out this issue...

Unit powers up, plays tapes, plays radio, plays Line in BUT..

Sound only comes out right side in MONO or STEREO....or at least 99%. When you slide the balance to the left a slight sound is heard.

Stranger even...If I switch to Expand or Bi-Phonic I get sound out both speakers, but if I slide the balance to the left NOTHING, left channel is dead. Back to center....sound from both channels.

I have opened the box and cleaned the contacts on the sliders and the switches, no change.

Also, right VU meter always works on Mono, Stereo, Expand and Bi-Phonic. Left VU meter sometimes works on Expand and Bi-Phonic. BTW, the left meter is also the battery gauge and that works fine. Right meter is tuning strength and also works fine.

Your thoughts....Thanks
 

jimmyjimmy19702010

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I think the Bi-Phonic mode mixes the signal from both channels via a delay so the dead channel would be getting fed signal from the other channel so when you slide the balance from the left over toward the right, it's introducing some signal from the right channel into the left speaker.?!

Just because you have some output on Bi-Phonic mode on that left channel, doesn't necessarily rule out an amp issue on that channel. At least you know the balance slider is working ok.

Do you have both channels via Headphones?

Hopefully it's just a loose connection somewhere.

Good luck,

James... :-)
 

Lasonic TRC-920

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jimmyjimmy19702010 said:
I think the Bi-Phonic mode mixes the signal from both channels via a delay so the dead channel would be getting fed signal from the other channel so when you slide the balance from the left over toward the right, it's introducing some signal from the right channel into the left speaker.?!

Just because you have some output on Bi-Phonic mode on that left channel, doesn't necessarily rule out an amp issue on that channel. At least you know the balance slider is working ok.

Do you have both channels via Headphones?

Hopefully it's just a loose connection somewhere.

Good luck,

James... :-)
I just tested with head phones and it's on one channel.

Damn, sounds like I got an amp issue :bang:

When I had this apart on the bench, I was moving things around to see if there were any crackles or pops, but nothing. No sign that there maybe a loose connection.
 

Northerner

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Apr 16, 2012
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Could be the classic knackered JVC volume slider ie a break in the strip?

My M70 had similar symptoms other than you obviously don't have the biphonic function so you couldn't try that test.

I'd take out the volume slider and check its resistance. On the M70 I seem to remember its a 50k one. Mine had a hairline crack at one end fixed with a tiny amount of silver conductive paint.

Someone with more knowledge might have a better idea but this would be my guess :-)
 

blu_fuz

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That sounds exactly what was happening with the 838 Floyd tried to work on for me. After a lot of cleaning and testing, still no dice :-(
 

BoomboxLover48

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I had so much trouble with the function switch, biphonic, stereo switch and all. Switches are all connected via levers from the front. I would give those switches a good cleaning. I had the very same issue and switch cleaning solved it. I don't think it is the balance slider. No!
:yes: :yes:
 

baddboybill

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I had so much trouble with the function switch, biphonic, stereo switch and all. Switches are all connected via levers from the front. I would give those switches a good cleaning. I had the very same issue and switch cleaning solved it. I don't think it is the balance slider. No!
:yes: :yes:


Usually if switch is dirty or corroded mono can still be heard from both channels :hmmm: I'm guessing bad volume slider ;-)


Bad Boy Bill
 

Lasonic TRC-920

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I have cleaned the top sliders very good, sprayed them and blew them out with compressed air and they move smoothly with no crackle. The Bi-Phonic switch and level meter switches are harder to get too. I did spray cleaner ON them and clicked the switch back in forth, but I don't think that did anything.

All I did to test the balance was to unplug the speaker plug from the amp board and connect a test speaker to the out leads, checking each channel, but it was the same. Seems the left isn't working....


BoomboxLover48 said:
I had so much trouble with the function switch, biphonic, stereo switch and all. Switches are all connected via levers from the front. I would give those switches a good cleaning. I had the very same issue and switch cleaning solved it. I don't think it is the balance slider. No!
:yes: :yes:
I will give this another try. I didn't go deeper because, as you guys know, these older radios are PACKED from corner to corner and to make things worse, they assembled them layer by layer soldering in components as they went, so you have to reverse the process to get to some of the items inside, like the Bi-Phonic switch.

When I got this box about 5 years ago it was all working, but like you guys said, the top controls were dirty and caused issues and the Bi-Phonic switch has always been an issue. It maybe time to get out the soldering iron and GO DEEP!

As always, thanks for your guys help :-D
 

Superduper

Member (SA)
Chris,

There's so many different methods of failure that it's impossible to say for certain. Did you check whether you are getting normal output (both channels active) through the line-out jacks?

Also, if you are able to get sound out of both the left/right channels in any one of those 4 amp modes (M,S,E,BiPh), then the amp is OK. That's because the speakers are each connected to the respective amp, and if you can get sound, then both channels of the amp module itself is fine. However, that's not necessarily any better news than anything else being wrong because it still requires diagnosis and repair. Sometimes, because the amp module itself is non-serviceable, it's easier to just swap out the amp module than do the component to component diagnostics. Now, in Stereo mode, the amp expects it's own audio input signal. Clearly, it's not getting it. In the expand and biphonic mode, the processor will bleed a signal with a sound delay criss-crossing into the opposite channel (BEFORE the amp), which is why in these modes, you get audio. This tells me one of your channels is dead someplace, possibly in the preamp. Whether or not you get an audio signal via line out will tell us whether the issue is before the preamp, or after.

My guess based upon the idiosyncrasies and known issues with this model is that either the bass,treble or balance slider is bad. A break in any one of these sliders will affect the sound, and there are more than 1 failure mode in these sliders resulting in different symptoms. So sticking my neck out, rebuild the slider board including those 3 slide pots will likely (85% +/- based on educated guess) fix your problem. Yours isn't the first 838 with this problem. Many have the same issue. I have one here right now (belongs to different member) in queue waiting for fix.

Good luck.
 
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Alberto

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Superduper said:
Chris,

There's so many different methods of failure that it's impossible to say for certain. Did you check whether you are getting normal output (both channels active) through the line-out jacks?

Also, if you are able to get sound out of both the left/right channels in any one of those 4 amp modes (M,S,E,BiPh), then the amp is OK. That's because the speakers are each connected to the respective amp, and if you can get sound, then both channels of the amp module itself is fine. However, that's not necessarily any better news than anything else being wrong because it still requires diagnosis and repair. Sometimes, because the amp module itself is non-serviceable, it's easier to just swap out the amp module than do the component to component diagnostics. Now, in Stereo mode, the amp expects it's own audio input signal. Clearly, it's not getting it. In the expand and biphonic mode, the processor will bleed a signal with a sound delay criss-crossing into the opposite channel (BEFORE the amp), which is why in these modes, you get audio. This tells me one of your channels is dead someplace, possibly in the preamp. Whether or not you get an audio signal via line out will tell us whether the issue is before the preamp, or after.

My guess based upon the idiosyncrasies and known issues with this model is that either the bass,treble or balance slider is bad. A break in any one of these sliders will affect the sound, and there are more than 1 failure mode in these sliders resulting in different symptoms. So sticking my neck out, rebuild the slider board including those 3 slide pots will likely (85% +/- based on educated guess) fix your problem. Yours isn't the first 838 with this problem. Many have the same issue. I have one here right now (belongs to different member) in queue waiting for fix.

Good luck.
Norm, is there a way to jump out the Bass, Treble, etc sliders, bypassing the slider mechanism so that the signal would be at full 100% adjustment? Just to see if the dead channel comes alive after jumping out slider switch.
 

Lasonic TRC-920

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Feb 16, 2010
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Superduper said:
Chris,

There's so many different methods of failure that it's impossible to say for certain. Did you check whether you are getting normal output (both channels active) through the line-out jacks?

Also, if you are able to get sound out of both the left/right channels in any one of those 4 amp modes (M,S,E,BiPh), then the amp is OK. That's because the speakers are each connected to the respective amp, and if you can get sound, then both channels of the amp module itself is fine. However, that's not necessarily any better news than anything else being wrong because it still requires diagnosis and repair. Sometimes, because the amp module itself is non-serviceable, it's easier to just swap out the amp module than do the component to component diagnostics. Now, in Stereo mode, the amp expects it's own audio input signal. Clearly, it's not getting it. In the expand and biphonic mode, the processor will bleed a signal with a sound delay criss-crossing into the opposite channel (BEFORE the amp), which is why in these modes, you get audio. This tells me one of your channels is dead someplace, possibly in the preamp. Whether or not you get an audio signal via line out will tell us whether the issue is before the preamp, or after.

My guess based upon the idiosyncrasies and known issues with this model is that either the bass,treble or balance slider is bad. A break in any one of these sliders will affect the sound, and there are more than 1 failure mode in these sliders resulting in different symptoms. So sticking my neck out, rebuild the slider board including those 3 slide pots will likely (85% +/- based on educated guess) fix your problem. Yours isn't the first 838 with this problem. Many have the same issue. I have one here right now (belongs to different member) in queue waiting for fix.

Good luck.
Thanks for the reply Norm,

I did test the signal at the external speaker out and the left was dead there. I will have to test the RCA OUT and see if I get signal from there and I will report back. Obviously, I don't have the know how or tools to test the sliders other than to say "They seem to be working!" :dunce:
 

Superduper

Member (SA)
Chris, the bass treble balance and volume pots are all "stereo" pots. That means internally there are two completely separate controls operated by a single knob. Each/any can fail internally on either side and not affect the other. The headphone test is only good to check for blown speaker or bad jack. If no preamplifier signal is getting to that amp channel, you'll get silence since amplifying zero is still zero right? However in expand and biphonik mode, the signal processor bleeds a signal from good channel to the input of dead channel thereby appearing to bring it back to life. In reality, you aren't hearing stereo, just channel A program on A channel & channel A program again (with a delay) on B channel.
 
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Lasonic TRC-920

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Superduper said:
Chris, the bass treble balance and volume pots are all "stereo" pots. That means internally there are two completely separate controls operated by a single knob. Each/any can fail internally on either side and not affect the other. The headphone test is only good to check for blown speaker or bad jack. If no preamplifier signal is getting to that amp channel, you'll get silence since amplifying zero is still zero right? However in expand and biphonik mode, the signal processor bleeds a signal from good channel to the input of dead channel thereby appearing to bring it back to life. In reality, you aren't hearing stereo, just channel A program on A channel & channel A program again (with a delay) on B channel.
Yeah, that totally makes sense and is EXACTLY what I am experiencing.

I hooked up the 838 RCA OUT to the M70 RCA IN and I'm getting stereo signal to the M70.

So, does that sound like the Amp is good but one of the stereo sliders is bad?
 

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Superduper

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Could be. As I said, most likely even but could still be a break in the audio signal path somewhere else. In my mind, my money is on the slider board and other alternate possibilities just to cover all bases.

BTW, you can confirm my theory by using only 1 line-in channel (on the working side). Then set to expand or bi-ph and voila, both channels play with only 1channel as input, even as stereo mode remains dead on one side. Again this demonstrates the cross mixing of the wide modes. Really I'm only saying to observe this to help you better understand the mixing and how a dead channel in low level (preamplifier) can be mistaken for dead power amp.
 

Lasonic TRC-920

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Superduper said:
Could be. As I said, most likely even but could still be a break in the audio signal path somewhere else. In my mind, my money is on the slider board and other alternate possibilities just to cover all bases.

BTW, you can confirm my theory by using only 1 line-in channel (on the working side). Then set to expand or bi-ph and voila, both channels play with only 1channel as input, even as stereo mode remains dead on one side. Again this demonstrates the cross mixing of the wide modes. Really I'm only saying to observe this to help you better understand the mixing and how a dead channel in low level (preamplifier) can be mistaken for dead power amp.
Yes, exactly as you said. Line in, produces one channel on mono and stereo, switch it to expand and bi-phonc and it comes out both speakers, if you slide the balance to the right, nothing changes, if you slide it to the left NO SOUND AT ALL.

The M70 still plays in stereo with the line in into the 838, then line out to the M70.
 

chhchca

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Sep 6, 2016
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I revisit this topic from time to time, hoping to see any further info or updates on this issue. My unit is suffering same but the dead side is the right side.
 

Superduper

Member (SA)
chhchca said:
I revisit this topic from time to time, hoping to see any further info or updates on this issue. My unit is suffering same but the dead side is the right side.
If you read the thread closely, you'll understand that there is never 1-single definite solution to a failure that exhibits a particular symptom because there are multiple possible failure modes that can exhibit the same or similar symptoms. However, as I already expressed, I'm pretty certain that failed sliders are the most likely cause, since they are known to be failure prone on this and similar models.
 
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JVC Floyd

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That's what's wrong with mine a bad slider the reason I know is because if I push on it hard enough to balance comes back on but it won't stay on . I was considering bypassing the balance slider all together maybe removing it and just putting the jumper wires but I don't even know if that'll work. I always thought a balance control was stupid anyway all they do is cause problems.
 
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