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  #1  
Old 02-17-2018, 03:45 PM
ELzekio ELzekio is offline
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JVC CX 710 VHF/UHF tuner repair.

Hello everyone, now that I’ve had some extra time, I thought I’d re visit this JVC CX 710 (the 610s big brother ) that I’ve got, and see if I can get any luck on finding the issue for the vhf-uhf tuner from y’all here at Video-Karma. I believe I posted a thread in boomboxery a while back seeking help for the fm tuner and tv tuner.

I’m not very concerned about the FM tuner, but if I could at least get vhf picture on this unit threw the tuner, I’d be more than happy.

One suggested that the tuners were out of alignment, which could very well be the issue, but I don’t yet have the experience of using an oscilloscope to help with tuner alignment. To me, it also seems a bit unlikely that both the fm, am, and vhf/uhf tuners would all go out of alignment, but I suppose you never know.

What I have done and know:

- This unit has composite video and audio inputs, when using the composite inputs I am able to get a good picture and good audio with NO ISSUES at all. This tells me the CRT is able to process a picture and display it, meaning the issue is somewhere in the VHF/UHF tuning.

- When i inject either a VHF OR UHF signal in to the unit. it is pretty much unable to tune it in.

In the YouTube video I have linked below, I am injecting a vhf channel 4 signal threw into the unit from the VCR and into the TV 75ohm input. as you can see it is pretty much unable to process the input.

If you really work the tuning wheel into a single VERY precise spot, you sometimes can get a barley viewable and blurry picture that of course is not watch able. (As shown 13 and 30 seconds in to the video)

I have AFC on and so IF the unit was working correctly, it should automatically snap in to the perfect picture when you get the tuning dial anywhere around the right area.

https://m.youtube.com/watch?v=lpnzJPUfmkg

Also, as you can see in the video, when I get the tuning dial around where it should be, I am able to get pretty decent audio out of the speakers, but as I said the video doesn't want to come in.


I have repeated this test using a BLONDER TONGE AGILE MODULATOR (am 60 550), and tried injecting different vhf and uhf channels, but no matter what channel I broadcast on, I still get the same result of unwatchable signal even though it SHOULD be perfect.

I am also quite sure that there are NO physical connection problems with the antenna or 75ohm input because when I directly inject the VCRs vhf signal directly into the vhf input on the television’s tuning box as shown in the picture below, I get the SAME result as always (fair audio, but picture not coming in as shown in the link to the video above.)

EFB0811B-D993-4A6F-A6FC-A40480F35D39.jpg

5C6B4237-E6E9-4DB4-A2A5-77833E1847C8.jpg

I checked quite a few voltages around IC 101, but I have yet to find any that differ significantly from spec.


Here is a Link to the Schematic I Scanned. This is for the main tv board. That includes the vhf and uhf tuning. I can scan the other side of the schematic if necessary.

https://drive.google.com/file/d/1s0K...ew?usp=sharing

So what do you all think. Any chance there is anything I can do that does not involve a complete overhaul tuner alignment?

None of the caps have any visible leaking electrolytic. Think there could be one that can cause these problems?

Are there Any pots that I could safely adjust without an oscilloscope? Ive got a fluke multimeter.

I will try and get the full survice manual uploaded as soon as I can.

Last edited by ELzekio; 02-17-2018 at 04:24 PM.
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Old 02-17-2018, 06:48 PM
ELzekio ELzekio is offline
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Manual

Well I tell you what, scanning and putting together a 50 page service manual is a lot harder work then it may seem. here it is:

it can be downloaded from this google drive link, just like the schematic.

https://drive.google.com/file/d/1rJx...ew?usp=sharing
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  #3  
Old 02-17-2018, 07:20 PM
zeno zeno is offline
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Looks like IF AGC overload. I looked at manual & the schematic is missing.
Probably on page 46 which is BLANK !
Anyhows check all the voltages around IC101 first.
There is also a "noise" control that is probably the RF AGC control.
Try turning that.
If you post the schematic separately I will look at it to.

73 Zeno
LFOD !
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  #4  
Old 02-17-2018, 07:40 PM
ELzekio ELzekio is offline
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The link to the schematic is on the first post.

This link here.

https://drive.google.com/file/d/1s0K...ew?usp=sharing

When I download the service manual from the other link page 46 is there. It contains the TV PC board patterns, so I don’t think there shouldn’t be any issues.

Let me know if you are getting everything correctly.

Last edited by ELzekio; 02-17-2018 at 07:52 PM.
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  #5  
Old 02-17-2018, 08:49 PM
ELzekio ELzekio is offline
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So I injected the VCR channel 4 VHF signal threw the standard coax input on the side of the unit and fine tuned the tv to the single spot where a very very blurry signal would come threw as shown in the YouTube video I posted.

I then read the voltages of IC 101. These are my results:

Pin 1: 4.336 V
Pin 2: 4.360 V
Pin 3: 6.296 V
Pin 4: 9.57 V
Pin 5: 6.414 V
Pin 6: 6.534 V
Pin 7: 4.110 V
Pin 8: 7.73 V
Pin 9: 7.76 V
Pin 10: 4.119 V
Pin 11: 11.32 V
Pin 12: 3.842 V
Pin 13: 0.038 V
Pin 14: 10.90 V
Pin 15: 4.378 V
Pin 16: 4.377 V

I noticed to the right of ic101, there is a label called noice with a pot just beside it, is this what you are referring to?


F3EFA294-15A2-41EF-A8E3-79A9DAEC0E4D.jpg
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  #6  
Old 02-18-2018, 03:28 AM
colorfixer's Avatar
colorfixer colorfixer is offline
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I have a 14" set that does the same thing on a blonder tongue or a NES game. It does work fine however on a VCR.
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  #7  
Old 02-18-2018, 09:35 AM
zeno zeno is offline
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Noise is RF AGC try it.
Also try turning down the modulator if you can. If not unhook it & put
4' of wire on it & go OTA with it. . It will probably clear up.
OR hook a VCR to it.

If its not an external problem odds are its the IC.
Pins 3 & 4 are used. Pin 4 shows 2 voltages. One is for zero signal
the other is strong signal. It should change when mod is switched on
& off.
Also run the set & see if the IC is getting hot. Should be just warm.
You should also check the resistors off pins 3 & 4 before changing the IC.

Thats about all I can do long distance, maybe others have ideas.
Good luck & let us know how you do.

73 Zeno
LFOD !
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Old 02-18-2018, 01:07 PM
ELzekio ELzekio is offline
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For the tests I’ve been doing so far while this thread has been posted, I have been useing a vcr connected to the CX 710s coax 75 ohm input and NOT my blonder tongue.

*when ever I did though try broadcasting over the air with my blonder tongue I would still get the same results I get when having a vcr directly connected to the unit.*

While having the vcr connected again and having the vhf tuner tuned to where it should have the strongest signal, I felt around but i did NOT notice the IC 101 chip getting hot.

I really don’t think anything is wrong with the IC itself. The voltages as posted above are fine, except for the fact that pins 4 and 14 are still showing the N.S voltage for No/Low signal even though I had the vcr signal connected to the unit and tuned to where the signal is supposed to be strong.

I then tried adjusting the noice control according to the service manual, but it did not make a any meaningful difference in the picture at all.

As i mentioned before I am confident the signal is getting to the TU-01 tuner, because I tried injecting it directly into it and got the same results as always. If IC 101 is good, then it is possible that the TU-01 tuner has a problem?

Is it common for those tuner boxes to go bad?

Last edited by ELzekio; 02-18-2018 at 01:12 PM.
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  #9  
Old 02-19-2018, 09:40 AM
zeno zeno is offline
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Tuners were pretty good. Most symptoms are snowy, dead, loss
of a band. Yours shows classic IF overload. Washing out, buzz,
tearing etc. RF AGC fail gives snow & or jittery pix usually.
You can test that at the AGC pin for signal/no signal changes OR
if you have a little bias box unhook the AGC & set the voltage.
If it were the olden days I would give an estimate for the IC
& lots of labor. While the IC is out I would check the resisters in
the area ( dont have to lift them usually).

73 Zeno
LFOD !
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  #10  
Old 02-19-2018, 10:06 AM
ELzekio ELzekio is offline
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Yea I suppose I will get an IC on order and maybe a few caps in the tuning area for good measure.

Do you have a specific recommendation on the type caps for a tuning circuit on a tv boombox like this that I can get on mouser? I usually use ELNA silmic 2 caps and Nichicon FGs when recapping stereo equipment, but I think those are only rated up to 85 C, and may not be the best for this application.

Not trying to be nit picky, just want some that are a good fit for this environment.


I will also take another look at the resistors when I get the IC out.

Last edited by ELzekio; 02-19-2018 at 01:00 PM.
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  #11  
Old 02-19-2018, 02:45 PM
zeno zeno is offline
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Quote:
Originally Posted by ELzekio View Post
Yea I suppose I will get an IC on order and maybe a few caps in the tuning area for good measure.

Do you have a specific recommendation on the type caps for a tuning circuit on a tv boombox like this that I can get on mouser? I usually use ELNA silmic 2 caps and Nichicon FGs when recapping stereo equipment, but I think those are only rated up to 85 C, and may not be the best for this application.

Not trying to be nit picky, just want some that are a good fit for this environment.


I will also take another look at the resistors when I get the IC out.
I am inactive at this time so not sure who & what to use. 85 deg are fine.
since caps are smaller I would double the voltage unless there is a space problem. Most of it probably uses 25V caps so go to 50V etc. If you want to recap fix the problem FIRST then do a few at a time checking it as you go. Mistakes happen & there is nothing harder than finding a self induced problem.

C U later
Zeno
LFOD !
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  #12  
Old 03-01-2018, 11:01 PM
ELzekio ELzekio is offline
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Well I was able to replace the IC 101 with a genuine TA7607AP, As well as replaced most of the caps in the tuning section, but it looks like there has been very little to no improvement with the condition

These are the voltages on the new IC I installed.

These voltages were once again taken when I had a vcr connected to the 75 ohm coax input and the unit tuned to where the signal was best coming in on the unit.

Pin 1: 4.329V
Pin 2: 4.323V
Pin 3: 5.800V
Pin 4: 9.57V
Pin 5: 6.65V
Pin 6: 6.336V
Pin 7: 4.057V
Pin 8: 7.69V
Pin 9: 7.72V
Pin 10: 4.058V
Pin 11: 11.31V
Pin 12: 4.028V
Pin 13: 0.038V
Pin 14: 10.91V
Pin 15: 4.330V
Pin 16: 4.331V

I will keep slowly replacing some caps and checking the progress with every few.
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  #13  
Old 03-01-2018, 11:18 PM
ELzekio ELzekio is offline
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Here is 2nd better demonstration video after replacing the components.

https://youtu.be/JksDOedlbVo
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