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HELP! Low voltage reading


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#1 baddboybill

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Posted 25 November 2017 - 06:10 PM

Well I’m working on a VZ2000 and had got deck fully working and clean up done. Only issue was the tuner dial light was burned out. I checked voltage and got 18v reading. Was pretty surprised but decided to try a mini lamp that was actually less than 12v. Not sure what MA reading for it was. Anyway everything was working great but when I tested light, which switch also activates battery power, and then all lights went dim and has been dim since. I mean I figured the bulb I used would have blown just if incorrect voltage. But now I’m only getting about 3.8v at the wires for tuner light. And nothing else powers up just a very dim led power on light. I checked scheme but can’t seem to figure out what went out.

#2 JVC Floyd

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Posted 25 November 2017 - 06:51 PM

Bill I can't remember if there's a fuse on the power board or not but I would look there. I would poke around the power board and look for dry or bad solder joints sometimes they shift around just working on these things. Also see if you can trace the light circuit back to the source and look for bad fusible resistors.

#3 baddboybill

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Posted 25 November 2017 - 07:09 PM

Thank you Floyd. There is a fuse but it is working. I tried with batteries as well with no luck. Just low voltage. I can’t even find in scheme which is actually the hot lead going from power board so can’t even test that. Power switch is actually hooked to power board. I know nothing was loose cuz all I did was hold the light bulb where other bulb is. I didn’t even solder it on.

#4 Transistorized

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Posted 25 November 2017 - 08:18 PM

I had this happen one time with a different model stereo. Ended up being a bad electrical connection at the transformer. Can't remember if it was a dirty voltage selector switch or if it was dirty contacts on the DC (barrel plug) input on the back. Either one could cause a low voltage issue.

 

Another place I would look is at that switch. You may want to remove power from the radio and ohm out the switch to see if it is restricting power. Powering the bulb through that switch could've arched a contact within it and now the switch is not sending full power from the transformer to the stereo circuitry.

 

I'm not nearly as experienced as you guys but I would check voltage going in and coming out of the transformer first. Then look at that switch second.



#5 baddboybill

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Posted 25 November 2017 - 09:17 PM

I had this happen one time with a different model stereo. Ended up being a bad electrical connection at the transformer. Can't remember if it was a dirty voltage selector switch or if it was dirty contacts on the DC (barrel plug) input on the back. Either one could cause a low voltage issue.

Another place I would look is at that switch. You may want to remove power from the radio and ohm out the switch to see if it is restricting power. Powering the bulb through that switch could've arched a contact within it and now the switch is not sending full power from the transformer to the stereo circuitry.

I'm not nearly as experienced as you guys but I would check voltage going in and coming out of the transformer first. Then look at that switch second.


Thank you for the input. I really appreciate it. I actually have checked everything you mentioned and tried batteries as well. I believe it’s somewhere on the power rail or actually on power board but can’t really find it in the scheme.

#6 baddboybill

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Posted 26 November 2017 - 07:48 AM

Ok so this is where I’ve narrowed a low voltage reading down to. I’ve done readings on a few transistors on power board and came up with these readings.
701b 16.11
701e 18.2
701c 2.80
702b 17.23
702e 18.1
702c 17.97

Notice collector side is supposed to have high 16.7v reading according to scheme but only has 2.80v reading. There is a resistor between base side 702 and Collector side 701. I tried changing resistor with used resistor I found with no change. So I’m Hoping someone can help. Here is the scheme for area.Attached File  730BE1AE-C603-486F-9FC6-1EFB40AB4BD4.png   339.66K   1 downloads

Also readings on Q706 are as follows
B 2.67v
C 2.79v
E 2.12v

#7 baddboybill

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Posted 26 November 2017 - 08:00 AM

Attached File  22843BC8-9C59-4A5F-A3B4-FC33FD40DF96.png   453.92K   2 downloads

#8 Superduper

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Posted 26 November 2017 - 09:15 AM

Check R710/716 array, make sure not shorted or values not changed. Then check Q701 because values look suspicious if those resistors are working proper. Start there.

Oh and if possible, check PS board with output connectors disconnected if possible to see if the issue is with the PS, or if you have a short after the output that is dragging down voltage.

#9 baddboybill

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Posted 26 November 2017 - 09:27 AM

Thank You Norm, what is PS board? Also I kinda figured Q701 was off but what else would I check on it?

#10 baddboybill

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Posted 26 November 2017 - 09:51 AM

Weird R710 and 716 are not in series but it’s actually R710 and 711 in series and R716 goes to Q704 base. Also I’m getting about 16v on all of them

Mmm....If PS board is power supply board then that’s what I’m working on.

Ok I understand

OK I disconnected the power board and checked and yeah I’m getting the correct readings now I’m getting correct readings on Q701

So now I’m stumped.

#11 baddboybill

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Posted 26 November 2017 - 10:28 AM

I’m wondering if the IC 902 is shorted as this chip is for the leds for power and also seems to be for the dial light.

Attached File  AF37A5AB-4F9F-4764-BD58-FFB6963538C4.png   326.26K   6 downloads

Attached File  6CA17702-8504-4C9A-80ED-0F1C91890DC1.png   253.88K   5 downloads

#12 Superduper

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Posted 26 November 2017 - 11:22 AM

Ok so that’s why I said to test the power supply disconnected. You worked on dial light, are you sure you didn’t bridge a solder joint?

#13 baddboybill

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Posted 26 November 2017 - 11:50 AM

I don’t believe so as my eyes
are not as good as it used to be. I did check 902 and looks to be like low-voltage 2.38 or so volts going into leg 4 which I guess should be 10.2v

while the deadbulb was hooked up I held the two leads to the joints to test it and that’s when everything went dim

#14 Superduper

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Posted 27 November 2017 - 11:44 PM

Bill, I really hate to say this and please don't take this the wrong way, but this isn't the first time you've tested something live and something goes kaput.  So I suspect that your test probes are too thick, not sharp so they slip on the joints, shaky hands, whatever.  The point is that it's easy to fry something when boards are live.  I recommend that you get yourself a good set of micro spring hook probes.  They have small hook shaped ends that hook on and don't let go.  I use them all the time in locations where I don't feel 100% comfortable that something might slip and maybe cause a small puff of smoke.  I'm serious.  They aren't expensive and will end up saving you tons of headaches the more you use them.  Plus the good thing is that once the hooks are clipped on, it leaves you hands free.

 

Now as to your VZ2000, something is dragging down the voltage.  You mentioned you checked IC902 and the voltage is low, but we already know that.  The question is whether A the supply at the pin is low or B the IC itself is internally shorted dragging down the supply.  Why you focused on IC902 I don't know, but I am guessing that anything connected to that supply rail right now is going to be showing low voltage.  You need to find the source of the short or drain.  There are a couple ways to do this.  The simplest for you right now is to start disconnecting to isolate the problem and narrow down possibilities.

 

Based on your original measurements, C:Q701is showing low voltage condition with PS  hooked up but normal voltage disconnected.  Looking at the circuit diagram, that rail also powers 2 connectors:  CNS403 to tape deck, and CNS501 to microcomputer.  Disconnect one or the other and see if the voltage returns to normal.  If voltage is normal with one of those connectors hooked up (and not the other), at least you've narrowed it down.

 

Follow this same technique to further isolate.  Once you've narrowed it down to the problem board, then you are getting somewhere.



#15 baddboybill

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Posted 28 November 2017 - 06:13 PM

Thank You Norm when I get some free time I will check

#16 Ghettoboom767

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Posted 04 December 2017 - 02:30 PM

Wow,good luck on this Bill.

#17 baddboybill

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Posted 05 December 2017 - 07:30 AM

Think I’ve narrowed it down to IC amp chip. ;-)

#18 JVC Floyd

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Posted 05 December 2017 - 09:32 AM

Bill I have some amp chips in my scrapper vz 2000 e , I'm not sure if they are the same as the USA vz 2000. I haven't removed them yet but if you need them let me know .

#19 Transistorized

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Posted 05 December 2017 - 11:37 AM

Think I’ve narrowed it down to IC amp chip. ;-)

Wow. Who would've thought. Do you think it was just a coincidence that this happened while checking the bulb?



#20 Superduper

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Posted 05 December 2017 - 11:50 AM

Doubtful, amps don’t fry unless the outputs are shorted, which can cause damage quickly.

#21 JVC Floyd

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Posted 05 December 2017 - 07:08 PM

From my experience with car amp a blown ic is usually burnt out and you can tell by looking at it , or better yet smelling it lol.

#22 baddboybill

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Posted 05 December 2017 - 09:19 PM

Bill I have some amp chips in my scrapper vz 2000 e , I'm not sure if they are the same as the USA vz 2000. I haven't removed them yet but if you need them let me know .


Not 100% sure just yet Floyd but I’ll let you know. Thank You


Think I’ve narrowed it down to IC amp chip. ;-)

Wow. Who would've thought. Do you think it was just a coincidence that this happened while checking the bulb?

Not sure yet still checking

Doubtful, amps don’t fry unless the outputs are shorted, which can cause damage quickly.


Haven’t had much time but one chip seemed shorted. Hopefully I’ll have more time this weekend.

#23 baddboybill

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Posted 30 October 2018 - 05:42 PM

Finally after a year I figured it out with lots of help from superduper (Norm) and some parts from JVC Floyd. It ended up being a transistor on ps board. Thank you guys!!!

#24 JVC Floyd

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Posted 31 October 2018 - 11:03 AM

Awesome!.

#25 Funkateer38

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Posted 19 November 2019 - 04:18 PM

Hi, I have the same problem with a vz-2000. Do you remember the changed transistor?