Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for repair

[UNITRAX]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

I don't have an isolation transformer for the oscilloscope. That's why I asked for a hint how to connect the probe and gnd. I don't know if I measured correctly.

I assume that the VU meter in the right channel is working properly

0.2V/DIV
0.1ms/DIV
probe for C244
I got 150mV?

Way too much on the left channel.
1V/DIV
0.1ms/DIV
probe for C144

I'll remove the UPC4557 from the pcb and measure the resistance without the circuit.

[Per Hansson]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

Please watch EEVblog #279 - How NOT To Blow Up Your Oscilloscope!

[UNITRAX]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

Thanks.
99% of decks have an isolated power supply. My home power supply is so old it only has 2 wires. Active and neutral without mains earth reference.

Does this mean that after the transformer of the tape recorder I can measure everything safely?

[budwich]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

Does the meter adjust variable "POT" cause anything to change on either display? Verify the full resistance across the resistor along with each "portion". Compare against right and left.

[Per Hansson]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

Does this mean that after the transformer of the tape recorder I can measure everything safely?

No, you can only connect the ground clip to what is true ground.
True ground is when your multimeter shows you 0.00ohm from chassis/ground clip/ground in wall socket to whatever you measure.

An example: ATX PSU secondary side, provides maybe 25A on 5v line.
Consider what will happen if you connect ground clip there?
Sure it is only low voltage so no big explosion but it will still carry enough energy to melt traces inside the scope because you are shorting 5v to ground...

[UNITRAX]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

No, you can only connect the ground clip to what is true ground.
True ground is when your multimeter shows you 0.00ohm from chassis/ground clip/ground in wall socket to whatever you measure.

Wasn't sure to witch one I can safety connect.

An example: ATX PSU secondary side, provides maybe 25A on 5v line.
Consider what will happen if you connect ground clip there?
Sure it is only low voltage so no big explosion but it will still carry enough energy to melt traces inside the scope because you are shorting 5v to ground...

Is ATX PSU a linear power supply or a switched-mode voltage converter?
I think its not linear stabilizer with an isolated transformer. It's rather high-frequency switching power supply. But i might be wrong.

Does the meter adjust variable "POT" cause anything to change on either display? Verify the full resistance across the resistor along with each "portion". Compare against right and left.

In playback mode, the VR151 potentiometer does nothing. In recording mode, the left Vu meter goes up but is weak. It shows about 30% to compare with the right one.

[UNITRAX]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

I measured the resistance of the main pot 2x20k R101 and R201


pin 1 and 2
from 19 goes down to 0

pin 3 and 2
from 0 goes up to 19.6

R101,201 seems ok.

now VR151 and VR251

VR151

pin 1 - 2 gnd
from 0 goes down to 0.9

pin 2gnd - 3
all the time 24k!

VR251

pin 1 - 2 gnd
from 0 goes down to 0.8

pin 2gnd - 3
from 1.1 goes down to 0

[budwich]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

while measuring the resistance of 2-3 adjust the vr151. Does the resistance change?

[UNITRAX]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

while measuring the resistance of 2-3 adjust the vr151. Does the resistance change?

Nothing changes, it's still 24k.

[budwich]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

well, not sure what to make of that. What kind of variable resistor is it? (picture?) 24K does not align with overall resistance and any "sub section" should change with the adjustment.

Also, please indicate which pin number is connected to where in the circuit.

[R_J]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

VR151 must be bad, Check across the two outside pins (not the center wiper) it should read 1K. 24k suggests it is almost open.
Does it look like this?

[UNITRAX]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

You both have a point. It;s the VR151 that is bad.

When I washed the VR151, the resistance dropped to 3K. I've already ordered a new one for replacement.
Vu meter works during play, but still weak. I will write if everything is ok after replacing the adjustable resistors.
You guys are amazing.

[budwich]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

you may have more "onion skins" still (ie. faults elsewhere not allowing proper levels).... but once you get a replacement (might consider replacing both) since you indicated the other one didn't change the meter level. Further, the service manual lays out adjustments that should be done once you have the components replaced. Good luck, hopefully, you get it "whistling".

[redwire]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

With the open (wiper) trimpots, I will clean them by dragging a piece of paper under the wiper. It acts like gentle sandpaper to clean oxide off the wiper.
Or use a spray cleaner containing PPE like DeOxit, that works very good.

[UNITRAX]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

you may have more "onion skins" still (ie. faults elsewhere not allowing proper levels).... but once you get a replacement (might consider replacing both) since you indicated the other one didn't change the meter level. Further, the service manual lays out adjustments that should be done once you have the components replaced. Good luck, hopefully, you get it "whistling".

I ordered more adjustable resistors with different values. There are also playback, record VR gain or VR sensitivities in the circuit.
It's better to change them in pairs.

With the open (wiper) trimpots, I will clean them by dragging a piece of paper under the wiper. It acts like gentle sandpaper to clean oxide off the wiper.
Or use a spray cleaner containing PPE like DeOxit, that works very good.

already spryed a bit of contact pr, but resistance went up to 30k and now down to 9k. I'll do your trick with a piece of paper.

I changed the capacitors C101, C110, C109, C121, C124, C126, C137, C141, C63 with new ones. The same in right chanel.
I changed the drive belt and lubricated the cassette mechanism.
Counter belt is also new.
I was worried about weak heads, but the sound is great, clear and powerful. I'm surprised.

[R_J]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

The problem with these controls in where the leads get crimped to the resistive component, that is where they get intermittent and cleaning doesn't help. If the resistance across the outside pins is 9k~30k cleaning the wiper won't fix it.

[UNITRAX]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

VR151, 251 already changed
The left Vu meter has started and works with the right one.

Thank you so much everyone for your helpful words.
This community is priceless.

[CapLeaker]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

Good that this is resolved. But I did ask about VR151 in post 4. LOL

[UNITRAX]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

This deck had a lot of problems, mainly with the mechanics. It was poorly assembled and several springs were missing. Poor condition of electronic and contact elements. The record switch, all VR and main potentiometer were greasy with "miracle" fluid. The previous owner must have been a hard smoker, totally everything was covered with a yellow coating.
I measured the resistance after your 4th post, but I thought the differences were due to the damaged UPC4557.

I spent too much time on it, but now all functions are working, it's been playing for 2 days.

[sam_sam_sam]

Re: Damaged system of the VU indicator in the JVC KD-A11 tape recorder Request for re

You might have spent to much time on however this is one more item that did not end up in the landfill and you have an item should give you some years of enjoyment hopefully