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

[budwich]

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

sorry I don't understand what you have posted.
are there wave forms or just DC voltages? Not sure that you are using the scope correctly.... "randomly" connecting the ground of the probe to a circuit can be damaging as this is inserting a ground / short into the circuit (comment related to " connected between the -,+ c143").

[UNITRAX]

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

Only when recording is turned on, I can hear the 1kHz sound from the generator on the output
The right vu meter connected to the oscilloscope when I turn on 1kHz,
The voltage drops by about 120 - 140mV

Left vu meter to the oscilloscope. The voltage drops by only about 5mV

The oscilloscope connected between -, + C143, shows nothing.
same with the C243

Oscilloscope between Vr151- gnd and C143 - negative = 0.5V

Oscilloscope between Vr251- gnd and C243 - negative = 20mV

[petehall347]

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

have you tried recording yet ?

[budwich]

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

good work... just to confirm, are you referring to the outputs of the 4557? You need to check the output just after the caps in question (just after the 4557 outs) to see what is being delivered to the meters.

[UNITRAX]

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

"... then it would be the diodes or 4.7uF capacitor C143 or 47uF capacitor C144. Why are you messing with the 22uF parts, there's none in the meter circuit.
The VU meter rectifier diodes D151, D152 are OA90 germanium diodes. So they have a low Vf around 0.34V@1mA on a multimeter.
Notice it is a bit strange the meter/cap are reversed polarity, the (+) goes to GND.

The usual problem with cassette decks is a dirty REC-PLAY switch. This long switch I move back and forth 100 times, and use pot/switch cleaner with PPE additive. Don't get any spray on the belts!"

C 143 4.7uF/50V and C144 47uF/25V I changed to new ones.
Their polarity is marked on the PCB, both top and bottom. According to this, the old and new capacitors were soldered.

Measured diodes with a multimeter:
D151= 0.310V
D251= 0.324V

D152= 0.342V
D252= 0.326V

I know the issue of the recording switch, I checked its conduction, seems ok. I switched it many times to be sure. I know the tape mechanism is a different story. I ordered new belts, the old one is already weak. After the replacement, I will adjust the mechanism.

excellent pickup. That cap is likely bad IF that is what he took out of the system. It appears that the OP was just going by the fact that he got a reading with the meter as opposed to looking at the schematic for what the values actually are... maybe.

I saw the polarity reversed in the diagram after you mention it. PCB has descriptions of components and solder pads. I hope the old ones were installed correctly, because that's how I put the new ones in.

ok.... as you planned, a scope of the area with a nice 1khz input signal will likely quickly point to where the signal disappears (ie. after which component).

The LINE IN inputs a 1kHz Sine signal

The oscilloscope connected to the output, the results of Lch and Rch can be seen in the pictures.

I only have basic information from his service. I don't have an isolation transformer and the oscilloscope is connected to the same wall as the tape recorder.

Volts/Div = 0.2V
Time/Div= 0.1ms
Probe= X1

[budwich]

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

excellent pickup. That cap is likely bad IF that is what he took out of the system. It appears that the OP was just going by the fact that he got a reading with the meter as opposed to looking at the schematic for what the values actually are... maybe.

[redwire]

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

[...] The sound level is equal on both channels, headphone and output. Therefore, I say that the damage is in the circuit. [...]

If there is good audio (not distorted, good level) at the headphone jack, then it would be the diodes or 4.7uF capacitor C143 or 47uF capacitor C144. Why are you messing with the 22uF parts, there's none in the meter circuit.
The VU meter rectifier diodes D151, D152 are OA90 germanium diodes. So they have a low Vf around 0.34V@1mA on a multimeter.
Notice it is a bit strange the meter/cap are reversed polarity, the (+) goes to GND.

The usual problem with cassette decks is a dirty REC-PLAY switch. This long switch I move back and forth 100 times, and use pot/switch cleaner with PPE additive. Don't get any spray on the belts!

[budwich]

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

Those seem ok. NOTE: in your impedance check of the inputs to the op amps, your 2/3 measure is the one of concern because that is the input associated with the left channel which you say is not working correctly.

[UNITRAX]

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

In the first photo, the value of the capacitor desoldered from the circuit.
The second photo shows a new capacitor.
The new ones had higher ESR and Ω
Tomorrow I can change them for new ones, or order capacitors with better parameters?
Thank you all for your involvement. I will also need tips on how to use the oscilloscope properly.

[budwich]

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

ok.... as you planned, a scope of the area with a nice 1khz input signal will likely quickly point to where the signal disappears (ie. after which component).

Hopefully, your "better" means lower values although I am somewhat "concerned" at your description of "better esr and resistance"... hmmmm.

Having said that, what capleaker just posted, only one upc4557 (ie. basically no right and left board), then your impedance test of the two sets of inputs DOES appear to point at an issue associated with the input "area" of the apc4557 either with connections coming towards it or within the chip itself. If you take it out of the circuit and check those pin pairs, that will tell you more about where the problem lies.

[CapLeaker]

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

I just checked the schematic and the board. The UPC4557 uses one side for one channel and the other side for the other. There are no two separate op amp IC's for each channel. I would look hard at caps in series (DC blocker) like C143. When these go high in ESR in a circuit like this, you loose audio volume. Other than that you could take UPC4557 out, solder in some wires to the board and wire up UPC4557 channels up in a manner that left is right and right is left. If the problem indeed is the one side of the op amp, the problem should be now on the other VU meter.

[UNITRAX]

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

Thanks for the correction, I'm still new to electronics and I make mistakes everywhere.
I don't remember the exact esr of each capacitor, but I had new ones ready for replacement. The desoldered ones had better esr and resistance than the new ones.

[budwich]

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

quote: "The UPC4557 resistance on the left and right outputs is not the same
legs 2 - 3 180 kOhms
legs 5 - 6 2.4 Mega Ohms!!!"
Those are NOT output pins. They are signal input pins to the op amps. The main reason for the difference... likely is that if you look at the schematic, it appears that only one of the op amps is used in the circuit.... as there is likely a upc4557 for each side, if I understand the posted manual correctly.

Anyway, when you said that you tested the caps with a tester, what did the ESR reading say for the caps in question?

[UNITRAX]

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

the circuit between the VU meter and the UPC4557 is pretty simple. Since you got two channels (left and right), compare them to each other with an oscilloscope...
Did you check VR151 if it's open? Check if the audio coming out of both channels are equal in volume. You got to figure out is it only the VU meter (circuit) that isn't working correctly or is one channel low in volume causing the VU meter to read low. Read page 11 and 12.

I checked VR151, the left VU meter does not respond to adjustments.
The Vu meter is OK, the fault is somewhere in the system. Tomorrow I will borrow an oscilloscope and then I can take measurements.

See what signal you get out of the headphone jack because that is also used to drive the VU meters. It is the op-amp output.
I had one of these JVC decks, built so good all metal buttons.

The sound level is equal on both channels, headphone and output. Therefore, I say that the damage is in the circuit.

I agree with the quality of the materials used to build this tape recorder, it is armored.
Very similar to Technics RS-M10 and Panasonic 616

what does this mean? your previous posted impedance measurements of the outputs showed "similar" readings for either output. Hence, the question. Please explain.

How did you check the output capacitors from the upc4557... maybe the capacitors are having a high / higher esr than is acceptable.

When I measure Vu meters not connected to the circuit. The left and right have the same resistance, the vu meters readings are the same, so I say the gauges themselves are working perfect.

I checked the following with a tester of electronic components:
Capacitors
C141, C142, C143, C144, C902,
C124, C126.

Diodes
DI51, DI52.

Transistors
X63,X64,X65,X66
X101, X102, X103, X104.

The tester shows that they are fine.
The UPC4557 resistance on the left and right outputs is not the same
legs 2 - 3 180 kOhms
legs 5 - 6 2.4 Mega Ohms!!!

[budwich]

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

I did it at the beginning, both VU meters are ok, the resistance result is identical and the tips stand up under the ohmmeter. Note that the resistance at the first output of the upc 4557C opamp is several mega ohms. Does this indicate its damage?

what does this mean? your previous posted impedance measurements of the outputs showed "similar" readings for either output. Hence, the question. Please explain.

How did you check the output capacitors from the upc4557... maybe the capacitors are having a high / higher esr than is acceptable.

[redwire]

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

See what signal you get out of the headphone jack because that is also used to drive the VU meters. It is the op-amp output.
I had one of these JVC decks, built so good all metal buttons.

[CapLeaker]

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

the circuit between the VU meter and the UPC4557 is pretty simple. Since you got two channels (left and right), compare them to each other with an oscilloscope...
Did you check VR151 if it's open? Check if the audio coming out of both channels are equal in volume. You got to figure out is it only the VU meter (circuit) that isn't working correctly or is one channel low in volume causing the VU meter to read low. Read page 11 and 12.

[UNITRAX]

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

I did it at the beginning, both VU meters are ok, the resistance result is identical and the tips stand up under the ohmmeter. Note that the resistance at the first output of the upc 4557C opamp is several mega ohms. Does this indicate its damage?

[jiroy]

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

JVC KD-A11 Cassette Deck

Both channels play cassettes. The problem is that the left VU indicator does not work.
I desoldered and checked the resistor R143.

Capacitors
C 141, C142, C143, C144, C902,
C124, C126.

Diodes
DI51, DI52.

Transistors
X63,X64,X65,X66
X101, X102, X103, X104.

The tester shows that they are functional

IC901 UPC 4557C
Measured with an ohmmeter
between the pins
4 - 8 charges to 840 Ohms

2 -3 charges to 180 kOhm

5 - 6 charges to 2.4 MOhm

4 - 1 19.2 KOhm
4 - 7 19.2 KOhm

When turning on the device, the indicators should stand up for a moment. The right one does it correctly. The left one gets up, but much less than the right one.
The indicators themselves work fine. The ohmmeter shows the same result in both gauges.

I don't know what to do next.

If I were you , I would check the internal resistance of the VU meter itself ,then try to resolder its pins if on board.Try that first and if it didn't work ,repost .

[UNITRAX]

JVC KD-A11 Cassette Deck

Both channels play cassettes. The problem is that the left VU indicator does not work.
I desoldered and checked the resistor R143.

Capacitors
C 141, C142, C143, C144, C902,
C124, C126.

Diodes
DI51, DI52.

Transistors
X63,X64,X65,X66
X101, X102, X103, X104.

The tester shows that they are functional

IC901 UPC 4557C
Measured with an ohmmeter
between the pins
4 - 8 charges to 840 Ohms

2 -3 charges to 180 kOhm

5 - 6 charges to 2.4 MOhm

4 - 1 19.2 KOhm
4 - 7 19.2 KOhm

When turning on the device, the indicators should stand up for a moment. The right one does it correctly. The left one gets up, but much less than the right one.
The indicators themselves work fine. The ohmmeter shows the same result in both gauges.

I don't know what to do next.