General information
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Job Number |
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0304-81 |
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Equipment type |
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VCR |
Made in: |
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Maker/Model/Chassis |
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Panasonic |
NV-370 EN |
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Dates 2003 |
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In |
Repaired |
Out |
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Thu.10.Apr |
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Comments |
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Abandoned for some years. |
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Symptoms observed
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1 |
Every now and then,
Video looses power momentarily with the following symptoms: + Switch Led goes off
and on (blinks) for one, two or up to 6 times + Counter display also
blinks as above. + The capstan (and
hence idler) moves left an right in the initial 4 seconds + Sometimes the VCR
goes dead and it will come alive again after some time?!?! + When dead (no
LED/display) a ticking is heard from one of the motors( I believe the
capstan) + Clock display always
lighted on even when VCR is dead. |
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2 |
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3 |
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Repair notes
Full repair description and observations |
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1 |
I suspected that fault was in Power Supply section. I immediately replaced C1101, C1102, C1103, C1104 in power supply. I also took advice and changed
R1101 with a 0.22Ohm using a 1.0 Ohm and 0.33Ohm (quarter watt) in parallel. This
did not solved the problem Further readings... On Capacitor C1101
(rated 4700uF / 16V) there is 15V On the other 100uF /
63V there is 56-60V |
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2 |
Some more reading
regards this power supply... Here are dc voltages comine
out from the plugs when the VCR was 'dead'. Voltages rounded up and dc. Plug P1001: 15, 0, 24, 60 V Plug P702: 12, 0 V Plug P1002: -23V, -23V, -31V, Plug P7502: -31, -23, -23, 0, 60V -23V are fake voltages
becasue these are actually ac voltages and read 4.2V ac. Schematic says 3.8V
ac so I believe they are fine. However most other d.c Voltages are high, especially the 22-25V on C1102
which should be aroun 18V
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3 |
It is evident that I
have some voltages higher then expected. I changed R1101 and R1102 but with
no luck.
So hopefully it should
be somewhere in Voltage regulation circuit. |
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4 |
There where two main
regulated voltages a 5V (via Q1, 2SD636) and a 12V (via Q2 2SD1275+heatsink).
The problem was that the 12V was not there. The unregulated 20V from supply
where there but the base should have 13.6V dc via R8 (6.8k) but it was 0V. R8
was hot and had a large voltage drop on it. I repalced R8 but still got 0V on
base. Following the circuit I found that the base voltage was controlled by
another transistor Q6004 near microproccessor circuit. The 12V supplied by the
voltage regulator were being shunted to ground by Q6004 because it was biased
as such. The base of Q6004 should be supplied 0V from from microprocessor, so
that Q6004 would act as on 'open switch' and the 12V are not sent to ground.
However there where 4.6V on the base and together with the 4V on the emittor,
the transistor acted as a 'close switch' and the 12V were not present. I checked the voltages
on a good microprocessor of another working NV370, and when off, this pin had
4.6V and when on it became 0V. So why the microprocessor of the faulty VCR is
not being 0V on switch on?? So now the fault was
somewhere with the microprocessor. I checked supply voltage and it was
present (5V pin 64). I suspected that the microprocessor is not working well.
I decided to change the MN15342VGC3 from another NV370, and to my surprise
but also joy, the fault remaianed with the good processor. I placed back the
original processor. By the way, when I placed the suspected processor on the
other good NV370, the VCR operated well - hence confirming that the IC is
fine. Now I suspected that
the IC is not receiving a signal to start it up, and so it remains in standby/inactive
state. Unfortunately the pin responsible for power on//off (hence pin 22) was
5V when OFF and 0V when ON. Thus the IC was getting the start signal. The
problem was getting very frustrating but yet challenging. I checked all the
voltages on the IC and compared them with the good one (from the other good
NV370). I just find very few different voltages. However one of them was
critical. On the faulty VCR, pin 16 was 0V, while on the good VCR pin 16 was
4.5V. Pin 16 is the RESET function of the IC!!!! So the problem is now clear
- the microprosserr is on However I did not want
to stop there and see why pin 16 is receiving 0V. On ots own (removed from
circuit) this pin had 4.5V. First I checked a capacitor across pin 16 and
earth (50V iuF) but was found ok. Without this in circuit the fault was still
there. Then I followed from where pin16 gets the 4.4V (or better to sat from where the reset signal is trigerred by being
0V). This came from a circuit around Q6002 and Q6012 (both 2SD636), (near
audio / V-lock circuit). The didoes, resistors
and transistors in question were all ok. The collector of Q6002 should be
4.5V (which goes to the reset pin) but was always 0V. The emittor was to
earth. Thus this transistor was receiving a sort of signal to reset?!?! I was
getting a bit bored by this fault now, and was going to leave the reset pin
unsoldered (I wonder if would have been any problems if I did so!?) However, there was stll
an electrolytic capacitor, C6029 (10uF, 16V) to check. In the diagram it was
stated to be 33uF, 25V. Anyway I removed it and it looked healthy. I checked
with capacitor meter and found only 6uF instead of 10uF!! I repalced it with
a 10uF 50V and to my hapiness I got 4.4V on the collector Q6012. I soldered
back Reset pin 16 of the processor, and the Video came back to life, fully
operational and never went dead again. Looking at the circuit
diagram I still did not understand what was going on exactly, but probably,
the capacitor was becoming short circuit, and the 5V on one of its side
bypassed to the base of transistor Q6012 and biased it wrongly. |
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Repair summary |
C6029 (10uF, 16V) lower capacitance (6uF)
causing MicroProc. to RESET Mode |
Other measurements and technical comments
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1 |
N/a |
