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JVC HRD-750 VCR - early NICAM

 
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Re: JVC HRD-750 VCR - early NICAM

Post by AidanLunn » Mon May 05, 2014 10:24 pm

I keep on blowing the CPs is because I sometimes find it a bit fiddly keeping the pins of my meter still when measuring on the STK or CN pins. I'm usually OK with measuring them, though, seems I had a couple of accidents yesterday, though.

Measuring CN1 pin 1 with my other meter, an Eagle Y122KB, it's also 17.5V.

Minus the post regulator voltages, the results are:
CN1
1) 17.6V (uni-t meter)
4) 17.5V. The input of L21 should measure 16.3V.
5) just 8V.
6) 8.7V.

CN2
2) -0.5V
6) 50V

Will the o/c CPs cause the wildly different results, as the difference is confusing.

EDIT: I'll post the post-regulator voltages when I have the CPs replaced.
Last edited by AidanLunn on Mon May 05, 2014 10:51 pm, edited 1 time in total.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by AidanLunn » Mon May 05, 2014 10:44 pm

D24, C13, C14 and C21 have also been replaced.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Refugee » Mon May 05, 2014 11:02 pm

We need to make sure the battery in the meter is good before we go any further. A new one would be best.
A waveform on the scope will tell us if it is tripping.
A photo of the front of the scope showing the names and numbers on the controls will help us get you a trace of the transformer secondary waveform.
I think it is a bad sense or capacitor or perhaps the opto isolator/regulator if the meter battery is fresh.
Bad capacitors send ripple back to the control chip on the live mains part and make it switch on and off all the time. It is looking for clean DC.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Red_to_Black » Mon May 05, 2014 11:14 pm

AidanLunn wrote:Measuring CN1 pin 1 with my other meter, an Eagle Y122KB, it's also 17.5V.

Minus the post regulator voltages, the results are:
CN1
1) 17.6V (uni-t meter) should be around 16V, so not too bad with O/C CP802

4) 17.5V. The input of L21 should measure 16.3V. as above with O/C CP803


5) just 8V. Something amiss here (see below) needs to be about 17V, this is an input to the STK chip for the sw 12V line
6) 8.7V. looks OK with CP804 O/C should be about 7.4V when loaded properly

CN2
2) -0.5V something amiss here This should be -30V
6) 50V will be OK (should be 45V) because the supply is still not loaded properly


Will the o/c CPs cause the wildly different results, as the difference is confusing. See above in red

One anomaly here is pin 5) of CN1 is connected via R20 (8.2K) to pin 6) of CN2, we have a large volt drop across that resistor, either the resistor is high or the regulator STK chip is pulling it down, you might want to re-check the measurement of that one to see if that is the case.

EDIT: I'll post the post-regulator voltages when I have the CPs replaced.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Red_to_Black » Mon May 05, 2014 11:21 pm

Refugee wrote:We need to make sure the battery in the meter is good before we go any further. A new one would be best.
A waveform on the scope will tell us if it is tripping.
A photo of the front of the scope showing the names and numbers on the controls will help us get you a trace of the transformer secondary waveform.
I think it is a bad sense or capacitor or perhaps the opto isolator/regulator if the meter battery is fresh.
Bad capacitors send ripple back to the control chip on the live mains part and make it switch on and off all the time. It is looking for clean DC.


Ref the diagram is further back in this thread, basic regulation is carried out on the primary side so no opto, the regulation of this part is fairly loose and rudimentary , it uses secondary regulation for the critical rails via an STK chip giving the always 5v and 12V and switched 5V, 12V, and motor 12V rails.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Refugee » Mon May 05, 2014 11:51 pm

That is like the basic circuit that is offered on the better data sheets for the chips.
I was expecting a VCR to have a bit more stuff live side of the power supply.
There is me being more used to working without manuals with the chip data on a screen close by :ccg

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Red_to_Black » Mon May 05, 2014 11:54 pm

No, this one is relatively simple, it is a quite nice JVC design unlike the horrid Thomson FV30, 31 and 32 machines of that era, when I get my scanner working again I will e-mail you a better scan. :thumb

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Refugee » Tue May 06, 2014 12:20 am

Don't make too much fuss about scanning as I now have given the early circuit a better look and confirmed that it is like a basic mobile phone charger on the live mains side.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Red_to_Black » Tue May 06, 2014 12:22 am

Already sent. :cca

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Refugee » Tue May 06, 2014 1:09 am

The file in in and ready for study in the morning.
I am off to do another thread now then bed.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by AidanLunn » Wed May 21, 2014 12:44 pm

Apologies for the delay

The new CP fuses have arrived but I'm postponing this until there is a "gap" in the repair of my 485U e.g. if awaiting parts for that etc.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Refugee » Thu Aug 21, 2014 1:00 am

Adrian has come round with this VCR.
The CPs have been fitted on the track side for the time being as not all are the correct value.
There was another one down by the legs of the STK multi regulator that also was blown.
The 30V problem was the 82 ohm fusible resistor on the power supply.
The VFD heater was cold checked by measuring 5 ohms across the 10 ohm fusible resistor and then the correct 10 ohms with the plug disconnected from the power supply.
Do not bother measuring the 3.9V AC with any meter. The DVM read over 20V :aah
An AVO 9 measured 1.2 volts due to the inductance on the internal components.
Don't bother :aao

Adrian had the diagram with him and a look at that took me to the timer board.
Q1 was missing all but the ends of its leads :cch
There was also another CP there but it was OK.
I had a quick look in my stock but did not have a 2SD whatever it was but I checked the data on the web and it said VCE 100V.
I have got plenty of BF469s so I put one in and powered it up and it looked good.
To check the stereo sound I had to run a long audio cable across the workshop to my valve amps and we got good cinema quality sound :aad
It took about an hour to fix it.

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