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JVC HRD-750 VCR - early NICAM

 
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JVC HRD-750 VCR - early NICAM

Post by AidanLunn » Thu Apr 17, 2014 1:51 am

After sitting on my "round tu'it" pile for a while, I thought I'd tackle what should be an easy fix whilst parts for my two TV projects come through.

This VCR had no life in any lights on the front panel, bar the VU meters, wouldn't reproduce anything at all from the standard audio stack and there was not a peep of any life on the SCART video input - output is fine. Otherwise, the machine was fine.

On CN2 on the PSU board:
Pin1: ground
Pin 2: -30V DC (measured as 52V)
Pin3: 3.9V AC (measured as 15V)
Pin4: also 3.9V AC (measured as 1.2V)
Pin5: Ground
pin6: 45V (52V)

I think I have routed this all down to diode D24 (o/c), which is type RD8.2EB2, unfortunately there are not any sites on Google that seem to sell these. What substitute would be suitable?

Ferguson version of this VCR is the FV33H (an example of which I also have in my collection).

 
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Re: JVC HRD-750 VCR - early NICAM

Post by malcscott » Thu Apr 17, 2014 10:14 am

8.2v zener ??

 
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Re: JVC HRD-750 VCR - early NICAM

Post by sideband » Thu Apr 17, 2014 10:39 am

Yes probably just a standard 8.2v 400mW zener diode. Probably safer to use a 1 watt type just to be sure.


Rich

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Terrykc » Thu Apr 17, 2014 10:47 am

It is, as Malc suggested, an 8.2V 500mW zener diode - and is obsolete!

Full data here if you want it : http://www.alldatasheet.com/datasheet-p ... .2EB2.html

but any half watt or larger 8.2V zener should do.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by AidanLunn » Thu Apr 17, 2014 12:47 pm

Thank you, I wasn't sure whether it would have to have a partcular diode as it's printed on the (Ferguson) schematic - that suggested to me that only that type or equivalent would be suitable. It's nice to know that the repair should be easier than that.

I can't see how this fault could affect the SCART input and audio stack issues, but I haven't studied the schematic in that much detail yet, all I can gather is that CN2 plugs straight in to the oards relating to the front panel. As the machine obeyed commands perfectly, this suggested the problem was down to a rail or two on CN2, as the power for the operation buttons is mainly derived from CN1 rails, which I have not bothered to test.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Terrykc » Thu Apr 17, 2014 1:06 pm

AidanLunn wrote:Thank you, I wasn't sure whether it would have to have a partcular diode as it's printed on the (Ferguson) schematic - that suggested to me that only that type or equivalent would be suitable ...

Aidan, no different to a transistor marked on a schematic as, say, BC107, for which there are large numbers of alternatives.

You are right to be wary - especially with power devices in Line Output and SMPS circuitry because there may be specific aspects of the device which make it hard to substitute.

But just Googling RD8.2EB2 would have given you several hits pointing to data sheets that would reveal that it is nothing more than a vanilla flavour zener ...

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Cathovisor » Thu Apr 17, 2014 1:24 pm

My first VHS machine. I never thought I'd see it on a vintage radio and TV forum!

 
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Re: JVC HRD-750 VCR - early NICAM

Post by AidanLunn » Thu Apr 17, 2014 2:51 pm

Mine's in pristine cosmetic condition, not a single scuff on the thing. It's even got the warranty card with it :)

I don't know if it's just me, but I also really like the design of this generation of JVC VHS machine, as VHS doesn't usually appeal to me. Very B&O-esque.

I do seem to collect a lot of these JVC decks though, for admiration of just how well built they were!

There was one of this generation of JVC deck that had a sideways loading mechanism, I've been after one of those for yonks.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Red_to_Black » Thu Apr 17, 2014 4:16 pm

On this model C14 1uF @ 50V should also be replaced at the same time, although this is the start up capacitor I have had zeners fail because of this capacitor too, it may be prudent to replace any other Electrolytic capacitors in the PSU as well, IIRC when some other elecs fail the PSU voltages can creep up.

I cannot find the previous scan I had of the FV 33H Psu, I will have another look at the manual later on to double check which one it is, I often mix model numbers and which machine is which these days, I must be going senile. :aaq

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Red_to_Black » Thu Apr 17, 2014 5:59 pm

AidanLunn wrote:After sitting on my "round tu'it" pile for a while, I thought I'd tackle what should be an easy fix whilst parts for my two TV projects come through.

This VCR had no life in any lights on the front panel, bar the VU meters, wouldn't reproduce anything at all from the standard audio stack and there was not a peep of any life on the SCART video input - output is fine. Otherwise, the machine was fine.

On CN2 on the PSU board:
Pin1: ground
Pin 2: -30V DC (measured as 52V)
Pin3: 3.9V AC (measured as 15V)
Pin4: also 3.9V AC (measured as 1.2V)
Pin5: Ground
pin6: 45V (52V)

I think I have routed this all down to diode D24 (o/c), which is type RD8.2EB2, unfortunately there are not any sites on Google that seem to sell these. What substitute would be suitable?

Ferguson version of this VCR is the FV33H (an example of which I also have in my collection).


Right found the scan for the FV33-H. :thumb pins 3 & 4 are not referenced to ground, only to each other, as this supply is generated from a seperate winding, this is the AC supply for the VFD heaters.

I am not really sure why pin 2 is showing as a positive supply, as pin 2 is on the same winding (a centre tapped winding) on the other half of the same supply winding of that feeding pin 6, and fed with the rectifier in reverse. edit: unless D24 was S/C ?, I noted you said it was O/C ? :aab
Attachments
FV33H Psu.png

 
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Re: JVC HRD-750 VCR - early NICAM

Post by AidanLunn » Fri Apr 18, 2014 11:16 pm

I've replaced the zener, I haven't tested it tonight yet as it's getting a bit late.

Also, while doing routine checks, I did notice that C13 had sunk to just 45uF! Replaced straight away.

One of the tags to D24 was burnt off whilst desoldering the original, so I practised for a few hours scraping down the trace to copper with a Stanley knife and then laying solder on some scrap PCBs from various set top boxes. I'm pleased to say that whilst not looking elegant, the repaired trace seems to work very well indeed.

All of the other caps tested for operability and capacitance, and no others seem to have any issues.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by AidanLunn » Sat Apr 19, 2014 6:20 pm

D24 seemed to be short circuit. My OP had a typo, it was -52V. Replacing this fixed that back to -30V. Both 3.9V AC rails are still unequal, so that narrows it down to R22, a fusible resistor, or the couplers C29 and C30, which both read at 0.002uF.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Red_to_Black » Sat Apr 19, 2014 6:28 pm

AidanLunn wrote:D24 seemed to be short circuit. My OP had a typo, it was -52V. Replacing this fixed that back to -30V. Both 3.9V AC rails are still unequal, so that narrows it down to R22, a fusible resistor, or the couplers C29 and C30, which both read at 0.002uF.


Measure across just the two AC points 3 & 4 (meter set on AC volts range) not to ground, they are not really referenced to chassis, they are an AC supply in their own right, measuring either one to chassis ground proves nothing, one side could have a lower impedance to ground by other components, but this is irrelevant and will only confuse matters.

This AC supply is only for the heaters in the VFD, one side is tied via R21 (3.3K) to other windings, this is probably to stop the nominal AC supply from floating too high wrt to the other windings of the transformer, R21 also doubles as the current limiter for D24 as this is a typical high-ish impedance supply for the -30V rail, this is only used again for the VFD (the display).
The AC supply quoted in the manual is only for across the heaters of the display.
If you try to measure this supply wrt to chassis you will get a difference between pin 3 & 4, this is normal, you will be chasing a non-existent fault.
Last edited by Red_to_Black on Sat Apr 19, 2014 6:49 pm, edited 1 time in total.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by AidanLunn » Sat Apr 19, 2014 6:48 pm

Thank you R2B, measuring the AC rails correctly read out 15V.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Red_to_Black » Sat Apr 19, 2014 6:58 pm

AidanLunn wrote:Thank you R2B, measuring the AC rails correctly read out 15V.

As this is a high frequency chopper supply the AC reading you get depends upon the meter you are using, it will not be an accurate reading with most digi meters.

If your other rails are "nominally" correct then the AC supply will also be correct, don't start chasing ghosts and non-existent faults.

If this heater supply was really 15V the display heaters on the machine would be lighting up like the bars of an electric fire! and/or burn out.

if the display is working normally then don't worry about that supply, it is unregulated and relies on the windings ratio of the chopper transformer to keep it in step loosely with the other supply lines, it will most likely be correct all other things being equal.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by AidanLunn » Sat Apr 19, 2014 7:42 pm

I have checked the rails on the regulator output, all were OK, however I did blow fuses CP803 and 804, still no change just before these were blown. This is after testing CN2 rails. Thanks for that, R2B. Shows I'll need an analogue meter!

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Red_to_Black » Sat Apr 19, 2014 8:07 pm

Hi Aidan,
I would not bother too much about trying to measure that supply, the information presented in the manual is only for information purposes :cch , you would rarely if ever have to actually measure that supply, and IIRC you end up with some bizarre readings if you try, even on earlier conventional mains transformer models you got some odd readings due to the way it is configured.

The only time you might have to investigate that supply would be if you had a machine that otherwise worked perfectly but had no electronic display, even then most problems were mainly around the minus 30V supply.

You will only end up confused with odd readings on that part of the circuit.

Arguably the DC supplies prior to the regulator should be nominally correct first, provided the DC rails are present and correct then the AC supply will naturally follow suit.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by AidanLunn » Sat Apr 19, 2014 10:16 pm

I've just tested across the 3.9V rails again, double checked that my meter was set to 20V AC, it now measures 5.9V. Considering R2Bs earlier statement, that sounds far more plausible.

It seems as though the fault could be on the switch/display board itself. This, however, wouldn't explain why the only sound it outputs is HiFi mixed down to mono, and no sound from the standard audio head stack, as well as the SCART input and front display problems. But is it worth me totally recapping the PSU anyway, considering the rails?

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Red_to_Black » Sat Apr 19, 2014 10:37 pm

Post up the voltages you have now on CN1, you can miss the AC ones off if you wish as you have confirmed they are present.

I will have a look at the display panel for you, although this will be for the Ferguson FV33-H and so may not match physically exactly, but the basic parts should still be present as that is how that display type has to work.

Also in the new post put up exactly what symptoms you now have, you can also post up any other voltages after the regulator. be aware that some of these are switched.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by AidanLunn » Sat Apr 19, 2014 11:12 pm

Red to black wrote:Post up the voltages you have now on CN1, you can miss the AC ones off if you wish as you have confirmed they are present.

I will have a look at the display panel for you, although this will be for the Ferguson FV33-H and so may not match physically exactly, but the basic parts should still be present as that is how that display type has to work.

Also in the new post put up exactly what symptoms you now have, you can also post up any other voltages after the regulator. be aware that some of these are switched.


Well I need to replace those fuses on the regulator so what they will measure now is irrelevant, but just before I did accidentally short circuit pins 1+2 and 2+3 on CN803 (regulator), they measured OK, as did pins 1+2 on CN804 (same board). All within +/- 2V of the value printed on the diagram. I haven't got the exact measurements to hand and I've retired for the night.

Now all of the supplies on CN2 on the PSU seem to be OK. I've still got something telling me to replace C29 and C30 as they're both double their printed values, but as they're such low voltage my mind is telling me it's extremely unlikely they're faulty.

Symptoms (pre-fuses blown):
1) No display on the LCD panel, no LEDs lit on same panel, bar the VU meters, which flash for a second when the machine is switched on from standby.
2) No video input on SCART, output is OK
3) No audio from normal audio head stack
4) Only audio from HiFi soundtrack, downmixed to mono
5) Machine seems to obey all commands from both front panel and remote except the button to switch between mono/HiFi/mixed audio.

Seems a bit of a brain-buster!

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Red_to_Black » Sat Apr 19, 2014 11:36 pm

Symptoms (pre-fuses blown):
No display on the LCD panel, no LEDs lit on same panel, bar the VU meters, which flash for a second when the machine is switched on from standby.
No video input on SCART, output is OK
No audio from normal audio head stack
Only audio from HiFi soundtrack, downmixed to mono
Machine seems to obey all commands from both front panel and remote except the button to switch between mono/HiFi/mixed audio.

Seems a bit of a brain-buster!


Not really, with supplies being missing you will have all manner of symptoms, depending which or which combination is missing what symptoms you actually end up with is anybody's guess.

Forget about replacing C29 and C30, these capacitors are not prone to failure in that position, and won't have any bearing on the symptoms you presently have.

The next step is to get the fuses or is that CPs ? replaced on the regulator panel before you can go any further, doing anything else at this point will be counter productive.
You may, or may not have already fixed the original fault, and/or added new ones :qq1 , until you have all the supplies present and correct after the regulator we are just guessing.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by AidanLunn » Sun Apr 20, 2014 3:20 am

That reminds me, where can I get replacement "CP" fuses for these JVC decks? A search of "JVC CP fuse" on Google yields nothing.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Refugee » Sun Apr 20, 2014 3:41 am

I have not seen the fuses in your set.
In the past I have replaced rectangular soldered in Japanese fuses with the cylindrical ones sold be Farnell/CPC and RS without issue. It is just type and rating and most importantly physical size.

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Red_to_Black » Sun Apr 20, 2014 11:25 am

Hi Aidan, Ref,

Fitting glass fuses may be a bit problematic due to layout of the PCB and physical size/space.

The ICP's (integrated circuit protectors) in this machine were the N-Type, this refers to the case packaging style, and is about the size of a TO-92 transistor.

To workout the current rating you multiply the N- number by 40mA, eg. an N-10 would be 400mA, an N-50 would be a 2000mA or 2A device.

There is also an F-type, which is electrically the same as the N-type, this also has the same current rating/numbering style as the N-type as above but in a different case style.

As an aside, when I was repairing these machines I only stocked/ordered the N-types, the reason being. this type would physically fit in place of either device, an F-type won't always fit in place of an N-type due to the length of the legs.

PS. the pictures below, the F-type is actually about the same physical size as the N-type, if not a bit smaller, the scaling of the pictures is deceiving. :)
Obviously these two pictured devices have different current ratings, N-type being 2A, and the F-type 600mA. :bba
Attachments
ICP N type.jpg
ICP N type.jpg (8.26 KiB) Viewed 536 times
ICP F type.jpg
ICP F type.jpg (8.14 KiB) Viewed 536 times

 
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Re: JVC HRD-750 VCR - early NICAM

Post by Michael Watterson » Sun Apr 20, 2014 12:14 pm

Picofuses likely fit. FEC & RS used to do them. They look like 1/8th W resistors. Bend one lead over ... ?

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