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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 2nd Nov 2016, 7:56 pm   #1
Vauxfan2k
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Default Aftermarket mod/repair on VCR main board.

Hey guys.

Got another fergie for the collection and whilst removing the bottom plate to inspect the belts I spotted this mod/repair (not the neatest way of insulating it).

Can anyone tell me what it is and it's function?

Thanks

Steve
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Old 2nd Nov 2016, 8:49 pm   #2
dazzlevision
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Default Re: Aftermarket mod/repair on vcr main board

It would be helpful if you gave us the model number of the VCR.
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Old 2nd Nov 2016, 9:38 pm   #3
Vauxfan2k
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Default Re: Aftermarket mod/repair on vcr main board

It's a 3v35, my second one as it's in better cosmetic condition than my other one. Not quite the 3v36 I was looking for tho haha.
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Old 2nd Nov 2016, 11:07 pm   #4
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Default Re: Aftermarket mod/repair on vcr main board

Well, it looks like a 5600 pF ± 2.5% capacitor, crudely wired across between the base and emitter of Q8. That would tend to slow it down turning on and off. I don't have a wiring diagram, though; so I can't say what Q8 does, or why it might need a low-pass filter on its base.

EDIT: Tanya came up with the goods Q8 is the -30V regulator transistor. The capacitor would reduce noise on the output. It could be a bodge to cover for a failing Zener diode D12.
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Old 3rd Nov 2016, 7:26 pm   #5
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Default Re: Aftermarket mod/repair on vcr main board

Quote:
Originally Posted by julie_m View Post
It could be a bodge to cover for a failing Zener diode D12.
Why not just change the zener diode?
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Old 3rd Nov 2016, 8:01 pm   #6
julie_m
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Default Re: Aftermarket mod/repair on VCR main board.

That is something probably only the phantom bodger knows for sure
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Old 3rd Nov 2016, 8:59 pm   #7
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Default Re: Aftermarket mod/repair on VCR main board.

Where is the zener diode located? Il have a go at replacing it as I have a machine for spares that I can rob it off.

Thanks
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Old 3rd Nov 2016, 9:25 pm   #8
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Default Re: Aftermarket mod/repair on VCR main board.

First of all, search on http://www.elektrotanya.com/ for "HR-D120" which is the equivalent JVC-badged model. Check that this matches your machine and the Q8 on your board is indeed the -30 V regulator transistor. If so, the Zener diode D12 will be electrically connected to the base of Q8. The part number suggests it has a PIV of 30 V, which would actually give -29.3 V but this probably is close enough to call a nominal -30.
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Old 3rd Nov 2016, 10:12 pm   #9
Vauxfan2k
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Default Re: Aftermarket mod/repair on VCR main board.

I have the service manual (genuine booklets) for this and the 3v36 so will dig them out and check. It would probably be fine crudely fixed as it is, but I'd rather do it properly.

Thanks for your replies
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Old 3rd Nov 2016, 10:20 pm   #10
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Default Re: Aftermarket mod/repair on VCR main board.

Fixing it "really properly" would entail applying an oscilloscope to the -30 V rail (or whatever that transistor is connected to, if I have made some terrible mistake and it isn't Q8 in the PSU) and observing what happens with and without the extra capacitor in circuit.
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Old 3rd Nov 2016, 10:24 pm   #11
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Default Re: Aftermarket mod/repair on VCR main board.

Just thought, I could just swap out the whole main board from the donor machine, certainly less hassle from an electronic point of view. Can't remember if all the wiring is plug and play though. I usually just open the board on its hinges to work on them not fully remove the boards.
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