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Old 26th Sep 2020, 3:17 pm   #1
dj_fivos_sak
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Default Head drum/head amp interference on Sanyo Beta VCR

Hi all. I am having a very annoying issue with my Sanyo VTC 5350 Beta machine. So as you probably already know, most of these VCRs usually have a metal RF shield that covers the heads and the head pre-amp circuit to prevent interference from magnetic fields and radio interference. My problem is that without the RF shield, the machine plays back with a great picture, but when the shield is in place, I get lots of intereference after a while. If I tap/slap the machine on the area above the shield, the picture returns to normal temporarily. What could be the cause of this? I can see that the double-insulated wires that connect the rotary transformer to the head pre-amp are touching the RF shield, but this appears to be as designed from the factory. The shield doesn't appear to be damaged in any way, as you can see from the pictures. Thanks in advance.

Fivos
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Old 26th Sep 2020, 3:28 pm   #2
Andrew2
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Default Re: Head drum/head amp interference on Sanyo Beta VCR

There's a lot of gain and limiting in these head amplifiers, and any stray coupling between input and output can cause oscillation. Screens can cause this unwanted coupling if they are not properly connected to the PCB and chassis at the right points.
Make sure any earthing springs are clean and that they firmly contact the places they are intended to. The same applies to all places where the screen is screwed down.
It's also a good idea to give the plugs a good wiggle to clear any oxidised connections.
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Old 26th Sep 2020, 5:34 pm   #3
dj_fivos_sak
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Default Re: Head drum/head amp interference on Sanyo Beta VCR

Hi and thanks for your reply. Indeed it seemed like an earthing issue to me so I decided to put this jumper wire (see attachment) in order to (hopefully) create a better ground connection between the head drum and the pre-amp. After doing so, the VCR played back a few tapes for a couple of hours without any interference in the picture so I believe I have solved the problem now. I also read this interesting advice in the technical section for the VTC 5000 on palsite.com (see second attachment).
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Old 26th Sep 2020, 5:44 pm   #4
G6ONEDave
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Default Re: Head drum/head amp interference on Sanyo Beta VCR

Can't remember if this is applicable to your model but there might be a pickup brush on the bottom end of the spinning drum shaft, which might not be making proper contact. If there is one it's accessed from beneath the deck.
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Old 26th Sep 2020, 6:13 pm   #5
dj_fivos_sak
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Default Re: Head drum/head amp interference on Sanyo Beta VCR

Dave. There is no pickup brush in this series of Sanyo decks, as far as I can see.

Fivos
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Old 27th Sep 2020, 11:04 am   #6
G6ONEDave
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Default Re: Head drum/head amp interference on Sanyo Beta VCR

I should have looked at the 2nd picture more carefully. The centre spindle earth pick up is rivoted to the inside of the cover. I would try pulling it slightly so as to increase the pressure contact with the centre spindle and also try a bare metal star washer on one or possibly all of the fixing screws for the cover plate. Ideally try fitting the star washer between the cover and deck chassis. Something else to check would be any capacitors that go between the motor supplies and chassis/earth. I should have mentioned to check that the pick up is not tarnished.
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Old 28th Sep 2020, 6:11 pm   #7
dj_fivos_sak
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Default Re: Head drum/head amp interference on Sanyo Beta VCR

Dave
I did try inscreasing the tension of that rivoted earthing spring but that didn't help at all. I also made sure all screws are as tight as they should be. The extra ground wire I added appeared to be the solution as the problem has gone away now, although there are quite a few caps in the motor drive circuit but I didn't think of testing them and I don't have an ESR meter.

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Old 28th Sep 2020, 8:45 pm   #8
nigelr2000
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Default Re: Head drum/head amp interference on Sanyo Beta VCR

I seem to remember that these had a seperate earth strap on the cover will have to pop the top off mine and check
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