Moving Coil Cartridge - Output Problems... - diyAudio
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Old 15th March 2012, 07:49 PM   #1
Ron AKA is offline Ron AKA  Canada
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Default Moving Coil Cartridge - Output Problems...

I pulled a really dumb mistake. I had my moving coil head amplifier out and when I put it back I connected it up backwards. So the output of the low level battery operated Marcof head amp was connected to the moving coil cartridge, and the input of the head amp was connected to the phono cartridge input of my Hafler pre amp. There was a bit of a pop when I turned it on and no output of course.

I then connected it up correctly, and have what appears to be normal output from the right channel, and zero output from the left channel. I then switched the left and right input connections to the head amp. With them switched I get full output from the left channel, and what seems to be almost full output from the right channel.

Makes no sense to me. Any ideas what could be going on?
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Old 16th March 2012, 05:58 PM   #2
Ron AKA is offline Ron AKA  Canada
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I have done a bit more definitive testing. I unplugged the left input cable to the head amp. With the right input only I get full volume from the right channel, and zero volume from the left. When I plug the right channel only into the left head amp channel I get full volume out of the left and nothing from the right. Concluded the right channel of the cartridge and the head amp and my phone stage in the preamp are good. Also the left channel of the head amp and preamp also seem good.

Plugged the left channel only from the cartridge into the left channel of the head amp. I initially got nearly full volume from the left channel but it seemed to slowly drop down to 50%. I then shut of the head amp and volume went to zero. I switched the head amp back on and got nothing from the left or right channel. Thought there may be a battery problem in the head amp so I changed them. Tried same thing again and got nothing out of the left channel whether it was plugged into the left or right channel of the head amp.

I'm thinking I have a short in the windings of the left channel of the cartridge -- fried. Kind of sad as this is a recently rebuilt Fidelity Research cartridge FR1-MK3F. It has silver windings, and now has a ruby cantilever.

Your thoughts?
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Old 16th March 2012, 06:08 PM   #3
SY is offline SY  United States
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Before proceeding, make sure that the head amp isn't putting out DC from its inputs (making a bad situation worse). Put 10R resistors across the input and measure the voltage across them. It should be zero. If it isn't, don't plug any cartridges into it, even for diagnostics, before it's repaired.
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Old 16th March 2012, 11:13 PM   #4
Ron AKA is offline Ron AKA  Canada
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Here is a circuit drawing for the Marcof head amp. I would have connected the cartridge to the output shown on the right ! My cartridge specs say it has a load impedance of 10 ohms. Would it be safe to connect a digital MM to measure the winding resistance?

Click the image to open in full size.
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Old 17th March 2012, 10:23 AM   #5
SY is offline SY  United States
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Well, it has a capactively coupled output, so you got that goin' for you. I don't *think* there's a problem using a DMM to check coil DCR (that's different than load resistance) and make sure it's the same for both channels, but if you want to be extra paranoid, see if you can beg/borrow a preamp from someone to substitute for yours for a trial, and plug the cartridge into it. And I mean the whole preamp, not just a head-amp- it's always possible that your problem is the preamp.
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Old 17th March 2012, 04:47 PM   #6
Ron AKA is offline Ron AKA  Canada
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Quote:
Originally Posted by SY View Post
Well, it has a capactively coupled output, so you got that goin' for you. I don't *think* there's a problem using a DMM to check coil DCR (that's different than load resistance) and make sure it's the same for both channels, but if you want to be extra paranoid, see if you can beg/borrow a preamp from someone to substitute for yours for a trial, and plug the cartridge into it. And I mean the whole preamp, not just a head-amp- it's always possible that your problem is the preamp.
Since the right channel from the cartridge works normally when I plug it into the right channel of the pre preamp or the left channel of the pre preamp, doesn't that validate that both channels of the pre preamp and the rest of the amp system are good?

My only issue is when I plug the left channel of the cartridge. It now doesn't work when I plug it into either the left or right channel of the pre preamp. Unless I somehow got a coincidental bad connection in the turntable and tonearm wiring it seems to me the only location left for the problem is the left channel of the cartridge.
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Old 17th March 2012, 05:33 PM   #7
SY is offline SY  United States
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Don't discount that possibility! I had symptoms like that once and it turned out that the little plug deal at the bottom of the tonearm had wiggled loose. The fact that the problem progressed is interesting- if you fry coils, it's dead from the get-go.
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Old 17th March 2012, 07:15 PM   #8
Ron AKA is offline Ron AKA  Canada
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OK some good news and bad news. The good news is that I think my cartridge may be OK. I pulled the pre preamp out and connected all the cables where I can see and work with some room, and one at a time wiggled them. I'm now convinced I have a bad RCA phono plug on the left channel cable from the cartridge.

The bad news is that it appears to be a special cable for the Grace G-707 tonearm. I belive Grace is out of business, but non OEM replacements are available on the internet, but for prices ranging from $120 to double that. I'm thinking I will just buy a couple of gold plated RCA plugs (originals were not) and replace them. Should work I think?? Here is what the tonearm end plug looks like - the beige one. It also says Grace on it.

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