fault finding on my ortofon mca 10

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cressy
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#1 fault finding on my ortofon mca 10

Post by cressy »

Hi guys, i bought an mc 10 cart that came with an mca 10 mc head amp. After playing for about an hour the music faded out down to the volume of an mc cart playing through an mm phono stage. The cart is fine and abit of testing confirmed that the head amp has failed. In the instructions is mentions that when the unit is switched off that it becomes straight through so that an mm cart can be used without having to unpug it. Presumably so users with arms with detachable headshells can swap between carts.
Switching the unit on, the battery meter shows full charge but there is no gain. Switching off there is a switching pop through the speakers but no change at all in the operation.
I have had a look inside and there is no evidence of a failed component. The cap bank shows no evidence of leakage.
There are 4 transistors in the in the unit 2 marked bc 485 and 2 marked bc486.
The switching pop leads me to think that the switch is ok, but the fact that both channels went down at the same rate at the same time to me means that the issue is connected to the psu rather than an individual component.
Plus the fact that something still comes out of it when its off suggests that the circuit is being bypassed even though it is on.

Any ideas?
Cheers ant
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cressy
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#2

Post by cressy »

Found another transistor in there in the psu, i'll see if there is a voltage after it....
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Dave the bass
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#3

Post by Dave the bass »

Is the switching btween 'amplified' and 'bypass' manually switched Ant? If it fades it could be a switch problem. I used to get that on some 70's/80's selector switches that had started corroding or got durrrty.

Or, resistor going high in value after the unit has been on for a while possblee?

DTB
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cressy
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#4 Re: fault finding on my ortofon mca 10

Post by cressy »

I had forgotten about the mca 10, found it while cleaning out a cupboard. So i decided to have a poke around with a meter. Found out what was wrong with it, quite how i missed it before i have no idea.
One of the psu caps in the bank of 6 was passing dc so was buggered, rather than replacing 1 i replaced them all and it now works perfectly.
It was a face-palm moment when i figured out what was wrong..........
I think i'll also dig out the nuvistor phono stage too and have a play.
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