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Old 26th November 2003, 03:59 PM   #1
pete.a is offline pete.a  United Kingdom
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Default Cartridge failiure

Im having a right nightmate at the moment trying to locate a fault. Iv'e built a op-amp phono stage, well a kit anyway thats worked fine for a couple of year's or so i thought!
On a couple of occasions iv'e had one channel of the cartridge go down..ie no sound at all..when iv'e switched on..and Len gregory's MM mkii are expensive to say the least.
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Old 26th November 2003, 04:08 PM   #2
pete.a is offline pete.a  United Kingdom
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Default Cartridge nightmare's

Sorry i forgot this bit...

My pre amp is passive diy home brew, which has a selector switch made up of three wafer's. One for channel and the other operates a row of blue led's, just so i know what's in use ie cd,tuner, phono..etc...
Could it be that DC from the led wafer nearest to the phono input is some how tracking across, and feeding back through the phono stage into the cartridge, bearing in mind that only one channel blows.
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Old 26th November 2003, 05:13 PM   #3
Netlist is offline Netlist  Belgium
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Pete
Your nightmare could be a DC problem. Only accurate measuring can tell. Do you have DC blocking caps at the input?

/Hugo
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Old 27th November 2003, 06:56 PM   #4
pete.a is offline pete.a  United Kingdom
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Default Input caps

Hi Hugo,

Blocking caps on the input to the pre or phono stage ??? Niether would be the answer to that.
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Old 27th November 2003, 07:06 PM   #5
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Posting a small schematic could perhaps clarify things.

/Hugo
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Old 2nd December 2003, 05:53 PM   #6
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In case someone could comment on the original question, here's the schematic of the preamp.

/Hugo
Attached Images
File Type: jpg phonopre.jpg (68.7 KB, 357 views)
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Old 2nd December 2003, 07:12 PM   #7
dhaen is offline dhaen  Europe
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Hmm.... failure of either 15 volt regulator that supplies the OPA137 could cause a DC presence on the input connector.
Much more likely IMO is some intermittent short circuit.
Another thought is intermittent SCR latchup occuring on the OPA137 during power-up. C23 and C24 (10uF) are for ripple rejection, but also cause slow power-up. If they are not the same value, ie one has dried up, it could cause a problem. Also, check to see that the power wiring is identical between channels.
Hugo's idea of input caps sounds good to me, but don't make them too big
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Old 2nd December 2003, 07:32 PM   #8
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Quote:
Originally posted by dhaen

SCR latchup
Pardon my ignorance... What is SCR?

If it is DC that kills your cartridge, the investment of a decent cap
(Even a bunch of caps to compare) will still be cheaper than new cartridges.
Anyway, I would try and measure the DC values at start-up and shutdown with a good DMM.

/Hugo
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Old 2nd December 2003, 07:37 PM   #9
dhaen is offline dhaen  Europe
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SCR= Silicon Controlled Rectifier.
It's a 4 layer device that, once switched on, cannot switch off until the power is removed.
The elements of some integrated circuits can behave in this fashion when reverse biased. this can happen when a supply rail goes missing, or is slow to appear.
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Old 2nd December 2003, 08:00 PM   #10
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Never to old to learn.
Thanks

/Hugo
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