BOZ stops working when adding pot-ground - diyAudio
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Old 30th May 2007, 06:02 PM   #1
kmj is offline kmj  Sweden
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Default BOZ stops working when adding pot-ground

A stange thing has happend to my "simple-no-trouble-passtime-pre". I needed a compact preamplifier and my choice landed on a BOZ driven by two 2x12V 3,7VA PCBtransformers (yes, it's enough) and a friend of mine made me a copy of his PCB using this setup.
Aside from using bigger capacitors I used the same setup as him but adding three pairs of inputs and two pairs of outputs and this far everything was alright. When every component was soldered in place I/we hooked it up to a scope but nothing went through so we started to look for the fault. We didn't find it but only the symptoms... If we look at the original schematic, Mr Pass has noted som measuringpoints and I got 60V all the way to the component designated P102. This I suppose indicates that the transistor isn't working as it should

Anyway, I measured the resistance across R103&R105 and got approx 10K (should be 110K) and next to my pre the original my friend made was and it measured 110K at the same place.
I removed input, output and potentiometer (2x10K on the input) and hey, it measured correct! I reconnected the signalwires and it still works and lastly, the groundwires from the pot. Now it didn't work again so I found the source of the problem but how do I fix it? Each of the pots two groundleads only disables it's own channel.
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Old 31st May 2007, 02:23 PM   #2
kmj is offline kmj  Sweden
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No one that knows what it could be due to or am I to confusing in my description?

I only have the layout of the coppertraces but no silkscreen or similar so theres no pictures that can be of use.
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Old 31st May 2007, 03:51 PM   #3
steenoe is offline steenoe  Denmark
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Sounds like you have the wiper of the pot mixed up with the gnd or something.
Check out the pot with your DMM and see if it functions as you suppose.

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Old 31st May 2007, 03:53 PM   #4
kmj is offline kmj  Sweden
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well, that was my first thought as well but the fhing is that the board is made with this particular pot in mind and the ofter example of the preamp has it mounted the same way. Strange...
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Old 31st May 2007, 04:18 PM   #5
steenoe is offline steenoe  Denmark
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I still think you should check out the pot. In and ground are static and the output will vary with the setting of the pot. The wiper is output.

Steen
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Old 31st May 2007, 04:32 PM   #6
kmj is offline kmj  Sweden
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checked, it is an alps 549G (the one from ELFA) on both and they are placed with groundpins at the same place. The output (middlepin) is also placed at the same holes and the only difference is that I use extensionwires (no short) instead of mounting it directly on the board. I have also placed my inputcaps BEFORE the pot instead of after.
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Old 1st June 2007, 01:14 AM   #7
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Quote:
Originally posted by kmj


I have also placed my inputcaps BEFORE the pot instead of after.


NOT ACCEPTABLE!

Behind the input cap, there should be no pot. Otherwise, the pot works as an additional DC divider (when the pot's ground is connected), reducing the mosfet gate voltage and so turning off the gain mosfet.



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Old 1st June 2007, 06:08 AM   #8
kmj is offline kmj  Sweden
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Quote:
NOT ACCEPTABLE!

Behind the input cap, there should be no pot. Otherwise, the pot works as an additional DC divider (when the pot's ground is connected), reducing the mosfet gate voltage and so turning off the gain mosfet.
CRAP! I thought that it would be better to have them there to prevent possible DC from reching the pot, I'll switch the wires immediately
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Old 1st June 2007, 06:44 AM   #9
kmj is offline kmj  Sweden
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I changed the wires and now I get the correct values, I'll try it out in an hour or so and get back to you
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Old 1st June 2007, 08:12 AM   #10
kmj is offline kmj  Sweden
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Well, I get music out of one of the two channels but there´s heavy static on it, but the main problem is solved.

Thanks!
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