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.
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.
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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