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Dumb questions for smart people
Old 29th May 2020
  #1
Gear Nut
 

Dumb questions for smart people

1. What happens to a preamp, if the FET is not biased correctly? What kind of faults would a person be able to audibly hear?

2. I have an old Mic Preamp with badly torn traces on the PCB. Would it be ok to use thin wires to re-route the trace?

3. If my mic preamps power supply - toroidal transformer goes bad...how screwed am I? I hear they are custom made for each product, so buying off the shelf ones is nearly impossible.
Old 29th May 2020
  #2
Transistor biasing is done for stability at operating point of the circuit. It stops thermal runaway and therefore avoids damaging the transistor.

When you say torn traces do you mean that they are no longer conducting properly? If so rerouting the trace is possibly an option - could be quite tricky though.

Yes if the transformer goes bad it could cause damage to the circuit if it does not have an internal fuse. If it has a fuse it should just trip the fuse.
Old 29th May 2020
  #3
Gear Nut
 

Quote:
Originally Posted by mrfantastic View Post
Transistor biasing is done for stability at operating point of the circuit. It stops thermal runaway and therefore avoids damaging the transistor.

When you say torn traces do you mean that they are no longer conducting properly? If so rerouting the trace is possibly an option - could be quite tricky though.

Yes if the transformer goes bad it could cause damage to the circuit if it does not have an internal fuse. If it has a fuse it should just trip the fuse.
When detaching some capacitors, I ended up pulling up the trace from the pcb. So I thought about getting a wire and soldering one end to the component thats soldered to the PCB, and then soldering the other side to the capacitor. This would completely bypass the trace, but still would connect the two components. My only issue is, what gauge wire should I use? The PCB trace looks pretty thin, and the wire I have in mind is about twice as thick as the PCB trace. Would this end up blowing anything up?
Old 29th May 2020
  #4
Ahh ok - so just a trace repair to 1 part is reasonably standard procedure. Yes if go pin to pin to bypass the damaged trace you're fine. You can get proper trace repair wire - a solid core kind of thing but is nice and thin and is coated so that you don't cause any short circuits if laying across the PCB. You won't blow anything up as long as you avoid shorting anything.
Old 29th May 2020
  #5
Gear Nut
 

Quote:
Originally Posted by mrfantastic View Post
Ahh ok - so just a trace repair to 1 part is reasonably standard procedure. Yes if go pin to pin to bypass the damaged trace you're fine. You can get proper trace repair wire - a solid core kind of thing but is nice and thin and is coated so that you don't cause any short circuits if laying across the PCB. You won't blow anything up as long as you avoid shorting anything.

I found some thin copper wire, but it's not coated. I was thinking of coating it with nail polish afterwards to protect it. So the gauge of the wire being different wouldn't really mess anything up right?
Old 29th May 2020
  #6
I'm not sure about the nail polish coating but as long as you keep it away from any that it could short to even with electrical tape you should be fine. Or get hold of the correct wire to make it the best job. The wire gauge shouldn't really mess anything up.
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