guitarmike2107 wrote:I have seen phenolic boards bubble, but that was after to much heat and solder, I don't think it was actually the board bubbling but the resin.
As far as I know there is nothing in a paper based phenolic that would cause conduction, the last time there were conducting boards was due to them using graphite or similar to make the FR4 boards black. i.e. boards don't become conductive, it would be a manufacturing fault and would likely affect the whole board I would think?
Your top connections look fairly clean/no bubbling
Did you solder the underside wires after soldering the top connections?
Since I supplied the board it better bloody not be conductive!
I soldered the under board connections first, then I soldered the wires onto the board, then I installed the board into the chassis and then I soldered the components onto it.
I didn't have time to check any voltages earlier but I did measure resistance using the 20Meg setting on my meter from the board to ground and couldn't find anything. I'm just clutching at straws with the board conducting idea as there really isn't any reason for this to occur! Components check out ok, wiring checks out ok so what on earth could cause this?!
The only other idea that may help is making that audio probe Martin mentioned. Least then I could find where the audio starts going bad inside the amp. Either that or wait for my scope to come back so I can use the spectrum analyser to see if there is anything unusual. Or take the board out and rewire it.
Ok when I poked my meter around the board, couldn't find anything. I found that when I touch the treble cap with all controls on 0 I measure 1.2mA, then when I raise the treble it goes up to 2.4mA, if I raise the bass it goes up to 14mA. I lifted the treble and bass caps off the board, disconnected the wires from the pots and used jumper leads to connect the pots to the components and it was no different.
I did find that if I touch the now empty turret I can hear the tap come through the speaker, same with the empty turret I left by the V1 bypass cap in case I wanted to go split cathode. Guessing this is just my meter lead acting as an antenna.
That doesn't sound right, tapping an empty turret should not do anything, unless you have a microphonic tube or cap somewhere that is reacting to the vibration..
Just tried it again and its not the noise its the connection as I still hear it when I gentley place the probe on it. I even grounded the other end of the probe and it still made a noise when I touched it. The other turret near V1 only does it when the volume is up.
There is quite a bit of dirt on top of the board on closer inspection but I think its bits of resin off the board itself. Do you think its time to remove the board to see whats going on?
Don't know about anybody else, but I'm running out of ideas. Idle voltages look fine, but something goes awry when a large signal is applied. I think by now you have tried replacing about every capacitor on the board, which would be the most likely place for such a failure to occur. What's left? Wire insulation breakdown?
Well I just thought, I wonder if the entire audio range is affected. I mean I'm not sure the amp sounds great with just the middle up. So maybe I'm just noticing the issue more on the high end.
I've removed the board now. Can't see anything wrong underneath. The only thing I found was around each turret is a wet looking material that I can scrape away with my finger nail and there was a lot around the NFB resistor and HT Dropper resistor turrets.
Its hardened and I'm sure its not flux as its white when I scrape it off. I'm sure flux is sort of yellow. Can't smell it, hayfever sufferer am afraid!
I'm ordering a new board from Mike at Modulus, only this time I'm using the G10 FR4 Board so there is much less chance of me killing it with my soldering iron .
.