Britannia Debug Help

[pryde]

Hello folks.

Built up a fabbed 1776 Britannia pcb and am getting no signal except for some "hiss" that changes when turning all pots.

First oddity is the Q1 trimmer (5k) only adjusts between 8.2 and 8.9v on the test pad with the 2n5457 oriented per the layout. If I rotate Q1 180 degrees the trimmer will adjust to 6v ok BUT the D1 LED lights up (not D2) but still no sound other than some hiss 

I swapped in another 5k trimmer, a couple new LEDs for D1/D2 (5mm diffused red) and several 2n5457s with no change in the behavior.

Note: The other test point voltages are good for the j201's.

Trying to get this part sorted first and see where it takes me.

Any ideas? Thanks.

   

[gtr2]

hmm.. someone else emailed me about a similar problem on Q1's biasing...

I haven't had the issue with the two I built but I'll look into it.

Josh

[pryde]

hmm.. someone else emailed me about a similar problem on Q1's biasing...

I haven't had the issue with the two I built but I'll look into it.

Josh

Hey Josh,

I tried other value trimpots and ended up with a 50k to get the Q1 biasing in the 6v range. A 20k only gets it to 7v at full CCW. A 50k will give enough range.

The circuit is working ok after all this. Strange indeed.

[midwayfair]

hmm.. someone else emailed me about a similar problem on Q1's biasing...

I haven't had the issue with the two I built but I'll look into it.

Josh

Hey Josh,

I tried other value trimpots and ended up with a 50k to get the Q1 biasing in the 6v range. A 20k only gets it to 7v at full CCW. A 50k will give enough range.

The circuit is working ok after all this. Strange indeed.

That's really, really strange. You might want to make sure you didn't accidentally use a 3.9K or something for R3. A 5K pot should be overkill even for out-of-spec 2N5457s. I guess all that really and truly matters is that your gate is still at ground and you're getting sound, though.

[pryde]

Thanks Jon,

I did check R3 early on as I suspected that but it was in fact 390R.

I installed at least 10 different transistors with no difference in biasing using a 5k trimmer.

I am not sure why this was/is happening 

[miter53]

Don't know if this is relevant in your case, but I just wired up my Britannia build and I had the same high voltage at Q1. When I switched to a 5457 that wasn't purchased at Tayda, it was fine.

[gtr2]

I'd have to ask but I'm pretty sure the other individual that was having an issue was using a Tayda 2N5457....  I only have them from stock I borrowed from smallbear...  Hmm...