Acetylene Overdrive. Updated with bias issue

[Prudhomb]

I just ordered this. Is there a particular voltage I am trying to bias to and if so is that on Q5? if so what pin?
I see on the schematic 4.2-5v labled, 4v measured. Is 4v the goal?

thanks

 

[PapaBear]

Looks to me like he's saying they're labeled to bias at 4.2-5.2, but when he traced it measured 4.0, I would start at 4.0 and tweak it up using your ears from there

 

[Robert]

Yep, Barry is correct.

The text actually should read "4.5 - 5V" and "4.5 - 5.2V" respectively.

The original used fixed resistors, not trim pots, but the actual voltage measured on the Collector of Q1 and Drain of Q5 was 4V. The measurements were taken while powered on a 9V supply.

 

[PapaBear]

Yep, Barry is correct.

The text actually should read "4.5 - 5V" and "4.5 - 5.2V" respectively.

The original used fixed resistors, not trim pots, but the actual voltage measured on the Collector of Q1 and Drain of Q5 was 4V. The measurements were taken while powered on a 9V supply.

So, on Q1 there is no trimmer, best to socket that R6 5K6 resistor or use a trimmit?

 

[Robert]

You certainly could but I'm not sure that it's necessary. I just installed 5K6 like the original in that location.

 

[PapaBear]

I just ordered this. Is there a particular voltage I am trying to bias to and if so is that on Q5? if so what pin?
I see on the schematic 4.2-5v labled, 4v measured. Is 4v the goal?

thanks

Where to measure depends on if using through hole or SMD, through hole it's the left leg facing component side, SMD it's the top pad on the side with 2, best I can tell

 

[Robert]

You can measure on the far left leg/pad of Q5 either way, the SMD / through-hole patterns are connected regardless of which one is used.

 

[Robert]

This is irrelevant but I found it entertaining...

Out of curiosity I referenced this pic of the original PCB (from a demo video) just to see if a different value was used for R6...
It appears random resistor values were installed throughout the entire PCB just for the photo.

Touché.

 

[Prudhomb]

thanks for the replies, I should be able to bias this with my new pocket oscilloscope I just got! Just need to follow your directions for the correct pin out. All my stuff is ordered and on the way, I went ahead and sourced a 113, though I saw in another thread a 112 will work. Unfortunately Tyada did not carry the the 113 or the 2n5089. Found them on antique electronic supply, with a reasonable shipping rate. Im sure mouser has them, but there flat shipping of $10 is a killer if you only need one part. Thanks for soldering those j201's so I dont have to search all over for them!

 

[Prudhomb]

lol on the random resistors

 

[Prudhomb]

Oh and I grabbed a couple of OPA2134 to upgrade the op amp as well

 

[Prudhomb]

Trim pot is super touchy on the sweep. Measuring at Q5. Barely moving it at about the 2/4 position goes from 1v to 8.

Then if I get around 5 it holds for a 10-15 seconds and drops to 1-2v. I got it to hold at about 5.6 for a few minutes but a gental shake of the board it dropped to 1v.

The pictures attached it dropped from 5 to 1 didn’t touch it.

Is this a wonkey trim pot? Is it somewhat temperature sensitive? Or is this normal behavior?

 

[Prudhomb]

Swapped out the opa2134 for a couple 4558d I had laying around. The bias seems to be less Touchy but I am still getting the occasional piping crackle sound

 

[G.G.]

are folks just setting the Input trimmer by ear?

 

[Prudhomb]

I did adjust by ear in addition to the voltage. I’m getting this hissing cracking sound. I’m wondering if I don’t have a bad transistor or bad transistor solder joint. I think I need a day or two away from the circuit to decompress

 

[G.G.]

When I did the first test-fire on my build I was getting a lot of hiss and radio station broadcasts when I touched the gain pot (or possibly the boost pot, can't quite remember). I noticed I hadn't soldered one of the leads to the Boost switch.