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Slow Loris Bias off I think

Started by Craig Beauchamp, February 11, 2012, 12:28:31 AM

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Craig Beauchamp

My Slow Loris I built is not biased (I think) properly and I'm not sure what to change. When I hit a chord hard I get sound that doesn't sound great. If I push a pedal in front of it turned up it works great until it decays then gets weird again. I built two at the same time and they both do the same thing.
Here the voltages
308 IC
Pin1: 8.71V
Pin2: 8.74V
Pin3: 8.45V
Pin4: 0
Pin5: 4.2mV
Pin6: 8.83V
Pin7: 9.43V
Pin8: .1mV
2N5457
Drain: 9.43
Source: 1.173
Gate: Never stabalized and I got tired of waiting for it to stop slowly dropping.

madbean

Voltages are completely wrong on the IC. You should have 9v on pin7 and about 4.5 on pin3.

Check that you have used 100k for both R10 and R11. The junction of those two resistors forms a voltage divider which should result in about half the supply voltage, or around 4.5v.

Craig Beauchamp

They are 100k. Brown/Black/Yellow.

mgwhit

Make sure you didn't accidentally swap R11 (100k) and R3 (1M), which are right next to each other on the board.  The value you're getting on pin 3 is very close to what I would expect to see if you had 1M in R11.

Craig Beauchamp

That's not it either. I just checked. In fact, I had socketed R3 in one of them and toyed with it prior to asking for help. Currently R3 is 1M and R11 is 100k, but no love. What would I change in the power section to drop the votage of VC? It's high on Q1 as well as the IC (right?).

Craig Beauchamp


maysink

Need some pics. Both sides of the board.
[nothing to see here]
-e

Craig Beauchamp

#7
trying to shrink the photo file size to 1000kb. Do you want me to pull the pots off the backside for that picture?

Craig Beauchamp

#8
Here's one side. I'll get the other as soon as I can get the pots off.
Mods are C8 now two .0022 in parallel
C12 now 1uf (didn't change this on the other board and it has the same problem)
multiple diode changes (they worked on a RAT I modded)
used UA308HC (small bear said it worked the same as LM308)

madbean

Can't really tell: which way is the tab on your metal can IC pointing? Tab should be pin8, IIRC.

madbean

Quote from: Craig Beauchamp on February 11, 2012, 01:59:56 AM
That's not it either. I just checked. In fact, I had socketed R3 in one of them and toyed with it prior to asking for help. Currently R3 is 1M and R11 is 100k, but no love. What would I change in the power section to drop the votage of VC? It's high on Q1 as well as the IC (right?).

Supply is correct on the IC and transistor. It's definitely the bias supply causing the problem. We just don't know yet what's causing the bias supply to be so high.

Craig Beauchamp


mgwhit

Are you sure your R10 is 100K?  Doesn't look Brown-Black-Yellow to me.  I can't see your R11, but I'll take your word for it.

Craig Beauchamp

Yeah it looks like brown-black-gold-gold. Which doesn't exist that I know of. (My other yellows look yellow, not gold) I'll swap it for another one that I'll check with my meter when my kid takes a nap just in case.

Craig Beauchamp

That was it! Good eye and thanks so much for taking a look and helping. I guess there is such a thing as brown-black-gold-gold. No idea what it means though. Do you know? I put it on the meter and it wobbled bettween 1.3 and 1.5 ohms.
Anyways, thanks everyone who took the time to look and offer suggestions.