- This topic has 7 replies, 4 voices, and was last updated 5 months ago by
.
-
Posts
-
Greetings, First, since y’all will ask, attached are photos of my build. 1) Device passes audio. level is roughly unity. Tonal quality is fine but has a slightly filtered treble. Controls do nothing at all. 2) Voltages on Q test points are Q1D=6.9, Q1S=4.4, Q1G=3.2, Q2C=5V, Q2B=1.4V, Q2E=1V, Q3C=5V, Q3B=1V, Q3E= 1.2V. VA is about 7 Volts. VB = 3.6Volts, VC = 5 Volts.IF i remove IC 2, VA increase to 8 volts. 3) looked at and measured values of components where possible. All seemed either “right” or explainable. 4) originally put pots in wrong (DOH!!) clean-up was a mess… as you can see, not my finest hour.
My suspicion is that since U1 LS13700 passes audio, it is OK. U2 TL074 seems to control the “action” of which i have none. When I remove the chip, the voltage rail goes up a healthy bit. This makes me question that neighborhood. Yer thoughts? Wisdom? Rude comments?
Thanks!!
Nobody is rude here or I will let them know to knock it off. 😉
The first thing that catches my eye is the diode which needs to be a germanium type and yours appears to be a silicon diode.
The forward voltage of a 1N34A germanium diode is typically around 0.3 volts. This is lower than the forward voltage of silicon diodes, which is typically around 0.7 volts. And yes, there are many folks selling silicon diodes as 1N34A germanium particularly on eBay.
A 1N34A Germanium should look something like this:
and not like this
If you have extra diodes you can test the forward voltage for yourself which is critical for the SWAH to Wah.
Again it needs to be around 0.3 volts.
Hi Barry, Thanks for the reply. 1) that is the diode D3 that shipped with my kit. I recall that it was set apart for this special task, but I can’t find any evidence of that now. sigh… 2) I just measured it with the “semi” function of my meter; it measured .24. The rest of the diodes measured in the .5-.6 V range. I also measured DCV to ground from each side of the D3.1 side is 1.51 V to ground the other is 1.38 V to ground. difference is .13 volts which makes little sense to me…
Further thoughts? Thanks!
I was not aware it was from a kit so if it was shipped with your kit then I expect it is tested and known for the correct forward voltage.
Did you dial it in properly as it is not the type of pedal that works in every setting. You have to dial it in for your specific guitar. The original this is based on has similar instructions.
I have just now noticed the potentiometer lugs. You should carefully re-flow them so the are filled in, and not open. Have a bit of extra solder handy when re-heating.
Here are two different pot images for example;
While the pots are bent up I would re-flow all solder joints for components just to be sure.
Then I would bend the pots back in place and re-flow the pots from the components side of the PCB.
touched up pot solder joints from both sides of PCB. Examined for other components w/ similar looking joints and touched those up as well. No difference. I tried all sorts of combinations of settings to hear if SOMETHING is happening. I tried various pickups and instrument level settings. no changes. I guess I will unearth my bench and scope and see if I can follow the trail… Thanks.
John your voltages are low and should be closer to 9 and 4.5 for VA and VB. Could be a problem. Looks like R28 is correct so thats not the problem. I’m trying to read the color bands to check the resistors but the photo is a bit washed out. Check C9 and C10. If voltages are good and still no wah you could try lowering R17 to 22K. That is the value of an older doc version for the SWAH. Also there was a note on the doc that it could even be lowered to 10K. Lowering will give more sensitivity. I’m sure you know already that a light touch pick won’t get picked up by the effect. So pick hard to get her set up. I’ve made a couple stubborn ones. Once dialed in though very cool.
What’s the 074 pin voltages with it both in and out of socket
Did you take your transistor voltages with all pots CCW what transistors did you get with the kit
Measure across D3 with no power to the circuit in both directions it should only conduct in the forward direction as you know
Your Q2 and 3 base and emitter voltages are very low compared to those in the build doc so I’d suspect a problem prior to Q2 base which in turn will effect the current going up to pins 1 and 16 of the LM13700 state variable filter the thing that makes it wah explained here by My7of9 (Barry)
https://revolutiondeux.blogspot.com/2012/07/mad-professor-snow-white-auto-wah.html?m=1
As Playsforfun says you can’t see the resistor colour bands clearly so it’s very difficult to be 100% on those so check every resistor value by colour band if you haven’t already to make sure they’re the correct value
- You must be logged in to reply to this topic.