The component tester I ordered arrived today and I just tested the three transistors. They range from 311 hfe to 429 hfe. From what I understand hfe is how much gain they have. What does the Uf voltage mean? What do I look for when using this machine? Is it more to tell me if a transistor or other component is broken or out of tolerance? Thanks
They're fine looks like the fwd voltage of the base emitter junction so about right for a silicon transistor at around 0.7v
Have you got your transistors pushed as far into the sockets as you can get them they're the same pitch as SIL sockets but defo much deeper I can't find a datasheet for those sockets so don't know if it's a spring type mechanism that locks them in place
From what I could find they're designed to be used with 0.1" headers which would normally lock in place as you push them in and header pins would be a lot thicker
I was really hoping it was the sockets that were the problem but I just removed the plastic socket housing and soldered the transistors directly to the socket pins and get the same result Q1 2.2,1.6,1.03
I even continuity tested the transistor legs and the socket pins/legs and they’re definitely attached well as there’s zero resistance.
Im stumped.. why does attaching the transistors into the circuit pull down the voltage so much.. it’s clearly way lower in Q1 than it should be. I really don’t know what could be the issue. Any other ideas before I toss this pcb out the window?!
Checking further resistance; Q1 collector connects to Q2 base. That’s what it should do. But when I check Q2 collector to Q3 base it shows -11 on the meter. What is negative resistance? So it is connected but there is some resistance? Is that would it should be? It’s also showing -13 and -10 on the Q3 collector and emitter when connected to Q2 collector as well... is that anything suspicious?
Q2 collector and Q3 base don't directly connect there are several components between the two that would affect resistance readings
You can check resistance on your transistors by measuring from a reference point
Without any power to the circuit
For example Q3 emitter you can see it has R14 going to ground so your reference point would be ground to Q3 emitter where you would expect the value of R14 470R the accuracy of your reading would depend on your meters input impedance and whether it would load the circuit down etc
Q1 collector we've already checked referencing from a VREF point where you got just over a 100K what you'd expect so no high resistance affecting it there
All you're basically doing is measuring resistance along a path or trace you can see from the schematic what it should be but as stated there are other considerations all components will have an element of resistance, there may be paralell resistances etc
Series resistance is easy enough you simply add values paralell a little more complex and series and paralell takes a wee bit more maths
With your build all you can do is continue your checks you have made some progress
You may be getting minus meter readings because your leads are simply inverted swap them round and see
I'd personally at this stage continuity check everything around Q1 to make sure there are no bridges and good connections eg on the schematic you can see R5 connects to Q1 base and C1 with the other side going around to R4 check that then on the pcb you can see R5 is next to R2 and R6 where you dont want a connection so check there's no connection there
I'm far from an expert but as I said it does appear once Q1 is active in circuit the voltage drops it may well be something simple that's gone over my head so hopefully someone with a better knowledge than me may instantly know what's going on!
I checked the continuity and resistance of everything this afternoon. I went through each resistor and every one is the correct value/resistance it’s supposed to be, as well as the circuit has continuity where it’s supposed to.
I plugged in the pedal again and jammed a little bit and it does sound pretty good. The volume has to be cranked to reach unity but it does sound pretty good if you find the right gain and eq setting.
I’m sick of looking at this thing at this point and I’m gonna call it done for now unless any new info comes to light like you said. I’ll leave this post up in case anyone else has something to suggest that we both overlooked.
I want to thank you so much Mcknib for all your help and time with this. I feel like I’ve learnt a lot through this troubleshooting process that will help me a lot moving forward with my next builds. I have a stack of other pcbs I am excited to get to, fingers crossed they’re come together much more smoothly. Cheers.
Always good to take a break and come back a little saner it can definitely get frustrating, what I'd do is give both sides of the pcb a good clean with alcohol and a toothbrush I'm starting to think if it sounds good it may well be your meter affecting voltage readings what model of DMM is it
I've been able to have a closer look at your pics and zoom in on my pcb is there a bridge at volume lug 3 and the black ground wire that'd certainly affect the volume if it is making contact
Here’s the meter I use. Unfortunately what you pointed out was just a little piece of metal flake or something on the pcb and wasn’t affecting anything. Brushed it off and nothing changed. I’ll try the toothbrush and achohol suggestion tomorrow!
The impedance is fine so it will be pretty close to the actual voltages you've read
I don't think you're that far away from sussing the fault
Out of curiosity when you get the 4v reading does it increase the volume
As you say you're better stepping away from it and coming back to it with fresh eyes I don't think cleaning it will fix it, there is a possibility it will but it's more to get a better look at it without all the gunk clogging it up
I cleaned the pcb with alcohol and a toothbrush. Didn’t fix anything unfortunately as you suspected but it’s easier to see now. Posted some more pics above, if perhaps something looks suspicious.
I can’t test the audio signal at 4V right now because I was getting the 4V with Q1 transistor unplugged from the socket, and it’s now soldered directly to the socket legs. I could unsolder it if you think it’s important to try? I will try more probing with the audio probe next.
Nothing jumping out maybe a wee bit too much solder on your jumpers I'd also check if you haven't already that you have continuity with components leading off where you have your jumpers I'm taking it where you required jumpers did physically have pulled pads? did you actually solder your jumpers directly to the component legs particularly the jumper Q1 collector to R2 looks like you did but hard to see
I'm gonna have a look back through again kinda lost my way a wee bit maybe concentrating too much on Q1 collector you did say earlier volume was good up to Q3?
Good idea doing a re probe I think you've just invented a new DIY phrase haha
So probing the audio signal, everything seems okay until Q3. Collector of Q2 is louder and more distorted than the input jack signal. But then when I probe base of Q3 there is no audible audio signal, it just pops when I touch it.. Then Q3 collector has audio signal but it’s basically the same volume and gain as the input signal. So it’s lost it’s volume and gain that it had at Q2.
Ok looking at the components in the EQ section after Q2 going into Q3, R7 is super quiet, R10 is about level with input but is distorted, R8 and R9 are super quiet like R7 (barely audible). R3 is loud and matches the collector of Q2. C5 is quiet. Is there anywhere specifically I should check?
It's just a case of tracing from where the audio's ok to where it fails and finding where the signal loses volume
So if it's good going into a component and not coming out you'd look around that area and reflow solder joints etc
Audio probing basically tells you what area your problem is in not what's causing it whether that be a short, faulty component, dry solder joint or whatever you need to investigate reflow joints see if that fixes it and so on