Audio Circuit Issues With Oscilloscope Screen Caps. - Page 1
Forum section: electronics / beginners

author: Ungolian
author_rank: Regular Contributor
timestamp: April 12, 2021, 10:51:46 pm
content: Hello all, I'm having a rather odd problem with an audio circuit. It's a 3 transistor clean boost that can also do clipping. I'll try to be as precise as possible, but I'm relatively new to this, so I'm sure I'll leave something out. Anyways, here's some screen caps. I'm using a 1k sine, but something is dropping it down to about 65hz, and there is some random oscillation and crackling. The oscillations even happen when the effect is bypassed, and the treble, bass, gain, and volume pots change the frequency and volume of the oscillation (which I thought was a grounding issue, but no luck). I also figured it might be a capacitor issue, so I replaced every electrolytic, and the same thing happens. There are also some film caps that I haven't replaced yet as the board is part of a kit and I don't want to risk pulling any pads off. This isn't a modulation effect, and yet the voltages on the collectors of Q1-Q3 modulate by about .5v from 4.1v to 4.6v in a totally random manner. It's a 9v effect, and I've tried both battery and adapter power with the same results. Maybe it's a bad solder joint? I went over everything. Twice. I replaced all 3 transistors as well, and again, same thing. Replaced the bypass switch. Nope. I've made this particular circuit before, and have never had this happen. The randomness is maddening. It will work perfect fine for a few seconds, then go back into oscillation. Which again makes me think of capacitors. I didn't think the smaller film caps could cause that because they discharge too fast. I included a shot of the input cap where it goes into the base of Q1, and even without any amplification, the random oscillation is there. So I shortened the input wire, thinking it was some kind of heterodyning. Nope. But whatever it is, it looks like it's coming in directly at the input. Maybe a B+ issue? But I replaced the filter cap, and even enlarged it. I've tried different input cables and audio input methods.The screen caps are just strange. I've looked at this circuit before with my oscilloscope, and it will vary the amplitude of the wave form, and clip it asymmetrically, but never change the frequency. I'm at my whit's end. What am I missing here? It pretty much has to be one of the components I didn't replace yet, but could a resistor, pot, or film cap cause this?Thanks in advance!

author: Jwillis
author_rank: Super Contributor
timestamp: April 14, 2021, 01:40:17 am
content: It's much easier to diagnose a problem if you include a schematic of the circuit your working one which includes where you are taking the measurements in relation to the ground reference/references . Are you using a common ground reference when taking measurements? This can make a big difference to what is showing on the scope .

author: bob91343
author_rank: Super Contributor
timestamp: April 14, 2021, 04:21:22 am
content: What is a screen cap?

author: floobydust
author_rank: Super Contributor
timestamp: April 14, 2021, 04:52:34 am
content: I think OP means "with oscilloscope screen captures".

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 14, 2021, 04:56:25 am
content: The number one rule of troubleshooting is: Do not randomly replace parts.Just don’t do it. Not even one. Never replace a part in a design that you know has worked in the past until you have verified either in the circuit or out of the circuit, that part is actually defective in some way. New parts, unless acquired from some Shenzhen bargain bin, are not suspect. It is your work that is suspect.So it’s a a kit. Even more reason to blame your work. The sooner you accept the fact that with a kit, 99% of the time, it’s something you did wrong, will you become more successful at kit building.Did you install the parts in their correct locations? It is incredibly easy to swap a 2.7k resistor with a 27k. Did you install an electrolytic cap backwards? A transistor, a diode backwards? It is very hard to look at your own work objectively. But, that’s what you need to learn. It can be difficult. Everybody makes mistakes. Everybody.Then of course there is your soldering. It’s probably way worse than you think, especially if you’re using a cheap iron and lead free solder.The most important things you can post are:Photographs. Of your board. Top and bottom. Well lit and focused.

author: Ungolian
author_rank: Regular Contributor
timestamp: April 16, 2021, 11:51:27 pm
content: Thanks to all that replied. I had to put everything down and get away from this stuff for a few days, that's how maddening it was... After going back in with a clear head, I found the problem. I'm using right angle PCB mount pots, when soldering one of the pots, I clipped a cap with the iron, and decided I couldn't have that wart on my nice new shiny board. So I changed it. In doing so, I had to bend one of the pots forward. Aluminum pot lugs have more give to them than the solder holding them in place. There were some small cracks/gaps at the base of the lugs, and re-flowing the solder fixed the problem. After monkeying around and replacing electrolytics, all 4 pots had that problem. Lesson learned. The vast majority of problems with stuff like this is usually off board connections, or things that move like switches (or pots...). I totally overlooked the one thing I was moving the most- the pot lugs.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 17, 2021, 12:30:06 am
content: Quote from: WattsThat on April 14, 2021, 04:56:25 amUnless, the original component manufacturer has changed the specifications since the circuit was designed!"Been there, done, that, got the tee-shirt!"On one occasion, I was doing some mods on a TV Transmitter remote/auto control system.This unit used several monostable multivibrators to provide a set of delays.It was convenient, whilst modifying an unrelated part of the unit, to remove these devices.Unfortunately, one fell on the floor, & as is not unusual in such cases, "disappeared from the ken of man!""Who cares?-----plenty in the store!"I grabbed a couple, stuffed them in the sockets, & proceeded to complete the mods, & test the control unit.It didn't work!It turns out that the original chips of that part number had been found to be unreliable at providing short delays, & had been redesigned to optimise such use.We, however, were using it to provide a long delay, in which use the modified device had become prone to early "time out" when using the same CR time constants as before.The IC maker, (National Semiconductor, if I remember correctly), foresaw this problem, & produced a new monostable, similar in all respects, except optimised for long delays.This, of course, had a different part number!No Internet back then, so after a lot of research, I found about the change, purchased the new chips, tested the control unit, which now worked properly, then changed all the schematics & documentation to reflect that change.This was an"in-house" design, with only two such units in service, so we could "cover all bases".Just imagine if there had been thousands out there, with an ongoing production run!Quote