Hello all!
Major newbie here still. I have my oscilloscope and I’m getting slightly better at reading schematics. I understand that a capacitor blocks DC and that a diode blocks AC. Caps also can act as filter capacitors or coupling. Rectifiers convert AC to DC.
It may seem that I’m oversimplifying but that’s honestly what I understand so far. I just haven’t been able to get a decent explanation that my brain can understand on any of these yet. I was a teacher, a technical instructor actually, so I know how to take highly technical concepts and break them down into easily digestible lessons and that’s what I’m hoping to find here.
That being said, my current task is a busted phono board on a Marantz 2230. AUX works perfect but when I use phono, it’s barely audible. So I thought I’d break out the oscilloscope and try my hand at tracing the audio signal. I fed my signal generator a 1khz signal into the phono input and started testing. The inputs, J701 and J703 look great. They match my control signal. So far so good. Then I check the very first cap, C702, and the side connected to the input ,obviously, looks great, the other side, totally wonky signal on the scope. It went from a nice sign wave to something resembling the Rocky Mountains. So I tested the other side. Same result. So either I’m doing something wrong, not understanding what I’m seeing, or both of those caps are equally bad, and I’m doubtful that they’re both bad.
My question is this: where can I learn how to trace an audio signal so that I can intuitively know where a problem lies? I’d love to be able to understand that when an audio signal goes through a certain type of component, I would know what it should look like on the scope. For example those capacitors. Or maybe what would the audio signal look like going into a transistor? B, C, E what should I expect to see on the scope?
Any places I can go to learn this? Any specific MCL videos that go over this? I am watching the videos but I’m starting at number one and moving on from there but if I could skip ahead to learn the signal tracing bit, I’ll do that and then go back to the older videos.
I’d greatly appreciate any advice y’all have for a super eager learner.
Thanks!
If you are a patreon member video 115 may be helpful.
Rocky mountain peaks. On my recent amp repair an under biased transistor would create the rocky peak sign wave. Low/under voltage on the base of the transistor.
Check voltages, DC, and check surrounding resistors for tolerances.
@rs-electron I will check that episode out! Thank you!
@onedamnote not coming through. Try Imgur if you’re just needing to post a pic.
Not sure if that’s supposed to be a sine wave or not? Looks more like a sawtooth.
-Tracing An “audio signal” is no different than tracing a line voltage or “voltage”.
Although a sine wave is often the first ‘attack’ at tracing a signal in order to diagnose it’s throughput,
For a true test of any amplifier you need to send a square wave thru it and look for ripple.
But For now lets just stick to sine waves
In looking at your oddly sawtooth shaped test pattern one thing that is very prominent is that the wave is falling apart only on the positive peak and not the negative.
And what should be a smooth ramp both on the upswing and the down swing is rather rough.
I’m no tech but if I had to haphazard a guess is that DC is getting into and thru your signal path which indicates a capacitor, and that cap may or may not have been strained via the resistor directly upstream being fried.
Desolder a leg of the upstream resistor and check it’s resistance, Desolder a leg on the upstream capacitor and check for leakage.
-I have no clue why the distortion is manifest so badly only on the positive peaks..
Irregardless of that, make sure that whatever your driving the audio input with is not higher than 0db or unity gain (0.775 volts)
As stated earlier, I’m no tech so Perhaps another one of the more advanced electro techs here can expound on what is riding the waveform as well as to why you would see only positive peaks with huge distortions.
Shot in the dark here. Since it only happens when you hit max amplitude, it might be an amplifier/mixer turning on at that level? Or a bad solder joint acting as a mixer?
If you have the schematic handy, post it up. I would suspect capacitor or some kind of breakdown leakage. Especially since it is only on the positive ramp, and has a modulation appearance, something on that side of the circuit is not functioning. Also worthy of testing, try various frequency inputs and see if there is any change in the peaks. That is my thought process.
Radios + Tubes + Scopes + Cars= Nothing better!
@amrench Following a good and well organized troubleshooting technique I would do the following:
Start with some “proofing facts” first, as in: verifying that the signal source (your Vinyl record player whatever that is) is outputting the right amount of clean signal and that is not @ fault (itself) and also checking the audio RCA cables (for proper continuity and low resistance). Perhaps feeding the signal to another (known working) amp. would be the easiest method. Then move to the Marantz2230 but would check the 35.6V phono stage power supply (P800 Module)-verifying for “cleanness” of the supplied voltage (minimum AC ripple, voltage level etc.) then move on to the Phono stage itself but first would measure all DC voltages on each transistor as stated on the schematic and if all of the above are per specs and “sound” then and only then would finally start following the signal path.
Latest Post: hi all im new Our newest member: The Tow Rat Recent Posts Unread Posts
