Tracing signal with scope help

eizner23

eizner23

Super Member
I am learning the ropes of tracing a 1khz sine wave using tektronic tds420a scope.
I am diagnosing a dead left channel in Kenwood ka5500. I can trace the signal to the volume pot with unit powered down, but when I turn the unit on with DBT the protection relay trips when I have the clip of the probe on chassis while probe on part I am expecting signal. Take out o scope probe unit powered up fine. this amp does not have earth ground on its power cable, so i believe this would be save. What am I doing wrong here?
 
Register to hide this ad
The only thing I can think of right at the moment is to tell you to check your outlet earth ground.
 
If everything is working fine, I'd set the scope to "AC coupling".

The ground clip probe is connected to mains ground, and to the 2nd ground clip in a dual trace scope. You can't connect a scope ground to anything as you do with a DMM. Please search some videos about "oscilloscope safety" and "oscilloscope ground", perhaps you can understand better what's going on with your setup.
NO AFFILIATION:

I use my scope with a 2 prong adapter to lift the ground, knowing this is not "safe", but my unit is a digital scope with all plastic case and knobs, no risk of touching metal parts. This way I never short anything to ground.
 
The schematic that I am looking at for the KA-5500 does not show a protection relay in the power supply section. I do see one that disconnects the speaker positive outputs from the drivers. That relay is driven by GND via Q28, which is not the same as the chassis ground.
 
I agree with that being a good video to watch - watch it twice.
However, simply put - yes, the scope ground can be connected to the chassis, assuming the chassis is also grounded, which is true 99% of the time.
 
If the chassis is not grounded, then it should still be OK. If you're blowing breakers or something, I would suspect you have a line to chassis leakage problem which needs to be fixed.

Its also possible that the dim bulb is limiting voltage enough to cause stability issues. Solid state can be stupid like that. Try a larger bulb if you're running less than 100 watts.
 
Thank you for the feedback everyone. I did watch the video @elnaldo posted but in a different scope thread in preparation to my tinkering. I will watch it again.

I will try an isolated ground adapter on the scope plug although that doesnt seem to be an issue according to the video. I think I am using a 100w bulb but I need to confirm. I will try once w/o the DBT. DBT never lites up. I will do some research on setting scope to "AC coupling" mode.
 
I've got a TDS460, and do not know why I did not catch this from the start, but make sure the scope coupling is NOT in 50 ohm mode on your TDS420A if you are in DC coupling.
 
Last edited:
I agree, If the scope has a 50 ohm input, that will short most of the circuits. I wouldn't disconnect the DBT until you figure out the problem with your setup, check the scope is not shorting something.
 
I took the 100w DBT out and scoping points with the unit on doesn't trigger protection. With DBT the relay won't engage until 10 seconds or so, then it clicks and the right VU meter pegs, clicks again, repeat. Maybe a sign of something not right in the right channel of amp section.

Using A.C. coupling mode at 1M allows me to see the signal now. Thank you for the heads up on that. The waveform looks a little fuzzy, maybe my scope needs to be serviced or calibrated, or that I'm using a 100mhz probe I mistakening purchased for this 200mhz scope.

20171218_193614_1513648693950.jpg
 
For FM work, even a 60 Mhz scope would be enough, so your probe will be fine. 2 things:

1. Turn the scope on and leave it be for half an hour or so, then press the "Shift" button followed by the button with the word "Utility" above it and run the "System Cal" function to normalize the scope, then calibrate the probe using the square wave output on the scope.

2. Get on youtube and watch some of this guy's videos and pay attention. W2AEW
 
Ya, agreed on scoping the input. Be aware switching power supplies, fluorescent lights, and other devices can be noisy and can create fuzz.
 
Another option would be to plug the amp into an isolation transformer. That would also eliminate a possible ground fault/short condition. Your amp will be "floating" like in the video.
Did you verify all your power supply voltages are correct before trying to trace a signal? You cannot expect a good audio AC signal without the proper DC supply voltages. Also check the DC balance/offset and bias/idle current at the power amp outputs. Use your DC voltmeter for these measurements.Then, once you KNOW your DC voltages are set up correctly check for a good AC signal halfway through the signal path, e.g. where the pre-amp connects to the power amp. If the signal is good there you know the front half is ok and the problem is after that point. If bad there your problem is before that point. If you have the identical input signal on both channels you can compare the 'same' point in the circuit between left and right. Forgive me if this is too basic, but you did say you are "learning the ropes of tracing a 1khz sine wave."
PS. I learned my first electronics lessons (and wrote my first FORTRAN programs on punch cards) in Madison many years ago :biggrin:

EDIT: more info
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom