Help: adjusting amp with oscilloscope

[Jody]

Hello,

I don't have much experience with oscilloscope measurements, but I have to complete the adjustment procedures for a JVC amplifier. I'd like to make sure that I won't screw up something, particularly causing a short through ground, so your help would be appreciated.

I have completed the idling current and offset adjustments with my Fluke multimeter. This amp features a voltage selector circuit and switchable gain (0, -6 and -12 dB), the adjustment of which requires use of oscilloscope.

At that step, a 7-ohm resistor is connected across the speaker terminals (+ and -), a 20 Hz signal is applied at the input, and the Volume is adjusted until 34V RMS is obtained at the output / across the resistor. Then the trimming resistors for L and R channels should be adjusted at the point just before clipping.

I am uncertain of the instructions: where should I connect the scope, how and should I de-ground it? The amp's plug for 220V has 2 pins instead of 3, but the manual says that the  probe and the case should be disconnected from ground, so that the instrument and the amp are electrically independent. Well, aren't they already independent? This confuses me a bit. Looking at the instructions, I infer that the measurement with the scope should be done across the 7-ohm resistor, and not across the control points in the circuit (they are marked in the manual, see below)

So, how should I approach this and connect my scope's probes?

1) Should I get an isolating transformer, isolate the unit, and then connect my probe's tip and ground across the resistor?
2) Should I get a differential probe? (too expensive)
3) Should I use my X and Y probes in differential mode (X-Y), connect the probes' tips across the resistor, and the ground crocodile tips at the GND of the unit?
4) Perhaps I misunderstood the instructions in the manual? I attached the manual so that you can check yourself if I got it right.
Please check out "6. Adjustment Procedures", section "Voltage selector circuit adjustment"

Here is the service manual in PDF --> https://ufile.io/iuvch (just click "Slow download")

Thanks.

[DrDiode]

hi,

An isolating transformer would work, remember to isolate the amp not the scope.  I have had some luck using 2 scope channels and applying the "difference " math function.

[Jody]

Not sure if this is what they meant in the service manual, not even sure I got it right. Is there any alternate way to approach this, without isolating transformer and without differential probes?
Would be nice if someone elaborate.

[The Soulman]

  I believe this was poorly translated from Japanese(?) to English, not
to sure what is the intent of this adjustment, I'd guess maximum gain before distortion?
But for that they don't have a input voltage level stated.
In this case I'd say if it ain't broke, don't fix it.
Or if there is some slight level difference (a few percent) between the channels you can adjust that here.
Just put in a constant 50Hz sine wave signal and measure the output voltage with your fluke, turn up the volume till it reads 20 Volt, then insert signal to second channel and measure its output voltage, should also be close to 20 Volt, if not adjust.
Shouldn't make a lot of difference between loaded and unloaded output.

[Jody]

Still, how should I observe this on the scope? Yes, it seems like maximum gain before distortion to me, as well.

I think the circuit could be out of alignment, and I just want to check to make sure. The idling current was somewhat out of alignment when I got this amp, it's from 1983/84.

[coromonadalix]

Dr diode you're wrong

You must use an isolated transformer with the scope, nothing else,    it would serve when servicing a tv, an amp  etc.. etc ..  you may encounter hot ground in certain cases, and the scope would be isolated, and protected, some old tv's had an positive ground on the case.


@Jody

In audio, the reference signal is 1khz at 0db, you can check, cassette decks, open reels, preamp amp  etc ...
A scope will be useful for distortion, crosstalk and signal quality checks, a DMM will help to check adjustments like : biais transistors adjustments, regulators adjustments, if you have dc at the outputs, with the more recents DMM  having DC+AC  you will know the health of the psu(s) inside fast enough.

Ex: at my job we had an defective series of smps, outputting 48volts dc BUT an added 40volts ac appeared too, we had lots of touchscreen problems with an embedded computer ?? the new DMM having dc+ac measurements showed that pretty fast.

For a 20-30 years amp, you may or will face a capacitor recapping due to aging and parts going out of specs, because of that, the noise floor will be higher than normal, an hisss or hummm may be heard, when no sound. 

Had this problem with a Sony Esprit series cassette deck, recapped the psu with slightly higher values, and the background hiss noise dissapeared. Still going strong after 15 years.

@Jody   search and google how to use an scope, they are great tutorials out there, even I admit  with recent models you just hit auto setup and everything is adjusted, just need to set the probes value x1, x10, x100  ... before use.  I have a tds2012b  that i love with an auto voltage setup, since i play with different voltages outputs but the same working frequency.

[P90]

you isolate the DUT, not the oscilloscope.