| Electronics > Repair |
| Audio… (Amps, THD, channel separation) with analog equipment (scopes, etc.) |
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| Fried Chicken:
--- Quote from: Kleinstein on December 28, 2024, 08:06:17 pm ---The observed 120 V peak to peak are some 43 V RMS. So still quite a bit lower than mains voltage. Higher power amplifiers, especially when made for 8 ohms can indeed provide quite high voltages. They may have a warning about potentially hazardous voltages at the speaker output. The amplifier output is normally low impedance. So the difference in the loading should not be that relevant. An amplitude dependent effect would point to a linearity problem - maybe also loudness enabled. --- End quote --- DOH, of course mains voltage is 120VRMS. I didn't think about that. My head is spinning slightly. To convert from Vp-p to VRMS you divide by two and then divide by the square root of 2, correct? There's no loudness enabled, the amplifier is set for 8Ω speakers, the pots for the volume control flatten out at the top 1/32nd of a turn so they should be working fine (bypass, 0Ω or whatever). |
| Kleinstein:
The difference measurement on the scope also includes the linearity of the scope input amplifier. Chances are the audio amplifier (at least if not broken) is more linear than the scope. With the audio output, one may have to generate the difference in a different way, e.g. with a transformer or connecting 1 scope probe between the 2 outputs. |
| Fried Chicken:
I just shorted the input. The service manual says I should have 127 dB SNR with the input shorted, and 120dB with the input not shorted. I could not see a discernible difference when shorting the inputs to ground, however the Left channel (inexplicably louder channel) is sitting at 1 mV with some signal at 100mHz? The right channel at 0.75mV. I swapped the probes to verify, and the reading stands, but later dicking with the probes on the scope changed the amplitude. I think I'm beyond what the scope/probes can do. Maybe I need better probes. Scrolling now down the service manual, there is an "Idling adjustment" that should be at 10mV +/- 1 mV DC. Doing so for the left channel I get a hovering from 9.33mV to 9.49mV so in spec, but I brought it to hover around 10mV DC. Checking the right channel, however, it was hovering at 11.7 mV DC, so out of spec. Result! I now brought it back into spec. I really don't know what exactly I'm adjusting here. What does the idling adjustment do? The next adjustment is to push "Auto Class A", so the unit will go into Class-A exclusively under a certain output level. This brought the idling to ~89mV on the right channel and ~84 mV on the left channel. Spec says "more than 20mV DC". Ok. Issue has now emerged, disabling "Auto Class A" brings the idling voltage down quite a bit and it took a while for it to rise again... then again yamaha said "leave on, no input, for 2 minutes prior to testing", ok. It went back within spec. Last adjustment the VU meters: select 2 Ohm speakers, apply no load or an 8 Ohm load, put a 1 kHz signal... Read a 28.3V output on the speakers and adjust so the 0dB LED "Adjust to the lights up point". reduce signal by 1dB to 25.2 V and "Confirm the 0dB - LED fades out". I assume a vRMS, and for this I used my Fluke 45. My power resistors are very hot right now. Weirdly, the left channel indicates higher output than the right channel, even though it sounds quieter. This is even after setting the idle adjustment. |
| Kleinstein:
Aiming for some 10 mV sounds like the adjustment of the standing current (drop at an emitter resistor) for a class AB amplifier. 10 mV+-1mV looks quite small tolerance. Normally I would not complain about 12 mV. AFAIR the rule of thumb for a class AC is something like 0.5 kT/e ~ 13 mV per resistor. This setting could effect the cross over distortion a little, but amplifier should still work OK if not totally off. The settings can be a bt temperature sensititve. This can cause the slow settling. |
| Fried Chicken:
I am about to blow a fuse here trying to interpret the difference of signal between Ch1 and Ch2... The right channel looks lower output than the left, but.... When I see the signal on the screen, it looks like Ch2 is smaller... but if I invert Ch2 and then add the two signals together, then boost the sensitivity of the reading and/or increase the signal from the function generator. I figured it out, but now looking at the difference of signals, the left channel being normal, starting on a rise and crossing the 0 at the zero: As I turn up the output, I get what I initially expected, the left channel slightly pulls the signal up from zero (at the left side of the axis)... as I keep increasing volume, however, suddenly the inverted right channel signal becomes dominant and the inverted signal becomes dominant... What's *really* weird, is that this happens not only when I do the output adjustment, but even when I increase the volts/div sensitivity of the scope. If someone has the patience to interpret this, and either explain something in my knowledge that's missing, or has an explanation for this phenomena, I would be very grateful. I'll post pictures/a video to go with this post shortly. [edit update] I flipped the board over and noticed a number of bothersome things: when I did the recap I didn't do a particularly good job. I only had a flux pen and not actual flux I could apply onto the board. As a result there were some extra solder whiskers floating around and a number of my solder joints could probably have been better. I went in and cleaned all of them up, reflowed almost all of them, re-checked the biases and repeated my measurements from before. I'm not sure it electrically made a difference, because I didn't take notes of my measurements, and the phenomena still arises where the left channel pulls stronger at lower voltages (~2v) and as I crank it up to ~20v output the right channel pulls stronger (when taking the difference between the channels). However.. measuring with the Fluke 45, I get 20.12v on the left and 19.9v on the right (this is maxing my function generator when attenuated -20dB). This doesn't make sense. When looking at the difference of channels, and inverting the right channel, as I crank the voltage up, the waveform begins to follow the inverted right channel suggesting it is "stronger". My ears agree: the soundstage sounded shifted to the right. The Fluke 45 DMM disagrees. This phenomena stays even at different frequencies, although my ears hear a frequency dependence to this phenomena. |
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