Doesn't really matter. Think mine is RCA/Phono which has a plug in adapter to BNC and the scope probe plugs into that.
I have checked some of the voltages and most seem to be ok. Ones that are within less than +/- 5% of the values on the schematic I have considered as ok. (I'm not sure how much lenience I should give)
The voltages at R18 seem fine, the same with D2 and D3. However, the voltage at R7 which should be -111mV measures as -176mV and slowly drops over time. I'm not sure if it would ever reach -11mV, as I haven't held it on for long enough, but it would take quite a while. So, I'm not sure if this is way out, or whether it would eventually drop to the correct voltage?
One of the voltages at T3 is about 5-6% out, but one of them is 20.5mV when it should be 16mV, so this is about 22% out.
Would the added decoupling capacitors affect any of these readings?
The differences on my board to the schematic is that R6 has been changed from 120k to 62k. R11 is 120R but is listed as 47R, but someone on here has said that might be intentional by Quad.
Would it be wise to remove some of the components near the incorrect voltlages to properly test them?
It seems to be working great now!
Ignore the above. I changed R13 to a 27R, so the input sensitivity is now 1V. I would have thought that would only affect the amount of volume I need on the preamp, but it has got rid of the loud hum, speakers banging when I turn it on and gets rid of the distortion that was present at all volume levels.
Thanks everyone for all of your help! I'm just going to do some more tests before I finally finish with this.
Ignore the above. I changed R13 to a 27R, so the input sensitivity is now 1V. I would have thought that would only affect the amount of volume I need on the preamp, but it has got rid of the loud hum, speakers banging when I turn it on and gets rid of the distortion that was present at all volume levels.
Thanks everyone for all of your help! I'm just going to do some more tests before I finally finish with this.
Just to add some ideas for testing:
A signal generator needs to have very low distortion to be useful for evaluating audio gear, since it needs to have at least an order of lower distortion than the D.U.T. Also, since there is no measurement capability in just the generator, you also need an oscilloscope of some type which has a high resolution screen and >5MHz bandwidth for at least visual comparison on a video screen of some suitably scaled, hi-res type.
The FG100 generator design is based on an obsolete XR2206 analog chip and its datasheet app. note that dates back to 1972. It's specified to have better around 0.5% or better sinewave THD after a calibrating adjustment at a specific frequency. That's good enough for the type of generator and for casual listening checks but nowhere near good enough for hifi audio measurements or comparisons where you are typically looking for differences down at 0.1% and below and you probably don't need instruments to tell you what 1% or more distortion sounds like. There's more to this and there are better digital and analog oscillator designs and products out there but not unfortunately, at such cheap, throwaway prices.
Still, if you need high purity audio signals, there are plenty of websites offering anything online from a single, 1kHz reference tone to comprehensive processing software for amazingly low distortion tones and comprehensive envelope formats from chirps to gated tone generation to calibration tones and sound mixing resources. Surely, free is cheap enough too: Audacity 2.0.6 Released | Audacity (R) This software will blow you away with its capabilities but it doesn't take long to get a 1 kHz signal of any level which plays continuously, stepped levels, panned, periodic, chirps or whatever format you like. All you need to add is a decent DAC (don't use a tube type as you need precise tones for measurements, not the added effects)
A signal generator needs to have very low distortion to be useful for evaluating audio gear, since it needs to have at least an order of lower distortion than the D.U.T. Also, since there is no measurement capability in just the generator, you also need an oscilloscope of some type which has a high resolution screen and >5MHz bandwidth for at least visual comparison on a video screen of some suitably scaled, hi-res type.
The FG100 generator design is based on an obsolete XR2206 analog chip and its datasheet app. note that dates back to 1972. It's specified to have better around 0.5% or better sinewave THD after a calibrating adjustment at a specific frequency. That's good enough for the type of generator and for casual listening checks but nowhere near good enough for hifi audio measurements or comparisons where you are typically looking for differences down at 0.1% and below and you probably don't need instruments to tell you what 1% or more distortion sounds like. There's more to this and there are better digital and analog oscillator designs and products out there but not unfortunately, at such cheap, throwaway prices.
Still, if you need high purity audio signals, there are plenty of websites offering anything online from a single, 1kHz reference tone to comprehensive processing software for amazingly low distortion tones and comprehensive envelope formats from chirps to gated tone generation to calibration tones and sound mixing resources. Surely, free is cheap enough too: Audacity 2.0.6 Released | Audacity (R) This software will blow you away with its capabilities but it doesn't take long to get a 1 kHz signal of any level which plays continuously, stepped levels, panned, periodic, chirps or whatever format you like. All you need to add is a decent DAC (don't use a tube type as you need precise tones for measurements, not the added effects)
Interesting, I'm going to look into using my computer as a signal generator. My DAC measures 0.0013% THD+D at 100Hz, so it might be well suited to it. I need to look up what it measures at higher frequencies.
I've used Audacity forever, for vinyl ripping etc, but that's cool that you can use it for that. Thanks for pointing that out.
I know that using a signal generator and scope is useful for diagnosing issues with an amplifier, but are they of any use for an amp that 'seems' to be fine? Probably just to look for areas you might be able to improve in the circuit?
I've used Audacity forever, for vinyl ripping etc, but that's cool that you can use it for that. Thanks for pointing that out.
I know that using a signal generator and scope is useful for diagnosing issues with an amplifier, but are they of any use for an amp that 'seems' to be fine? Probably just to look for areas you might be able to improve in the circuit?