I have been spending yet more time with the small scope and have found this when looking
Measuring the signal in GND with the probe with the GND at the RCA 3cm away, is this just the probe ?
If Imeasure the signal across rage RCA - quiet
Measuring the signal in GND with the probe with the GND at the RCA 3cm away, is this just the probe ?
If Imeasure the signal across rage RCA - quiet
Measuring the signal out +ve with the probe and probe GND to the -Ve signal out
Adding a cap across the + and -ve 0.1uF I can short most of the noise
Not sure what this means ?!
Adding a cap across the + and -ve 0.1uF I can short most of the noise
Not sure what this means ?!
I'm seeing 3.57 and 2.42 MHz on the display. I am not familiar with that scope - what is that? If those traces are at those frequencies, then you have a serious issue with oscillation coming in from somewhere.
To confirm this, try the following experiment: clip the probe's ground lead to the probe's input. This creates a small loop. Ideally, you should have a flat trace on your scope, even with the scope on maximum sensitivity. Move the probe with the loop around your circuit, like a metal detector. If you see high-frequency oscillations on the scope, it is a sure sign that there is oscillation or that something is spraying HF around the place.
To confirm this, try the following experiment: clip the probe's ground lead to the probe's input. This creates a small loop. Ideally, you should have a flat trace on your scope, even with the scope on maximum sensitivity. Move the probe with the loop around your circuit, like a metal detector. If you see high-frequency oscillations on the scope, it is a sure sign that there is oscillation or that something is spraying HF around the place.
Andrew, hi
Yes the scope is showing 3.57MHz around the input circuit and nearer 2.4MHz on the output. This is just one of the amplifiers, both are still noisy albeit this one marginally noisier.
I haven't replaced the 27pF caps on either but I did take off and test one set (interestingly the amp that doesn't seem to be showing oscillation) the majority of the components have been changed on the amp. I guess I should start by replacing the 27pF caps. Perhaps Polystyrene rather than ceramic discs ?
Any other components that might be adding noise I wonder, all electro caps apart from the 10,000uF have been changed,
Yes the scope is showing 3.57MHz around the input circuit and nearer 2.4MHz on the output. This is just one of the amplifiers, both are still noisy albeit this one marginally noisier.
I haven't replaced the 27pF caps on either but I did take off and test one set (interestingly the amp that doesn't seem to be showing oscillation) the majority of the components have been changed on the amp. I guess I should start by replacing the 27pF caps. Perhaps Polystyrene rather than ceramic discs ?
Any other components that might be adding noise I wonder, all electro caps apart from the 10,000uF have been changed,
Using the loop shows this around the oscillating amp over the input stage. 0.1-0.2mV and around 3.0MHz looking at the time and osciallations
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Changing components isn’t the way to go. Do both channels show the same problem? Remember to check this, you have to remove power from the one channel, take a look at the other, then do same on the other channel. HF will couple into everything.
To get a baseline, turn everything off except your scope. Use the loop to check for HF noise. If none, switch things on one by one until you are 100% sure you have the culprit piece of gear.
As remarked earlier in this thread, HF like this sprays into everything and gets modulated by any mag fields around, which then subsequently gets demodulated by the B-E junctions in the amplifier and that then manifests as hum-buzz.
But, let’s make sure we know what we are dealing with as it’s easy to get carried away.
Can you post the full schematic up again?
To get a baseline, turn everything off except your scope. Use the loop to check for HF noise. If none, switch things on one by one until you are 100% sure you have the culprit piece of gear.
As remarked earlier in this thread, HF like this sprays into everything and gets modulated by any mag fields around, which then subsequently gets demodulated by the B-E junctions in the amplifier and that then manifests as hum-buzz.
But, let’s make sure we know what we are dealing with as it’s easy to get carried away.
Can you post the full schematic up again?
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Andrew, hi
The amps are mono so only one channel each amp. both units have the 230v fan and VU meter removed. One now has a separate 22V supply (15VA toroidal transformer 4 way rectifier and 2,200uF cap) highlighted in red and I removed the 1K2 7W resistor. The unit was not any quieter than the voltage drop resistor and needed a link to the central signal GND to avoid additional hum
The amps are mono so only one channel each amp. both units have the 230v fan and VU meter removed. One now has a separate 22V supply (15VA toroidal transformer 4 way rectifier and 2,200uF cap) highlighted in red and I removed the 1K2 7W resistor. The unit was not any quieter than the voltage drop resistor and needed a link to the central signal GND to avoid additional hum
Only one amp exhibits the 3.5MHz oscillation (Which happened to be the one with the new anti thump circuit supply)
Yes this unit has the separate LPS which connects to the board but it was noisier with no connection from 0v to the main ground, highlighted in green the other LPS just feeds the 12V fan with the thermal switch before the supply
Now with this connected it's still not as quiet as the one with a voltage drop resistor and appears to be oscillating
I added this after disconnecting the GND trace as a GND loop without
Now with this connected it's still not as quiet as the one with a voltage drop resistor and appears to be oscillating
I added this after disconnecting the GND trace as a GND loop without
Okay I have re-checked the 27pF capcacitors and they measure 27pF, I also removed the separate power supply and reconnected the 1K0 drop resistor and the oscillation is still there what should I look at next !?
I believe it to be a good practice to twist signal and supply wiring so these have a degree of reducing the influence of radiated electrical fields. To do this one needs to have wire that is reasonably flexible. There is a lot of single core heavy gauge wire in the photo graph. In a manner of speaking this appears to be glued to the pcb with solder. The ideal is to have a mechanically strong anchor point for circuit wiring before using any solder. That could be achieved by strapping the wiring with plastic straps of using pin in terminals or solder tags.
We know from what you have described that you have a diode bridge for of rectification for your transformer. The dc output of the bridge rectifier appears to be 22 volts. For use with a 7815 regulator that input should be a minimum of 23 volts according my Databook. an 18 volt rectified voltage should be around 25 volts depending on the load applied. What is the regulation percentage for the new transformer?
We know from what you have described that you have a diode bridge for of rectification for your transformer. The dc output of the bridge rectifier appears to be 22 volts. For use with a 7815 regulator that input should be a minimum of 23 volts according my Databook. an 18 volt rectified voltage should be around 25 volts depending on the load applied. What is the regulation percentage for the new transformer?
Mjona, thanks,
I took some more measurements and found that two resistors were seemingly out of spec R17 and R18 were measuring 420 Ohms on the board all the others close to the 470 Ohms (466-471). I removed the resistors and they did measure 470 Ohms off the board. So I suspect TR10 and TR11 might be one of my problems with one of the amps.
I suspect this is just one more challenge in getting the amps useable for me. I think I will just go all out and replace the main caps as well as they are 20 years old but do measure fine. Then all the caps excusing the 27pF and three film on the antithump circuit have been replaced and all the power resistors
What's the best way to test the BUZ 905 transistors
I took some more measurements and found that two resistors were seemingly out of spec R17 and R18 were measuring 420 Ohms on the board all the others close to the 470 Ohms (466-471). I removed the resistors and they did measure 470 Ohms off the board. So I suspect TR10 and TR11 might be one of my problems with one of the amps.
I suspect this is just one more challenge in getting the amps useable for me. I think I will just go all out and replace the main caps as well as they are 20 years old but do measure fine. Then all the caps excusing the 27pF and three film on the antithump circuit have been replaced and all the power resistors
What's the best way to test the BUZ 905 transistors
Normally one starts with the filter caps and the loudspeaker relay as these both are generally selected for lowest cost and the first to deteriorate over time. Quite some issues may arise from both. Often when replaced owners complain that sound character has changed as the deterioration went slowly and one gets used to it. Of course they blame the new parts, that is why they are laymen.
Nice graphs again but it simply can not be that a separate PSU for the 7812 causes all the stuff you experienced. You are not using wireless stuff very close to the audio setup do you? And why use audio caps for non audio purposes?
BTW the casing you use for mains filtering are made of plastic. Are you aware that the same casings are made in cast metal? I use those for the same purpose. A few millimeters of metal shields pretty well.
The copper rods in the fuse holders are an indicator of ignorance/incompetence to many technical people. You better not post such practices.
Nice graphs again but it simply can not be that a separate PSU for the 7812 causes all the stuff you experienced. You are not using wireless stuff very close to the audio setup do you? And why use audio caps for non audio purposes?
BTW the casing you use for mains filtering are made of plastic. Are you aware that the same casings are made in cast metal? I use those for the same purpose. A few millimeters of metal shields pretty well.
The copper rods in the fuse holders are an indicator of ignorance/incompetence to many technical people. You better not post such practices.
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Jean-paul, I appreciate your knowledge and 'help' but I have no idea why you always want to put others down ?
As it happens I think I have indeed found the problem, I used Claude AI and have found via checking the 470R resistors that two came up at 420R in circuit union removing and investigating I found the problem was with one of the T905 MOSFETS that measures 4K between drain and gate
As it happens I had some used Nichicon FG caps so that we me being frugal not buying any new caps !
Or maybe I use the Forums for insight and assistance and prepared to tolerate those who might be able to help, just in case they do. I believe (maybe misconception) that I am confident and comfortable enough to take no personal issue with it. This is actually my choice, this may well change if AI offers more insight than those who are harder work to correspond with.
Ever been in a board room where no one is prepared to raise a question or issue for fear of being ridiculed ?
I have never taken this option and never would, your approach would actually encourage a lot to do so if you were CEO or chair, which I suspect you aren't (no offence intended BTW)
Ever been in a board room where no one is prepared to raise a question or issue for fear of being ridiculed ?
I have never taken this option and never would, your approach would actually encourage a lot to do so if you were CEO or chair, which I suspect you aren't (no offence intended BTW)