Hi Alan,
You can make an RF probe with a couple caps and a diode.
Take a 1N4148 (or equiv) signal diode, to the cathode, connect a 0.01uF capacitor. Take this junction to the positive input on your meter. The other side of the diode can go to another capacitor to block DC, another 0.01uF should do it. The negative lead goes to the first capacitor and the negative of the meter, the positive lead is the second capacitor.
My first attempt at drawing with "paint". Forgive me.
Your meter will indicate a positive reading if high frequency signals are present. The meter will not be calibrated, so don't worry about the exact reading. An oscilloscope is really what you need here.
If this network is damaged it may cause the amplifer to oscillate. There is probably another reason for this. I would check the soldering on all small caps off the supplies in the area. Also make sure the heatsink is properly grounded.
-Chris
You can make an RF probe with a couple caps and a diode.
Take a 1N4148 (or equiv) signal diode, to the cathode, connect a 0.01uF capacitor. Take this junction to the positive input on your meter. The other side of the diode can go to another capacitor to block DC, another 0.01uF should do it. The negative lead goes to the first capacitor and the negative of the meter, the positive lead is the second capacitor.
My first attempt at drawing with "paint". Forgive me.
Your meter will indicate a positive reading if high frequency signals are present. The meter will not be calibrated, so don't worry about the exact reading. An oscilloscope is really what you need here.
If this network is damaged it may cause the amplifer to oscillate. There is probably another reason for this. I would check the soldering on all small caps off the supplies in the area. Also make sure the heatsink is properly grounded.
-Chris
Attachments
If I may ask a question.
On the heatsink are two large copper clad power amps, Toshiba 2SC5198. Each of these as you probably know has three pins.
There's a wire goes from the middle pins of both of these and is soldered on to one of the pins of the large caps C802. Does it matter which side of the cap the wire is soldered to and if had soldered it to the wrong side when I replaced the large caps could that have cause a problem?
On the heatsink are two large copper clad power amps, Toshiba 2SC5198. Each of these as you probably know has three pins.
There's a wire goes from the middle pins of both of these and is soldered on to one of the pins of the large caps C802. Does it matter which side of the cap the wire is soldered to and if had soldered it to the wrong side when I replaced the large caps could that have cause a problem?
Hi Alan,
Looking at your diagram, that lead goes to the positive of C801. It is the positive supply.
Measure the DC voltage there, you should get around +38 VDC.
An oscilloscope is almost as useful as your meter. An audio oscillator will round out what you need. A frequency counter can be had later, but is also useful.
-Chris
Looking at your diagram, that lead goes to the positive of C801. It is the positive supply.
Measure the DC voltage there, you should get around +38 VDC.
An oscilloscope is almost as useful as your meter. An audio oscillator will round out what you need. A frequency counter can be had later, but is also useful.
-Chris
Hi Alan,
First confirm the wire is connected to the correct spot. Next, measure the overheated resistor, you should probably replace it. Then it's off to make the RF probe and try that.
An oscilloscope would be your best friend right now. If you know someone with one, try to get them to look at your amp with it. The oscillation may be rail to rail, so set the range high on the 'scope.
-Chris
First confirm the wire is connected to the correct spot. Next, measure the overheated resistor, you should probably replace it. Then it's off to make the RF probe and try that.
An oscilloscope would be your best friend right now. If you know someone with one, try to get them to look at your amp with it. The oscillation may be rail to rail, so set the range high on the 'scope.
-Chris
Hi Alan,
The wire is installed correctly. The middle leg of the transistor is the collector. In your amp, the supply is connected to this terminal on the output transistors. Same for the driver transistors.
Your amplifer is likely oscillating. Carefully check the small capacitor's solder joints. It is possible some of these have failed and gone open. The capacitors in the zobel appear to be okay ('cause they are passing current).
-Chris
The wire is installed correctly. The middle leg of the transistor is the collector. In your amp, the supply is connected to this terminal on the output transistors. Same for the driver transistors.
Your amplifer is likely oscillating. Carefully check the small capacitor's solder joints. It is possible some of these have failed and gone open. The capacitors in the zobel appear to be okay ('cause they are passing current).
-Chris
On this schematic. I disabled the resistor circled in blue because it was heating up
I was checking the voltages for each of the transistors circled in red. They checked out correctly.
I also read the voltage correctly at other circled red points.
Unfortunately when I tried to test the transistor in green I must have shorted it. (Those things are small).There was a small spark and now when I switch the amp on the fuse at the transformer keeps blowing.
I've now taked the transistor circled in green out of the pcb but the fuse still blows so I guess I've damaged something else as well. Oh the fun continues. I do intend buying more tools, esp an oscilloscope but in the meantime I'm practising.
Regards
Alan
I was checking the voltages for each of the transistors circled in red. They checked out correctly.
I also read the voltage correctly at other circled red points.
Unfortunately when I tried to test the transistor in green I must have shorted it. (Those things are small).There was a small spark and now when I switch the amp on the fuse at the transformer keeps blowing.
I've now taked the transistor circled in green out of the pcb but the fuse still blows so I guess I've damaged something else as well. Oh the fun continues. I do intend buying more tools, esp an oscilloscope but in the meantime I'm practising.
Regards
Alan
Attachments
Hi Alan,
The outputs on that channel have likely shorted. Change the set and the drivers on that channel. At least check the bias transistor, I'd just replace it to be sure. Make sure the two 100 ohm resistors are still in the land of the living.
You have to put the resistor back in (blue circle). It is required. That channel has a fault, I'm guessing oscillation. Check the small capacitors in that area to be sure they are soldered in properly, or that the leads are still attached to the body of the part.
-Chris
The outputs on that channel have likely shorted. Change the set and the drivers on that channel. At least check the bias transistor, I'd just replace it to be sure. Make sure the two 100 ohm resistors are still in the land of the living.
You have to put the resistor back in (blue circle). It is required. That channel has a fault, I'm guessing oscillation. Check the small capacitors in that area to be sure they are soldered in properly, or that the leads are still attached to the body of the part.
-Chris
Hi Chris
Thanks for the input. I may leave off the project for a while until I get a better iron, an oscilloscope and also until I study some practical electronics. (If you can recommend any sources I would be grateful but of course there's lots of info on the web)
I think I know what you mean by the drivers, the bias transistor and the outputs but I'm not 100% certain. I'm starting to feel hobbled by my lack of basic knowledge and lack of understanding of the terminology.
For example, the outputs? Is that output transistors? And if they are indeed shorted does that mean they would draw too much current and thus the fuse at the transformer would blow?
Oscillations? Somehow high frequency oscillations can occur in the circuit causing the output resistor to overheat. The Zobel network serves to dampen oscillations and if it's not doing it's job that's what happens. It seems the caps should be looked at if the network is not functioning?
Thanks for the input. I may leave off the project for a while until I get a better iron, an oscilloscope and also until I study some practical electronics. (If you can recommend any sources I would be grateful but of course there's lots of info on the web)
I think I know what you mean by the drivers, the bias transistor and the outputs but I'm not 100% certain. I'm starting to feel hobbled by my lack of basic knowledge and lack of understanding of the terminology.
For example, the outputs? Is that output transistors? And if they are indeed shorted does that mean they would draw too much current and thus the fuse at the transformer would blow?
Oscillations? Somehow high frequency oscillations can occur in the circuit causing the output resistor to overheat. The Zobel network serves to dampen oscillations and if it's not doing it's job that's what happens. It seems the caps should be looked at if the network is not functioning?
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