14-10-2016, 06:23 PM
Not necessarily.
High gain would be caused by the resistor between the output and the inverting input going high or O/C. But, as I said, high resistance in the lower "limb" - the resistance between the inverting input and ground - would reduce the gain; all the way to unity if O/C. Now, resistors are generally pretty reliable, but as well as that resistance, there is the series capacitor I mentioned, who's job is to reduce the gain to unity at DC. Well, we all know what electrolytic capacitors do
I've no idea if this is the problem, but it fits in with some bits of the description (low gain, oscillation), and is certainly something I've seen before. Most solid-state power amps are meant to run at 20-30dB, so become oscillators if you turn down the gain. And the reported frequency of 700kHz suggests Nyquist loop oscillation rather than localised parasitic HF/VHF oscillation that emitter followers love to do.
In addition to all that, there is plenty else to check. But it's an easy one to eliminate before going further - simply change those 2 caps. Though frankly, I'd change them all, as there's no real point trying to apply logic until you've replaced all the components that are known to wear out. Some of those Japanese capacitors can be amazingly reliable, but other types aren't.
High gain would be caused by the resistor between the output and the inverting input going high or O/C. But, as I said, high resistance in the lower "limb" - the resistance between the inverting input and ground - would reduce the gain; all the way to unity if O/C. Now, resistors are generally pretty reliable, but as well as that resistance, there is the series capacitor I mentioned, who's job is to reduce the gain to unity at DC. Well, we all know what electrolytic capacitors do
I've no idea if this is the problem, but it fits in with some bits of the description (low gain, oscillation), and is certainly something I've seen before. Most solid-state power amps are meant to run at 20-30dB, so become oscillators if you turn down the gain. And the reported frequency of 700kHz suggests Nyquist loop oscillation rather than localised parasitic HF/VHF oscillation that emitter followers love to do.
In addition to all that, there is plenty else to check. But it's an easy one to eliminate before going further - simply change those 2 caps. Though frankly, I'd change them all, as there's no real point trying to apply logic until you've replaced all the components that are known to wear out. Some of those Japanese capacitors can be amazingly reliable, but other types aren't.
