I wonder if I can tap anybody’s knowledge on something I can’t work out?
I’m repairing a Quad 520f power amp that has a mains Soft Start circuit that has a few failed components. How it operates is clear enough, with an RC time delay feeding the base of a transistor, switching in a relay coil, the contacts of which by-pass a current limiting resistor.
The bit that confuses me is the voltages. The bridge rectifier is fed by one of the (two) power transformer primary coils which is connected to the mains. In other words, the rectifier input is connected to the AC mains (240v AC) in series with one of the primaries, so 120v AC. After rectification this results in over 160v DC, yet C2 is rated at 60 volts, R1 is 0.5W (I think) and the relay has a 48 volt coil.
I can’t think why this circuit doesn’t burn out the second power is turned on, yet it seems to have worked fine for 40 years… until now.
Any thoughts?
I’m repairing a Quad 520f power amp that has a mains Soft Start circuit that has a few failed components. How it operates is clear enough, with an RC time delay feeding the base of a transistor, switching in a relay coil, the contacts of which by-pass a current limiting resistor.
The bit that confuses me is the voltages. The bridge rectifier is fed by one of the (two) power transformer primary coils which is connected to the mains. In other words, the rectifier input is connected to the AC mains (240v AC) in series with one of the primaries, so 120v AC. After rectification this results in over 160v DC, yet C2 is rated at 60 volts, R1 is 0.5W (I think) and the relay has a 48 volt coil.
I can’t think why this circuit doesn’t burn out the second power is turned on, yet it seems to have worked fine for 40 years… until now.
Any thoughts?
Yes, this relies on the base current flowing through the 470k dropping some voltage.
But it's perfectly ok to rate C2 at 200VDC to avoid possible cascading failures.
C2 must also have low leakage. This circuit seems, however, to be designed somewhat marginally.
But it's perfectly ok to rate C2 at 200VDC to avoid possible cascading failures.
C2 must also have low leakage. This circuit seems, however, to be designed somewhat marginally.
I thought the transistor base would be expecting just a few volts, can 470k drop 168v by that much?
Yes, I’d be inclined to use a higher voltage capacitor, but a 200v cap won’t fid on the board, plus the existing 60v cap has been working fine for decades.
Yes, it does seem quite crude, but it is used in an industrial grade power amp made by Quad. I’d have thought they knew what they were doing. And it’s been doing its job for 40 years.
My question is not so much “how can I fix it” but more “why is it working?”, “where are all the volts going?”
My question is not so much “how can I fix it” but more “why is it working?”, “where are all the volts going?”
When the transistor is "ON" this will hold/limit the voltage across it (B-E) at ~0.7v plus a small v drop across the relay coil and in comparison R1 is a very large value and so the largest v-drop will be across it, so C2 should never see a high voltage.
When the transistor is "ON" this will hold/limit the voltage across it (B-E) at ~0.7v plus a small v drop across the relay coil and in comparison R1 is a very large value and so the largest v-drop will be across it, so C2 should never see a high voltage.
icesteve and rayma are both spot on. When everything is working, C2 will see around .7v + 48V coil. C2 can easily see more if tran fails. I think they should have put a R across C2 and reduced the 470K some to make more of a resistive divider to set the base voltage. As it is,they are relying on the gain of the tran to be pretty constant. Looks like the coil is 4K-ohm and say the tran has a beta of 50 makes the 4K looks like around 200K or a voltage division of around .3 x168 is 50V. Higher beta though starts to push up the voltage, lower beta pushes it down. Relay coils often have a wide range of acceptable voltages (the 48V probably works from 40 to 60 volts) so the circuit can be pretty sloppy, which it is.
