Four of them are used as two parallel pairs of 1000uF total for the two output coupling capacitors.
That should be ok to increase somewhat. I can't locate where the others are in the schematic,
but it says that are used as parallel triples.
Luxman SQ-1220 Manual - Solid State Integrated Amplifier - HiFi Engine
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If they connect to the speakers then they are coupling caps and if they are in parallel then you have an effective 1000uf cap. Fitting two 1000uf would give a 2000uf value which is fine for speaker coupling duty.
Coupling caps
Thank you I will increase the value, the main reason for me changing these caps is because when you turn the amp on it kicks 9vdc out for a few seconds out of each set of speaker terminals and when you turn the amp off it kicks about 15vdc out that slowly goes down but takes a few minutes. Is it likely that these caps are the main cause they are 47 years old.
Thank you I will increase the value, the main reason for me changing these caps is because when you turn the amp on it kicks 9vdc out for a few seconds out of each set of speaker terminals and when you turn the amp off it kicks about 15vdc out that slowly goes down but takes a few minutes. Is it likely that these caps are the main cause they are 47 years old.
It may be worse after increasing the capacitor sizes, if the problem is in a different part
of the circuit instead. Give it a try though, they are old enough to be replaced anyway.
Ian,
Is this with a load (loudspeakers) unconnected? If so, this will be normal and has nothing to do with capacitor quality. The caps need charging up/discharging at power up/down and without a loudspeaker or equivalent load resistor, charging/discharging current is through several k.ohm only, thus several seconds.
Yes it is acting in that way with no load, I'm not using any good speakers with this at the moment as it blew a HF on a cheap speaker, or I think it did as the speakers were ones I was using to test out amps it could happened on a different amp, but I daren't use it on my main speakers surely this would be a bad idea with all that DC about. Also it says LUX corporation on the front of my SQ1220 (circa 1968). I have found out that it is the same amp as a SQ301 (circa 1965). AS far as I know the only difference is the choke and the SQ301 was a prototype and their is a lot more information on the net about the SQ301.
If these are the output coupling caps then doubling the value will increase the effect of the DC shift on turn-on. They may or may not be the cause of the DC shift, but they will certainly pass it on more effectively to the speakers. Try to solve the actual problem. If not, you should consider reducing the value of the output caps so at least the speakers see less of the problem.
That is normal for a single polarity supply.
The coupling capacitor is there as a DC blocking capacitor.
It start out when OFF at zero Volts. During the power ON, it charges up to approximately half supply voltage.
To charge up, it needs current.
Most competent designers include a start up charging circuit that is not through the speaker and include a shut down discharging circuit that is not the speaker.
I can't recall ever seeing a commercial mainstream AC coupled amp with dedicated additional circuitry to silently charge/discharge the coupling cap. Not to say there isn't one out there... just never seen it. I have seen amps that ramp the voltage to the input stages to minimize thump but that is something different and its not a perfect fix.
You need to understand the theory of caps and their charge and discharge characteristics. There isn't really 'lots of DC', there is a predictable and calculable current that will flow at power on and power off. Back in the day no one bothered much.
You need to understand the theory of caps and their charge and discharge characteristics. There isn't really 'lots of DC', there is a predictable and calculable current that will flow at power on and power off. Back in the day no one bothered much.
There is lots of DC at turnoff precisely because the cap is the only thing blocking the DC from the speakers. At turn on it has to charge up through the speakers, and at turn off it has to discharge through the speakers. This is what you are seeing. Making the caps larger will increase this effect.ianjones said:
If there is a protective circuit then this may provide a short across the speakers during turn on and turn off. If so, it may be faulty. That is where you should look. Find a circuit diagram.
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