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Could someone explain how the standby and filter cap configuration works in the amp?
How can the negative end of the 10uf/450 be connected to B+?
How can the negative end of the 10uf/450 be connected to B+?
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When the STANDBY switch is in "shorted" position, there is 0V in the capacitor (so its polarity is not important). When the switch is open, on the positive end of the capacitor the voltage is higher than on the negative end - I don't see the problem. Why do you think it is incorrect?
Mark -
I just didn't understand this design. Thanks for explaining.
There is a problem with the amp just not sure what at this point. Right now I'm only seeing HV on the plates of the el84 and nothing else.Comment
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Don't understand why they used such a big cap across the standby switch.
It sure will kill the dreaded "standby off pop" , but on later turning on, there will be a localized explosion at the switch contacts, which will pit and erode it quickly.
Shorting a high voltage cap (with a lamp in series) is what camera flash does, by the way, just to have an idea of the high voltage and current, short duration pulse means.
When somebody shorts a PSU to discharge it, *steel* chassis and screwdriver gets pitted and with a blue/black area around where the spark hit, go figure; imagine the tiny metal contact inside a switch.Juan Manuel FaheyComment
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maybe this is why the switch is doing something weirdComment
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I don't know why you are focusing on the SB switch.
If you have voltage to the EL84 plates, then the switch is fine.
Work your way down that impossible to read power supply schematic.
Do you have voltage at the next node (screen)?
Keep going until you find where the voltage drops out.Comment
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No voltage on screens and any of the other nodes.Comment
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FWIW I just had one of these and one screen resistor was open, essentially turning one EL84 off. The other EL84 was redplating badly.
If you have no voltage on the screens power off the amp and discharge the power supply and measure the screen grid resistors.Comment
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Screen resistors are not open, they are 100R each in circuitComment
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Have you checked that with a meter?Originally posted by pontiacpete View Post
If they're ok, then the 1k 1W resistor between the D and C nodes may be open.My band:- http://www.youtube.com/user/RedwingBandComment
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yes, I checked them with a meter.
Also, lifted the 1K dropping resistor R32 thinking it was opened, but unfortunately it's good.Comment
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If the schematic is correct, then voltage on the EL84 plates must result in VB+ either side of (a good) 1k R32; could you re-check plate, screen grid, D and C node voltages?My band:- http://www.youtube.com/user/RedwingBandComment
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If it is indeed 'good', then there must be a voltage on the other side of it. No?Originally posted by pontiacpete View Post
Does the other side to ground read megaohms?
If not, then C23 may be shorted.
You would be well served to isolate each supply & bring them online one at a time.(left to right)
Attached is a Velocette 12R power supply with voltage readings, in a crisper format.
trace-elliot-velocette-12r-Power Supply.pdfComment
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I took readings again and still the plates have 410v, the screens start at a little over 20v then slowly go down to 2.4v.
I check the resistance to ground on either side of C23 and it goes from mega ohms to open.Comment
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Thanks for help everyone. I found a trace from the standby switch to the 1K resistor that was opened. It didn't look opened
but with the meter it said open, so I ran a wire bypassing the trace,(which has a darken quality it)to the 1K and the negative
end of the 10uf/450 cap and now there's voltage around the board. It's not sounding so good but it's a start.Comment
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