375 VDC circuit

argonrepublic · 25th October 2009, 06:05 PM

I am trying to repair a guitar amplifier. This amp was a basket case. I have been somewhat succesful and have managed to get some circuits working.
This 375 Volt output is low. Open circuit, it gradually climbs to about 190 VDC. When I connect it to the power amp it plummets to about 20 VDC. It is easy for me to assume a short or excessive load. I guess it would be cool if I could rig up a dummy load to see what happens. Not really sure what I would use for this load.
Nevertheless, I would realy like to understand this circuit to test it and learn about it. I understand the diodes as rectifier, the caps closest to output seem to be voltage multiplier which I was reading about recently. If anyone could spare a bit of time to give me a short lesson, that would be great.
Thanks
I guess it would be nice if I knew would ac voltage to expect at the input of this circuit. Amplifier manufacturer seems to have no idea.

argonrepublic · 25th October 2009, 06:11 PM

Here is schematic

Glowbug · 25th October 2009, 06:13 PM

Maybe you can model it up in PSUD2 and get an idea for what it should be doing...

Other than that, can't really help, I'm learning these things too

anthony.taylor1967 · 25th October 2009, 08:39 PM

Hi Argonrepublic, I don't know that much as my knowledge is very rusty, some 20 years of rust, but you may have leaking caps or dried out caps in the supply. Have you measured how long the voltage stays up after switching off without anything connected to the supply or may be even leaky diodes. It would be cheap to replace just to see. Is there a short where it connects to the rest of the circuit are the valves ok.
At this moment I have ran out of ideas

What guitar amp is it?

Hope this helps - Anthony

Spiny · 26th October 2009, 08:38 PM

This is not a voltage multiplier circuit, its a simple bridge rectifier circuit followed by a CRC smoothing circuit.

Things to check:-
Before checking place a discharge resistor accross the caps and measure the voltage. (a 2K 1 watt resistor and a couple of crocodile clips will do)
disconnect the caps (12 and 13 ) and the resistor R13, measure its resistance It would not be unusual to find its gone high in value, a replacement will not be expensive. Use the diode test on your DMM to check all 4 diodes in circuit, as these are 1n4007's they are easy to get hold of and cheap.

If you have a method of measuring the caps (c12 and 13 ) see what they are like, otherwise with them discharged place your meter accross them on a high resistance range, it should climb in value hopefully going overrange. if it shows a stable resistance then the caps are leaky.

the input AC voltage will be roughly 375/1.414 (265 volts) measured on the transformer wires.

25th October 2009, 06:05 PM	#1
argonrepublic diyAudio Member Join Date: Apr 2005 Location: Simcoe Ont	375 VDC circuit I am trying to repair a guitar amplifier. This amp was a basket case. I have been somewhat succesful and have managed to get some circuits working. This 375 Volt output is low. Open circuit, it gradually climbs to about 190 VDC. When I connect it to the power amp it plummets to about 20 VDC. It is easy for me to assume a short or excessive load. I guess it would be cool if I could rig up a dummy load to see what happens. Not really sure what I would use for this load. Nevertheless, I would realy like to understand this circuit to test it and learn about it. I understand the diodes as rectifier, the caps closest to output seem to be voltage multiplier which I was reading about recently. If anyone could spare a bit of time to give me a short lesson, that would be great. Thanks I guess it would be nice if I knew would ac voltage to expect at the input of this circuit. Amplifier manufacturer seems to have no idea.

25th October 2009, 06:13 PM	#3
Glowbug diyAudio Member Join Date: Feb 2006 Location: Hillsborough, NC/McLean, VA	Maybe you can model it up in PSUD2 and get an idea for what it should be doing... Other than that, can't really help, I'm learning these things too __________________ Jim J.

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25th October 2009, 08:39 PM	#4
anthony.taylor1967 diyAudio Member Join Date: Oct 2009 Location: Manchester	Hi Argonrepublic, I don't know that much as my knowledge is very rusty, some 20 years of rust, but you may have leaking caps or dried out caps in the supply. Have you measured how long the voltage stays up after switching off without anything connected to the supply or may be even leaky diodes. It would be cheap to replace just to see. Is there a short where it connects to the rest of the circuit are the valves ok. At this moment I have ran out of ideas What guitar amp is it? Hope this helps - Anthony

26th October 2009, 08:38 PM	#5
Spiny diyAudio Member Join Date: Feb 2008	This is not a voltage multiplier circuit, its a simple bridge rectifier circuit followed by a CRC smoothing circuit. Things to check:- Before checking place a discharge resistor accross the caps and measure the voltage. (a 2K 1 watt resistor and a couple of crocodile clips will do) disconnect the caps (12 and 13 ) and the resistor R13, measure its resistance It would not be unusual to find its gone high in value, a replacement will not be expensive. Use the diode test on your DMM to check all 4 diodes in circuit, as these are 1n4007's they are easy to get hold of and cheap. If you have a method of measuring the caps (c12 and 13 ) see what they are like, otherwise with them discharged place your meter accross them on a high resistance range, it should climb in value hopefully going overrange. if it shows a stable resistance then the caps are leaky. the input AC voltage will be roughly 375/1.414 (265 volts) measured on the transformer wires.

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