A request from a non member

[richard296]

so as i don't make a mistake and understand exactly what you both are trying to tell me. i tried 12v first with a 5 watt 220 ohm resistor which i had. digi meter set on 20 volts no load it read 11.83v when connected it dropped to 11.67v then continued to drop to 11.60v over a twenty second period. would this be charging of the caps. i switched off until you say it's ok. What i did notice the speakers are kicking in when first switching on. i tried a few times to make sure. thank you richie

[Refugee]

That is normal for a computer power supply with no load on the 5 volt output.
Can you measure the voltages at both ends of the 220 ohm resistor to negative.

[pwdrive]

Ditto as refugee says, I'm assuming that your 220 ohm resistor is connected in place of the original missing resistor and that the 12 volts from your external PSU is connected to the rectifier side of that resistor.

If connected as above then the voltage on the rectifier side of the resistor should read somewhere near 12 volts eg: 11 to 12 volts, it's this voltage that is feeding the the PA board directly, if that voltage settles around that figure it would for the moment indicate that the PA stage is ok so far.

If the above is ok then it's the voltage on output side of the resistor that is needed in order to get an idea of the current draw of the circuit(s) it's supplying.

Lawrence.

[richard296]

placed 5 watt 220 ohm resistor in place of missing resistor. using another 12v power supply not computor one rated @ 20 amps. reading on rectifier side 12.8 v otherside 6.28v with a 15watt 150 ohm resistor this reading is 7.28v. finally i soldered the 15watt resistor in situe plugged into mains plugged in cd player amp sounds great. resistor started getting hot straight away couldn't touch it after a minute or so. just guessing time but ten minutes later sound is distorted. thank you richie

[pwdrive]

From the voltages you have given the current flowing through the 5 watt 220 ohm resistor you connected is approx 30 ma, this would give a power dissipation of approx .2 watts (200 mw)

The current flowing through 15 watt 150 ohm resistor is approx 36 ma, this would give a power dissipation in the resistor of approx .2 watts (200 mw)

Neither of these currents would cause the resistors to get hot.

Lawrence.

[Skywave]

Not that it's really necessary, but just for thoroughness, I agree with Lawrence's calculations. So, somewhere, there is something seriously in error, either in your tests or in our comprehension of those tests.

Neither of those currents would cause the resistors to get hot.

Quite: lukewarm, maybe, but not "hot". Are you really sure that those test resistors, rated at X watts are really X-watt rated resistors? And when you state that they are Y ohms, are you sure that they really are Y ohms? Perhaps you should repeat those tests and measurements again. And if the results are indeed the same, the only logical conclusion that can be drawn is that what we think you have there, in way of a circuit arrangement, is not what you actually have.

Al.

[pwdrive]

When you plugged it into the mains and the resistor got hot did you measure the voltage on both sides, if so the readings might help.

Lawrence.

[pwdrive]

Richie, a little info about the 150 ohm 15 watt resistor, it would need a current of .32 amps (320 ma) flowing through it in order for it to reach it's rated dissipation of 15 watts, at it's rated dissipation it would get hot.

Lawrence.

[richard296]

plugged into mains after soldering the 15watt 150 ohm ceramic/cement resistor in place.It's the same physical size as the resistor on the 7 volt side (see pics). The ac voltage on bridge was 48 volts and 9v on the other circuit. dc voltage on resistor tail next to bridge and earth 65.5 volts. other side of resistor was 50volts. On the other low voltage circuit it was 14.3 volts on both sides of resistor to earth. hope this helps richie.

[Skywave]

Original text within this post now deleted, since it crossed with Richard's post immediately above, rendering the text of my original post redundant.

Al.