A request from a non member

[Refugee]

220 ohms should dissipate just over 9 watts if there is a short circuit so 10 Watts is fine.
The final value will depend on when you begin to get audio out of the board.
It looks like it was the same size as the other resistor and that looks like a 10 what rating.
Judging by the black patch on the board I would guess that it has run at about 4 times its rating for a few minutes before failing.
Have you tested the amplifier with an MP3 player yet?

[richard296]

220 ohms should dissipate just over 9 watts if there is a short circuit so 10 Watts is fine.
The final value will depend on when you begin to get audio out of the board.
It looks like it was the same size as the other resistor and that looks like a 10 what rating.
Judging by the black patch on the board I would guess that it has run at about 4 times its rating for a few minutes before failing.
Have you tested the amplifier with an MP3 player yet?

can.t get a 220 ohm @ 10 watts local. the other resistor in power supply is 15 watt no resistance marks. I think i can get a 15 watt 150 ohm local will that do. I haven't tried the mp3 player yet i don't wan't to fire it up until i can get a resistor in power supply then i can move forward. thank you richie

[Refugee]

There is little point in fitting another resistor until we are sure it is not going to burn.
The first thing I would do is to sort out that mains wiring. Have you got anything that is mains rated. Perhaps an extension lead for a computer power cable?

Then the low voltage side can be powered from the transformer and the bench power supply can be used to power the 40V side of it. 30V will do for testing the amplifier with an MP3 player as it will most likely shut the power supply down well before full volume is reached but this will not matter.
You can then safely touch on resistors while watching the currant.
It should produce something from the instrument circuit even at 30 volts but I can't vouch for this.
Once the amplifier is tested I would expect it to work with the bench power supply feeding the instrument end of the missing resistor as this point on the circuit will be at a lower voltage such as 12 or 24 volts at less than an amp.
Then we can work out the value and power of the resistor for you from the currant reading on the bench power supply.

[richard296]

I understand refugee. I have bypassed 50v plug and tracks on pcb as advised. tools i have 12v transformer it also has the 3 and 5 v dc. And a variable ac mains transformer. niether have a meter for reading anything. thank you richie. does this mean i'm stuck until i get the correct equipment. advise please.

[pwdrive]

If you power up the output side of the missing resistor with your 12 volt dc supply it will be worth monitoring and noting the current draw with your test meter, as refugee says it should be less than 1 amp.

Lawrence.

[richard296]

i will try with the 12v, ps It's a computor power supply 12 + and 12v - can i use it to give 24v later or even now. do i put meter on volts or amps and do i put in in line with power supply or where do i place probes. thank you richie. to find the current draw i will have to take a reading before connecting to find difference am i correct.

[Refugee]

In the absence of a variable power supply I would go for a 10 ohm resistor and 12 volts.
Measure across the resistor. A computer power supply may well need a load on the 5 volt wire to make it stable such as a car brake light bulb.
You will need to isolate the earth if you are going for 24 volts from a computer power supply.
I would be tempted to go with its own transformer and start with 220 ohms and 10 whats for the resistor and measure the voltage across the resistor.

[pwdrive]

I'm not well up on computer PSU's as to regarding connecting up for 24 volts output, refugee or someone might know, Plus we don't know for sure what the mystery rail should be running at in voltage terms, 24 volts might or might not be ok so start with a lower voltage say 12 volts.

If using the 12 volts output from your PSU:

First double check that there is no short or abnormally low resistance in the circuit that the output side of the missing resistor is feeding, then:

Connect the -ve output from your PSU to the common -ve rail on the amps PSU, select a suitable DC current range on your test meter, connect the +ve lead of your test meter to the +ve output of your 12 volt supply and the -ve lead of your test meter to the output side of the missing resistor, I don't know what the DC current ranges are on your test meter but you need to select a range that will easily indicate a current draw of 100 ma to 1 amp DC.

I am assuming that your power supply can deliver enough current, the current draw expected is unknown at this point but as has been suggested we would not expect it to be more than 1 amp, probably half or a quarter of that if the circuits that your PSU is feeding are ok.

Switch on and keep a finger on the on/off switch in case you have to switch it off quickly! If the current draw shoots up to 1 amp or more switch off immediately and investigate.

Lawrence. Whoops, late again.

[Refugee]

Computer power supplies tend to start and run up to full voltage as soon is there is enough mains at the input and will not run below that.
These PSUs also have a standby wire that connects to the power button that will need to be connected to start it up.
They also monitor the 5 volt wire for regulation and if this does not have a load on it the output will all be poor or may even pulse on and off.
I would still put a pair of wires on the points where the missing resistor came from and connect a volt meter to them.
Then just dab a resistor across them and watch the volts. 220 ohms at 10W should be fine as a start.
We can work out the currant from the voltage reading. The resistor will get warm so keep them off anything that will get burned including your hands.
The unit has been powered up with the resistor burned out so no further damage will happen if power is applied again without the resistor.
If you have got a Variac then it can be run up that way with the test resistor in place.

[pwdrive]

There are some more pics here but I can't magnify them at my end:

http://www.swistak.pl/a15614083,DUOVOX-G...zona-.html

Lawrence.