A request from a non member

[richard296]

70 v was from the top of my head i didn't think i would get as much assistance. it's def 40v and 7v marked next to bridges. The 40v def goes direct to amp. refugee ii was unsure if i could fire it up without the missing resistor. pwdrive you say low res high current low vol. i am sure it's going to be 15 wat min t but have no idea of the resistance. if it can be fired up without the resistor safely i would rather try any advised resistor at my own peril. i have a mains varable power unit which i can use to bring the voltage up slowly.

[richard296]

refugee top marks pnp are 2n3055 i thought the amp was up to 100watt. no frayed wires the plugs do look bad in photo but the reason is. where they've joined 2 or more wires it protudes over the top of plug, all the other wires are single and are well below the survace. The tracks are very close especially where the solder joints meet any suggestions would be welcome. and finally the plug rating 50v it's only 40v and 7v coming from the tranny or is it higher at the plug after going through the caps

[Refugee]

I have found a Mullard circuit that is not too different to what I would expect you to have.
R18 and R19 will be 0.25 ohms and the speaker will be 4 ohms made up with the two 8 ohm ones you have.

[Refugee]

I was thinking of the in coming mains wires with 240 volts on them being on that tiny plug with the two tracks running all the way over to two solder pads. If they are mains I would certainly use another plug by the transformer and keep them off the circuit board all together.

[richard296]

thank you refugee. i got the picture now. I will do something to make that safer.

[pwdrive]

"pwdrive you say low res high current low volt "

That's only a guess on my part, finding out what the resistor supplies might help in determining what it's value should be, if it supplies anything on the PA board I wouldn't fire up the PA board without the resistor fitted.

Lawrence.

[Refugee]

That is exactly what we need to do either with a continuity buzzer or powering it up as it is.

[richard296]

I have found a Mullard circuit that is not too different to what I would expect you to have.
R18 and R19 will be 0.25 ohms and the speaker will be 4 ohms made up with the two 8 ohm ones you have.

If it's very similar to what i have. can we not guess at the values of the burnt resistor. Without the actual schematics it's going to be a guess anyway. I am positive your choice is going to be better than mine.

[Refugee]

I don't think the resistor is part of the audio amplifier at all.
I would consider it more likely to be powering a regulator on one of the other circuit boards.
A typical audio amplifier of that type just does not have a resistor that size on it at all.
The only high power resistors are the emitter resistors and they are showing up on the photo of the amplifier and do not look burned.

[pwdrive]

Like I said, I would try and find out what circuit(s) the missing resistor would have supplied, trace the connection back. Just a note, DC coupled PA stages can go puff extremely quickly if provoked.

Good luck

Lawrence.