Please help with correct capacitor replacement 1000uF or 100uF...

[ostrong]

Hi everyone, I'm somewhat of a newbie with electronics repair so please forgive me if there is an obvious answer to this question…

I'm attempting to repair an amplifier PCB out of an old PA system (Alto Elvis 15MA) which I got second hand, as is and not working.

I managed to find the schematic and I discovered that someone has removed a capacitor (I didn’t realise at first, I thought it was a blank because the joints and surface were very clean).

In the schem. diagram there are two conflicting capacitance values listed for the capacitor at location C15.
On the diagram itself it says 100uF and in the parts list it says 1000uF/ 25v

*I think* the capacitor is powering a 24v relay (K1-A/K1-B) so I’m not sure if this means it will need to be the 1000uf value...?

I have attached a screenshot of the two sections I am referring to and also a pdf of the entire schematic.

I don’t want to damage the PCB and so I’m hoping someone can tell the correct one to use please?

I greatly appreciate any help anyone can offer me.

Many thanks,
Ollie.

 

[Nigel Goodwin]

It's part of the speaker protection and muting circuit, either should be fine - it will just affect the speaker protection speed.

However, before you do anything else, check the voltage at the junction of R3 and R33 (output of the amplifier), this MUST be very close to 0V, if it's more than a 100mV or so either way there's on issue in the power amplifier. If it is higher then that, then it's most likely a LOT higher, and would kill a speaker if you connected one, and the speaker protection didn't work.

 

[ostrong]

Thanks so much Nigel, I appreciate your help.
The actual layout of the PCB is not quite as straightforward as the schematic, as R3 and R33 are far apart and I can't see how they are directly connected therefore I have no idea where the junction is that I need to test

I have attached the PCB layout, any chance you could please show me where I should be putting my multimeter probes to test the amplifier output?

 

[Nigel Goodwin]

Easy done, just measure the voltage at all four ends of them - they should all be the same, as they are very low value resistors.

 

[dr pepper]

Another way you can do it is to measure Dc volts across pins 2 & 3 on the relay, take care not to short them with meter leads.
It could be the o/p transistor(s) is shorted causing several volts (15v looking at the schem) at the amplifier o/p, causing the speaker protection to disconnect the speaker.

 

[Nigel Goodwin]

Another way you can do it is to measure Dc volts across pins 2 & 3 on the relay, take care not to short them with meter leads.
It could be the o/p transistor(s) is shorted causing several volts (15v looking at the schem) at the amplifier o/p, causing the speaker protection to disconnect the speaker.

That just shows the relay is triggered or not, so it could be a fault in the protection circuit - actually measuring on the emitter resistors checks the amp alone - and is easier to measure, as it's such an easy and obvious point to get to (any end of either of the two big resistors - and preferably either end of both if you can't identify the centre of the two).

It's a bit like the volume control in a radio, it's an easy and obvious place to start when fault finding as it's easy to find and get to, and also splits the fault to audio or before that.

 

[dr pepper]

If you look at the schem nige the points I mentioned are the o/p of the amp and ground.
Dc volts present here as you know would trigger the protection.
I'm not sure what I referred to as pin 2 is actually a relay pin come to think of it, it might just be the number of the faston connector.

 

[ostrong]

Thanks so much for your help Nigel Goodwin & dr pepper I managed to find someone to help me and it turned out to be a bad opAmp IC chip. That was replaced and the PA works again! So that's one less thing in the rubbish tip!!
I just popped back to give you an update and to say thanks again

 

[dr pepper]

Glad to see you got it sorted.

 

[shokjok]

A 1000 uF polarized capacitor could destroy the BC550 transistors, or their equivalent substitutes. I noticed your schematic lacks connections to the power supply voltages, creating a floating circuit.