Audio amplifier physical noise+possible popped capacitor

[Bassalisk]

Hello,

I have this audio amplifier from Vivax AM 100.

But it has somewhat weird problem. When you turn it on it starts buzzing but, not like buzzing on speakers but rather circuit starts buzzing, rattling noise.


There is also like wax residue on the board, which i don't think its normal. The capacitor on the picture is dented a bit.

The top relay is the only possible source of the noise.

Can anybody tell me what is the purpose of these 2 large capacitors and this relay? Maybe I can replace them and fix my amplifier!

http://pokit.org/get/img/4b11aec3633d2f7a3c760413d41f79ca.jpg [Broken]

 

[berkeman]

The brown residue on that bottom cap does look bad. I would guess that the caps are either power supply smoothing caps or audio coupling caps. Can you (carefully) touch the top of the relay to feel if it is the source of the buzzing?

 

[Bassalisk]

ok i found out that the brown stuff is industrial glue. So that's out of the picture.

The chattering is definately coming from relay.

I do not understand how is this relay connected to the capacitors and little transistors near. (There are 2 little transistors, not power ones(i think), that are behind those huge caps).

Do amplifiers have some sort of "general" circuit used to control relays for speakers?

I think some time constants aren't in order. But its too crowded near those capacitors. i'll try to draw a circuit diagram maybe that will help.

Im guessing those black boxes behind the top left capacitors are rectifiers?

 

[skeptic2]

I have seen power supplies in which initially the power supply capacitors are charged through a resistor. When the capacitors reach a certain voltage the relay activates, bypassing the resistor. The purpose of that circuit is to prevent the high capacitor inrush current from blowing the rectifier diodes or a fuse.

The circuit to activate the relay should have some hysteresis in it to prevent it from chattering but the designer may have expected the capacitors to charge rapidly enough along with the natural hysteresis of the relay to prevent the chatter. It is possible that the charging voltage is lower than normal or that the capacitors have become leaky and do not charge as rapidly.

 

[AlephZero]

Having a relay in a power supply circuit seems strange, assuming this is "consumer" equipment.

But high powered amps often have circuits which keep the speakers disconnected until the amp power supply has settled down, and disconnect them quickly when you switch off, to avoid transient thumps and/or oscillations getting to the speakers (which could damage them over time, as well as sounding horrible). Maybe that's what your relays are for.

It looks like there are separate two relays inside the plastic box (i.e. one for each audio channel) so if both channels are not working, that suggests it is a "common mode failure" of the power supply and delay circuit to both relays.

If the relays are "chattering" audibly, you can probably see the contacts moving (assuming there is nothing blocking the view, of course).

In which case, either the relay has died, or the circuit that powers the relay has died and isn't providing enough voltage/current to hold the relay on.

If that guess is correct, the two "large" caps are probably part of the signal path to the speakers, and nothing to do with the relay power.

 

[Bassalisk]

I see. Is it safe for me to test this live?

The caps are rated at 50V. I have oscilloscope, i would like to try and measure the signal waveform.

 

[skeptic2]

I think it would be a good idea to trace out the circuit before you begin testing. That way you know what you are testing. It always pays to be careful, even with 50V.

 

[oneamp]

Why is the white plug behind the rectifier burned and damaged?

 

[oneamp]

Another thing. If you already had the picture (which you do, since you have it open in windows photo viewer), why did you have to take a screenshot of it ;)

 

[Bassalisk]

Another thing. If you already had the picture (which you do, since you have it open in windows photo viewer), why did you have to take a screenshot of it ;)

I have this nifty program pokit, which uploads screenshot to web in a second, keeping the picture small. So the original jpeg was around 2 MiB and this screenshot too around 200 KiB or so.

The plug isn't burnt, its from this industrial glue.

I traced the circuit and it didn't make any sense. The base of the one transistor was connected to the controlling point(the solenoid) on the relay. I took oscilloscope and I had pure sine wave on the relay, which explains the rattling. Anyway the chattering stopped without me know what happened. It didn't fix the amplifier, I will have to do a lot more research on this to see if I can fix it.


Anyway, thank you all for your help, much appreciated.

 

[meBigGuy]

I would expect that the relay is designed to allow the amplifier to stabilize before connecting the speakers to the amplifier. I have an old pioneer that does that. Turn it on, then it waits a while and then there is a click and sound begins. I looked at the schematic, and it would be a chore to determine all the terms that contribute to the logic driving the relay but it seems to be be based on audio output, final amp bias, and a time delay.

I expect the transistor collector is connected to the relay. The center pin is not always the base.