Guitar amp hum no signal

Thread Starter

Rabbitbreth

Joined Jan 22, 2019
160
Hi,

My 50w bass amp has a loud hum with no signal .
The driver moves in and stays there .
No apparent broken solder that I can see .

Apparently this could be the filter caps gone but before I start messing around with anything I was wondering if anyone could confirm this diagnosis and have a look at the photo perhaps and tell me amp CB.jpegwhich components I should investigate or replace .

Thank you
 

MrChips

Joined Oct 2, 2009
30,486
What is the make and model of the amp?
First thing, disconnect the loudspeaker from the circuit otherwise you risk blowing the loudspeaker.

Measure the DC voltage at the output wires that go to the loudspeaker.
My guess at this point is:
1) Bad power supply
2) Bad output drivers

Let's see if we can find a circuit schematic before we do anything further.
Post close up and sharper photos so that we can read the numbers on the ICs and transistors.
 

Tonyr1084

Joined Sep 24, 2015
7,777
First, I'm certainly no expert on this subject. But if your speaker is pulling all the way in then it's seeing a DC current at some voltage. That doesn't sound like a cap to me, but again, I'm not the expert here on this subject. I'd suspect that either a bridge rectifier has failed or more likely your output amp transistor has shorted either to ground or to V+.

IF you were to reverse the wires at the speaker I bet it would push out instead of pulling in. That's a sure sign of a DC voltage reaching the speaker. As warned before - unplug it or you could burn it up.

I'd measure the PS final voltage output to the amplifier. If - JUST "IF" - if the + voltage is 64VDC AND you're reading 64VDC on the speaker the amp could be shorted. "COULD BE"! Not "Necessarily". You could have a short in a pre-amp somewhere. From there I get even deeper into the fog, so I'll let others who are more skilled guide you. For now, your speaker is not seeing an audio sine wave, it's seeing a steady DC current, quite possibly at max voltage. Which, as mentioned before, could damage the speaker if left in that position longer than you have already punished it.
 

Ian0

Joined Aug 7, 2020
9,502
Almost certainly a failed power transistor. Check for short circuits between Emitter and Collector (Collector is the case)
 

Reloadron

Joined Jan 15, 2015
7,452
Wouldn't rectified unsmoothed DC cause 100Hz/120Hz hum?
Why yes, now that you mention it. Thank you for pointing that out Ian. Had the line frequencies in my head and ... brain took a mental break. :)

This is where a scope really comes in handy.

Thanks Ian
Ron
 

Thread Starter

Rabbitbreth

Joined Jan 22, 2019
160
Hi everyone ,

thanks for all the input . I'm trying to find a schematic for you guys but it seems this company is now in Russia . Its a cheap Torque Acoustics 50W bass amp that I was given so as long as the speaker is not gone I'm going to see what I can do and maybe learn something. sorry about focus on pics . have just uploaded a few more ..with flash this time ..and specs on.

Have measured the DC on speaker , just for a second ..saw 19 volts .

I dont have a scope to measure hum but there's a audio clip if that helps .

ah .. it wont upload Mp4 file . but I would guess its about 100 / 120 hz.

thank you all.
 

Attachments

Dodgydave

Joined Jun 22, 2012
11,239
Looks homemade to me, If you have voltage across the speaker then one of the transistors on the heatsink has probably blown. There should be as near to zero volts across the speaker. Start by checking the those transistors.

IMG_20231012_103200.jpg
 
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Dodgydave

Joined Jun 22, 2012
11,239
Okay so what voltage is across the speaker then, it should be around zero volts, check the supply voltages across the two capacitors, .
 

Dodgydave

Joined Jun 22, 2012
11,239
So there is something definitely wrong with the supply, check the diodes near the fuses, it looks like a centre tapped transformer output, putting equal AC voltage on the yellow wires to the fuses, they should be equal near enough between ground and positive and negative from the bridge rectifier.

IMG_20231012_114832.jpg
 
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Thread Starter

Rabbitbreth

Joined Jan 22, 2019
160
So there is something definitely wrong with the supply, check the diodes near the fuses, it looks like a centre tapped transformer output, putting equal AC voltage on the yellow wires to the fuses, they should be equal near enough between ground and positive and negative from the bridge rectifier.

View attachment 304743
I tested those 4 diodes . 0.31, 16.8, 0.31, 16.8 . . apologies but I'm not clear on what to do with the AC reading ... or what the bridge rectifier is ( AC to DC converter right?) I need to disconnect the speaker first before taking these readings , yes ?
 

Dodgydave

Joined Jun 22, 2012
11,239
I tested those 4 diodes . 0.31, 16.8, 0.31, 16.8 . . apologies but I'm not clear on what to do with the AC reading ... or what the bridge rectifier is ( AC to DC converter right?) I need to disconnect the speaker first before taking these readings , yes ?
Okay to measure the supply, so on DC, put your Negative lead on ground and measure the Positive side on the capacitor, and keep your Negative lead on ground now measure the Negative side, they should be around the same voltage.

To check the diodes turn the power off, and put your multimeter on diode test and check across each Diode, it should read open circuit one way, and around 0.7V the other way.
 
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