keyboard amp loud hiss

Thread Starter

ninjaman

Joined May 18, 2013
331
hello
i got this amp recently. it was sat in my car overnight and got cold and possibly damp (could that be it?)
either way when i turn it on, initally it had a loud pop which i imagine from what i have read, is a large inrush current. the pop only seemed to happen when i switched it on. however in the video i have uploaded you do not hear the pop either of the times i switch on the amp. there is however a loud hiss. i am not too sure what this could be. i heard that removing the speaker and measuring with mV meter you get a certain reading. i tried that and got a mess of reading, all over the place, nothing solid. not sure what reading i should expect. i measured the resistance of the speaker and got 5.7 ohms. not sure what that reading should be but i thought its a bit of info.
any ideas or advice.
thanks
simon
EDIT: i have uploaded two pics of a dodgy looking cap, 330uF 200v, white stuff which i thought was thermal paste and a dark brown goo on the bottom where it was soldered on. i mesured the resistance of the cap and got 2M ohm connected either way round, using the beeper on my meter i got a tone for about two seconds then it stops, change leads around i get a tone again and then it stops. is this because the cap is charging to some extent and going from closed to open?
 

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scorbin1

Joined Dec 24, 2019
45
The capacitors in the pictures don't look horrible but could be the culprit. The sounds you describe definitely sound like a capacitor failing and outgassing to me. Could just be old capacitors, but possibly another failed component causing Cap failure. Perhaps another member can chime in with other things to check, or reinforce the recommendation to change caps.

The continuity setting(the beeper) applies a voltage to detect continuity, definitely sounds like it is charging the caps. Once charged the caps don't allow any current flow and the meter sees what it thinks is lack of continuity.
 

Thread Starter

ninjaman

Joined May 18, 2013
331
thanks for the response, the main thing is not buying/replacing components what are not required. i am looking at buying caps from china as they are the cheapest option and i dont want excess components laying around.
 

Dodgydave

Joined Jun 22, 2012
9,688
Ideally you need the service manual, as for measuring the DC voltage across the output with speakers disconnected, you need practically zero volts, anything over 500mV there's probably a fault, . What is the model number of the amp ?
 

scorbin1

Joined Dec 24, 2019
45
Be careful buying cheap capacitors for audio circuits. Unless it is just practice equipment, component quality can be a big factor in sound color and quality. If it is just a filter cap in the power supply section the effects will probably be minimum as far as sound quality goes, but can still be a considerable factor. If it's a filter cap in the amplifier section, it WILL almost assuredly have some effect on sound quality and color.
 

KeithWalker

Joined Jul 10, 2017
1,305
The capacitors are probably OK. It is quite normal for cheap amplifiers to give a pop when they are switched on. It is just the power supply charging the capacitors in the circuit. More expensive amplifiers have a built-in delayed muting circuit so that you don't hear that.
When you get the hiss, what do you have connected to the inputs? If you have nothing connected and the volume is turned up you can expect to get a hiss like that.
Connect the amplifier to a signal source and turn it on with the volume control turned down. Then turn up the volume until you get a comfortable listening level.
Don't worry about the speaker impedances. They are probably 8 Ohms. The amplifier will be able to work with 4 or 8 Ohm speakers. Just keep the volume down when you switch on and then don't overload the speakers or you will blow the amp and the speakers.
Regards,
Keith
 

Thread Starter

ninjaman

Joined May 18, 2013
331
the volume is down, like off. everything was turned down and the hiss was still very loud. nothing was plugged in
also i disconnected the inputs/outputs and controls so it was just the power supply board (whatever is on that) with the speaker coming off.
 
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Wolframore

Joined Jan 21, 2019
1,989
the volume is down, like off. everything was turned down and the hiss was still very loud. nothing was plugged in
also i disconnected the inputs/outputs and controls so it was just the power supply board (whatever is on that) with the speaker coming off.
sounds like a bias issue. I would change the caps out. First you need your power supply to be good. Those caps look like bypass for power. They are leaking, that’s not good in my book. If you want your amp to sound good, you need good components, power supply has huge impact on sound quality.
 

Thread Starter

ninjaman

Joined May 18, 2013
331
hello, i have replaced the cap, switched on the amp and only got a loud pop every two seconds or so. then the loud hush sound came back. could it be that the speakers are faulty? what else could i check?
thanks
simon
 

Wolframore

Joined Jan 21, 2019
1,989
Amplifiers are almost impossible to diagnose without test equipment. At least a decent multimeter, oscilloscope and signal generator.
 
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