Power amplifer crackling.

[Tony Kennedy]

Hello,

I am attempting to fix a power amp that has just started making crackling
noises on one channel (noise is always there, volume makes no difference to
the level of crackling).

I have searched on Google and it seems that I need to look for faulty
resistors or transistors at the preamp stage. Does anyone on the forum have
any additional/better advice for me (am I looking in the wrong place)?

Thanks in advance for any help at all, it is much appreciated.

Tony.

 

[Rheilly Phoull]

Hello,

I am attempting to fix a power amp that has just started making crackling
noises on one channel (noise is always there, volume makes no difference to
the level of crackling).

I have searched on Google and it seems that I need to look for faulty
resistors or transistors at the preamp stage. Does anyone on the forum have
any additional/better advice for me (am I looking in the wrong place)?

Thanks in advance for any help at all, it is much appreciated.

Tony.

I would look in the power stage since the volume contol makes no
difference. Conventional wisdom says check electrolytic capacitors then
transistors.

 

[Tony Kennedy]

I would look in the power stage since the volume contol makes no

difference. Conventional wisdom says check electrolytic capacitors then
transistors.

Cheers. Anything specific to look for? None of the components look 'expired'
(heat burns etc).

 

[Sofie]

Tony Kennedy:
Look for leaky driver transistors and leaky electrolytics..... and cracked
or cold solder connections...... when I say look, I mean "with test
equipment" .... DMM, Scope, ESR meter, etc.....not just a visual
observation..... bad components won't necessarily "look expired.. heat
burns, etc." .
The good news is that since you have the other channel that works OK, you
have a basis for test comparison and a source of substitution components.

 

[El Meda]

Cheers. Anything specific to look for? None of the components look 'expired'
(heat burns etc).

Heat the power stage transistors one by one with the soldering iron
tip, beginning with the smaller ones. The crackling noise will
increase when you heat the damaged transistor.

 

[Tony Kennedy]

Heat the power stage transistors one by one with the soldering iron

tip, beginning with the smaller ones. The crackling noise will
increase when you heat the damaged transistor.

Thanks for the advice, I will give it a go over the weekend.

Tony.

 

[Tony Kennedy]

Look for leaky driver transistors and leaky electrolytics..... and cracked

or cold solder connections...... when I say look, I mean "with test
equipment" .... DMM, Scope, ESR meter, etc.....not just a visual
observation..... bad components won't necessarily "look expired.. heat
burns, etc." . ok.

The good news is that since you have the other channel that works OK, you
have a basis for test comparison and a source of substitution components.

Good point.

Thanks for the advice, hopefully I will get round to having another go at
the weekend.

Tony.

 

[Tony Kennedy]

Look for leaky driver transistors and leaky electrolytics..... and cracked

or cold solder connections...... when I say look, I mean "with test
equipment" .... DMM, Scope, ESR meter, etc.....not just a visual
observation..... bad components won't necessarily "look expired.. heat
burns, etc." .
The good news is that since you have the other channel that works OK, you
have a basis for test comparison and a source of substitution components.

Thanks for the advice. I have just got round to looking at the amp again.

I have ruled out the power transistors (I swapped the preamp boards over,
the good preamp board is fine with both sets of power transistors).

Is there any specific place I should start looking (ie, look along the
signal path)?

And you mention driver transistors, do you mean the main power transistors?

Thanks again, any help is much appreciated.

Tony.

 

[Sofie]

Tony Kennedy:
The driver transistors are after the preamp...... they are what "drive" the
big power output transistors. If there is not a separate driver and bias
board after the preamp then this circuitry may be included on the preamp
board...... there will probably be some bigger transistors than the
smallest preamp transistors...... also somewhat physically larger resistors
than would be found in the preamp circuitry..... usually several low ohm
units from 0.1 to 100 ohms..... the big power output transistors are
usually near and around this circuitry, at least electrically. The base
connections of the big power output transistors will be electrically
connected near and around the emitter connections of these driver
transistors. There may also be pre-drivers and biasing circuitry to test
for noise. Be certain to look for faulty solder connections on the board,
noisy and leaky electrolytics, noisy and leaky semiconductors which includes
both diodes and transistors.
Are you having fun yet?
Good hunting.
--
Best Regards,
Daniel Sofie
Electronics Supply & Repair
----------------------------

Thanks for the advice. I have just got round to looking at the amp again.
I have ruled out the power transistors (I swapped the preamp boards over,
the good preamp board is fine with both sets of power transistors).
Is there any specific place I should start looking (ie, look along the
signal path)?

And you mention driver transistors, do you mean the main power transistors?

Thanks again, any help is much appreciated.
Tony.

- - - - - - - - - - - - - - -
..components.
..
..
..

 

[Tony Kennedy]

Thanks for the reply Daniel,

The driver transistors are after the preamp...... they are what "drive" the
....snip...
both diodes and transistors.

I think I have got that. So the place to start is with the big power
transistors and work backwards a bit and replace the trannies and caps I
find. I had started with the input (balanced, signal into two electrolytics
and then on to an opamp).

Are you having fun yet?

Oh yes, loads of fun. When I swapped the preamp boards round initially the
good one stopped working (I must have connected something the wrong way
round as two op-amps blew, but I managed to find that). The worst thing was
that the amp initially went wrong going to a gig on new years eve, the
mono/bridge switch took a bash (destroying the switch) so I could not use
either channel). It made for a very nervous new years eve using the backup
amp (which had been cutting out at rehersals, but didn't on the night (or
since)).

One thing about the board, there are components (labled Q1, Q2 etc) that
have 5 pins (I can find no semiconductors that are similar in my Maplin
catalogue). Do you know what these might be? I have uploaded a pic of the
board to
http://homepage.ntlworld.com/kennedylanduk/ramsa/ramsapics 003.jpg
the component is slighty to the left of the upper opamp (Q4).

Thanks again for the help, I really appreciate it.

Tony.