"static" noise from power stage

[MoonBassAlpha]

Hi folks
I'm repairing a Laney BC30 combo.
It reproduces the input signal fine, but there is a constant volume static-like sound coming out of it all the time, punctuated fairly frequently by louder pops.
My first thought was a dry joint somewhere so I went all over the usual suspects (output transistors, big caps, bridge rectifier) with an iron to reflow them - nothing doing. I removed the preamp input and bypassed the send-return with a patch lead - still the same.
I'm now thinking it could be an odd failure mode with a cap across the supply rails, or a protection diode across the rails going "soft".
There isn't an undue amount of mains hum either.
Any comments?
Cheers
Jules

[icastle]

[quote name='MoonBassAlpha' post='1039434' date='Nov 27 2010, 11:56 PM']Hi folks
I'm repairing a Laney BC30 combo.
It reproduces the input signal fine, but there is a constant volume static-like sound coming out of it all the time, punctuated fairly frequently by louder pops.
My first thought was a dry joint somewhere so I went all over the usual suspects (output transistors, big caps, bridge rectifier) with an iron to reflow them - nothing doing. I removed the preamp input and bypassed the send-return with a patch lead - still the same.
I'm now thinking it could be an odd failure mode with a cap across the supply rails, or a protection diode across the rails going "soft".
There isn't an undue amount of mains hum either.
Any comments?
Cheers
Jules[/quote]

PSU electrolytics breaking down perhaps?

[MarkBassChat]

The problem may be caused by several issues but I would never consider
[quote name='MoonBassAlpha']... an odd failure mode with a cap across the supply rails[/quote]before I analyse typical possible causes.
And if you have:
[quote name='MoonBassAlpha']sound coming out of it all the time, punctuated fairly frequently by louder pops[/quote]
this may be caused by failing capacitor in the signal path. So I would look at the power amp input capacitor and other small capacitors in the signal path (there are only 3 or 4 in this amp).
Of course by "lookig at" I mean replacing them because the failure is usually not visible.
If this does not help, I would look at the power supply.

Mark

[Phil Starr]

It could be so many things that it is going to be pretty nigh on impossible to diagnose over the net. Buy a can of freezer spray and systematically work along the signal chain. When you suddenly freeze the faulty component the fault will change and you have a culprit. There may be more than one. If it is a leaky cap then check any associated pot for pitting as it may have let in an unwanted DC voltage across the pot.

[zigmondo]

Assuming it's not a valve jobby? Valves would be the first thing to check if so(eg, any gone microphonic?)..
Perhaps the popping might suggest capacitor as above?
Power supply faulty? (given the constant sound you describe)

[MoonBassAlpha]

[quote name='zigmondo' post='1040660' date='Nov 29 2010, 12:26 AM']Assuming it's not a valve jobby? Valves would be the first thing to check if so(eg, any gone microphonic?)..
Perhaps the popping might suggest capacitor as above?
Power supply faulty? (given the constant sound you describe)[/quote]
It's transistor, circa '94, the power board has some surface mount stuff on the underside too.

[MoonBassAlpha]

Resolved today!
I replaced the 2 transistors on the input (long tailed pair), and the one following them (all smd) and all seems good.
The noise briefly went after I reflowed one of them, but came back after a few minutes, so I suspected it was in an odd failure mode.

I have to big up the service response from Laney here.
I asked for the circuits for this amp via email and got them back in 4 minutes! That takes some beating, no?
cheers
Jules

[icastle]

[quote name='MoonBassAlpha' post='1042840' date='Nov 30 2010, 07:59 PM']I have to big up the service response from Laney here.
I asked for the circuits for this amp via email and got them back in 4 minutes! That takes some beating, no?[/quote]

Very impressive - circuit diagrams can be very hard to come by from some manufacturers and often come with a price attached.