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Ev p 3000 power amp fault

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    Posted: 24 May 2011 at 8:29pm

Hi all... here's and odd fault...after repairing this beast of an amp ev p 3000 this odd fault popped up..

now, on using the file for voltage readings as out lined...this is what's happened...

A channel fully working with all correct voltages as ref the circuit lay out...bias for hot/cold setting is 15mv spot on float voltages 67+/- correct..then offset at speaker o/p 10mv...all is fine nice clean power...

Now, here's the odd one..B chan..voltages fine but when testing the offset that should be 10mv..it's at 300mvConfused... so I've swapped round A side to B channel to see if the driver circuit was the fault such as long tail circuit making the o/p high.. and nope both driver circuits are working using A channel change over....even when adjusting the offset pot to bring down the dc offset it doe's not come down
even fitted a new pre set pot to both side's...things are still the same...

I've checked each resistor/transistor/cap again and again even track to tack point to point on both side's thinking it may show different readings and there both the same via cold testing.. now on powering back up there's different offsetting's  it should be 10mv's and no highter..not 300mv's...

when powering up the power relays click over on both side's after it does it's safe protection test,
all voltages check spot on apart from the offset!!

matched o/p's fitted to both sideS all part's fitted via spec... some may say it's the long tail circuit to the amp? but, swapping round the block's and refitting the plug connections from the main pcb to the power stage's it's channel 2 that's acting odd...

Has anyone had a fault like this where you've gone over each part triple checking before powering up to ensure there's no over heating smoke happing...

any pointers would most helpful with this on going pain in the butt...

Kind regards. T.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 2011 at 9:06pm
So do you mean you've changed over the output modules and the fault follows the module, or stays on the same side? It isn't clear.

What was the original fault, an output module blow-up?

If you have narrowed the fault down to the 'repaired' output module, that suggests it hasn't quite been repaired yet!!

You might find its something as subtle as a tiny leak on a predriver transistor, something that will only show up with component-out tests. It is not good enough to check the components in circuit.

DO the bias hot and cold come in ok on the faulty channel? If so, are the trimmers in roughly the same position, or all the way to one side to obtain the correct bias? Sometimes one half of the bridge can lose its bias completely..meaning the offset will never acheive anything like zero. CHeck the aux bias supply rail is reaching the output module, it comes from the power supply card in the middle of the amp.

Some of the resistors in particular on the P3000 are only 1/8W and can blow open quite cleanly, so double check all of those too.

There are also some very fine tracks that can vapourise such that they leave the 'image' of the track imprinted on the board to make it look like the track is still there when it isnt!

Have a break, leave it for a couple of days and look again with a fresh pair of eyes!






Edited by kedwardsleisure - 24 May 2011 at 9:12pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 2011 at 9:19pm
I was going to say leaky driver transistor.  At this stage I tend to start changing ones which could possibly cause the problem because a few pence for a transistor is less than my time to try and do it analytically.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tan-2 Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 2011 at 10:49pm
Thanks guy's for your feed back on this matter...both channels bias up nice on the hot/cold setting to 15mv's even the pre set have been changed just in case.. yes it was rebuild due to being blown up...  every track has been check from point to point with the working side and there's no print damage to be found.. all damaged part's that was burnt/s/c o/c was replaced with same spec as the file says.. there's no damage to the pcb...

I've lifted each leg of channel A's resistors to match the replacement one's for fitting into b side.. there 1% types for this beast.. I've gone over each voltage reading from the good channel and it's the same as B channel apart from the different offset..300mv on B to high as to 10mv on A channel even the offset trimmer's have been changed in case there faulty...

it power's up ok and doe's the safe check before the relay's allow the o/p end's the connect...

it's a odd one this...



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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tan-2 Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 2011 at 11:19pm
it's an odd one.... if A chan is swapped round to B chan, B driver circuits works being linked to A BUT refitting the Side's to there rightful places's B chan o/p block still act's up with 300mv on the o/p end.. A channel is ok.. 


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Post Options Post Options   Thanks (0) Thanks(0)   Quote audiomik Quote  Post ReplyReply Direct Link To This Post Posted: 25 May 2011 at 12:03am
From faultfinding a different Amplifier with a similar fault; have you checked for oscillation at the output of the faulty channel?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 25 May 2011 at 12:11am
Also, does the output from the two channels look the same on the scope, same clipping points under load etc?  The current limiting circuits could potentially cause oddities like this too.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tan-2 Quote  Post ReplyReply Direct Link To This Post Posted: 25 May 2011 at 8:21am
@audiomlk, there's no oscillation at the o/p of this unit as it protect's it's self and will pull the o/p relay off...

@ceharden I will be scoping the o/p and see what happens, it's case of looking at it to long and going round the house's as they say...the current limiter as been rebuild in the o/p stage aswel..
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 25 May 2011 at 9:52am
Don't rely on the protection circuits for fault diagnosis.  The VHF sensing part is quite insensitive, you need to have a pretty large oscillation before it'll put the amp into protect.

I think watching the behaviour of the amp running into a load might give you some clues.

The P series has a DC servo to correct for any offset so there must be some reason why it is unable to detect and correct for it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tan-2 Quote  Post ReplyReply Direct Link To This Post Posted: 25 May 2011 at 12:46pm
Thanks for talking...now you can see how it's acting up...

new pcb more like for this old timer....more testing to be done here...I will post the out come...gin..


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Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 25 May 2011 at 12:55pm
What happens if you disconnect the ribbon cable going to the output board.  Is there still a DC offset?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 25 May 2011 at 1:04pm
Incidentally, the DC offset adjustment is purely for the VCA which does the limiting.  There is no offset adjustment for the output stage itself.

Also, does the offset stay the same whether there is a load on the output or not?

As I said before, your investigation has to centre around why the servo isn't able to correct the offset.  Eg, for starters, is the DC offset present on the input of the integrator and what is the integrator outputting?
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