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PPX 1600 channel issues

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simonp1100 View Drop Down
Young Croc
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 12 June 2018 at 5:35pm
Always start at the beginning, so check the XLR & jack inputs (just do this with a multimeter). Let me know how you get on.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote scott_fury Quote  Post ReplyReply Direct Link To This Post Posted: 12 June 2018 at 11:02pm
That makes sense to me as an electrician. Going from a known starting point and testing for live voltage or dead testing for continuity until I find the point where the circuit is shorting or broken.

Although I know how a resistor operates and how to recognise the symbol on schematic, my limited understanding means I don’t know why it is there and what it’s purpose is.

Maybe starting with the input and working through, following the signal path, may give me a better understanding of how an amp works? God knows, as an owner of a few knackeres PPX’s I’m gonna need to know how to fix ‘em!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote scott_fury Quote  Post ReplyReply Direct Link To This Post Posted: 17 June 2018 at 9:34pm
Well I haven't got much further with the testing. What I have done so far is to put a 1kHz tone into the input from my phone, measuring 0.121V, and then test it up to IC2.




I can measure the signal at R42 as 0.121V and then on the other side as 0.088V. I can also measure this 0.088V at the input pin of the op amp at IC2. I'm guessing that C22 and R42 are a high pass filter before the op amp, hence the reduction in voltage?

How can I measure that this signal is getting through the opamp (pin 1 or 8)? I'll need to power up the amp and measure the signal combined with the voltage from the rail?
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simonp1100 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 17 June 2018 at 9:53pm
You could just trying swapping the working chip on channel A to the faulty channel and see if it works o.k. or you could just inject a signal with a signal generator or a mobile phone with a volume control into the mono link molex connector and see if that works (if the chip is working correctly) as IC2 is being used twice in the circuit.

Still check the bridge switch as if this is faulty this will not send the signal to the rest of channel B amplifier circuit.


Edited by simonp1100 - 17 June 2018 at 9:58pm
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scott_fury View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote scott_fury Quote  Post ReplyReply Direct Link To This Post Posted: 18 June 2018 at 9:43pm
Originally posted by simonp1100 simonp1100 wrote:

You could just trying swapping the working chip on channel A to the faulty channel and see if it works o.k. or you could just inject a signal with a signal generator or a mobile phone with a volume control into the mono link molex connector and see if that works (if the chip is working correctly) as IC2 is being used twice in the circuit.

Still check the bridge switch as if this is faulty this will not send the signal to the rest of channel B amplifier circuit.

I’ve done a functional test on the bridge switch as well as for continuity and it seems to be operating fine.

I put a tone though the link on Channel B with nothing on output. So I then swapped the IC2 chips from A to B. The result was that Channel B worked fine, but Channel A produced a very quiet signal in comparison. So the faulty op amp seems to produce no sound on B and a very weak sound on A.

I’ll order some replacements and see what the results is. Should I replace all of them whilst I’m at it? Will any 4558 chip do?

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simonp1100 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 18 June 2018 at 10:12pm
good, looks like a faulty chip. Just replace with a MC1458P (Farnell 110-3044). I would just change ALL of them as they are only 42 pence each.

Clap
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Post Options Post Options   Thanks (0) Thanks(0)   Quote scott_fury Quote  Post ReplyReply Direct Link To This Post Posted: 18 June 2018 at 10:58pm
That’s great Simon. Thanks for helping. I also have a 1200 that’s only got one channel, so may buy a bulk load of chips in case it’s the same issue 👍🏻
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RootsExplosion Quote  Post ReplyReply Direct Link To This Post Posted: 21 April 2021 at 10:44am
Hello guys 


Big thanks for all the information here. 
We serviced a PPX1200 and got the same fault. We have R71 with specifications: 5K6, 2Watt. We measured 7,2Kohm on it. 
Replaced it, works fine now. 

Is theyre an explanation for the higher value? 

Grtz 


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