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Phono Pre-amp fault
Phono Pre-amp fault
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Old 29th January 2018, 09:40 PM   #1
Dunk02 is offline Dunk02  Australia
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Default Phono Pre-amp fault

Hi
I have a faulty phono amp that has seen little use and been sitting around in a drawer for a year. It is an Art Accessories DJ PREII.

The right channel has stopped working.

When putting a 1khz sine wave through the amp I can see the signal with a scope on the left channel but not the right, just ripple and noise. I cannot see any damage on the board. There is continuity at the phono socket.

The unit runs from a 9v wall wart.

Is there a another strategy to finding the fault, or is there a likely suspect component? I am not familiar with the circuit layout yet.

Thanks
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Old 29th January 2018, 09:59 PM   #2
MAAC0 is offline MAAC0  Portugal
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Check the rubber. It can be shortening a PCB trace Check also the switches for continuity. ... Use the scope to trace the op-amps. Have You tried swapping the phono leads ?
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Old 29th January 2018, 11:19 PM   #3
audiopip is offline audiopip  Australia
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Also I would check the soldering around any 'high thermal mass' components, regulators, switches, phono connectors. From the photo some of them do not look too good. Flow soldered boards with SMT components suffer badly from this problem and they may take several years for faults to develop. Or PM me, send me the board (I'm in Melbourne) and I will try and fix it for you!
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Old 30th January 2018, 12:10 AM   #4
Dunk02 is offline Dunk02  Australia
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Quote:
Originally Posted by MAAC0 View Post
Check the rubber. It can be shortening a PCB trace Check also the switches for continuity. ... Use the scope to trace the op-amps. Have You tried swapping the phono leads ?
Fault is the same with pcb out of case. Capacitance switches have no effect. No other switches. Done the lead swapping.
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Old 30th January 2018, 12:11 AM   #5
Dunk02 is offline Dunk02  Australia
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Quote:
Originally Posted by audiopip View Post
Also I would check the soldering around any 'high thermal mass' components, regulators, switches, phono connectors. From the photo some of them do not look too good. Flow soldered boards with SMT components suffer badly from this problem and they may take several years for faults to develop. Or PM me, send me the board (I'm in Melbourne) and I will try and fix it for you!
I just resoldered all connections in the backside of the board. No change.
I have a temperature controlled out air gun. What temp should it be set for the lead free soldered SMT components?

Thanks for the offer. I’ll keep that ace up my sleeve for the moment ��

Last edited by Dunk02; 30th January 2018 at 12:13 AM.
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Old 30th January 2018, 10:18 PM   #6
MAAC0 is offline MAAC0  Portugal
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Dunk02
The rubber I meant is that shiny rectangular thing You see on the right side of the 3rd photo glued to the PCB. There is a PCB track that goes near the RCA's.
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Old 31st January 2018, 01:23 AM   #7
Dunk02 is offline Dunk02  Australia
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Quote:
Originally Posted by MAAC0 View Post
Dunk02
The rubber I meant is that shiny rectangular thing You see on the right side of the 3rd photo glued to the PCB. There is a PCB track that goes near the RCA's.
Apologies, the rubber made out of metal! Thought you meanít the rubber on the case exterior. Am I looking for a broken track to the faulty channel caused by mechanical strain? I am not sure which track you mean.

I have probed the right channel output (100khz sine wave input) while stressing the board but nothing showed up.
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Old 31st January 2018, 01:35 AM   #8
Dunk02 is offline Dunk02  Australia
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I don’t even know if inputting a DSP sinewave as a test signal is the correct thing to do?? No prior experience with this.
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Old 31st January 2018, 09:42 PM   #9
MAAC0 is offline MAAC0  Portugal
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The track I mean is under the edge of the "metal thing" As the "metal thing" is connected to ground and over the track ...


Why 100Khz ? Music goes up to 25 Khz. I would try something in the 1 Khz audible range.


You have lots of Vias (Holes) and one of them could be faulty. I would test them for continuity or perhaps fill them with solder.


But I bet it's one of the op-amps. Have You traced them already ? Feed You signal and then test at each outputs if there is a signal.
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Old 31st January 2018, 10:36 PM   #10
Dunk02 is offline Dunk02  Australia
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Whoops, meant 1000Hz.

The track you mentioned is pretty beefy and I checked continuity - OK.

I think the board is covered in a light conformal coating so don’t know if heat gun reflowing if surface mount components is viable.

Testing opamps in circuit - that is a new technique I will have to learn. At the moment I am not entirely sure of the board topology. I can see the input power stage, some circuitry for LEDs, and three IC packages with lots of resistors and capacitors, and three transistors. Looks much more complicated than simple circuit diagrams I have seen. I presume the ICs are the opamps but cannot find a data sheet for them - ST GZ308.

Last edited by Dunk02; 31st January 2018 at 10:46 PM.
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