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Left Channel Dead - Help needed - Old 80's Proton Amplifier
Left Channel Dead - Help needed - Old 80's Proton Amplifier
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Old 13th February 2018, 07:27 AM   #1
jiot is offline jiot
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Default Left Channel Dead - Help needed - Old 80's Proton Amplifier

hello All

My old faithful 80's Proton AI3000 All in One Home AudioAmp/Cassette/CD Player has broken..left channel started to Pop/surge..then no sound out of the left channel at all...regardless of input selected.

However I was surprised to find the Headphones still work ?! (both channels fine)...
Anyone say what could be going on.
Looking at the amp circuit .. the 'line out' goes via the headphones jack sockets then onto the speaker terminal circuit board...which has upon it a cap/resistor in line with the L/R Speaker Output Terminals.

One thing I have tested so far with a meter is the resistance to ground (black negative) speaker terminal to the amps metal chassis...I was surprised to measure 4.5ohms...should it be nearer zero ?!..if so..could I just connect it the negative terminals to the chassis 'to make sure' ?

Any thoughts about what has been said ..and what I should check...many thanks in advance for responders.
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Old 13th February 2018, 08:59 AM   #2
mjf is offline mjf  Austria
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.....first i would clean the headphone jack with a cleaning spray or so what.........to be shure there is no contact failure.
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Old 13th February 2018, 09:04 AM   #3
Mooly is offline Mooly  United Kingdom
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Left Channel Dead - Help needed - Old 80's Proton Amplifier
The high resistance may be completely normal, I would not alter anything there.

Its impossible to advise without seeing all the circuit details but my advise would be to locate the output stages (are they chip or discrete ?) and do some basic DC voltage checks of the operating conditions.
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Old 13th February 2018, 10:32 AM   #4
jiot is offline jiot
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Thanks for replies mjf, Mooly

Im going to check the headphone jack socket contacts!

Ive Attached most of the circuit in question...if anything more of the Units circuit may be of use let me know and I will post it.

Mooly,,so happens I have another unit the same for spares..which works except for a faulty cd player control board...I checked its negative speaker terminal to chassis ground resistance...its 0.5ohm! so of course thinking something not quite right...but Im not sure where in the circuitry the connection to the metal chassis is made...so not sure ..still searching.
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File Type: jpg proton left channel cct.jpg (339.6 KB, 53 views)
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Old 13th February 2018, 12:00 PM   #5
Mooly is offline Mooly  United Kingdom
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Left Channel Dead - Help needed - Old 80's Proton Amplifier
Seeing the circuit, and knowing it works OK on headphones then I would go along with mjf's suggestion of carefully checking the headphone socket and also the soldering of the socket and all connections to and from that small sub board.

You could also check the DC voltage across each speaker terminal (with no speakers attached). There should be less than 100 millivolts DC present.
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Old 13th February 2018, 12:52 PM   #6
jiot is offline jiot
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Mooly Thanks again...and thanks to mjf's suggestion...cant be sure but after a good spray with cleaner/lube spray into the headphone socket and working a jack plug in-out more than a few times..in addition to finding a power supply socket lifting a track (resoldered) its working again !!!
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Old 13th February 2018, 01:12 PM   #7
Mooly is offline Mooly  United Kingdom
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Left Channel Dead - Help needed - Old 80's Proton Amplifier
That's good
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