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Old 7th Jan 2021, 8:50 pm   #1
bigfathairyvika
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Location: Cambridge, Cambridgeshire, UK.
Posts: 489
Default Yamaha C-4 Pre/control amplifier.

This C-4 came in with symptom of bad sound and fuse blowing.
It had been sold as having a capacitor upgrade and in full working condition.

On opening it was fairly obvious that someone who hadn't a clue had been at it.
The soldering that had been done was really poor with lots of dry joints and solder splatters.
Also evident was three transistors with heatsinks had almost left the board.

I re soldered the visually bad joints and removed all the solder splatter which was shorting a few tracks.
Next the three heatsinks were removed and the transistors re mounted.
This uncovered some cracked tracks which were repaired.
Two of the heatsinks were for the voltage regulation and although there were holes for screws , none had been fitted. Two screws each and all was well.
The third looked like it had previously been glued to the board and there was space for drilling two mounting holes in the pcb, so a quick drill and the heatsink was secured.

After checking the voltage adjustments a quick test and it was all working apart from the headphone output.

Re soldering all the pads in the headphone section cured this but left a bit of crackle on the balance pot.
A shot of switch cleaner and sweeping it backs and forwards a few times cured this.

On reassembling I noted that the headphone socket had previously been overtightened and cracked.
This needed careful dismantling to allow glue to be put in the correct place without gluing all the contacts together. The contacts are all silver plated and cleaned up a treat.

So, all in it was just solder joints that caused nearly all the faults.

A quick search on the internet found many other mentions of bad joints in this and other C series amplifiers. But most of them in this example were the result of badly done soldering.

On testing I did note that the headphone output is very much overly powerful. It could probably drive a set of full size speakers without much trouble.

Useful tips for this amp:
When setting the 0v offset voltages, note that they will drift all over the place all the time. This is normal apparently. But as the spec is +/- 200mv you have plenty of leeway.
The heatsinks run hot and this is normal, the 12v regulator section is dropping from 30v and only powering a couple of relays and three small lamps.

Mark
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Old 9th Jan 2021, 1:28 am   #2
Ted Kendall
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Default Re: Yamaha C-4 Pre/control amplifier.

Too common a problem, I fear. You frequently see "upgraded" hi fi on auction sites listed as "for parts or not working". I wonder why. Good save, though.
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Old 6th Feb 2021, 10:40 am   #3
vinrads
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Default Re: Yamaha C-4 Pre/control amplifier.

Well done getting it working again ,if you need to use the headphone socket you could fit a couple of resistors to limit the output , first use a potentiometer to establish the output that suits you ,measure the resistance and fit two resistors ,you may have cut the track to fit them ,good luck with it , Mick.
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