Yamaha CA-400
 

Hi all,

Well I thought I'd make a relevant thread for this amplifier. I bought it locally and it caught my attention being a nice bit of 70s teak finished kit and I could do with a good amplifier for my hifi. It was sold as having one channel missing but in reality the faults were much more severe.

I have now replaced all of the rectifiers, some were burned up and others were short so it wouldn't have worked very well in that state. One of the large 4700uF capacitors had a hole in the side and the other although in good physical condition was a dead short. I also had to replace the 10R 2W resistor which had burned up.

With all of the above done I applied a bit of power via the Variac, a suspicious warm smell soon eminated and I traced this to the replacement 'D705' rectifier getting hot. As this supplies the output transistors this will be my next area to investigate as something in this circuit is drawing excessive current. I believe the transistors fitted are known to be problematic so I'll start there and check the surrounding components.

It seems to be a very well designed amplifier so I look forward to hearing it working especially having never heard a yamaha amplifier of this age either.

Re: Yamaha CA-400
 

An update with the amplifier, I decided to replace all of the output transistors - reading online I found that it seems to be a common cause of failure with these amplifiers. As I'm gifting this one to my pals who have just moved house and wanted something like this for their hifi setup I thought it would be best of make it reliable.

Once these were replaced I powered up the amplifier via the Variac which was more promising this time, no high current draw or smells of heat. I plugged in some headphones and could hear a nice hiss with the volume turned up so I knew that it was alive now.

With this all done and the new transistors biased I connected up my turntable and had a half hour or so of listening to some records. All was good until some time later when the right channel started rustling heavily, too bad to just leave. This was traced to the first stage transistor, a Toshiba 2SA763. I went through all of my spares drawers to find something suitable for replacement and came across some BC327-25 transistors, with some bending of the legs and a bit of insulating I fitted this in place of the '763 and was then met with a very nicely working amplifier.

Rather than doing any more work I spent the remainder of the evening listening to more albums and was delighted by the quality of the amplifier, it works and sounds very nice. I did also replace 2x 22uF 16v capacitors as one had been replaced with a large 250v rated one, as the stereo board is two identical circuits side by side it was annoying me that it looked so out of place and both were swapped to a nice matching pair. So there we have it, quite a straightforward service with nice results, what more could you want!

The final steps are to clean up all of the metalwork and lots of attention to the wood casing. Some large strips of veneer are missing and one side panel had fell off, after much gluing and clamping this has been re-attached. The loose veneer has been stuck back down and some thin craft ply is currently being glued in place of the missing sections. Once this is all done it will be sanded level before sanding over the whole cabinet and re-veneering throughout.

Re: Yamaha CA-400
 

Toshiba 2SA763 noise, know problem see here
Top Ten Worst Transistors - noisy, failure-prone, whatever... and replacements
https://audiokarma.org/forums/index....ements.731653/

Re: Yamaha CA-400
 

Thanks for the link, I'll keep that handy as it's a real problem with some 70s amplifiers. There are a few more 2SA763 transistors used although they're okay. The other thing to note is a lack of mains earth with the amplifier, I find that having a 2 core mains lead and separate earth terminals to the rear is quite outdated for an amplifier made in 1975. For safety purposes I'll fit a new 3 core lead and earth the chassis.