Yamaha C-80 preamp problem

[Moondog55]

OK Just to let everybody know in advance that electronics isn't my strong point.

I seem to have a oroblem with intermittent fading and then return of volume, it gets soft and then louder again.
The local Yamaha certified repair techie has had a look at the unit and says he cannot find anything wrong with the unit.
It isn't any of the power amps as they work fine with other controllers, so I am sure the problem is in this Yamaha.
Any ideas?? I am not sure at this stage whether to sell it off or repair, up until now I have been very happy with it.
I have read the threads relating to C-80's and this problem did not come up.If I keep it I will follow the other advice and do some serious ventilation of the case.

regards
Ted

[Moondog55]

BUMP!!!

Problem is getting worse,have to do something soon; OK do I try and fix problem or just buy a new pre-amp?? I know it's not exactly a DIY query but my thing is speakers not the stuff further up the line.

Given the state of the worlds economy maybe I should just buy new and keep a factory in China busy, what a shame!!

[tomtt]

you might just set this pre-amp aside, and fix it later.





these are simple to build -

http://www.diyaudio.com/forums/showt...46#post1226446

[comicrcc]

Do both channel's fade in and out???? How about the phono input???? I have a C-80 and had power supply problems which caused noise and loss in phono input. The 6800uf caps and a few diodes were bad. Seperate power supply for phono. I will see if I can find my service manual for C-80.

[Moondog55]

Yes both channels at same time. I haven't used the phono input on over 9 months so can't say.

This has started recently and the repair bloke said it did not happen when testing it

[rocco gibralter]

Hi moondawg, I had a similar problem caused by overheating ...bias offset, Is there any distortion or erratic sound output?
Sounds like you might have a leaky or dry psu cap? if they arent working properly you get power dips on the supply side! cheap enough and easy to replace!

Never trust repair mens diagnoisis they mostly do the bare minimum ie, if you have a blown o/p transistor they will only replace the failed one not all of them...PLZ Dont fuel the Chinese economy, these things are built with a self destruct button, There are some decent valve amps now tho!

Best advice is to take off the cover and get stuck in armed with your digital multimeter..Unplug it first obviously! Start looking for obvious stuff like dry solder joints etc bulging caps, discoloured resistors etc...

if it all goes horribly wrong..think of it as a learning curve!

we all have to start somewhere!

[tvi]

Does the headphone output also fade?

The headphone amp bypasses the "invert amp" so if the headphone out is good then the problem is most likely with the "invert amp"

I'd first suspect the mute transistors, I have shadowed them in the attached pic.

If you need it I have the service manual, its 13meg though.

hope this helps

Regards
James

[comicrcc]

I would suspect the power supply because you said both sides do the same thing. Both invert amps are both bad. Could be but not as likely. The 6db flat amps are ahead of the invert amps any way. If you are pluged into the reg outs the inverts are out of the picture. The original unit had 4 leg 4700uf 35v cans. The power supply is pushing them very close to 35v. I could not find that size in a 4 leg that would fit the pc board holes. I had to get 6800uf 63v 4 leg caps. Takes 4 of them. C367, C368, C211 & C212. The only ones I could find were on EBay and they are taller than orig caps and you can not put cover back on. I also replaced the diodes in the supply's while working on it. The extra uf and voltage give a cleaner and less to the max on caps working voltage. I have not had any problems since then. Sounds great. There are + & - test bus points near supply. Check with scope for level & ripple noise.

[comicrcc]

Sorry I did not get back with you last night as I was watching the UF / LSU football game. I am a big UF fan. This preamp uses a lot of discrete parts not like newer units with dual opamps where 1 bad opamp could affect both channels. To have 2 bad transistors in the same place in both channels is very unlikely. If you had some test equipment you could insert a sine wave to both channels thru tuner input and check the output along the signal path. There is a known problem with this unit with the power supply and breakdown of the filter caps over time. I hope this helps.

[Moondog55]

Sounds like I need to replace the PS smoothing caps then,
I will start looking for a better replacement Any sugestions??