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Issues with a Yamaha C-50 pre

eteller

eteller

Addicted Member
Picked up a C-50 and have found left channel to cut out and the phono section plays really low (MM cartridge on the MM setting). Upon inspection I've found four caps that are toast, two 1000uF near the power supply and two covered smaller ones in the phono amp section, everything else looks fine. I'm curious on the odds of the damage being limited to these caps?
 

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eteller said:
Picked up a C-50 and have found left channel to cut out and the phono section plays really low (MM cartridge on the MM setting). Upon inspection I've found four caps that are toast, two 1000uF near the power supply and two covered smaller ones in the phono amp section, everything else looks fine. I'm curious on the odds of the damage being limited to these caps?
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What makes you think there bad..The glue around the base of them.:D
 
Left channel cutting out.Is most likely either a cold/cracked solder connection or a dirty switch contact.But it could be more serious.Usually not.
 
two covered smaller ones in the phono amp section,
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Those are not caps.Each one has six solder connections.Guessing they are a differential pairs for the phono pre-amp.
 
well if it's glue it's corrosive. I tried cleaning all the switches, checked all the solder joints on the bottom of the board. Pushed on all the components on the top of the board. Same issue, MC mode is louder than MM but cuts in and out, bad turn on/off thump.
 
bad turn on/off thump.
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Need a service manual.The thump is most likely excessive dc offset in the pre-amp.Might just be and alignment or..might be a couple transistors as well.
MC is routed through a MC phono preamp then to the EQ amp. The MM is routed straight to the EQ amp.Thats why the MC is louder than the MM.

The glue needs to be irradicated..And a recap wouldn't hurt.
Check your solder connections where the RCA phono jacks solder to the main board. These jacks have a bad habit of internally breaking the ground or center connection due to tight fitting phono plug IC's.

Wiggle the phono plug connections to see if you can locate a possible broken connection.
 
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Got a neighbor who is an electrical engineer. I bribed him and he came over and we re-flowed all the joints going to the RCA's (the left OUT jack is a little loose) and anything else that looked suspect. There are two little pots in the EQ amp section and no amount of cleaning will get those un-scratchy nor affect the low volume issue, you can turn them and that channel will cut on, till you switch someting. He recommended replacing the caps and to get a schematic/service manual which I can't find. The issues will clear up for a minute and you switch inputs or controls and it reappears but will sometimes switch channels. He had no idea on the thump, but you cleared that up.
 
eteller said:
Got a neighbor who is an electrical engineer. I bribed him and he came over and we re-flowed all the joints going to the RCA's (the left OUT jack is a little loose) and anything else that looked suspect. There are two little pots in the EQ amp section and no amount of cleaning will get those un-scratchy nor affect the low volume issue, you can turn them and that channel will cut on, till you switch someting. He recommended replacing the caps and to get a schematic/service manual which I can't find. The issues will clear up for a minute and you switch inputs or controls and it reappears but will sometimes switch channels. He had no idea on the thump, but you cleared that up.
Click to expand...

The low volume on MM input is going to depend on your cartridges output level.All my preamps act the same way as yours. I just need to turn the volume control up a little higher when using MM.Right now I'm using a HOMC cart and I have plenty of volume.
 
Glad you like it. The brochure was in fair condition, but I still had hours of work to do on it.
I hauled my C-50 out of mothballs to have another look at it. I had repaired a bunch of solder joints on the rca jacks when I got it, but now that I have better equipment, I see many suspect joints throughout the unit, particularly around the power supply. I'll be reflowing all the joints in the unit.
Also, I'm renewing the heat sink compound on the large transistors. One I examined looks like it had about the minimal amount; it was dry and had large gaps.
I may put this rig back into service, but I'll have to revisit my M-50 first.
 
avionic said:
The low volume on MM input is going to depend on your cartridges output level.All my preamps act the same way as yours. I just need to turn the volume control up a little higher when using MM.Right now I'm using a HOMC cart and I have plenty of volume.
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With this one I gotta crank it to 11 to get LOW volume, better on the MC circuit but the volume knob is still up 3/4 of the way and very noisy.
 
Here's a pic of the solder side of my C-50's power supply area:
attachment.php


Pretty ugly. Lots of shriveled and cracked joints.
 

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