[SOLVED] Yamaha C-4 problem: horrible distorted sound / hiss in left channel

[Mr. Yamaha]

Hi guys. I have a problem with my recently restored Yamaha C-4. It has been fully recapped with high q caps, trimmers were replace by multi turns and the VD1212 diodes were replaced by 2 1N4148's in series. The restoration has been done by a highly regarded professional here in the Netherlands.

The guy who restored it, listened to it for a day or 2 after the restoration and it played fine. When I picked it up, we listened together (also de-bypassed tone control) and it sounded great.

Reinstalled my complete system last week (C-4, M-2, NS-1000M) and it sounded off. Left side sounded dull. Well that turned later out be a half dead tweeter on my left NS-1000M. While it seems fine beforehand on my CR-800. Coincidence? Installed a 'new' tweeter and the system sounded great.

Back to the C-4: I listened for a week and then I started to notice nearly audible distortion sounds in the left channel randomly. They come and go.

Because I always listen to the C-4 in flat, so the tone control is always bypassed. But then, in order to find the problem, I de-bypassed the tone control and the distortion sound did almost break my ear drums. What the hell. Turned the volume knob to zero fast enough to prevent further damage.

The facts:

• Distortion sound in left channel
• Nearly audible when tone control is bypassed and audible through playing music.
• Awful audible when tone control is active and no music audible through that channel
• Turning the knobs of the tone control (bass, treble and freq turnovers) does not have influence on the sound.
• Balance completely set to right does not prevent the distortion sound to come out of the left channel.
• My C-4 runs pretty hot. When playing for 2 hours you can lay your hand on the right side (above volume), but it feels pretty hot. Left side also hot, but less than right. The repair guy said that some of the boards were pretty bad due to the heat it suffered and he had to make several bypass traces. But I've read that a C-4 always runs hot.

Any ideas on where to start? I can hold (and use) a multi meter and a soldering iron myself.

P.S. I know the repair guy should have a look, but he lives on the other side of the country and then I won't see my C-4 for a month.

 

[avionic]

Check the solder connections on the ribbon cables that connect the small tone control/headphone amp board to the front main board.

 

[Mr. Yamaha]

Check the solder connections on the ribbon cables that connect the small tone control/headphone amp board to the front main board.

Checked and reflowed (both sides of all ribbons). Reflowed some sketchy solder connections on every board.

Did not solve it unfortunately..

Can a bad transistor cause this somewhere?

 

[avionic]

Nearly audible when tone control is bypassed

Does the control amp make the same noise without a source connected? Say AUX without anything connected to the aux input jack?

Can a bad transistor cause this somewhere?

Yes..As can diodes.

 

[petehall347]

get in touch with the tech that worked on it ..he/she might remember something .

 

[Mr. Yamaha]

Does the control amp make the same noise without a source connected? Say AUX without anything connected to the aux input jack?

Yes..As can diodes.

Yes the noise is there without any source connected, but it seams at much lesser volume than with source connected. And again is pretty loud with tone control not bypassed and less loud with tone control bypassed. And also audible through headphones. I even hear some hiss in the other channel at high volume and balance does not change anything to it all.

It also looks he left some VD1212's in, or are there look a likes?

He replaced 2 in the headphone board, seems to be 0 left.
He replaced 0 in flat amp board, seems to be 3 left.
He replaced 2 in the function board, seems to be 2 left.

 

[avionic]

How about a photo of the printed board under those two heatsinks ?

 

[Mr. Yamaha]

How about a photo of the printed board under those two heatsinks ?

Here you go. Already reflowed those 12 legs and scraped away old solder flux.

And can you confirm that those VD1212's on the earlier photo's are VD1212's?

 

[Mr. Yamaha]

Any ideas?

 

[avionic]

can you confirm that those VD1212's on the earlier photo'

Yep. Thats what they are. Theres around 10 of them in there.

 

[avionic]

Does wiggling the wafers on the selector switches have any effect on the making the left channel act up ?

 

[Mr. Yamaha]

Does wiggling the wafers on the selector switches have any effect on the making the left channel act up ?

It does do something, but I'm not sure it influences the distorted sound. When I wiggle those wafers I hear cracks and pops in both channels. Even when I push on the board itself I hear those cracks and pops, so it's not specifically the wafers I guess. I also noticed that when the small board next to the headphone board is disconnected, there is no distorted sound in the left channel. I'm not sure what that tells us, beacause maybe I wouldn't have a sound at all with that board connected.

 

[Electone]

Re-flow the ground connections on the selector switches.

 

[Mr. Yamaha]

Re-flow the ground connections on the selector switches.

Thanks, that solved the cracking and popping of wiggling the selector switches.

But unfortunately did not solve my initial problem. I also notice a low frequency hum on the right side at high volume.

 

[Mr. Yamaha]

Some other things to check?

 

[avionic]

The fact that its so intermittent. Is going to make this real tough to find the issue.Even for veteran techs. I suspect you have a active device ( transistor or diode ) acting up when its warmed up.

 

[avionic]

Those photos of the foil side of the circuit boards look pretty filthy. Which they all usually do get very dirty. I stand the chassis on its side on a puppy training pad. Then using several cheap 2" paint brushs and a small bowl of acetone. Start at the top and brush down with copious amount of acetone. After several treatments the boards will look almost like new.Gets rid of all the old crusty flux,dust and dirt.

 

[Mr. Yamaha]

The fact that its so intermittent. Is going to make this real tough to find the issue.Even for veteran techs. I suspect you have a active device ( transistor or diode ) acting up when its warmed up.

Can a VD1212 cause my problem? One VD1212 measures nothing: the tester (cheap Chinese though) says: "nothing detected". Other VD1212's measure fine.

Those photos of the foil side of the circuit boards look pretty filthy. Which they all usually do get very dirty. I stand the chassis on its side on a puppy training pad. Then using several cheap 2" paint brushs and a small bowl of acetone. Start at the top and brush down with copious amount of acetone. After several treatments the boards will look almost like new.Gets rid of all the old crusty flux,dust and dirt.

I will do that, since it's open now, thanks for the tip.

 

[avionic]

Can a VD1212 cause my problem?

Yes.. I use two 1N4148 diodes wired in series. For a substitute.

 

[Mr. Yamaha]

Little update here. I replaced the remaining 6 VD1212's with 6 x 2 1N4148's in series. While I was at it, I replaced known trouble makers TR511 till TR514 and TR619 till TR622 with KSC1845's.

But the problem is still unchanged.

Other options to check? Should I measure voltages according to the electric scheme? I'm not sure how to measure those voltages at the right places on the boards.