• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Got a Yamaha M4!!!! But needs work

lha1992

lha1992

Kenwood Collector
Subscriber
Well, since getting my Yamaha CR-1020 almost a month ago, I really like Yamaha!!! And after looking at what else was to offer, I saw the M4 and C4 system. I really liked them!!

So, yesterday afternoon I went on CL and saw a supposedly broken M4 for sale and said that it played in the left, but made a popping sound or something in the right. I called the guy and he said it was mine, and he brought it over all for $20!

Now for the bad, the amp was HORRIBLY dirty. Had rust or something on the chassis and parts of the amp and lots of dust. Took me about 3 hours to clean it all up, had to take the heat sinks and some other boards out to clean it better. Looks really good now.

So, I decided to hook it up and connect my Kenwood KA-9100 integrated as the pre-amp and try it out. I get a signal on the amp's meters, but it is intermentant and puts out very low sound. It does play in the left and you have to shove the volume on the pre-amp sometimes to make the right work, but then it would soon go out. I did clean the leveling pots in the back with deoxit too.

So, what should I do? Which things to check first? I kinda checked the DC offset, but I hooked the DCM into the speaker connectors and got 2v on the left and 200mV on the right and turned the pot and got that down, but these directions in the service manual are a little confusing.

Here are some pics of the amp:

Before cleaning:
http://s840.photobucket.com/albums/zz323/lha1992/?action=view&current=SANY0100.jpg&newest=1
http://s840.photobucket.com/albums/zz323/lha1992/?action=view&current=SANY0104.jpg&newest=1
http://s840.photobucket.com/albums/zz323/lha1992/?action=view&current=SANY0107.jpg&newest=1
http://s840.photobucket.com/albums/zz323/lha1992/?action=view&current=SANY0108.jpg&newest=1

After cleaning:
http://s840.photobucket.com/albums/zz323/lha1992/?action=view&current=SANY0113.jpg&newest=1
http://s840.photobucket.com/albums/zz323/lha1992/?action=view&current=SANY0118.jpg&newest=1
http://s840.photobucket.com/albums/zz323/lha1992/?action=view&current=SANY0119.jpg&newest=1
http://s840.photobucket.com/albums/zz323/lha1992/?action=view&current=SANY0123.jpg&newest=1
http://s840.photobucket.com/albums/zz323/lha1992/?action=view&current=SANY0120.jpg&newest=1

Thanks
 
Register to hide this ad
It does play in the left and you have to shove the volume on the pre-amp sometimes to make the right work
Click to expand...
Sounds like the speaker protect relay(s) contacts may be burnt-pitted or tarnished.
 
Does speaker B sound the same as speaker A on the right channel? It has two seperate spkr protect relays..
 
Last edited:
No just M4, but soon hopefully i'll get a C-4.

Anyway, I just went and checked to see if speaker B works, and its a no go. :sigh:

It sees the music coming it because the meters are going and everything, but no output. And for some reason, sometimes while theirs no Pre-amp hooked the left meter will start bouncing. I need to check and adjust offest and bias too, do you think that may resolve it? But the speaker relays engage. Their are 2 speaker relays right? One for speaker A and one for speaker B? Because I hear no click when I power up, but hear a click when I push the speaker A or B button.

So, what should I do next?
 
Well I adjusted bias, idling, and offset to what its supposed to be. Both channels had over 100mV but everything is at 1mV. I accidently adjusted the right channels DC offset pot instead of the left which is what I was measuring and I was wondering why it wasn't moving when I turned it from point to point. I didn't ruin the outputs do you think? When went to see the offset on the right it was more than 200mV which is what I was reading.

Anyway, IT WORKS!!!!! It plays in both channels!!! I think I need to deoxit the leveling pots because when I moved them the right channel went out.

But, it works and it sounds GREAT!!
 
Last edited:
Good job..:thmbsp: I think your outputs survived the oops!
 
I think I need to deoxit the leveling pots because when I moved them the right channel went out.
Click to expand...
Those pots are 30Kohm with carbon resistance pads.I would suggest not using Deoxit D5 but rather Deoxit DL100. D5 has a solvent that may wash away the carbon. DL100 has no solvent. 30K pots are hard to find.
 
Thanks Avionic for helping me!

I still hear static in the left channel for some reason, I guess the leveling pots. Sometimes I have to use the volume on the PRE to force it to come back.

Forgot to say, this was an early early M4, built 10/12/79.

The only thing it needs is some lights for the switches and i need to put a new grounding plug on it (my fault, broke when I was taking it off).
 
I still hear static in the left channel for some reason,
Click to expand...
Swap your pre amp inputs around.If the static moves to the right its in the preamp.
 
avionic said:
Swap your pre amp inputs around.If the static moves to the right its in the preamp.
Click to expand...

Well your right on the pre-amp. The static moved to the right.

But now bad news has come up. I was checking offset again just to be sure it was okay and checked right channel, came up to 4mV. Then the left, came up to -133mV, and adjusted all the way to the end of the pot and got it down to -60mV. Then I touched the molex connector and the DMM said more than a volt and the amp went into protect and won't come out. It did come out for a little bit then it went back into protect.

If I touch any molex connector the left channels meter will bounce and bounce. The meter will bounce with nothing being touched at all. I really don't know what is wrong with it now. I may try again later, but I don't want to risk ruining the amp. At least I know the protection circuits are doing what they are supposed to do.

So what should I do?
 
lha1992 said:
Well your right on the pre-amp. The static moved to the right.

But now bad news has come up. I was checking offset again just to be sure it was okay and checked right channel, came up to 4mV. Then the left, came up to -133mV, and adjusted all the way to the end of the pot and got it down to -60mV. Then I touched the molex connector and the DMM said more than a volt and the amp went into protect and won't come out. It did come out for a little bit then it went back into protect.

If I touch any molex connector the left channels meter will bounce and bounce. The meter will bounce with nothing being touched at all. I really don't know what is wrong with it now. I may try again later, but I don't want to risk ruining the amp. At least I know the protection circuits are doing what they are supposed to do.

So what should I do?
Click to expand...
Set the balance pot to about the center of its sweep. Check for bad/cracked solder connections on the circuit boards
 
Last edited:
avionic said:
Set the balance pot to about the center of its sweep. Check for bad/cracked solder connections on the circuit boards
Click to expand...

Okay then. I'll power it back up and set the DC balance pot to the center of the sweep and see if it will come out of protect.
 
Well, I just turned the DC balance pot on the left channel to the center of its sweep and it came out of protect. The bad is that it has a DC offset of -2.20v.

Edit: just touching the trimmer to bring the DC down the relay clicked and went into protect.
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom