Yamaha M-45 (M45) Repair; Popping, No Output

[ic-racer]

Yamaha M-45 (M45) Repair; Popping, No Output: DSBG

I have used my M-45 for about 20 years, but yesterday it went pop now it is on the repair bench.

Initial symptoms: intermittent loud popping. Every 10 minutes. (I don't know if there was any sound output because I have 2 amps powering 2 speaker sets and I was in another room when I heard the popping. I suspect there was no audio. All powered down.

Today, no sound from either channel. Some crackle when output turned to max but otherwise silent.

On Powerup:
Protection light comes on then goes off as normal and relay clicks on.

Voltage drop across the 0.22 resistors is now 0.1mV (Was 4.0mV a few years ago when checked) (Class A = off; Didn't check with Class A= on)

DC offset at speaker outs: 80mv both sides (was 5-6mV a few years ago when last checked).

Nothing looks fried, no 'smell.'

That is all I have so far.

I have the M-40 service manual but not the M-45

 

[avionic]

Take a clear photo of the power supply area of the Main circuit board. I've had the large main caps in these babies go south before.Measure your rail voltages on the center pins the output transistors.Should be a stable voltage,Not sure of the absolute value probably somewhere around + & - 56 volts.

 

[ic-racer]

Take a clear photo of the power supply area of the Main circuit board. I've had the large main caps in these babies go south before.Measure your rail voltages on the center pins the output transistors.Should be a stable voltage,Not sure of the absolute value probably somewhere around + & - 56 volts.

I'll check those things and post a picture. I was also thinking about the power supply. Seems the lights on the meter and the power for the relay are intact but I have not figured out if those come off separate from the main rails.

 

[ic-racer]

Supply voltage 63V + and - at each of the transistors on the heatsink.

 

[ic-racer]

Inside picture of the amplifier in question.

 

[ic-racer]

The two big and two smaller electrolytics have 1meg or greater across the terminals, so neither are shorted.

 

[whoaru99]

Do these have the DSBG problem?

 

[brutal]

Check the two 4.7Ohm resistors near the muting caps (center, large) and anything else the glue may have gotten to.

I just recapped one of these for a friend and DSBG had nearly destroyed a leg on one and had corroded the groundplane busbar. That snot gets conductive when it darkens.

 

[ic-racer]

Check the two 4.7Ohm resistors near the muting caps (center, large) and anything else the glue may have gotten to.

I just recapped one of these for a friend and DSBG had nearly destroyed a leg on one and had corroded the groundplane busbar. That snot gets conductive when it darkens.

OK, the two 'medium sized' caps in the center of the board have it. Back in 2007 when I was going over the amp I saw that stuff there and wondered what it was. Now I know its DSBG.

 

[ic-racer]

OK now I see, there are about 5 resistors around that area. I suspect at least one is trashed with corroded leads, or partially shorted by the conductive nature of the stuff.

So I have some work to do. I'll replace the buggered up resistors and see if that fixes it.

 

[ic-racer]

Wow, this place is fantastic. I have not been in an amplifier for about 4 years and I log in here and in less than a day I have my answer.

Here is a resistor that is open circuit. The lead oxidized away to nothing from the DSBG.

 

[ic-racer]

I suspect those green resistors are metal film resistors and would be replaced by the ones that are now usually blue, yes??

 

[avionic]

I suspect those green resistors are metal film resistors and would be replaced by the ones that are now usually blue, yes??

You suspect correctly.I use metal film 1/2 watters.May want to closely examine those jumper wire ends as well.

 

[whoaru99]

I think those caps you've take out might be a couple of the Black Gates. You might just want to recap the thing while you're at it (aside from the large main caps). I used Elna Silmic IIs in my M-80 and probably will again when I get around to doing my two M-40s.

 

[brutal]

Replace everything you can that has been corroded by the glue.

Pull the bus bar and clean it. I use a Dremel wire brush most of the time.

I save clipped cap leads for most of the jumper replacements, but on this amp they are all very long so I used some magnet wire I had laying around. Worked great and looks pretty cool as well.

 

[avionic]

I use a Dremel wire brush most of the time.

:scratch2: I use scotchbrite and elbow grease.:thmbsp:

 

[brutal]

:scratch2: I use scotchbrite and elbow grease.:thmbsp:

What, no naval jelly?

On second thought, that stuff might eat right through the copper.

 

[brutal]

While I was in it, I got rid of the crappy floppy RCA connections. What a daft cost cutting measure.

 

[ic-racer]

This whole DSBG stuff reminds me of a situation I ran into a few years ago. I was tracking down what I though was a bad resistor in a PCB tube guitar amp (in the picture below.) The flux from around the tube sockets (which were soldered to the PCB) became conductive. The symptoms were clicking and popping and poor sound.

After removing the "bad" resistor I, by chance, discovered the PCB was conducting between the traces.

The middle of the picture is the "After" cleaning image and on the right is a tube socket "Before" cleaning. My impression at the time was that impurities in the air became imbedded in the old flux and became some novel new passive component.

This was my first case where cleaning a PCB made a dramatic improvement in the amplifier sound. In this case the residue could be completely dissolved with alcohol.

 

[ic-racer]

This is what I ordered from Allied:

Carbon Film 10K 1/4 w
Metal Film 4.7R 1/2 w
Illinois 1000uF 100v 5% good to 105 deg C. The lead spacing is 7.5mm whereas the original was 10mm.

Mouser had Nichicon 1000uF (80V like the original voltage rating) but it was on Back Order and it still had the 7.5mm spacing.