A few questions about M-85 operation...No class A change and not impressive o...

69sixpackbee

Well-Known Member
I am new to the Yamaha camp and suffice to say, so far I am not very impressed. Please help to sway my opinion. I wanted a big amp so I went for the M-85. I have it paired up with a CX-800 Pre and a T-1 tuner. I have everything hooked up accordingly. For now I have some old boom box speakers on the wall to check it all out. Even if I wind up the separate volume pots along with the volume knob on the preamp the output is less than stellar. I figured that this 270 WPC monster would knock them off of the walls! Hardly..it is clear and fairly loud but my little Realistic receiver blows this one away! I even had my iPhone initially hooked into the inputs via the headphone out jack and figured I would get decent sound initially before I went and hooked up the rest of the separates. Let me explain a finding. I opened it up and checked the voltage idle adjustment per the service manual. The initial reading was a tad over 1.5mV on both channels. I twisted the pot quite a bit but I got it at a rock- steady 15.5mV. Great. Now when I engage the "Class A" auto button the readings were supposed to double (30mV). NOT!! Still 15.5mV. The indicator LED shows it is in "A" mode but it obviously is not working. Tried to get DeOxit in the switch. No difference. The service manual is not much help as the schematics are all but clearly legible. Right now, I am pretty much convinced I have a $500 boat anchor and am hoping that someone can give me a direction to look. I really can't fault the guy I bought it from as he said it worked fine with his Kappa 8's .

Thanx for any and all help!
Bud
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Well sixpak. First your going to have to verify all the power supplies are up to snuff. And getting to the correct destinations.
 
I did. Thanx!

From my understanding, the "Auto A" button shifts the circuitry in the current bias section to "double" the idle voltage and monitors it until the threshold is reached and it will "shift" back to the nominal as demand increases. I have been squinting like hell to even try to read the voltages at the pinouts but to no avail. Until I get a better copy I am kind of stuck. I am trying to wing it but without any real experience with the Yammie's I'm like a blind man in a movie theater.

Thanx,
Bud
 
Pinouts?

You sure you're measuring in the right place? IIRC, you have to measure at the emitter resistors.

Regardless, no opinion is going to be swayed on a broken piece of gear. Sort it out and you will be rewarded.

Have you found the SM through one of the links in the forum sticky?
 
Pinouts?

You sure you're measuring in the right place? IIRC, you have to measure at the emitter resistors.

Regardless, no opinion is going to be swayed on a broken piece of gear. Sort it out and you will be rewarded.

Have you found the SM through one of the links in the forum sticky?


Oh, yeah. I got the SM albeit it is not the best copy. The schematic resolution is useless so I am prodding around as best as I can. The images show where I am checking the voltages as per the manual. Once again I get a steady 15.5mV at the test points and when I engage the Auto Class A switch it might bounce up .3mV that's about it. Tonight I had my iphone hooked up directly to the inputs this time while I was testing it further. I have an app that outputs test tones and this is the same way I set up other amps, usually in cars. There is sufficient amplitude, I think, to tickle the amp for a good test. I played a 1Hz tone and wound the pots all the way clockwise and I measured 7.45V at the speaker posts. I am using good Bose 301 speakers this time and when I have the volume maxed out there is not a lot coming out of this amp. What's there is clear but it barely is enough to get exited about. Here again, I don't have anything to go by do I will just report my findings.
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The heat sinks get warm as expected. The voltages that are going into the amp board for the xfrmr are around 65v. The voltages coming from the outputs are 90 and 35. Seem to be okay from what I can gather. This is not a very complex unit compared to the many I have been into. Aside from the obvious non-responsive "A" circuit I would think that this amp should scream and yet I can carry on a conversation at a normal tone of voice in its presence.
I must have a lemon that is going to need help.

Thanx,
Bud


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I just purchased a very good copy that I can actually read. Now I will try to bonehead through. More than likely, however, I will probably just end up sending it back for a refund. I have enough projects and this Yamaha is just a boat anchor as currently is. I'll mess with it for a little while but I don't know. All I can say is all of the functions, aside from the auto A switching circuit are in working order. But when there is barely enough volume coming out with a good input going in something is out of whack. The LED meters might be saying I have 200 Watts coming out but my ears say otherwise. 7.4V out at the speaker terminals is not 200W of power!




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I would check the components in "Auto-A" block, there are not too much components involved. In my M-80 there was an open resistor in this section.

I would also check if the voltage is dropping anywhere between the output of the main board and the binding posts.
 
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BTW, thanx to all for their help so far. I am going to yank the main board and start by removing the dreaded Sony glue and move forward from there. I'll try to compile a cap list and check the transistors and the diodes. I'll just spend a few hours at the component level and plod along.

Thanx,
Bud


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I just purchased a very good copy that I can actually read. Now I will try to bonehead through. More than likely, however, I will probably just end up sending it back for a refund. I have enough projects and this Yamaha is just a boat anchor as currently is. I'll mess with it for a little while but I don't know. All I can say is all of the functions, aside from the auto A switching circuit are in working order. But when there is barely enough volume coming out with a good input going in something is out of whack. The LED meters might be saying I have 200 Watts coming out but my ears say otherwise. 7.4V out at the speaker terminals is not 200W of power!



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What are your input level controls set at ?
Around those larger green nichicons is usually a lot of dried glue and a lot of corroded/oxidized component and jumper leads. Don't forget to check the jumper leads , in between the two copper buss bars, integrity. They as well as the buss bars are usually corroded.
 
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Hi, Avionic,

I had the iphone hooked directly to the input and it was max volume. Prior to that I had the CX-800 preamp attached to it and I twisted the volume knob on the pre nearly full. I had both volume pots on the M-85 maxed out fully CW.
I will start pulling the boards out tomorrow nite.
Thanx again!
Bud


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It's disconcerting that both channels exhibit the same symptom, but my first real M-80 issue that required surgery was low output on one channel and was 100% related to DSBG on one of the blackgates in that channel.
 
It is disconcerting to say the least. I was all exited to get one of these famed M-85's (and I paid five-bills for it too!:tears:) just to get it home to find out that an amplifier that is known for ear splitting, thunderous power barely has enough "oomph" to make my small boom box speakers happy. Yeah, you can say I'm slightly pissed:thumbsdn: Just yet another project to put on the bench...o boy.

I mean even a portable CD player or anything for that matter connected to the inputs should be sufficient to make this thing rumble! I have numerous amps over the years that I use/test in the same fashion so I am certain that this Yamaha is no different. I will be using it eventually with the CX-800 preamp, a T-1 tuner, EQ-70 and the PX-3 TT. Not all "matching" components per se, but Yamaha components none the less.
 
What I find curious is. Both channels are effected. Have you tried all three speaker binding posts. Does it have any signs of speaker post damage?
It has to be something common between the two amplifiers -- Power supply or possibly the relay circuit board. Damaged binding posts often damages the relay board as well. Worth looking over the solder connections where the binding posts solder to the relay board.
 
Hi, Avionic,
When i received the amp the boys at FedEx did a wonderful job of making sure that they broke the "C" speaker + post and caved in the left side cover that you remove to mount the rack handles. I repaired those cheezy posts and everything is well once again That was the only damage I saw. ALL 3 sets of speakers sound the same. The relays work fine as well. When I first power it up the protection LED is on for about 5 seconds and then it "clicks" and all is well as normal.

BTW, Norman at Analog Alley said to tell you "Thanks" for the referral :)
 
Disparaging a shipping damaged amp.

Where's that facepalm smiley when you need it?
 
It is disconcerting to say the least. I was all exited to get one of these famed M-85's (and I paid five-bills for it too!:tears:) just to get it home to find out that an amplifier that is known for ear splitting, thunderous power barely has enough "oomph" to make my small boom box speakers happy. Yeah, you can say I'm slightly pissed:thumbsdn: Just yet another project to put on the bench...o boy.

Did you buy it as a fixer-upper, or was it supposed to work?

If the latter then why have you not asked for a refund if it's such a POS and you don't want to deal with it? Beginning to sound a bit fishy....
 
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