Yamaha M-70 protection relay issue

zombirdy

New Member
I purchased a nice Yamaha M-70 from ebay, knowing it was broken. I'm a bit of a rookie at repairs but I'm very tech inclined and I've built some of my own audio gear.

The rifa cap that plagues these amps was exploded and the three 6800pf rifa caps next to it were all spider webbed (self healing?) The protection board connected to the psu board was covered in dried glue, some of which was black and crumbly like charcoal.

I replaced the exploded cap with an X2 panasonic, and the 6800pf caps with matching Y2 panasonics from the same series as the X2. There were two other little can caps that got replaced as well.

Now on the protection board I replaced three large cans, one of which looked like it was a 1mm from popping its lid, and all the resistors (glue corroded them) plus one zener diode. (apparently the stock resistors are carbon film, i used metal film 1% tolerance)

When I turn the amp on (cold start), the protection light comes on, and a few minutes (5-7 estimate) it shuts off and the amp works fine, music is incredibly clear and bass is heavy, nothing wrong with the channels.

Here's a couple of strange things, if I play music while the amp is warming up, you can hear sound after 5 or so minutes and then a few seconds after the protection light shuts off.

Now if I play music for a bit, and shut the amp off then back on, the relay clicks within a couple of seconds, as I'm assuming it would under normal circumstances.



TL;DR all capacitors are replaced on the power supply and protection circuit, corrosive glue has been 99% cleaned off. Protection relay has a lengthy delay before shutting off and resuming normal amp function.
 
Protection relay has a lengthy delay before shutting off and resuming normal amp function.
Actually its the other way around.
"Protection relay has a lengthy delay before energizing (setting)and resuming normal amp function. "
I know your refering to the protect LED though.


Now if I play music for a bit, and shut the amp off then back on, the relay clicks within a couple of seconds,

That sure sounds like a dried up cap .Or I suppose it could be a relay driver transistor with a thermal issue or a cold solder connection.
 
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What does your DC offset look like for the first 5 min.from a cold start?
 
What does your DC offset look like for the first 5 min.from a cold start?

Hoping I got this correct:

negative lead to ground and positive lead to TPO?

left channel keeps climbing in voltage, right before the relay clicked i think it hit 30mw

right channel was at negative voltage then hit .5mw

this amp was serviced before, i believe the pots and the speaker connects in the back were cleaned, not much else, i don't see a single component in here that doesn't look out of place like it was new.

I've already replaced all the caps in the power circuit.
 
Hoping I got this correct:

negative lead to ground and positive lead to TPO?

left channel keeps climbing in voltage, right before the relay clicked i think it hit 30mw

right channel was at negative voltage then hit .5mw

this amp was serviced before, i believe the pots and the speaker connects in the back were cleaned, not much else, i don't see a single component in here that doesn't look out of place like it was new.

I've already replaced all the caps in the power circuit.

"millivolts"
Not enough voltage to trigger the protect circuit.
Once it gets to the relay "click". Tap around on the protect and PS boards and see if the protect relay resets and the LED illuminates.Looking for possible cold solder connections.
 
What is the "power circuit"? Do you mean power supply or amplifier. Check for cold solder joints, these amps are known for them.
 
Sorry, I was very tired when making this post, so my terms are a little off.

I switched the unit on, and jiggled/poked/prodded as many components as possible, nothing activated the protection.

Are there any specific places I should look? should I re-flow the solder on the power supply even if it seems fine?
 
Do you have two multimeters ? Have you done step 1 and 2 of the PS adjustment procedure in the service manual? Page 7 of the service manual.They need to be adjusted together because the two pots enter-react with each other. If this does not improve your condition. Check solder connections(reflow) for relay driver transistors TR172 and TR173. Might even have to replace them if there temperature sensitive.
http://www.hifiengine.com/manual_library/yamaha/m-70.shtml
 
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I did a little up close inspection on the main amp board and the board that has the two DC adjustment pot switches, I am seeing some very suspicious solder joints on both boards, little rings around the middle of the solder.

The amp has gotten to the point where the relays click, music plays, and the protection light stays on for a couple of seconds after this process. I have poked and prodded every transistor and almost every other part in this unit but nothing seems to activate the protection.

I will be patiently re flowing as many solder connections as possible very soon, then I will post back with my results.
 
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