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Looking for AK Tech or advice - Yamaha M-70

S

SilentHatch

New Member
Greetings All,

Been doing some reading over the last months and year since I joined. I purchased an M-70 restore kit from eBay in hopes of repairing this device (in protection), but I just do not have the time, so I'm willing to spend some money on it since I spend the rest of my time working.

Some background, my dad got a C-70 / M-70 preamp/power amp combo with Magnepan MG-I speakers (no stand, so anyone that has a lead on those, let me know) back when he was in the Army.
He gave them to me (working) in 2011 timeframe and they worked but I moved out and they went into the basement. In any case, that's when the M-70 stopped working and I haven't had the time or monetary means to rectify the issue yet, and I've moved a couple times since then with the C-70 working (not in protection) and M-70 not working (in protection). I picked up the T-70 last year for a great price since I also like listening to the radio. Both the T-70 and C-70 work, as I can hear the radio coming out of the C-70.

First, I'm located in southern Maine, and I don't mind driving if necessary.
Second, it would be neat if there were someone from AK local that could look this over for me (paid, of course) and restore to working order.. or hold my hand as I look at it.
Third, I looked on Yamaha's website for authorized dealers and the closest is 70 miles (one-way) and their website mentions nothing about repairs.

I have a multimeter, some old janky soldering iron that I don't think actually works, and a 20 year old roll of solder. Sure, I could try to do this myself but the probability that I make the situation worse is.... likely. But if you want to hold my hand as I try to do it, sure. I'm home today/tomorrow and can take a look at stuff (or run to the nearest big box store to try to get some stuff).

In any case, I'd like some advice from you all or maybe someone in the area to see if they could take a look at it. And if anyone knows some good bookshelf or good tower speakers (been eyeing those klipsch tower speakers that have been on sale for like $249) to put with this, please share.

All advice appreciated.

I've tried to include some pictures that I just took a moment ago (just plugged in for the first time in a while, and turned on). Last time I had it working, all the volume level indicator LEDs in the center panel worked on the M-70 still.

Respectfully,
SilentHatch

PXL_20221229_104439861.jpg
PXL_20221229_104439861.jpg

PXL_20221229_104533096.jpg
PXL_20221229_104533096.jpg
 
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Welcome aboad Silent
First thing you need to do is check for substantial DC voltage at the amplifier(s) outputs. This needs to be measured at a testing point prior to the protect relay contacts.
 
Find TPO for the left and right channel. Ground your neg. test lead to chassis ground. Probe TPO 's with the pos. test lead. Ideally your looking for zero volts or less than 100mv..

Test Point Output
 
avionic said:
Find TPO for the left and right channel. Ground your neg. test lead to chassis ground. Probe TPO 's with the pos. test lead. Ideally your looking for zero volts or less than 100mv..

Test Point Output
Click to expand...

Thanks avionic - what setting should I be putting my multimeter to?

I'll do some research on my end tonight and see if I can find them quick. Thank you
 
I don't have any DeOxit here - if I do need to order that, can someone let me know the exact one to order? I think from my reading there was some talk about red and gold being a good combo? Might be getting ahead of myself, but rather get it ordered and on the way beforehand.
 
Okay @avionic
I did some testing and took some pictures so you can see what I was doing and where I was testing.
I did three tests, and the camera wasn't on my side so the numbers don't read what I was getting with both hands on the leads.

The black lead was going to a screw connected to the chassis, I couldn't find anything specifically called chassis ground.

M-70 Status | Left Channel | Right Channel
unplugged | 0.000V | 0.000V
plugged pwr off | 0.000V | 0.000V
plugged pwr on | 0.000V | 0.000 to 0.006V

PXL_20221230_014232585.jpg PXL_20221230_014400344.jpg PXL_20221230_014805674.jpg PXL_20221230_014826700.jpg
 
SilentHatch said:
Okay @avionic
I did some testing and took some pictures so you can see what I was doing and where I was testing.
I did three tests, and the camera wasn't on my side so the numbers don't read what I was getting with both hands on the leads.

The black lead was going to a screw connected to the chassis, I couldn't find anything specifically called chassis ground.

M-70 Status | Left Channel | Right Channel
unplugged | 0.000V | 0.000V
plugged pwr off | 0.000V | 0.000V
plugged pwr on | 0.000V | 0.000 to 0.006V

View attachment 2752967 View attachment 2752968 View attachment 2752969 View attachment 2752970
Click to expand...

Are you measuring with the meter set to AC voltage or DC voltage? It should be set to DC voltage.

And measure it powered on.

Athanasios
 
SilentHatch said:
Okay, had to hit the select button to change to DC... Thanks @Nashou66

left channel = 32.50 and dropping.
right channel = -0.940
Click to expand...

Is that 32.5 Vdc or mVdc Meter should be in mV range

If it's 32.5 there is something definitely wrong for that channel, Also the Right channel is too high as well. the left will definitely put it into protection, as would the right but that may just need to be dialed in with a DC offset adjustment if there is one. I dont have the Schematics for this unit infant of me.
 
Nashou66 said:
Is that 32.5 Vdc or mVdc
If it's 32.5 there is something definitely wrong for that channel, Also the Right channel is too high as well. the left will definitely put it into protection, as would the right but that may just need to be dialed in with a DC offset adjustment if there is one. I dont have the Schematics for this unit infant of me.
Click to expand...


Here's the electronic version I'm using:
https://medias.audiofanzine.com/files/yamaha-m-70-service-2-480494.pdf

One sec, let me go check the MM to be sure - I'll edit this post

edit - it's Vdc for both, it's auto changing and starts in mVdc and then goes to V when I test.
 
on page 7 s the location for the DC offset trimpots. Do you have mini graders to attach to your probes? If not try to attach the GND probe ( black) to somewhere on the chassis, maybe a GND screw or the negative speaker out. and measure the TPO while adjusting the trimpot to get close to 0 volts. most likely the 32 volts won't adjust but maybe the .9xx volts will.

Athanasios
 
avionic said:
Plus +32.5vdc ?
Click to expand...
If so. You need to pull and test these four transistors for starters.

TR135,139,143 and 147. They will be larger transistors mounted on one of the Large aluminum heatsink.
 
avionic said:
If so. You need to pull and test these four transistors for starters.
Click to expand...

Okay, guess I'll have to go buy a soldering iron tomorrow - the one I have is not temp controlled. I cannot test them while they're in the board, correct?

Nashou66 said:
on page 7 s the location for the DC offset trimpots. Do you have mini graders to attach to your probes? If not try to attach the GND probe ( black) to somewhere on the chassis, maybe a GND screw or the negative speaker out. and measure the TPO while adjusting the trimpot to get close to 0 volts. most likely the 32 volts won't adjust but maybe the .9xx volts will.

Athanasios
Click to expand...
Should I adjust that now or after I test those transistors?
 
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