Yamaha M-70 Thread

Bratwurst7s

In The Frying Pan
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I have an M-70 that I bought last year, it's been sitting patiently waiting to rise to the top of the priority list. It just got there.

I paid about 1/3 of the going price around here and Knew when I bought it that it had problems. Supposedly it has been sitting in an attic for 20 years and has problems "but not with the output transistors". OK. I have a bag of "yeahright" handy. But for the price it was worth the gamble. I think. Perhaps Hope.

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General appearance is quite good, just 2 small scratches on the left upper face corner. Inside, the only thing that stands out is that in the right rear of the main pcb one cap and a zener are non original. Not too dusty, doesn't smell like nicotine, damp, cat piss or smoke.

The "Power C Board" up front has some DSBG and one small cap that's leaking but no signs of corrosion that I can see at this time. It has the Rifa cap(s) in the power supply that needs to be replaced.

Cheers,
James

edit 29-Feb-2020: I made a replacement parts list some time ago but failed to post it here, so I'm doing so now, for the use of Audiokarma members. It represents a considerable amount of work. While it is being provided to help other members I do not provide it to be copied and monetized.
 

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So after a first visual inspection of the interior the only thing that stands out is that Nichicon FG and replacement zener in back on the right. I assume the right channel has problems.

I did buy an original service manual last year. Among other things it looks like I'll need to build a 200w 8 ohm test load. And it's probably time to buy a second DMM that is true rms. I do have a signal generator.

DSC04815.jpg DSC04816.jpg DSC04817.jpg DSC04818.jpg DSC04819.jpg

It looks like all of the outputs are original, I can't tell yet concerning the emmiter resistors. But all of that is suspect.

Cheers,
James
 
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So, now we get to the ugly part. Time to remove the bottom plate and see what's going on down below. First glance didn't look bad at all. No burnt traces. But there are a number of rather shiny solder connections that don't look original. Not bad but not original. Then I took a look at the front pcb that holds the main filter caps and bridge rectifiers. :eek::(

So that was a big WTF moment. Really? Someone took a drill and severed the traces to the 18000µF caps? REALLY? :thumbsdown::no::crazy:

DSC04830.jpg DSC04832.jpg DSC04833.jpg

So that was a bit of a downer. I'm now at the point of developing a plan for going forward. I have to find out if there is a problem in the high power section of the PSU or in the high power section of the outputs or both. At this time I'm assuming that all of the outputs are blown and that electrically this thing resembles a certain radioactive city in the Ukraine.

That's all at the moment. One thing that I did do today was order some filter caps from Mouser. That's kind of jumping the gun but they only had 5 pieces of the UCC KMH 22000µF/80 that I want and I don't want to wait 16 weeks for a restock. I ordered a pair of KMH 12000µF/80v with them. I won't be ordering any other parts until I start to get a feel for how bad the damage is. Obviously I'll have to jumper those traces, I don't imagine that I'm going to find a spare board anywhere.

Cheers,
James
 
Thanks Bert! Working on it...

2 hours after my order with Mouser I got a shipment notice. Mouser+FedEx is the best. So far the record time from "last click on the order" to "parts in my hand" is 42 hours and 26 minutes. Think about that. Order comes in, parts picked and packaged, shipped from Texas, USA to Munich, Germany in 42 hours and 26 minutes. That's humpin'!

There are several threads of thoughts going through my head on how to approach this.

I don't even want to turn it on until the Rifa cap+ has been replaced, as well as the DSBG and leaking cap. With the X power supply I can't use a dim-bulb-tester to start it. So I have to think about a Variac. And normally I want to find and correct all of the problems before going into a general re-capping etc. And at this point while I suspect that the right channel is probably bad I also don't know if I can trust the power supply. Talked that one around in a circle didn't I. o_O

I think that I will first check out the power supply. Clean it up, deal with the obvious issues and check the components. Next probably de-solder the main output transistors and remove them with the heat-sinks. Check those transistors and then decide what to do from there. I'm not sure if this is the best plan of attack but that's what I'm starting with. I'm also going with the expectation that at least the entire right channel is suspect.

Cheers,
James
 
Replace that largest Rifa PIO cap. Its a fire hazard in the 220v model.
index.php
 
Is a dim bulb always a no-no on X power gear? I've used one on several M-60s, an A-1020 and an A-1000 with no problems.

The damage to the traces is insane. I wonder what in the world they were thinking. At least there is a lot of width to work with to make repairs.
 
Is a dim bulb always a no-no on X power gear?
At this point I'm "on the fence". I know I've said " no " in the past. Its pretty nerve wracking not using one. Did I mention B-6's have the x-power supply.I had to rebuild 1 channel in each of my B-6's.
 
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At this point I'm "on the fence". I know I've said " no " in the past. Its pretty nerve wracking not using one. Did I mention B-6's have the x-power supply.

Initial power up after major repairs is my least favorite part of the whole process by far. I think I'd probably have to pay someone to plug in a B6 I repaired without a DBT because doing it myself would give me a heart attack.

The DBT definitely saved me some grief when I was repairing that M-60 where someone had replaced resistors with whatever they had handy. Talk about weird voltage measurements when you have something like 4.7ohm subbed in for a 33k or other higher values in a few random places.
With the "repairs" that this M-70 has done to it I have a feeling there could be surprises like that hiding around.
 
2 hours after my order with Mouser I got a shipment notice. Mouser+FedEx is the best. So far the record time from "last click on the order" to "parts in my hand" is 42 hours and 26 minutes. Think about that. Order comes in, parts picked and packaged, shipped from Texas, USA to Munich, Germany in 42 hours and 26 minutes. That's humpin'!
A little off post but, best time to Melbourne is about 60hours, order monday 10pm, arrive Thursday 10am (normally thursday 2pm) with UPS.
It takes 4 days to get a parcel within australia, Unbelievable.
 
. I think I'd probably have to pay someone to plug in a B6 I repaired without a DBT because doing it myself would give me a heart attack.
I think I have like 350 tied up in three B-6's. So I still have 1 more to finish up.
 
Needs to be and XY rated "across the line" safety cap.

Thanks Avionic. Understood. That cap in my M-50 blew up the first time that I turned it on when I moved over here.
I did a general search concerning this amp and came across this thread...
http://www.audiokarma.org/forums/index.php?threads/smoke-from-m-70-what-to-do.376536/#post-4747576

The cap that you and Brutal recommended was this one...
https://www.mouser.de/ProductDetail/Panasonic-Electronic-Components/ECQ-U3A474MG/?qs=sGAEpiMZZMvQAn3cSZoTr1%2b0Ye1BktuzFq%2bm14xyTdw=

And for the 3 small 6800pF Rifa caps I'm looking at this one...
https://www.mouser.de/productdetail/panasonic/ecq-u2a682kl?qs=sGAEpiMZZMv1cc3ydrPrF0%2bjlB8SXIRuFjuQdipUHOg=

Question:
On the bottom sub-extention of that board there is a huge 0.1µF/450vac axial lead electrolytic, C611.
I was thinking of using one these to replace it...
https://www.mouser.de/productdetail/arcotronics-kemet/r73tw3100se00k?qs=sGAEpiMZZMv1cc3ydrPrF42R8ZpcOXPb/VGN6n3zWPo=
https://www.mouser.de/productdetail/epcos-tdk/b32686a7104k000?qs=sGAEpiMZZMv1cc3ydrPrF1u7m5EY228Lht/U8VTV1W8=
Do you think that they will serve?

Thanks,
James

edit: the parts list shows C611 as an oil cap.

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A little off post but, best time to Melbourne is about 60hours, order monday 10pm, arrive Thursday 10am (normally thursday 2pm) with UPS.
It takes 4 days to get a parcel within australia, Unbelievable.

That's pretty darn good considering the distance.

Cheers,
James
 
Initial power up after major repairs is my least favorite part of the whole process by far. I think I'd probably have to pay someone to plug in a B6 I repaired without a DBT because doing it myself would give me a heart attack.

The DBT definitely saved me some grief when I was repairing that M-60 where someone had replaced resistors with whatever they had handy. Talk about weird voltage measurements when you have something like 4.7ohm subbed in for a 33k or other higher values in a few random places.
With the "repairs" that this M-70 has done to it I have a feeling there could be surprises like that hiding around.

We have a huge test bench at work with a variac built in (Regeltrafo). But it would be a pita to drag the amp in there to use it. Next month I would be able to afford to buy a variac (ca €150). I just have to decide if it will get used enough to make the purchase worthwhile.

Cheers,
James
 
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