Yamaha M-70 Thread

[Bratwurst7s]

So I decided to take a step towards troubleshooting and desoldered all of the right channel outputs that are mounted to the heatsink and pulled the assy.

Heh, it's been a while since I used my little parts checker to test a transistor. I'm thinking that "B=" is hFE and uF is, wtf, don't remember. Or do I have that backwards.

But the 2 drivers and all 8 outputs seem to be good.
TR134: B=107, uF=625mV.
TR132: B=138, uF=630mV.

TR138: B=87, uF=569mV.
TR142: B=88, uF=572mV.
TR146: B=82, uF=572mV.
TR150: B=69, uF=570mV.
TR148: B=63, uF=551mV.
TR144: B=58, uF=577mV.
TR140: B=54, uF=556mV.
TR136: B=67, uF=556mV.



Cheers,
James

edit: or, if they aren't "good" at least they aren't shorted.

 

[Bratwurst7s]

It always happens to the best. You just have to find the key to follow the right path. It will not be difficult because you are really "Bravo" and you have with you the most powerful weapon: patience

Thank you Beau Geste, That was a very kind thing to say. All moral support is appreciated.

Cheers,
James

 

[Beau Geste]

Thank you Beau Geste, That was a very kind thing to say. All moral support is appreciated.

Cheers,
James

 

[Beau Geste]

So I decided to take a step towards troubleshooting and desoldered all of the right channel outputs that are mounted to the heatsink and pulled the assy.

Heh, it's been a while since I used my little parts checker to test a transistor. I'm thinking that "B=" is hFE and uF is, wtf, don't remember. Or do I have that backwards.

But the 2 drivers and all 8 outputs seem to be good.
TR134: B=107, uF=625mV.
TR132: B=138, uF=630mV.

TR138: B=87, uF=569mV.
TR142: B=88, uF=572mV.
TR146: B=82, uF=572mV.
TR150: B=69, uF=570mV.
TR148: B=63, uF=551mV.
TR144: B=58, uF=577mV.
TR140: B=54, uF=556mV.
TR136: B=67, uF=556mV.

View attachment 1092043 View attachment 1092044 View attachment 1092045

Cheers,
James

edit: or, if they aren't "good" at least they aren't shorted.

James , i have the same tester device too!!It saved me and saved my money a lot of times!!If i can give you an advice , i suggest this another device tester . It's a Op Amp tester .https://www.ebay.com/itm/Op-Amp-Tes...409919?hash=item1a3a6d957f:g:lkQAAOSw-YVXmhT3 It's a must for people like us that play with the Op Amp

 

[Bratwurst7s]

James , i have the same tester device too!!It saved me and saved my money a lot of times!!If i can give you an advice , i suggest this another device tester . It's a Op Amp tester .https://www.ebay.com/itm/Op-Amp-Tes...409919?hash=item1a3a6d957f:g:lkQAAOSw-YVXmhT3 It's a must for people like us that play with the Op Amp

Interesting. Thanks, I'll add that to my list.

Cheers,
James

 

[OMGCat!]

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

I'm not sure if they are available in 220v but Sencore and Heathkit (among others) made those variac/isolation transformer all in one setups that usually have a meter built in as well as a fuse holder in front you could use to limit current to whatever you felt is safe.
Otherwise I'd just DBT it and not worry.

 

[Bratwurst7s]

I'm not sure if they are available in 220v but Sencore and Heathkit (among others) made those variac/isolation transformer all in one setups that usually have a meter built in as well as a fuse holder in front you could use to limit current to whatever you felt is safe.
Otherwise I'd just DBT it and not worry.

I've only just started looking into it. When I get back to work next week I'll ask the electrician profi's. There might just be a portable one lying around there that I can borrow. Or enough parts to be found to just make one.

Cheers,
James

edit: there are several models available in 230v. I'll be making a list.

 

[Bratwurst7s]

OK. So I finally noticed that the SM parts list and schematic show Toshiba 2SA1146 and 2SC2706 as the outputs, O or Y rating. From my limited experience Yamaha usually shows alternates in the parts list but doesn't here. I've been able to find pdf's of the original Toshiba data sheets but only aftermarket data sheets for the Sanken's that don't show what different ratings they were made in. I'm still looking the data sheets over but the Sanken's look like a Yamaha sub, the specs look similar. The solder points on the outputs looked undisturbed also.

Looking the boards over carefully I'm starting to see where different solder points have been refreshed. The funny thing is that all of the parts in the amp are equally dusty and old/original looking except for that one cap and zener. Before I disturb anything else I'm going to make copies of the schematics and map out in color which parts/points have obviously been disturbed to try to get a feel as to what was the heck they were doing.

In the background I'm making a list of caps etc to order. I'll include zeners and perhaps add to my collection of transistors and diodes as a precaution. I need to research relay and trimpot replacements. I suppose that it will be a good idea to identify bridge rectifier replacements in case there is a problem there. Next weekend I'll place a Mouser order. And I need to decide on speaker terminals and input sockets.

Mouser/FedEx are working at speed. 44 hours 3 minutes from order acceptance to parts in my hand. I've used the UCC KMH filter caps in several amps now. These are 5 lug, with 3 dummies. The big ones will be a drop in fit after removing the 3 dummy lugs. The small ones will need 1 dummy removed and a small tip bend on the others and they will be a drop in fit. Nobody carries the 12000µF in 63v so I went up to 80v. I like that they are 105° caps, which is what I will be using everywhere possible. As many elcos as practical will be switched to Wima film.

Most of that is just planning and housekeeping. As soon as I have the safety caps and an idea of which parts of the amp have been fooled with in the past I'll make decisions on where to start but I'm thinking that I have to carefully check the entire power supply. I'm still trying to get a feel for what happened and what might be wrong, and why someone felt the need to sever those filter cap traces.

Cheers,
James

 

[Bratwurst7s]

The cap and zeener diode that someone has changed are in the protection circuit. D132, (RD6.2EB2) and C151, 47µF/10v. No idea what was installed for a zeener but the FG that they put in is 33µF/100v.

Cheers,
James

 

[Bratwurst7s]

The items/connections that I can see that have been obviously been re-soldered or touched-up:

On the PSU capacitor board, almost everything. R801, 802, 803, 804 and 805. Jumpers 816, 817 819 and 826. The HB + & -, LB + & - and E wires were resoldered both on this board and where they connect to the main pcb. However the 2x bridge rectifiers and 4x main filter cap connections look original (dull grey and slightly concave).

On the main pcb:
R317, 319, the common junction of R169 & 171, R 179, 135, 297, 301, 305, 309, 180, 136, 298, 302, 305 and 310.
TR111, 117, 123, 125, 104, 106, 108, 110, 112, 114 and 116.
The post where the cable from the speaker on/off switch connects has been resoldered.
TR157 (big 2sc2773 Sanken) connection looks very suspect, blobs of solder that curve in underneath and don't look like a good connection.
And D132 + C151 in protection as mentioned in the last post. From the stain on the pcb it looks like C151 was leaking before it was changed, perhaps that corroded the diode also.

C702 (10µF/10v) on the 12v stabilized PS board is leaking and D703 (WZ120 zeener) is corroded. It looks like that board has been removed but I can't see the back side until I take it out.

There might be more things that they soldered, it's tough to tell. But after a careful look it doesn't look like any parts were changed except for the 2 that I mentioned. They did not do anything to the pre-driver board that I can see, all of those connections look original.



It looks like there might have been a protection issue and they were shotgunning solder points to find a suspected bad connection. Taking the drastic step of severing the 18000µF cap traces is bizarre. Orange spots on the drawing are re-solder spots and purple are bad parts and the cut traces.

Because I will at least clean the heat sinks and install silpads I think that I'll remove the left output bank. I recall reading that I can temp install a resistor on the output pads for initial testing but I can't remember exactly what and which pads to bridge. (480 ohm ?w, ?or?). Can someone help me out with that info?

I have to decide if I should de-solder the bridge rectifiers and check them. Seems like that will be "yes". The question after that is weather it makes sense to re-install the old ones even if they are good. Possibly not, so I need to find potential replacements.

I'm very slowly trying to understand the schematic, at least in a block diagram sort of way. It looks like the protection circuit is being powered by the stabilized 12v PSU. So if that 12v board has a problem the protection block would be dead. Is this true?

I'm making progress on a buy list. I'll be buying far more than I hope that I need but the rest goes towards building up my benchstock.

Cheers,
James

 

[zaibatsu]

You can bridge the B/E pads with a 470Ω, 2W (or greater) resistor.

Edit - on your tester, I think "B" stands for Beta (hFE), and "Uf" is Vf, the forward voltage.
The hFE measurement probably isn't accurate/relevant for output transistors since the tester is measuring at too low a current, but other than that it looks like yours are healthy as you say.

I'm sure you'll have this thing repaired and rebuilt beautifully after a few more pages.

 

[Bratwurst7s]

You can bridge the B/E pads with a 470Ω, 2W (or greater) resistor.

Edit - on your tester, I think "B" stands for Beta (hFE), and "Uf" is Vf, the forward voltage.
The hFE measurement probably isn't accurate/relevant for output transistors since the tester is measuring at too low a current, but other than that it looks like yours are healthy as you say.

I'm sure you'll have this thing repaired and rebuilt beautifully after a few more pages.

Thanks Zaibatsu! That's what I needed.

Also, I just found this very useful thread that has a few more answers...
http://audiokarma.org/forums/index.php?threads/m-70-needs-help.487049/

Cheers,
James

edit: and this thread...
http://audiokarma.org/forums/index.php?threads/any-yamaha-m-50-experts-out-there.650961/

 

[Bratwurst7s]

I had a productive day today. Starting with rottalpha's wonderful Transistor Substitution thread and then working the AK search function I've been able to find subs/links/sources for a large amount of the parts in the amp. Imho that thread should be made a sticky, it's a valuable resource. Expanding it with diodes, zener diodes and bridge rectifiers would make it even more so.

http://audiokarma.org/forums/index.php?threads/transistor-substitutions.688515/

I copied all of the parts in that thread and then went on a hunt for diodes, zener diodes, bridge rectifiers and more and made a text file of them. Still working on trim pots and resistors but am compiling a pretty good parts sub list now.

Cheers,
James

 

[Oldsansui441]

Watching your build with interest Bratwurst7s, looks like you have most of your parts list sorted . I see you mentioned the notorious power supply cap on the M-70, here is a photo of an M-70 l used to own (was like this when l bought it), did fix it up though.

 

[Karl vd Berg]

Weird that the C151 cap value was lowered to 33uF in such critical part as the protection circuit. Normally the very same 47uF cap, perhaps 16V or 25V, will be used.

And can you confirm that caps C104~C107 are still 100uF/6.3V? The D103, D104, D137, D138 Zeners are the same RD6.2EB2, then you can use 100uF/16V ones, too.

 

[Bratwurst7s]

Watching your build with interest Bratwurst7s, looks like you have most of your parts list sorted . I see you mentioned the notorious power supply cap on the M-70, here is a photo of an M-70 l used to own (was like this when l bought it), did fix it up though.

I think that I have found answers for subs for almost all of the transistors, diodes, zeners and bridge rectifiers in the entire amp. I'm close to posting a tentative list. I still need to find data sheets and start asking some questions to confirm. Note that I'm not planning on replacing all of those parts needlessly. But I am planning for the potential need.

That board looks like someone took a flamethrower to it.

Cheers,
James

 

[Bratwurst7s]

Weird that the C151 cap value was lowered to 33uF in such critical part as the protection circuit. Normally the very same 47uF cap, perhaps 16V or 25V, will be used.

And can you confirm that caps C104~C107 are still 100uF/6.3V? The D103, D104, D137, D138 Zeners are the same RD6.2EB2, then you can use 100uF/16V ones, too.

The schematic shows 47µF/10v. The parts list shows 33µF/16v. You know how these Yamaha amps are. There is almost always something that is wrong. Either 33µF was what was originally installed or the unknown tech went with the parts list instead of the schematic. It would be great if someone that has an M-70 or has worked on one could confirm what is supposed to be in that spot.

C152 is 100µF/6.3v in both schematic and parts list and that's what is installed, looks original. D134 & 133 don't look like they've been disturbed. See photo #3 in post #3. C152 is to the left of the relay, D134 is on the right of the cap.

C105, 106, 107 and 108 are all 100µF/6.3 and look original. The diodes don't look like they have been disturbed.



So far the only parts that look 100% like they have been changed are C151 and D132.
The vote's not in yet as to if the output's are original. After scouring the search function it looks like the correct Sanken subs for 2SA1146/2SC2706 should be 2SA1186/2SC2837 instead of the 2SA1106/2SC2581 that are installed now. I have 6 pairs of NJW0302G/NJW0281G on hand and will be ordering more, just to have that covered if need be.

Cheers,
James

 

[Karl vd Berg]

After scouring the search function it looks like the correct Sanken subs for 2SA1146/2SC2706 should be 2SA1186/2SC2837 instead of the 2SA1106/2SC2581 that are installed now. I have 6 pairs of NJW0302G/NJW0281G on hand and will be ordering more, just to have that covered if need be.

Looks like Reichelt.de have the 2SA1186 and 2SC2837, James.

The 2SC2837 (pdf datasheet) has fT of 70MHz and Cob of just 60pF, instead of 30MHz and 400pF, respectively, for the NJW0302G/NJW0281G.

The 2SA1186 (pdf datasheet) has 60MHz and Cob of 110pF, still lower Cob than the NJW0302G/NJW0281G.

Digikey.at also have them

https://www.digikey.at/products/de?keywords=2sa1186
https://www.digikey.at/products/de?keywords=2sc2837

Cheers

[edit] - If original outputs are 2SC2706 (fT 90MHz, Cob 130pF) and 2SA1146 (70MHz, Cob 220pF), these are extremely fast devices...

 

[Oldsansui441]

That board looks like someone took a flamethrower to it.

Yes it wasn't pretty at all. Luckily the rest of the amp was in great shape.

It would be great if someone that has an M-70 or has worked on one could confirm what is supposed to be in that spot.

I can confirm that the capacitor responsible for the protection relay timing is indeed a mauve Matsushita low impedance 33uF 16v. l had to replace this on the one l had as the relay timing was all over the shop. I bag and tag all old parts from units l have worked on, not sure why as no one ever wants them when l move a unit on .

 

[Bratwurst7s]

Yes it wasn't pretty at all. Luckily the rest of the amp was in great shape.



I can confirm that the capacitor responsible for the protection relay timing is indeed a mauve Matsushita low impedance 33uF 16v. l had to replace this on the one l had as the relay timing was all over the shop. I bag and tag all old parts from units l have worked on, not sure why as no one ever wants them when l move a unit on .

This is very helpful! Thanks a lot for looking. So I could use a Panasonic FM low ESR 33µF/25v for that.

Cheers,
James