Ongoing saga of the Yamaha CR-n40s

Just dropped in. I have original service manuals for 640, 840, 1040 and 2040. I will PM for email instructions. The scan will be fairly large.
 
No real progress, yet.

On Sat, I should be done building my DBT, which will allow further diagnosis.

I’ve also captured the internal schematics of the predriver and power modules in LTSpice. The idea here was to get a better understanding of what’s in the modules and what functions are done external to the modules.

Regards,

Rob
 
I have a CR-840 that needs a selector knob, the tuning light and I think the output IC's and or the pre-amp IC's. Are the pre-amp IC's more prone to failure than
the output IC's?
 
Are the pre-amp IC's more prone to failure than
the output IC's?

My own belief is the opposite. I think predriver failures are usually knock-on damage caused by the output failing first, and only if the output pack also blows one or more emitter resistors. There are always exceptions, though, and I'm sure others may have different ideas.

For sure there is a school of thought that holds that the predrivers run too warm and should have had an external heat sink. (I'm not convinced. The lion's share of the heat comes from a passive component - a resistor - in the predriver, and what's left is split between two transistors. There would be no problem for those components to deal with the heat pretty much forever unless they were woefully under-specified.)

Cheers,

chazix
 
Thanks, its been a yr since I diagnosed the issue, so cant remember anything except reading somewhere the preamp IC's were NLA.

I see here in this thread the STK0060 MK2 will sub for the output IC's, which is great. But was there a sub found for the preamp IC's ?

My own belief is the opposite. I think predriver failures are usually knock-on damage caused by the output failing first, and only if the output pack also blows one or more emitter resistors. There are always exceptions, though, and I'm sure others may have different ideas.

For sure there is a school of thought that holds that the predrivers run too warm and should have had an external heat sink. (I'm not convinced. The lion's share of the heat comes from a passive component - a resistor - in the predriver, and what's left is split between two transistors. There would be no problem for those components to deal with the heat pretty much forever unless they were woefully under-specified.)

Cheers,

chazix
 
But was there a sub found for the preamp IC's ?

Not as such - the predrivers made for Yamaha combine a voltage amplifier and an output current-limiting function, which as far as I know is unique combination. You can probably find knockoffs on ebay and take your chances whether they're fit for purpose, or you can PM me for a sure-fire but fairly expensive solution. But it's really best to not replace an original predriver unless you are sure it is bad. In this thread you can find a method for testing a predriver with the output pack removed.
 
Joining this thread to report on A CR-1040 I got in for repair. Read this thread a few times and measured the voltages on the Pre driver and found high voltages on pins 13/14
on IC 602. So I went to the Power Modules and measured the resistance and found one power module to seem faulty. This was also confirmed by one of its Emitter resistors reading over a few Kohms. SO out came the module and replaced the Resistor and powered it up and relay clicked. Good so far.

Some one was in this before as the solder pads were peeling and broken on that Power pack and looks like a crapy job on the other one too,

49003525456_00db83f740_b.jpg


I like to make sure the holes are filled with solder, Just me.

49003730727_beae3b2798_b.jpg


So I added 330 ohm resistors as mentioned earlier in this thread to check if the Pre drivers are ok.

49003525461_88fb874780_b.jpg


Tested with Head phones and got sound from both channels.

So I ordered from UTSource 5 power packs just to make sure I get one that works . At only 5 bucks each and 30+ total shipped why not. And I'll have 4 in stock incase another
comes up for repair.



You guys here ROCK !!!!!

Will report back once I get the PM installed.

I'll also check the resistance between the respected pins as per this thread mentions before installing so as to not blow out a pre-driver.

Athanasios
 
Thanks for that post and pix Nash. Interesting you bought (5) to make sure to get 1 that works. Wonder if thats by design of the Chinese. Create lots of fake semis, so theres a scare and
good chance what you buy will be garbage, and eventually it will result in the buyer doing what you have just done. I'm seeing more and more semis that have incomplete datasheets, I think are done that way intentionally. The idea is to make it appear there is transparency on some of the new products where parts are available, but when you take it down to the last detail, they are leaving out crucial info
that you need to test the IC. You can just "buy a new one" or in your case 5 or more, but if the IC by design is intended to be an enigma, its not going to be a solution. An engineer I bough this up with
said its likely intentional, to protect the mfr from copyright. Its hard to make a case when they dont have a back channel thru the distributor and the IC is a few years old, and there is a new Chinese product that you can buy to do the same thing for less or equal than what the cost of the product you bought 3 years ago. Sorry for the rant on fake Chinese semis, my comments probably belong on another thread specifically for it, but I dont frequent this site as much as I would like to.
 
Thanks for that post and pix Nash. Interesting you bought (5) to make sure to get 1 that works. Wonder if thats by design of the Chinese. Create lots of fake semis, so theres a scare and
good chance what you buy will be garbage, and eventually it will result in the buyer doing what you have just done. I'm seeing more and more semis that have incomplete datasheets, I think are done that way intentionally. The idea is to make it appear there is transparency on some of the new products where parts are available, but when you take it down to the last detail, they are leaving out crucial info
that you need to test the IC. You can just "buy a new one" or in your case 5 or more, but if the IC by design is intended to be an enigma, its not going to be a solution. An engineer I bough this up with
said its likely intentional, to protect the mfr from copyright. Its hard to make a case when they dont have a back channel thru the distributor and the IC is a few years old, and there is a new Chinese product that you can buy to do the same thing for less or equal than what the cost of the product you bought 3 years ago. Sorry for the rant on fake Chinese semis, my comments probably belong on another thread specifically for it, but I dont frequent this site as much as I would like to.


I have bought many times from UTSource for rare Semis and so far so good , not one bad one. But you never know. And I probably will come across another 1040
that might need repair so why not have some in stock if they are so cheap?

And for that Matter I have gotten bad parts from Mouser, digikey and Newark all respected companies. Sometimes it just happens.


But thanks for the post as well.
 
Ok Everyone, the Amp Modules finally came in and they work Great and sound pretty damn good.

The replacement runs warmer than the original I left in, but that might be due to the amount of thermal compound is on the original yamaha part, that looks like was replaced
by someone else. Its loaded on there, I was always taught to use just a very thin layer to fill the imperfections of the part and heat sink
yet allow the heat to transfer quickly to the heat sink.

new parts heat sink is at 92 F° and old side is at 84F° . Maybe i'll re apply the thermal on the original part .

here is what they look like

49160946166_ddb46b9714_b.jpg



I got DC offset to close to zero on both sides. is there a bias adjustment for this amp or idle current? Seems to draw a lot of power, 67 watts in idle.


Athanasios
 
is there a bias adjustment for this amp or idle current?

No. That is supposed to be preset within the output stage modules.

With the original Yamaha-branded modules, you could expect to see 30-40mA of idle current (per channel) in a CR-1040. In my experience, aftermarket modules have nearly zero idle current when cold, so I'm surprised that you're seeing the aftermarket parts run warmer than the original. Were your temperature measurements made after running for a while with a load?

Let's suppose for a moment that both channels are dissipating about the same power within the output modules. This should be true, if they are driving enough of a load that it swamps whatever idle current they have. Given that, the heat sink that runs warmer must have better thermal coupling to its module. If that reasoning holds up, then I think you're right to suspect that there is too much thermal paste on the original module, and your minimalist application is indeed better.

Congrats on the repair!

chazix
 
No. That is supposed to be preset within the output stage modules.

With the original Yamaha-branded modules, you could expect to see 30-40mA of idle current (per channel) in a CR-1040. In my experience, aftermarket modules have nearly zero idle current when cold, so I'm surprised that you're seeing the aftermarket parts run warmer than the original. Were your temperature measurements made after running for a while with a load?

Let's suppose for a moment that both channels are dissipating about the same power within the output modules. This should be true, if they are driving enough of a load that it swamps whatever idle current they have. Given that, the heat sink that runs warmer must have better thermal coupling to its module. If that reasoning holds up, then I think you're right to suspect that there is too much thermal paste on the original module, and your minimalist application is indeed better.

Congrats on the repair!

chazix


Yeah I think its due to the thermal paste on the original. It looks like it was replaced at some time maybe with the one that I replaced.
Both had way too much grease in my opinion.

So for now UTsource had these Darlington Power Packs and they work fine as of now.

Thanks Chaz,

Nashou
 
The white paste is DOW 340 compound, you can buy it in a big tube online probably and have a lifetime supply. MSDS / hazmat data says to chuck it in the garbage after a yr as it
claims there's a shelf-life, but what happens I have seen is is an oil separates from the compound if left a long time on the shelf and all you have to do is stir / remix it and it seems fine.

Too much on an STK probably isnt a big deal as long as the mounting screws are tightened down securely. Usually if too much is applied, it squeezes out the edges of the device
and whats left inside performs the job fine, and you just carefully wipe the edges with something to absorb it and its all done.

I think what people do is they see how often compound dries out when they replace components, they figure apply a lot, then it will dry out over a longer time and the component will last longer.

On a side note, I still have the CR-840 here and wanna get it going someday. I read these dont fetch a good price on the market, but here's one just for parts that seems to get about $100.00 or so:
https://reverb.com/item/32650185-audiophile-yamaha-natural-sound-cr-840-stereo-receiver-60-watts

Wondering what a decent price to pay for one of these is when its all fixed up and working well?
 
The white paste is DOW 340 compound, you can buy it in a big tube online probably and have a lifetime supply. MSDS / hazmat data says to chuck it in the garbage after a yr as it
claims there's a shelf-life, but what happens I have seen is is an oil separates from the compound if left a long time on the shelf and all you have to do is stir / remix it and it seems fine.

Too much on an STK probably isnt a big deal as long as the mounting screws are tightened down securely. Usually if too much is applied, it squeezes out the edges of the device
and whats left inside performs the job fine, and you just carefully wipe the edges with something to absorb it and its all done.

I think what people do is they see how often compound dries out when they replace components, they figure apply a lot, then it will dry out over a longer time and the component will last longer.

On a side note, I still have the CR-840 here and wanna get it going someday. I read these dont fetch a good price on the market, but here's one just for parts that seems to get about $100.00 or so:
https://reverb.com/item/32650185-audiophile-yamaha-natural-sound-cr-840-stereo-receiver-60-watts

Wondering what a decent price to pay for one of these is when its all fixed up and working well?
I would not and have not paid much more than $100 for working 640s or 840s. Good luck with your repair.
 
@jbailey830 - Thanks. Seems the cost of the new STK's probably will be close to that with shipping and all. Id really prefer buying from a US supplier but Chong Kong seems to always come up as the only choice out there.

Couple of questions to the people posting here, one is the prototype pre-amps. Seems like they were a success, and is the inventor of them selling them? Other question is, lets say your STK's are shot, is it probably a good move to replace the pre-amps while you are at it, as they are likely gonna eventually go bad sooner or later? I would guess these series stereos are probably a good 30-40 years old now.
 
Couple of questions to the people posting here

You can PM me about predriver repros if you like. I don't have a strong opinion that predrivers should be proactively replaced, and I don't do that myself - but then, I know I can come up with them when needed.

HTH,

chazix
 
Ok sounds good. I guess at this point I ought to do more tests, so I powered up the stereo for a better diagnosis.
The FM tuning meter sways a bit to the left, the power light lights but no sound at all on either headphone jack nor speakers.
None of the meters move at all in AM or FM with the input selected as tuner. I also made sure the muting is off.
This almost seems like the power supply is down. The 2 fuses check ok. I guess the next step is slave in the 330 ohm resistors on the pre-amps and check for sound in the headphones?
Or maybe some basic DC voltage checks are in order first. I see original IG 02970 Yamaha stamped out put IC's in there and IG 02940's as the predrivers. Nothing looks like its been worked on
so far but I have not dropped the bottom cover off yet.
 
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