jbailey930
AK Member
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.
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
But was there a sub found for the preamp IC's ?
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.
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
I would not and have not paid much more than $100 for working 640s or 840s. Good luck with your repair.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?
Couple of questions to the people posting here