Yamaha CR-640 Blowing the main fuse instantly

[redpackman]

I got a Yamaha CR-640 a few weeks ago. Went through it and redid all the lights with the properly rated bulbs, by the way. Got it all spiffed up. It was playing well. Sweet sound. Balanced the DC offset too and got it to just around zero on both channels using the service manual as a guide.

Playing it the other night. Then suddenly, poof, it died after having played awhile. I took off the cover and the main fuse inside the unit (F801 on the HifiEngine service manual download) was blown. OK. Put in another. No delay now. It blew instantly. Dead short? Checked out the transformer by disconnecting its leads to the board(s). Gave it power. No fuse blowing, so the transformer, presumably is OK.

Now what? Power diodes? I'm a newbie and any input would be appreciated.

 

[redpackman]

Update: Took out the two large caps and tested them. They are not shorted and seem to be in the right range. I tested the rectifier diodes and they too checked out OK (removed from the circuit). I put the unit on a dim bulb tester which allowed it to operate without immediately blowing the fuse, BUT the bulb sure lit up the room the whole time. With the unit powered up, I checked the voltages on the two Darlington Yamama IG 02980 "Power Amplifier" modules. When comparing the two IG 02980's with each other, some of the pins registered similar readings, but some were quite divergent, particularly in the center of the units. One of them probably blown? Is that a legit assumption, but which one?

Grrrr.

 

[Binkman]

Be patient... been a busy couple of weeks for most of us. retrace your steps as an initial suggestion. full lit dbt means a short. I'd discharge the mains caps and do a continuity test in the traces of the power supply.
bink

 

[chazix]

My guess is that you're correct in concluding that one of the two power packs is bad (emitter-collector short in the output transistors). If that's true, I expect you'd be able to tell which one is passing excessive current under the DBT rig by measuring the voltage across the emitter resistors. That amounts to measuring the voltage between power pack terminals 3 and 8 (the third terminal from either end).

On the good power pack, the 3-8 voltage should be pretty close to zero under a DBT rig - it definitely shouldn't be more than a few tens of millivolts.

On the bad power pack, the 3-8 voltage might be in the whole-volt neighborhood - a large fraction of whatever the DC power supply is producing under the DBT.

Best not to run the receiver for very long, even under the DBT. The emitter resistors will be passing a lot of current, and if they blow, there will be damage to the pre-driver. It would be a good idea to replace the emitter resistors that are connected to the bad power pack, even if they still measure OK.

Hope this helps,

chazix

 

[redpackman]

Thank you to Binkman and chazix, I will check that out! In doing some comparative checks last night using the DBT, but without your counsel, chazix, comparing pack to pack I did notice that pins 3, 5, 6, and 8 had quite different readings in the two darlingtons. The other pins had readings on par with its twin. I'll now check the 3, 8 comparison. Can you tell me which component in the CR-640 is considered the "pre-driver." I'll give that some attention too.

Update: Using the DBT, one Darlington had a reading of .5v between pins 3 and 8, and the other had readings of around 0v . Have I found the bad unit?

 

[chazix]

which component in the CR-640 is considered the "pre-driver

The pre-drivers are the two stand-up modules, IG 02950. They are responsible for all the voltage gain in the power amp section, while the power packs are responsible for current gain.

In my opinion, it's next to impossible to make any meaningful checks of the pre-drivers when there is a fault somewhere else in the power amp section. (It's possible to make a rough functional test after a suspect power pack has been removed, but it's pretty fussy, involving tacking in some extra resistors to complete the overall feedback loop around the power amp.)

Cheers,

chazix

 

[redpackman]

Thanks, Chazix. I wrote my question about the pre-drivers before looking again at the unit itself. The pre-drivers are clearly marked. Sorry I troubled you with that I told you I was a newbie.

 

[Binkman]

newbie? lol... I'm still a newbie at 64. No dumb questions here (DIY) we can morph and ask any question. 'there are no stupid questions'.

 

[chazix]

Agree with Binkman - no here!

It does occur to me, though, that you might not be aware that there is a Yamaha-specific forum. Over there, you can't throw a virtual stick without hitting a discussion of pre-drivers and power packs and other CR-x40 trivia.

 

[redpackman]

Thanks. My plan right now is to get a replacement for the power amplifier and install it (IG 02980). Actually it's a Sanyo STK 0050 II which is supposed to be an equivalent. What are the chances if I install it and there are still components on the board that are defective or have been damaged by the original IG 02980 failure they will damage the new unit too? I don't want to just buy and install a new part just to blow it instantly. I will use a DBT on start up and check voltages. ON the other hand what are the chances that this'll correct the issue.

 

[chazix]

I take it that you've confirmed that the problem is a bad power pack, and identified which one is bad.

What are the chances if I install it and there are still components on the board that are defective or have been damaged by the original IG 02980 failure they will damage the new unit too?

Anything is possible, but in my experience, there's very little risk of that happening.

I do think it would make sense to replace the two emitter resistors that were connected to the failed power pack. The emitter resistors are the large 0.47 ohm ceramic resistors. I think the originals are 5 watt parts. The fact that the fuse blows means that your original emitter resistors must not have failed, but they will have been overloaded, at least briefly.

Failure of the emitter resistors won't damage the power pack, but it could damage the pre-driver.

ON the other hand what are the chances that this'll correct the issue.

I think the chances are excellent.

You should be aware that any currently available new STK 0050II will be, in a sense, a counterfeit. Sanyo hasn't made them for ages. It would be a good idea to get the replacement from a trusted supplier, which probably means "not ebay". I'm afraid I'm not up to date on who to suggest as a supplier - the suppliers that I've ordered from recently seem to have run out of them. I see that audiolabga.com says they still have a couple, but I can't personally vouch for them.

 

[redpackman]

Thanks for the input. I'm afraid I am getting them from eBay. They CLAIM to be Sanyo "old stock." We will see. I've identified the bad one using a DBT (because at full power the fuse immediately blows). Using DBT technique and measuring between pins 3 & 8, I believe I have found the bad one. I was told (hats off to Chazix) that the voltage should be low between those two. It is on one, but on the other (DBT) it's around 500 mv, which I believe is higher than it ought to be. I'll probably replace both of them with my "old stock" Sanyo's. I'll check out the .47 ohm resistors too.

 

[chazix]

Cool, I think you're well on your way to a victory dance.

One clue as to whether the parts you get are real Sanyos is if they run with a decent idle current, which is what the pin 3-to-8 voltage reflects under normal conditions. Once the shorted power pack is removed, I expect you would find that the good one's 3-8 voltage is something like 30mV. If the replacement also measures in the same ballpark, you're probably found genuines (and I and probably other AKers would love to hear where you got them).

In my experience, reproduction power packs don't run any idle current at all when cold (3-8 voltage is zero). That's not the end of the world, since the Yamahas are a "high feedback" design which will largely compensate for the lack of idle current. (And even the repros do start to exhibit a smidgin of idle current when they're warmed up.) But, if you feel like taking the trouble to replace only the failed power pack at first, and you find that the replacement part does not produce idle current, then I think it would be a mistake to replace the working one.

Cheers,

chazix

 

[redpackman]

Update: I pulled the Power Amplifier Darlington I suspected. It's the one with the higher DC voltage (500 mA) (using a DBT) between pins 3 & 8. I don't yet have its replacement. After pulling the component, I hooked up the receiver through the DBT again. I turned it on and the bulb glowed brightly at first, but then in a second calmed down to about a third of the glow it had been with the bad component in it. I left it on probably for 20 seconds or less. Does it hurt the unit to run it without the IG02980 in it? Would I be able to plug it in without the DBT to see if now the fuse holds instead of blowing instantly as it did before? Or is that dangerous?

 

[redpackman]

I pulled the .47 ohm resistors in front of the bad Darlington. I tested them both. They look good and test on my DVM at .5 ohm. According to the parts list these aren't just 5 watt .47 ohm resistors. It says they are "metal plate resistors" with a .47 ohm rating, and I assume a 5 watt rating. I'm not sure one can find these just anywhere. Are they kind of a fuseable link/resistor that will melt (metal plate) if the current overloads? How much risk am I taking reusing them? The only thing that I see (thus far) that took a hit from the Darlington going is the fuse...nothing else obviously burned. it was black and it blew immediately upon power up.

 

[chazix]

If we assume that the failed power pack was the only problem, there's no hazard in running the receiver without it.

I'm a little surprised that the DBT stayed lit, though. What wattage is the bulb? Offhand, I would expect a 60W DBT would be big enough to let CR-640 to power up without much glow, and be stable enough so that the protection relay would engage. Did you hear the relay click?

Some voltage checks might be in order, to try to confirm that the removed power pack was the only problem. Best to stick with the DBT for this. I would suggest checking:

• DC offset on the presumed-good channel should be near zero.
  
• Bulk DC supplies (available at the large caps, or at power pack pins 2 & 9) should come up to a large percentage of their normal-operation voltage, which is around + and - 40V.
• High-voltage regulated DC supplies should also be in the + and - 40V ballpark. These are available at the emitters of TR801 and TR802, or at pre-driver pins 6 and 15. If you probe the pre-driver pins, use extra care to avoid probe slip.

If that all looks OK, I think there is minimal risk in powering up without the DBT.

There's nothing all that special about the emitter resistors. They are not meant to act as fuses (although they sometimes end up doing exactly that!) Standard metal-film parts - really, just about any kind other than wire-wound - can replace them. It's not crucial to replace them, given that they still test OK - I just like to do it out of an abundance of caution when a power pack has failed short-circuit.

Cheers,

chazix

 

[redpackman]

Found a donor machine. It had one good Yamaha Darlington in it. Took it out and installed it in the original receiver. Started everything with the DBT using a 60 watt bulb. Heard the relay click. Checked it out. Voltages above seemed to be in range. Set the DC offset. Plugged it in without the DBT and it works thus far. Rechecked the DC offset on both channels.

I'm still a little skittish because the first Darlington blew when it seemed to be working fine for no apparent reason other than age. I know I'd set the offset on both channels and it was well within 5 mv +/- on both L and R channels, but it blew anyway. I hope this new/used Power Amp Darlington and the other one that's original to the 640 will work for awhile now.

Thanks again, to all, for all the help.

I can't do my happy dance yet. I'm going to see how it runs tomorrow. I've thought I had something solved before only to have it go south after a day or so. We'll see.

 

[Binkman]

Darlingtons come in many uses.. you're being output.. hopefully the pair will work. However.. if it blows then search for a matched pair.

Here's a sample explanation... think this outfit maybe commercial.. just came across it.

 

[chazix]

(I think when redpackman says "Darlington" in #17, it's a reference to an IG 02980 power pack, not discrete Darlington transistors that might need to be matched.)

It is unsettling when a failure doesn't seem to have any cause, isn't it? But at least in my experience, with bipolar output stages of various kinds, sometimes rail-to-rail shorts "just happen".

My guess is that redpackman's receiver is good to go. Nice work!

chazix

 

[redpackman]

A question for Chazix or anyone else out there. Chazix wrote, "
There's nothing all that special about the emitter resistors. They are not meant to act as fuses (although they sometimes end up doing exactly that!) Standard metal-film parts - really, just about any kind other than wire-wound - can replace them. It's not crucial to replace them, given that they still test OK

Well, I'm working on another unit that has a failed 5w .47 ohm metal film resistor. They're not easy to come by. Why not a wire wound, 5 watt, .47 ohm resistor? Watts the difference, in this application between metal film and wire wound if the other specs are the same?