CR-840 puzzler

[RayN]

OK, I've been through all the CR-840 threads and I'm pulling my hair out. I have a unit that turns on, lamps light, but the relay will not engage. Obviously there's no output on the speakers and the headphone jack is dead as well. All the voltages on the output ICs are good as are the voltages on the predrivers, with the exception of pin 15. According to what I've uncovered it's supposed to show 41.2VDC and both channels on this unit show 4.3VDC. There are no obviously burnt components. Any thought on where to turn next? I'll list the voltages I measured below:

Predriver

1 - 0.3
2 - 0
3 - 0.3
4 - 44.4
5 - N/A
6 - -45.9
7 - 2.4
8 - 0
9 - 0
10 - 0
11 - 0
12 - 0
13 - -2.9
14 - 2.9
15 - 4.3

Output

1 - -3
2 - -48.4
3 - 0
4 - -1.9
5 - -0.7
6 - 0
7 - 1.7
8 - 0
9 - 48.4
10 - 2.8

Thanks
-Ray

 

[mbz]

pin 15. According to what I've uncovered it's supposed to show 41.2VDC and both channels on this unit show 4.3VDC.

Need to check the power supply. Working from the CR-640 sm (couldn't find the 840) check TR801 and TR803 b/c/e DC voltages
to chassis/ground. These TR's are for the regulated supply. For both the IC's and protection circuit.

Has the power supply been recapped?

 

[RayN]

Thanks for the reply. I forgot to mention that I was also able to adjust the DC offset voltages correctly as per the manual.

Here are the voltages I measured on TR801 and TR803.

801:
E = 4.3V
C = 62.9V
B = 4.9V

803:
E = 0V
C = 4.9V
B = 0V

 

[RayN]

Oh, the caps in the power supply appear to be stock.

 

[RayN]

Need to check the power supply. Working from the CR-640 sm (couldn't find the 840) check TR801 and TR803 b/c/e DC voltages
to chassis/ground. These TR's are for the regulated supply. For both the IC's and protection circuit.

Has the power supply been recapped?

So based on what I see 801 isn't seeing the correct voltages on the E & B.

 

[RayN]

So you put me on a good track here. The low voltage at the 801 B (and 803 C) seems to be related to R803. I've got ~41V on the other side of that resistor.

 

[mbz]

Suggest check resistance of R801 (2.2K) and R803(3.9k), incircuit should be ok, amp powered off/unplugged.
If they are ok (may be open or drifted high) then replace TR803 (2SC1815) with KSC1815 (cheap, readily available)

 

[RayN]

Suggest check resistance of R801 (2.2K) and R803(3.9k), incircuit should be ok, amp powered off/unplugged.
If they are ok (may be open or drifted high) then replace TR803 (2SC1815) with KSC1815 (cheap, readily available)

Those two resistors were fine, but I pulled TR 803 and, sure enough, shorted. I had a C1845 handy which is pretty close to the C1815 and we have some progress! The tuner section is now tuning to stations and the signal meter works. But the relay will still not engage. I now have ~6V on pins 3 & 4 on the relay, which I didn't have before.

EDIT: The relay IS engaging briefly. If I power down for a few minutes and then power back up the relay engages, voltage flows to pins 5-8 for 1 second, and then the relay clicks off. So it appears something is still kicking this into protection mode.

EDIT 2: And now the offset for the left channel is completely out of whack. I'm getting 6-7 volts at the test point. The right channel is fine, I was able to adjust it down to about 9mv. I'm guessing this is why it's going into protection.

 

[mbz]

Redo voltage measurements on IC pins as per post #1

 

[avionic]

Only CR-XX40 model, I care for is the 2040..Discreet amplifiers.. No stink'n " chip amplifiers "...

 

[RayN]

Only CR-XX40 model, I care for is the 2040..Discreet amplifiers.. No stink'n " chip amplifiers "...

Yah, starting to see why.

Redo voltage measurements on IC pins as per post #1

Yep, lots of changes, particularly on the left channel. Right channel looks good:

R
1 = 0.4
2 = 0
3 = 0.4
4 = -44.4 (was +44.4)
5 = N/A
6 = -45.9
7 = 2.7
8,9,10,11 = 0
12 = -2.9
13 = -3.2
14 = 2.9
15 = 45.5 (was 4.3)

L - Not so good
1 = 0.3
2 = 0
3 = 0.3
4 = -44.4 (was +44.4)
5 = N/A
6 = -45.9
7 = 9.2 (was 2.4)
8 = 5.8 (was 0)
9 = 0
10 = 6.6 (was 0)
11 = 6.5 (was 0)
12 = 2.5 (was -2.5)
13 = 3.6 (was -2.9)
14 = 9.5 (was 4.1)
15 = 45.5 (was 4.3)

Is this a bad predriver on the left side?

 

[mbz]

Need a bit of time to digest, trying to track down datasheet, to see circuitry inside, but yamaha proprietry...
I would NOT consider switching IC's with the working channel. Problem may be that that the power IC (IC703)
is feeding garbage back to IC701, so you may smoke another IC

 

[mbz]

Have you been able to find datasheets for IC701,2?
As a long shot you could try replacing the electrolytics that hang off IC701,2 for the problematic channel.
They are mostly 1uf/50V and 4.7/63V and ??? This will buy me a little time

 

[RayN]

I'll poke around the ECs. Thank you so much for your time.

 

[RayN]

In the meantime I went ahead and ordered a hard copy SM and schematic. It'll be helpful to see the correct information.

 

[chazix]

I've been shy about wading into this one because, to be honest, I don't like the odds of success for diagnosing this by remote control. My spidey sense tingles from things like changing polarities on what should be power supply rails.

But maybe it will help to establish a few things:

• Predriver pins 15 and 6 should be connected directly to regulated power supply rails. If those pins don't measure around +45VDC and around -45VDC, there is something wrong with the power supply.
• Predriver pin 10 should be connected directly to the output of the power amp section. So this confirms that there's a high offset at the output of the left power amp. It's plenty enough to keep the receiver in protection.
• Predriver pins 8 and 11 should be loosely coupled to the emitters of the final output transistors. For there to be any more than a very small fraction of a volt difference between those two pins is a sign that there's something pretty wrong with the output stage. So if those two readings were made at the same time, there is almost certainly a bad emitter resistor or two (the big white ones near the power pack), and that would be a strong hint that the power pack is bad as well.

In my reasoning, I've assumed that PCB traces are OK.

I don't see any particular reason to suspect a bad predriver at this point, though it can't be ruled out, either. There is a very limited amount of reasoning that can be done based on the predriver voltages unless the output stage is healthy, and I don't think the left side is. If you pore over the "ongoing saga" thread obsessively enough, there is a technique in there for checking the main function of the predriver with the power pack removed, and some resistors tacked in to complete the feedback loop.

HTH,

chazix

 

[RayN]

@chazix I went ahead and pulled the output ICs and soldered in some resistors as per the "saga" thread. I only had some 470R resistors but I figured they'd be ok. Same issues, protect mode, same weird voltages on the left predriver and 5.6V at the offset test point. Right channel is in spec. So, I pulled the left predriver and voila, no longer in protect mode and I get audio out of the right channel through the headphone jack.

I guess I'll take a chance on a fleabay Chinese predriver. Any way I can do some checking on the output ICs to verify if they're OK? I tested all the emitter resistors and they measured between 0.6R and 0.7R so they're a bit out of spec. Should those go too?

 

[mbz]

Would predriver pins 8&11 be used for overload protection?
And what would happen if these pins were "disconnected"...

EDIT: Thinking these pins are the source of the "offset" (happy to be corrected)

Not a big fan of ebay, there are a few x40's at
http://www.oaktreevintage.com/Yamaha_Stereo_Parts_Unit.htm

I'm not a huge fan of anyone that charges $30- for a used main filter cap...

 

[chazix]

Any way I can do some checking on the output ICs to verify if they're OK? I tested all the emitter resistors and they measured between 0.6R and 0.7R so they're a bit out of spec. Should those go too?

The service manual will have a diagram of the power pack internals, and you can check several of the transistor junctions based on that. Won't be able to prove goodness, but it can detect some forms of badness.

Conventional ohmmeters aren't very accurate for sub-ohm measurements - I think your emitter resistors are OK.

Would predriver pins 8&11 be used for overload protection?
And what would happen if these pins were "disconnected"...

EDIT: Thinking these pins are the source of the "offset" (happy to be corrected)

Exactly right that they are for overcurrent sensing. I don't think I can rule out the chance that the offset problem would go away if the overcurrent function was disconnected. To check that, @RayN could try de-soldering pins 7 through 12 (inclusive), probably with the help of some good solderwick to make sure the pins are no longer connected to the circuit board. With those pins disconnected, only the primary amplification function of the predriver would remain.

Oh! That reminds me that you should probably check whether the offset can be adjusted out.

Cheers,

chazix

 

[RayN]

I'll recheck the offset with the predriver removed. I also thought I should check the voltage on the vacated 15 pins to see if anything weird is happening with the predriver removed.

Thanks a lot guys! I got this for free so for me it's about learning. If I manage to get some sound out of it it'll be frosting.