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Yamaha CR-640 Problem: Chicken or Egg?

R

radlabguy

New Member
I'm the original owner of a Yamaha CR-640 (and matching turntable), which have served well for over 30 years. Last week the 640 made a big bump in one of our Advent speakers, then all was quiet. The old 640 lights up (ah, green) and the tuner works, but the protection relay never pulls in. A quick check of the left channel bias testpoint with my trusty Fluke 77 and it measures a few millivolts; the right channel bias testpoint is at -40 Volts. Not good.

The right channel IGO2950 driver chip has the following voltages on its pins:
Pin Volts
1 0.012
2 0.007
3 -0.147
4 -39.6
5 -39.0
6 -39.7
7 7.39
8 -40.4
9 0.0
10 -40.3
11 -33.6
12 -32.3
13 2.707
14 7.61
15 39.4

And the right channel IGO2980 output amp chip shows
Pin Volts
0 7.63
1 2.7
2 -40
3 -40
4 2.7
5 -21.25
6 24.16
7 6.3
8 -40
9 40

Since there appears to be feedback between the output and driver chips (through R718 and R720, as well as through the power amp output through the bias adjust network into pin 3 of the driver), how can one tell which of these chips is bad? If I pull one or the other, there's no more feedback and the the output level could go anywhere from rail to rail. Or am I missing something (and I could very well be missing something: I've repaired many many radios and test equipments from the 1920's to the 1990's, but never a hybridized audio beastie like the Yamaha). Any hints?

Thanks!
 
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Sadly, unless you can find a set of paks from a donor 640, it's a doorstop if they're bad.

Unobtanium.

I take it, if possible, you've tried adjusting the bias? Look at the forum sticky up top for links to manuals.
 
Yes, the bias pot doesn't change the -40 volt bias reading; either the driver or the output hybrid is bad.
The output hybrid is still available as a Sanyo equivalent; only the driver chip is no longer available. But I'd like to make sure that the driver is good before replacing a bad output hybrid.
I have the schematic, but thanks for the link pointer.
 
The big clue is the low voltages on pins 13 and 14 of the IG02950, it is probably still good. The output power amp can be replaced with an STK0050II (and the II is important).

Look slightly to the left of the IG02950 for a burnt resistor 390 ohms, it is in that feedback loop and sometimes gets toasted when the power amp blows.
 
Thanks ecluser and merrylander. I followed the link and even found merrylander's equivalent circuit for the driver chip (more fun with LTspice someday!) But I think the plan will be to do some more testing per ecluser's suggestions, and purchase an STK0050II to have on hand for the substitution. I'll let you know what I discover.
 
CR-640 repaired and rebulbed

Just to follow up on my CR-640 repair: I confirmed that one of the output hybrids was blown (and not the driver chip) by measuring the resistance between pins 8 & 9 (and pins 2 & 3) of the output chip. These pins connect to the output transistor emitters and collectors, and on a good chip should measure several hundred ohms. On the bad chip they were essentially a dead short. I used my homebrew ESR meter to measure these resistances as it puts out a millivolt-level signal and will not turn on any transistor junctions. I found the schematic for the STK0050II chip on the Georgia Audio Lab website.

By the way, the Georgia Audio Lab folks charge $16 for the replacement; later a friend pointed me to the Match-a-Knob site that has them for about $6. Other sites charge up to $48 for the same STK0050II !!

So a new chip went in, the bias adjusted to dead zero (using an analog high-impedance voltmeter, not some horrible digital thing), and boy was the sound golden again. This receiver has been in use for 31 years!

Oh, and I replaced the burned-out slide-rule dial lamps with superbright green LEDs and it looks great.

Thanks again for everyone's help on getting my old Yammie back in the groove.
 
Green LEDs in the Dial

Here's what the CR-640 looks like with the green LEDs in the dial. The green of the original lamps (meters, dial pointer) are blown out a bit in the photo, but they match the LEDs nicely in real life. When the original lamps start going out, I'll replace them with LED's too, but getting to the one in the dial pointer might be a real bear.

The YP-D71 turntable on top is still crankin' along too.
 

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If you look closely at the dial pointer you will see the metal piece the dial cord is attached to is between two plastic'horns'. These can be spread apart gently allowing the metal plate to pop out and give uou access to the lamp.
 
I got those LED's over a year ago for another application and I have no idea what the part number was. But I do remember that they were through-hole, green, with a clear lens, and rated 1500 mcd at 30 mA. Try to get LED's with the largest illumination angle you can -- I think the ones I used were 20 or 30 degrees. You can search for LED's like that on Mouser or Digikey. I used several series resistors totaling about 800 ohms, and measured the current at about 27 mA. Total resistor power dissipation is about 0.6 watts, so that's why I used several resistors in series. I just soldered the LED's and series resistors to the long PC board lands where the original bulb sockets were located. Good luck!
 
radlabguy said:
By the way, the Georgia Audio Lab folks charge $16 for the replacement; later a friend pointed me to the Match-a-Knob site that has them for about $6. Other sites charge up to $48 for the same STK0050II !
Click to expand...

Glad you got the 640 up and running. I find it very gratifying to solve a problem like this, especially when you're initially advised that parts aren't available.

This is the first time I've heard of "Match-a-Knob", so I went there and they do have some great prices. I've ordered a fair amount of parts from Electronix, so far so good, always thought their prices were fair, but "MaK" has far better prices on practically everything I looked up. An STK module I need is half the price. Any readers aware of the MaK parts quality and service reputation? Is this a matter of getting what you pay for?
 
I have bought STK modules from AudioLab and they were genuine, also from B&D Enterprises. Can't say as much for other suppliers.
 
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