Yamaha CR-620 Powers On, Does Little Else

cade12

Active Member
I recently picked up a CR-620 to spruce up with plans of donating to a buddy of mine that wants to get back into vinyl. I'm running into some problems that I'm hoping you guys can help with.

The unit powers on. Power, FM, AM, and Stereo lights all work. The dial and meter lights do not.

The relay does not click on after switching the unit on. I've done some searching and tried to diagnose this, but I seem to be hitting a wall. The transistor with the heat sink that is frequently the cause of such headache seems to be ok. I resoldered the leads, and it's measuring close to 30 V. Still no relay click.

Also, it seems that the idle current can be adjusted easily for one side, but the other stays at 1.2 mV regardless of changes made.

What else should I be looking at/for?

Thanks in advance for the help!
 
I may have found part of the problem. Both channels show similar voltage until I get to transistors 707 and 708. 707 is at close to 30 V, 708 is at 1 V or less. Loos like I'll be starting there.
 
Both channels show similar voltage until I get to transistors 707 and 708. 707 is at close to 30 V, 708 is at 1 V or less.
Need to be more specific. Measures 30v's on TR707 emitter? Base? collector?
 
I believe it was 30 V at E and C. Can't remember B off the top of my head. The 1 V on 708 was also E and C, I believe. I can double check when I get home.
 
Looking at the transistors so that the heat sink is behind it, power transformer is to the left, I think the legs are CBE, but correct me if I'm wrong. In either case, the readings from left leg to right for each side are as follows...

707:
-28.1
-26.2
-27.5

708:
-1.14
.97
-.51

I also noticed that the output transistors for the same channel are much lower readings than the other. However, I just slipped and shorted one of those when probing and blew the main fuse. Guess I'll be replacing that as well.
 
Looking at the transistors so that the heat sink is behind it, power transformer is to the left, I think the legs are CBE, but correct me if I'm wrong. In either case, the readings from left leg to right for each side are as follows...

707:
-28.1
-26.2
-27.5

708:
-1.14
.97
-.51

I also noticed that the output transistors for the same channel are much lower readings than the other. However, I just slipped and shorted one of those when probing and blew the main fuse. Guess I'll be replacing that as well.

Collector is the center pin.
I'd pull and check 707 it looks shorted.

Are these the outhouse shaped transistors pressed against the heatsink ? 2SC458's ?
 
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Ah, gotcha. I'll pull it and test it ASAP.

And yes, those are the ones. Any ideas about where to source replacements? The last set I got from a seller on the bay. May be just as well to go that route again.

Since I popped the fuse, do I need to look out for anything else when I replace it?
 
Ah, gotcha. I'll pull it and test it ASAP.

And yes, those are the ones. Any ideas about where to source replacements? The last set I got from a seller on the bay. May be just as well to go that route again.

Since I popped the fuse, do I need to look out for anything else when I replace it?

2SC1815 or KSC1815 curved side against the heatsink.
 
Looking at the transistors so that the heat sink is behind it, power transformer is to the left, I think the legs are CBE, but correct me if I'm wrong. In either case, the readings from left leg to right for each side are as follows...

707:
-28.1
-26.2
-27.5

708:
-1.14
.97
-.51

I also noticed that the output transistors for the same channel are much lower readings than the other. However, I just slipped and shorted one of those when probing and blew the main fuse. Guess I'll be replacing that as well.

For me, TR707 looks as good as TR708. Compare the voltage difference between each leg with the other legs, they are almost the same.

If the voltages on TR707 are low, close to the negative supply rail, it is because there is another problem in the amplifier. Something pulls the bias transistors toward the negative supply rail. It may be a shorted output transistor TR725 or/and his driver TR721. It may be the power transistor TR723 which is open, or his driver TR719, or resistor R749. You have a large negative voltage at the output of the left channel, and this large voltage is detected by the protection circuit, which keeps the relay open to save your speakers.
 
What would be your suggestion as a place to start, then? What should I test to try to pinpoint the issue?
 
With power off, check if there is a short between the output (coil L701 is a good place for the meter probe), and the negative supply (the collector or case of the output transistors 2SB531).

Check for a short between the collector and the emitter of TR721.

Resistor R749 is easy to check. If this resistor is open the output will saturate to the negative side.

If your meter can measure the hFE, you could measure the hFE of TR721 and TR719, because they are easy to pull out. If you pull out transistors be sure to pull good ones at the good place. Don't mix NPN with PNP
 
Out of curiosity, and after having seem this mentioned on another thread, I put in a new fuse, pulled the output transistors, and fired it up. The fuse stayed in tact, and the relay clicked over after a couple seconds as it normally should. Does this tell us anything?
 
T721 and T722 do not appear to be shorted.

When I test from the cases of the output transistors to the coils, I get 0 ohms for all of them.
 
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