Free stuff doesn't always work as well as planned...

jhaan92

A Work In Progress...
Just acquired a fairly decent looking CR-820 for the low price of free. Only one small glitch - doesn't power on!

Took a closer look at the unit last night, and found that the main fuse was blown. So I replaced it another fuse of the same rating, and it powered up for literally one second, and POOF! The fuse went out again. I am pretty certain that it is on the main power supply board, as the rest of the fuses were pulled and it did the exact same thing again.

I had initially suspected output transistors, but given the fact that the unit dosen't even have the chance to get past the power up protection, I was considering simply starting off with a re-cap of the power supply board.


Any suggestions on the best way to approach this? I took a look at the service manual, but while Yamaha built a great little receiver, the people who were designing the service manual felt that a trouble shooting flow chart wasn't worth the effort, but I digress...




-Jason
 
had initially suspected output transistors, but given the fact that the unit dosen't even have the chance to get past the power up protection, I was considering simply starting off with a re-cap of the power supply board.
Check the output transistors.:thmbsp:
 
I would assume that the output transistors are the ones attached to the side case of the unit and are slathered in heat sink compound? If so, then two test out at 151 ohms, while a third one tests out at 102 ohms, and the final one tests out at 36 ohms.

Or, if you prefer diode readings, two of them measure 689, the third one is 563, and the final one is 115. Should I just bite the bullet and replace all 4? I can't really see this being too expensive, but I do know that there is a leaking capacitor on the FM circuit board that will most certainly have to be replaced, hence the thought to just recap the entire unit...what do you think Dave?




-Jason
 
I would assume that the output transistors are the ones attached to the side case of the unit and are slathered in heat sink compound? If so, then two test out at 151 ohms, while a third one tests out at 102 ohms, and the final one tests out at 36 ohms.

Or, if you prefer diode readings, two of them measure 689, the third one is 563, and the final one is 115. Should I just bite the bullet and replace all 4? I can't really see this being too expensive, but I do know that there is a leaking capacitor on the FM circuit board that will most certainly have to be replaced, hence the thought to just recap the entire unit...what do you think Dave?




-Jason

Jason
Did you remove them from the circuit before testing ?
 
No, I didn't. I'll be re-visiting this project tonight. I'm going to take inventory of what caps I need as well.

Would there be any difference with pulling them to test?
 
You have to pull them and use the diode test of the meter to check them, see echowars post about transistor testing.
 
Good info. This will give me something to do after work tonight. It would be nice to see this unit running again. Never owned any of the 70's Yamaha equipment, nor listened to it, so I am interested in how it sounds.



-Jason
 
Yamaha's tend to be more of a neutral sound than most of their competition. They were considered a step up from a Pioneer or Kenwood and were a little more expensive. Some people think they are gutless at low volume but that's usually because they have the variable loudness incorrectly set. At medium and high volume levels, they are hard to beat for what they are.
 
Well, time to revisit this topic...

Ended up having a shorted output transistor. Got a set (factory Toshiba) and swapped them out. Same issue. Power on, instantly blows fuse. Suppose I'll have to build a DBT. What could destroy an output transistor?

All I know from the previous owner is that it worked fine one day, shut it off. Went to turn it on the next day, nothing. Sigh...if only I was as good at troubleshooting this unit as I am Carver amplifiers....



-Jason
 
Recheck the outputs and check the drivers as well. Drivers are TR119-122.
 
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Which drivers? Also, did they use any sort of glue to affix the main supply capacitors to the circuit board? Possible capacitor leakage, but i would like to consult to professionals before jumping to conclusions.

How's Iowa?
 
Amplifier failures tend to move backwards, taking out other devices. Put your meter on diode check range and check every transistor in-circuit at least B-C and B-E. You should get one diode drop. Just follow the schematic backwards from the output. Not sure about the 820, but usually you can power up without the outputs installed at all.

The other culprit might be the rectifiers and main filter caps. Check 'em. You should also get a few ohms, maybe 2-5 across the power plug with the switch on, and infinite ohms from either leg to the chassis.

I'll repeat it yet again- always find and fix the specific problem before shotgunning caps. Doing the recap first almost never works and usually complicates the repair far beyond what it should have taken.
 
Well, found a completely roasted resistor (I think) inside one of those insulator sheaths. All charred inside. The mate right next to it gave me a 5 ohm reading after I de-soldered it from the board. The burnt one gave me a 0 ohm reading. Going to have to find a replacement. Unit will now power up without blowing a fuse if that is removed. Unit is in protection mode however, so I won't be able to do further testing until it is replaced.
 
Dave, you aren't talking about TR719-TR722 are you? I still can't find the drivers you specified.

Had a spare 4.7 ohm (two actually) resistor in another parts (ct-f1250) unit, and i installed one. Blew a fuse instantly. Still have to track down the culprit, but sleep is calling me...gotta work in the AM. I will post to this thread after i do some more digging.
 
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Well, 721 for sure...shorted entirely. 719 as well - got a reading of .433 on the base & emitter, with current flowing in both directions.

I'll probably just bite the bullet and replace 719-722, just because I've already found 2 that were bad, and I only get an OL reading on the others. And of course, that 4.7 ohm resistor mentioned in a previous post.

Tested all of the diodes, and they seem to be in good condition, reading .643 or better, but no higher than .750. I haven't checked any of the ceramic disc capacitors yet...have to do that tonight after work.
 
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