Spent the day poking around in the CR-820

Mr. Creasote

Mr. Creasote

Member
With the 620 in the rack I finally opened the 820 up. Now I'm very ignorant about what Im looking at so try to interpret my descriptions here. It was clean.. very little dust. I noticed a lot of smeard heat sink paste around the heat sinks. Left hand side of the unit. I beleve these are the power transistors, right? I think these are TR723,4,5,6 in the circuit diagram. One looked fatter then all the others. Taking the sinks off I see from front to back a Toshiba 2SB555, Sanken 25C1116A, Toshiba 2SB557, Toshiba 2SD427. The diagram gives TR723,4 as D427 and TR725,6 as B557. The Sanken looks out of place and the mess of paste around it leads me to belive it is not original. I dont know if this means anything but the end the sanken is on is much warmer. These look like they plug in, two screws and they pull out. Now I'm wondering, should I find original Toshibas? Replace all 4 or just the Sanken? If Im way out in left field here let me know.
Thanks
 
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Sounds like you have a mis-matched pair in there. You can look up the originals and get a cross to the proper pair of parts. Pretty sure the b557 and d427 are what you want.
 
It looks like On-Semi makes them. D427=MJ15015G and B557=MJ15016G. And order silicon insulators at the same time.. Are the On-Semi good replacements? I see they are a spinoff of Motorolla.
 
I've heard their stuff is good. Much better than risking getting a fake that takes out your amp~
My Technics SU-8080 uses the same devices btw.
 
Mr. Creasote said:
I dont know if this means anything but the end the sanken is on is much warmer.
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Did you try to check and adjust the bias current in the output stage?

The 2SC1116A is a better transistor than the original 2SD427, and I would start by a bias adjustment.
 
OnSemi replacements are good. Whether those are specifically the best, having not looked at data sheets, I couldn't say. Hopefully Merrylander will see this and chime in for you. You can reuse the existing mica insulators, but having spares is good. There are no service manual around for that receiver so information contained in AK about tweaking will be important to you. One thing I remember about mine was the bulbs for meters etc are in series. Enjoy your fun.
 
Those OnSemis are fine as subs since the chances of locating real B557 and D427 are nil. The B555 is really for the CR-1020 mated with a D425, the Sanken is roughly equivalent to that Toshiba. Actually the Sanken is used in pairs in the CR-3020 I just blew one recently.
 
The other replacements I found are NTE brand... D427=NTE280 B557=NTE281. NTE seems the Brand that Fry's and Halted sells.. About $5 at Fry's... $10 at Halted!! :eek:
 
So I imagine having a matched set of power transistors is good. Having thoes odd ones in is probably why it got warmer up front. I may get two sets so I can meet the minimum order quota. lol

"ADJUST THE BIAS"

I'll get to it... Sheesh... :D Im still studying the diagram and searching AK for how its done. I have a Fluke 29 as my only piece of test equipment. I went through the DC Offset test and got odd results. Kept swinging tetween 2 and 4 mv. Never got steady. Not sure what that means..
 
Grab some new mica insulators while you're ordering~
At least you have a good multimeter, and know how to use it~
 
SoCal Sam said:
How are your NS-670's sounding with the CR receivers?
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Hi Sam

Im enjoying NS-670s a lot. Thanks! :thmbsp: The 820 drives them hard and can really pump out some volume. The 620 seems to crap out before the speakers do.. Lol.. I havent developed my audiofile ears yet but the 620 seems a little 'brighter' then the 820. At about 1/3 to 1/2 volume with the 820 they are sweet. Much over that and its too loud for my little room but still sound very clean. They rock with the CR-820. Im really happy with them. I havent set the room up for listining, its arranged more so we can watch TV. Lets see if I can get through a tune-up on the 820 with out distroying it. :D
 
The fact that your power transistors are not the "official" complementary pair is not the cause of a larger power dissipation in that pair. The heat dissipation is proportionnal to the actual bias current. Once you will adjust the bias current, both sides should be at the same temperature.

As a side note, the test points TP-3 and TP-4 are the emitter legs for the left channel, and the test points TP-1 and TP-2 are hidden by the speaker wires, near the transformer.
 
Thanks Merrylander. Thats what I was looking for. I think.. Ummm.. The idle current, Umm.. Are these the bias pots or the offset pots? I ordered the On-Semi transistors. I should have them by Friday. Once in I'll do the adjustments and go from there.
 
Okay.. I have the transistors. the MJ15015G replaced the D427 and the C1116A, The MJ15016G replaced the B557 and the B555. If Im wrong let me know! Now the idle adjustment.. Unit turned on.. no speakers and no volume and everything set flat? Turn the pots all the way down and go from there?
 
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