CR-2020 hum that changes with volume.

Robisme

Sansui Enthusiast
I am working on a CR-2020 that has a hum. It can not be heard with the volume at 0, but increases with the volume. The hum level maxis out at about 11 o'clock on the volume.

I have replaced the regulators on the power supply board and the two 1000uf 50v caps on the cap board.

When I use a preamp there is no hum.

I can here the hum on the 25v and -25v points on the power supply.

The receiver did not work when I started, and after replacing the regulators, it came out of protection and works, with a hum.

Someone else did the SB mods, and what a job they did.(in a bad way)

Any ideas?

Rob
 
Yamaha cr2020

OK but can you try to use the CR2020 like a preamplifier with another amp to get confirmation that your hum noise is coming from the preamp section of your receiver ? You just have to use the switch on the back of the unit to separate amp from preamp .
Come back with info :thmbsp:
 
I connected a preamp, per my OP, and there is no hum. So, it is from the preamp. I could connect it as a preamp, but I think what I have done narrows it down to the preamp section.

The 25 volt supply has a hum on it and the preamp is amplifying it.
Both 25 and -25.

I just took some voltage measurements and they are off.

The output of the power supply 25v is 23.7v. The -25 is -31.

The regulators are:
712: B = 38.5v, C = 39v, E= 26v. Should be 28.5, 38.5, 28.

715: B = -34v, C = -33.5v, E = -35. Should be -40, -28, -40.

Someone did quite a bit of work on this, and who knows what they may have done. The regulators were replaced with two different types, by mistake I think.

The regs I put in at least have it working.

Rob
 
D717--12v zener and D715 7v zener probably would not hurt to replace ,at least check there actual voltage. D717 should read about 13v on one side and D715 about 6 volt on one side.

Theres a couple fusibles that may have drifted. FR801 & 802 as well as R801 and R802.
Has this portion of the PS been recrapped.:D
 
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D717--12v zener and D715 7v zener probably would not hurt to replace ,at least check there actual voltage. D717 should read about 13v on one side and D715 about 6 volt on one side.

Theres a couple fusibles that may have drifted. FR801 & 802 as well as R801 and R802.
Has this portion of the PS been recrapped.:D

Thanks, I'll check those diodes. The SB mods had been done before it came to me. Fuse resistors had been replaced.

Rob
 
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