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Little help with Yamaha CR420

Hello everyone I got a CR420 recently and i have being working on it for a couple of weeks now. When I got her the Left channel was blown, actually when i power it up for the first time one of the transistor exploded (That was not funny). I have replace all the power transistors, the big heat resistant resistors, the exploded little guy and its counterparts and most of the capacitors except for 2 mysterious caps that I can see correspond to C413(4), C419(20) and C421(22). in the diagram and boards drawing, they show not catalytic but in reality they are. So that is my first doubt.

Now its working but the right channel sounds hiss-y (i don't know how to explain) and the sound comes and goes. i have no idea where the problem is. I checked the balance pot and all the switches and the problem is somewhere else... more days of fun ahead.

The other question i have refers to the 2 large resistors in the main board i think they are R514 and R505. I cannot for the life of me find a replacement. Any suggestions?

Re: Little help with Yamaha CR420

Hissing sound is often evidence of super sonic (inaudible) oscillation. This can destroy tweeter and amplifier outputs! You need an oscilloscope to verify. Typical causes are open 10 ohm 1 watt resistors in series with a cap (typical 0.1uf) located where the amp output goes to the speaker relay. Rarely the capacitor will be open (simply substituting one will be easier than testing the cap). If those parts are ok, clean and tighten every chassis screw that couples a circuit board to the steel chassis.

Re: Little help with Yamaha CR420

I have to say the level of knowledge here is way over my head. Thanks for the advise nakdoc you are right about the resistor-capacitor thing it's an open resistor in my case. I also found what i believe to be at the root f the problems. the power transistor are wrongly paired. I couldn't tell if it's from factory or someone before mi got in there but in one channel tr is far over specs and in the other side far under. I literally had to read all the datasheets from all the transistors in the schematic to find this. Last night another low frec. transistor blew same was blown when i got the ampli. I ordered the transistors, in a couple days when i replace them will post results.


Re: Little help with Rotel RX 802 Amp/Receiver

Hi all.

I just bought this unit and it sounds like it looks, brilliant.
The only thing that concerns me is that I cannot get any volume on either phono setting with the volume knob set anywhere to the left of the 12 o'clock position. Once I go past that position the volume comes up quickly.
This is not the case when using the tuner or playing a CD.
It is the same with both of my turntables, which play ok through my other system.
Any thoughts?
If there is a potential problem there that could get worse I want to get it sorted.

Re: Little help with Yamaha CR420

C413 is 33pf, C419 and 421 are 470pf; it is unlikely these capacitors are bad, so don't worry about replacing them. Resistors R514 and 505 are 470 ohm 5 watt; this is a common value. I would not replace them unless they are open.
Hard to say where hiss comes from; I would short an input (like Aux) to eliminate the possibility of external noise. It could be a dirty switch, poor ground in any amplifier section, a bad transistor, or a power supply problem like a bad regulator transistor.
Did you clean the tape monitor switch? This is a common problem.

Re: Little help with Yamaha CR420

Thanks for the advice Johnnysan. I did as you recommended and every switch I checked was in good working condition (The first thing I did was deep clean everything) I ended up checking them all. and discovered that all + - of each channel are "beeping" when I check continuity. I followed every line from the input and once I get to the relay is as if all sere the same line. Maybe I'm doing something wrong. Can a bad relay do that? I triple checked with the amp on and off. I hope someone out there can help.


Re: Little help with Yamaha CR420

I doubt that a relay will cause a hissing sound. I would suspect a preamplifier transistor.