Yamaha CA-800 Bias Problems After Cleaning

t_munz10

New Member
Hey Guys!

Some of you may remember, I was taking apart a CA-800 for cleaning to see if that would fix the very scratchy output.

I've cleaned all the pots and switches with Deoxit and have semi reassembled everything (enough to test). Looks like the cleaning helped a little, but I may still be in for a full recap and transistor replacements.

Now when I play music it is way clearer, almost static free. BUT it's waaaaaay to quiet and only one channel is working. Even at full volume it's not even near normal talking volume. I've check the biases on the main amp boards... and it doesn't look good.

I can set the differential and mid-point potentials correctly (15V and 0V respectively).

But when setting the idling currents using VR603 and VR604, while measuring on TP3 and TP4, the manual specs .047V for ClassAB and .8V for ClassA. I can adjust to a maximum of about .003V on the right (working) channel and I'm reading 0V regardless of adjustment on the left.

I didn't mess with the amp boards at all so I am a little confused as to what could have happened. Any ideas on what to troubleshoot here?

(EDIT: I should mention that I didn't measure the bias before cleaning, but the amp was much louder ... and noisier.)
 
Were both channels OK before? Other than scratchy?
 
Check Fuses F601 and F602 on the right channel driver board. With a multimeter.
 
Both channels were scratchy before. Right one seemed louder than left, but definitely had sound from both.

Just checked. Fuses are good on both boards.
 
Inspect the wire connections on the top edge of right channel driver board. Especially A,B,C and S. These are the classA/AB bias switching connections.
 
I replace these switches on all CA-800 and CA-1000 as part of a preventative measure.
Great, thanks I'll get some on order.

Found out the source of my issue...:mad:
Hadn't reattached the shafts to turn the function and tape selects, and I had tape select set to PlayB, not Source. I must have just been getting some crossover and that's why it was so quiet. After setting it to Source, turning it back to Play B no longer plays anything (left over crud from cleaning still in the switch?).

Now, I've got both channels outputting, right seems ever so slightly louder than left, but both are quite full of static. I've tested with the coupling switch set to main in and the static is still there if I skip the pre-amp + tone controls.

Interestingly, I left it running with music for several hours this afternoon, and eventually the noise just cleared up... tried back to just main in and it returned and now won't go away again even using the pre.

Sounds like I should overhaul the two main amp boards? Do you typically just do all the electrolytics and the transistors or do you spend time trying to find the culprit?
 
Now, I've got both channels outputting, right seems ever so slightly louder than left, but both are quite full of static. I've tested with the coupling switch set to main in and the static is still there if I skip the pre-amp + tone controls.

Interestingly, I left it running with music for several hours this afternoon, and eventually the noise just cleared up...
Most likely culprit the protect relay contacts... drop in replacement.
https://www.digikey.com/product-detail/en/MY2-02-DC12/Z10455-ND/252217
 
Replace all the E-caps on the bottom power supply board while your replacing the relay. Check the solder connections around the regulated power supply transistors ( Especially the two with the little heatsinks.) These are pass transistors and will run on the hot side. Cyclic power on/off over the years causes cracks in there solder connections.
 
I'd like to know if your regulated +/- 50vdc power supply is working?
 
I take it you still can not bias the amplifiers . Correct?
 
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