CA-810 Preamp Problem

Got ahold of a one-owner CA-810, my first ever Yamaha integrated. Immaculate condition, clean, original box, manual, everything a person would want in a vintage amp. Of course it had been stored for awhile, and on power up it had all the usual signs of dirty controls. So, the usual suspects were addressed: cleaned volume and other controls related to the tone section as well as the speaker switch. Audio muting was the most stubborn, but after a couple of cleanings that was no longer a problem.

Okay, both channels play w/o distortion, but there's still issues.

First, the volume control must be turned way up - 1 or 2 o'clock - to get even medium volume. Is this a characteristic of the volume control on this amp?

Second, there is a constant low level hiss on both channels, clearly louder on R. I think that channel also has slightly lower volume. Time to narrow down the problem.

First I ran the preamp from a Marantz 1060 into the Yamaha amp section. Wow, sounds good, channels even volume, don't even have to turn the knob up as high. Reversing this, ran the preamp from the Yamaha into the amp on the 1060. There's the hiss, same sonic problems. So now I know: the problem is in the 810 preamp.

I will mention that I did NOT clean the function switch for a simple reason: access is ridiculous, and before I go there, I came here to AK. HOWEVER, I did input signal on several of the sources (tape, aux, tuner) and all performed exactly the same, so I'm doubting the function switch is the culprit.

From those that know the 810 preamp section, is there any particular place you'd start? Any known weak components prone to failure that might be causing this unit to act this way? Not finding much on AK about this particular situation. Thanks!

For the volume issue, make sure the "Audio Muting" switch is set to the middle (0dB) position and the "Loudness" knob is set fully clockwise to "flat." See if that fixes it.

jheu02 said:

For the volume issue, make sure the "Audio Muting" switch is set to the middle (0dB) position and the "Loudness" knob is set fully clockwise to "flat." See if that fixes it.

Click to expand...

Yep, got that. Still, sometimes it's easy to miss the basics!

After testing all the transistors on the tone board, discovered two were bad: TR313/314, both 2SA844, one for each L/R channels. Since I didn't have recommended equivalents, installed a pair of 2SA733 which are darn close in nearly every category, per the data sheets.

Time to test. Plugged into the DBT and . . . Viola! The amp came to life, sounds great. No more hiss, sound level output reflects a more normal measurement against the volume control setting, channels equal.

Time to check idle current, which looks easy enough. What I can't figure out from the service manual diagram is where on the board to test the "mid-point potential". Can't see anything that looks likes "LO and RO".

Check this thread. There is a good photo of the bottom of the board with the points marked. :thmbsp:

http://www.audiokarma.org/forums/showthread.php?t=643874

try1256 said:

Check this thread. There is a good photo of the bottom of the board with the points marked.

Click to expand...

Thanks, just what the doctor ordered! Hadn't found that thread before.

A couple posts back I'd stated that I installed a 2SA733 pair to replace the failed 2SA844 pair. Checking the data sheet, the one spec that was not quite up to original was collector to emitter voltage. The 2SA733 is -50, while the 2SA844 is -55. All other specs are equal to or better.

Q: Is this material? Perhaps (likely?) I'm not understanding this, but given the voltage serving this board, seems like plenty of "headroom" . . .

If I'm reading the schematic correctly, looks like you shouldn't see more than 15V on the emitters.

Schematic shows 13.7V on the base, 0V on the collector, and 14.4V on the emitter.

Thanks John. You've just taught me something that I should have been able to deduce for myself. After you posted this, kind of a "D'oh" moment for me.