Workin' out imbalance tone-amp in Yamaha CR-840...

[vibroverbus]

So pokin around my CR-840 which was sounding OK but not great. Looks like a multiple-malfunction situation:

The left channel gain is slightly weak in the tone pre-amp circuit, requiring something to be done there right from the git-go. I've pinpointed it to TR-503/TR-505 area so should be easily resolved. Second issue is not as far diagnosed - getting a tiny 60hz AC leakage on same left channel, and this comes up somewhere in post pre-driver (thank jebus the pre-drivers seem OK...) and/or power-amp zone. Should be easily narrowed down.

Both cases are not really so perceptible by ear, except I kept having a nagging feeling the balance leaned right and sure enough, a little over 1dB down. The hum isn't really detectible but if it's easily fixed via a couple cap swaps I'll take care of it of course.

The only doc I can find for the Yamaha CR-840 is that one same widely copied around the interweb, poorly scanned, owner's manual. I've been getting by on that and using the apparently nearly-identical CR-640/1040 docs which frankly seem fine for layout and 'general principle' purposes.

If anybody has a tip for a real quality schematic scan, let alone a S.M. that would be great.

As always here's pics just for entertainment value. Since the 2040 and 1040 are both in full service now, if I can get the 840 up to speed we'll have almost a full house...

Tight little cluster-fest at the back of this pic is the tone-amp area... probably could replace all the bjt's and e-caps and just be done with it...

The famous pre-drivers, along with the piggy-back heatsinks I've stuck on all my questionable and unobtainium Yamaha IC's... (Also visible - the extra-log lamp leads some PO's tech soldered onto the wirewrap posts for some reason instead of just soldering new lamps to the ends of the existing leads...)

Looks good at night though...

Scope shot of the 2 channels post-tone amp circuit (BTW, confirmed it is not the volume pots - both channels track excellently out of the knob as they should since I replaced the dual pot with a new ALPS knock-off from China).

Jiggle-handed shot of the hum on the left side post power amp. Out of the pre-driver it looks fine...

 

[vibroverbus]

Quick Addition: Starting the parts list for the tone-amp re-do, it specs either 2SC1815, or 2SC2320, three per side... Anybody have good sub/upgrade recommendations for those? I haven't pored over the datasheets yet but will if nobody's got a good quick answer...

Edit: Just realized have some KSC945LBU on order for another project, might do the job - need to check datasheets... hope I got enough for sorting purposes...

 

[merrylander]

Check with B&D at www.bdent.com for possible subs.

 

[B3Nut]

Those heatsinks look like an interesting idea...I have an Onkyo TX-3000 receiver with an STK voltage amp chip similar to those (weren't the Yamaha IC's rebadged Sanyo STK devices?) I'd like to prolong the life of the unit as it's a very nice piece.

TP

 

[vibroverbus]

Yep. If you've read all the nightmares and struggles with those pre-driver voltage amps in the Yamma-world they are the number one creator of doorstops. Problem with the Yams is they seem to have been special designs contract mfr'd by Sanyo specially for Yamaha, so nobody has discovered a true equivalent. Issue is most of the Sanyo volt-amp-pre-driver STK's seem to be all-in-one stereo chips instead of mono chips with a bunch of other compensation & stability stuff added in. I know Rob/Merrylander has cut them open and mapped the schematic.

I think one or two guys built add-in boards, but the best hack-around solution for Yamahas seems to use a single of the later stereo STK's and map the pinouts across to the two OEM IC's. The other thing to do would be to just start fresh with a simple add-in board using something like one of the nice National IC's, which would save a lot of space vs. the ancient STK formats. If I ever end up with a Yamaha doorstop I'm thinking of trying an LM4702...

Your Onkyo should be easier, since most of those "open-market" STK's are still floating around. In fact those are what a couple of guys have used as mentioned to replace the dual Yamaha proprietary chips.