CurtPalme.com Home Theater Forum

[Nashou66]

Thanks Mac, i might have found an exact replacement source. One thing I noticed is your part is 8Mhz and the original is 120Mhz. I doubt it makes a difference but thought id ask.

Athanasios

[Nashou66]

I went and ordered the parts from HB electronics I ordered the 2SC1913's as well 4 of each. I never dealt with them so we'll see how it goes. Once I get it back to where it was before i shorted the transistor We can diagnose the DC on the outputs.

Thanks again

Athanasios

[Nashou66]

Ok I put the 2SA913 in and I have power and no smoke.

I hooked up A DVD player and put in a CD no sound. Turned off the A-1 with the CD playing and then powered it up
to see if I could hear the buzz my tenant said would happen before the protection circuit kicked it.
No Buzz or sound.

Where do I check next?

there is a small amount of DC on the speaker outs as mentioned earlier.

Athanasios

[Curt Palme]

So NOW... welcome to the world of Yamaha blowed up stuff!

What you'll most likely find is a leaky transistor, or some resistor open or off value, which is causing the DC offset. What I'd do now is input a sine wave from a CD or tone generator, and scope the output of each channel before the protection relay. Chances are, you'll find one channel good, and one bad. Then at least you can narrow it down. If both channels show the same distortion, then my bet is that you still have power supply issues.

[Curt Palme]

So NOW... welcome to the world of Yamaha blowed up stuff!

What you'll most likely find is a leaky transistor, or some resistor open or off value, which is causing the DC offset. What I'd do now is input a sine wave from a CD or tone generator, and scope the output of each channel before the protection relay. Chances are, you'll find one channel good, and one bad. Then at least you can narrow it down. If both channels show the same distortion, then my bet is that you still have power supply issues.

[Nashou66]

Ok. I have a test CD with some test tones. I assume a 1Khz tone would be fine?

I have some schematics that Mac sent me, they looked like photographed ones, so they are not that great but will help
more than going in blind. So where should I look for the Protection circuit? the previous owner of the schematics drew a signal path line from the Left MC and MM phono inputs.

These are definitely not as nice as the marquee schematics I'm used to.

And the layout of this Amp is very tight and you have to take it all apart to get to some areas. Sucks!!!

Thats how I popped the 2SA913!!

Nashou

[macgyver655]

Look for the "Disc switch" and make sure it's turned off.

[Nashou66]

[macgyver655]

Also make sure the "tape monitor" switch is off. Heck, turn off the tone control switch also.

[Nashou66]

Ok Weird, It seams to be working fine? No distortion. Clear on both Channels. Volume works great. Tone controls work.
So why did he have it setting the protection circuit off? There still that slight amount of DC on the outputs.

Athanasios

[Nashou66]

Oh One thing I did find when I removed the bottom grate. there was a screw between the grate and main PCB board LC68970/NA07129. I wonder if that was causing the Speaker circuit protect to kick In? Not sure where the screw was
because i took of the grate with it on its side standing up and it fell to the bottom.

Athanasios

[macgyver655]

Possible he has a bad speaker. A very small amount of DC on some amps is normal. Your .055 doesn't bother me. But your higher readings as you turn up the volume has me slightly concerned. I'm going to look over those schematics to see if there is a bias adjustment. Haven't really looked at them yet.

[Nashou66]

From the AudioKarma site they do mention adjusting the bias for left and right channels. I'll do some research as well. I was planing to re cap the power supply for sure and maybe some other areas. The main Filter caps are 18,000uf. Not sure if going larger a little bit would help out or not. I did about 4-6 pots on the boards throughout the amp , so those might have to do with the adjustemnt.

Athanasios

[Nashou66]

So since i found no issue with the A-1 i put it in my bed room and brough in an extra TT i have. Hooked it up and major hum and buzzing!1 CD owrks fine. so its the MM section of the pre amp not working right. I was able to wiggle the right phono jack and got the hum to go away but also lost the right channel. So I have to look at that .

Athanasios

[Nashou66]

Ok I got the hum to go away. The PHONO inputs are not hard wired to the phono section, they have a small board for the MM and MC and a Load input where you can pre load the cartridge with the amount of resistance the cart call for, kinda neat. on the back side of the board are another set of RCA jacks that have phono plugs that go to the board. I cleaned these up, re flowed the solder on the board and re installed. I now get no hum. But, If i press the DISC button that is suppose to have a direct path passed the tone controls i get a very weak signal to the one channel. I have to figure out which one as the speakers i have this set up in are in the bath and the unit is in the bed room with wiring in the wall( I never marked the plate left and right. If i turn off the
DISC button I get equal sound output. I have to put my test disc on later to see the channel separation etc.

So there is something wrong with the DISC section i'd think.

Hmm

Nashou

[macgyver655]

Probably switch contacts. Spray some WD40 in there and switch on/off around 50 times.....

[gjaky]

WD40? It is not the best for contacts, for lubrication it goes well, but for contact cleaning I'd use some more delicate stuff

[macgyver655]

[Nashou66]

Damn it!!! New problem!!!

I had it working pretty good, i was going to work on the low output on the Phono DISC button later and just use the AUX input for CD playback. I have a ADCOM speaker selector and went to hook it up. here is where i might have done some damage. I hooked up the amp to the speaker selector outs!! Idiot, not enough lighting where i was working and I was getting no sound. Realized what I did and re connected it correctly and now I have no Right channel. I have the speaker selector box on top of the CD player which was on top of the AMP. Some copper strands from the wire might have dropped into the amp, but I didnt see anything in there. If it did drop it was right over the Transformers and the vertical power board.

I found a place that has the service manual for 16 bucks I might download it to find all the test points and proper voltages at those points. It was sounding really nice too!!!

Athanasios

[macgyver655]

Have you verified no right channel using various inputs and without the switcher?

[Nashou66]

I took the Switcher out and still no sound from the AUX input. At work now so will try later after, 10:30pm, for the other inputs. Have to bring it back up from the shop. Nothing seemed suspect from a quick look over.

Athanasios