Yamaha A-960 troubles

Well I pulled the one half that was good out of one and put it not the case with the other good one.

So I have one good IC made form two bad ones. But I need to test those two 9.1 Zeners. Or just replace them and see what the voltage is before I put the
SK146's in.

Nashou
 
Ok this is where I am. With the Gerry rigged sk146( TR108 made from two good half's ) I got the proper voltages for that channel( Right).

But TR107 blows out the half that is connected to TR105 .

At the Gate of TR107( removed since bad) I get about -4.57 volts where it should be 0. and fort he source i get -11 or a bit more. it changes the longer i keep it there. it use to be stable at 13.66
the other half seem fine except for the Source since its connected to the other half of the N J-Fet.

I changed D109 and D110 to be safe that wasn't the issue, I also pulled TR105 and it tested good. But could it be bad or leaky causing the one half of TR107 to blow?

Reading back I think I pulled and tested almost every Transistor.

Not sure where to proceed from here. But at least I now know which half of TR107 is getting damaged.

Edit: Just to mention with TR108 removed and TR 107 removed I got the same -13.66 at the sources for both Transistors but the gates were different.

Nashou
 
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Ok I am trying to figure out the voltages for the half that is getting blown. SO I pulled TR 105 and
here is what I got :

TR107 has 6 pins row one connects to TR103 which is still populated.

On bottom side of PCB from right to left I believe the pins are

Row 1
D=.3 mV, G=7.6 mV, S=-11.14 VDC

Row 2( connected to TR105 removed)

S=-11.14 VDC, G= -3.97, VDC D=.5 mV

Now TR 105 ( pulled )

E=.5 mV, C= +24.75 VDC , B = -4.82 VDC

Looks like the -4.82 at the base of TR105 should be a positive voltage.



Athanasios
 
Here is a picture of the board layout from a manual I ordered. Much better than the scans from HFE.

22810473228_4a54cfba64_k.jpg

30988947925_c90cc7db4b_k.jpg
 
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One more thing guys. The Schematic is marked incorrectly . D109 and R117 are flipped . And so are their counterparts in the other channel.

D109 should be where R117 is and vice versa. I was getting voltage reading different at the same spot and it was driving me nuts when looking at the schematic!!!

The clear board layout helped me find the misprint in the schematics. I think this is why so many A-960's phono sections are not working out there!


EDIT here is the revised version

30887413402_5175b581f3_k.jpg


Nashou
 
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Ok So Mac has been helping me over at Repairalmostanything.com.

We did some test and I'll repost it here.

He had me remove TR107,103,and105 and test the voltages on either side of R115 and R116 this is what I got

R115 -11.09 and -4.9

R116 +4.04 and +189.3 mV

the +/- 4 voltages are on the transistor side of TR103,104

the mV readings are probably just my meter we think.

So then he had me remove TR108,104 and 106 and retest the voltages on R116

I got

-6.96( goes to TR104) and -13.7

So then he gave me more voltages to check.

"Looks like they are pretty close to even. Now more voltages to try to narrow this down. Leave all parts removed for now.
Gate of tr107 on 103 side_________5.4 mV
Gate of tr107 on 105 side_________-2.7V
collector of 103______24.87
collector of 105______24.76

Gate of tr108 on 104 side______7.3mV
Gate of tr108 on 106 side______-1.7V but it drops steadily stoped measuring at -1.5
collector of 104_____25.08
collector of 106_____25"


So after this he had me :

"I just want you to put in the tr103,105,104,106. Swap channels though and then do voltage test again. BEC of all 4 and the 2 gate pads for 107 and 108 on the power side. This time, before you plug it in to test I want you to rotate the "Phono Selector" knob back and forth a bunch of times to try cleaning the contacts and then leave it on the 47K setting and then test voltages."


Here is what I got:

TR103

E=0 , C=24.64, B=-7.04

TR105

E=0, C=24.80, B=-7.04

TR104

E=0, C=25.12, B=-7.02

TR105

E=0, C=24.9, B=-7.03

TR107

G=-2.081

TR108

G=-1.452


They seem close

So now I am waiting on some cheaper Jfet's the 2SK363 single's to test . I wanted to Match them but for testing I guess it doesn't matter.

Macs Advice:

Matching them is not important for now. We are more concerned about how the channel reacts with them installed and hope the problem is resolved.
As far as the voltages matching now, well this is the first time we tested this way. Each part seems to affect the voltages.
And each part is required to balance the + and - line voltages. Everything is artfully placed to achieve a balance of 1 voltage against the other.

We also don't know for sure if the phono selector switch was causing an issue. That alone could of been the problem for blowing that TR107.
And since that FET is tied directly to that phono selector switch I would not recommend changing it with the unit powered up.
I would shut it down for a minute before changing that selector. FET's are very sensitive and a spike when changing
that selector could cause a failure.

Read more: http://repairalmostanything.com/thread/491/yamaha-960?page=3#ixzz4Q6Dc5nI2


So this is where I stand. Weird how just swiping the parts to another channel got me different voltages as I have removed those parts a few times
so I know its not bad solder joints. Hmm a mystery. I ordered 8 of the JFet's , 1.40 each compared to 15 for the 2SK146 dual units!!!

Hopefully the circuit will be ok when they are installed.

Thanks Mac so far !!!

Athanasios
 
Ok Installed the 2SK363's in both channels and still no change for the -B1 voltage still at 10v. But the
SK363's did not go bad. I am getting the 4 volts on the Drains but the Gate on the side that goes to TR105 has a -1.2 voltage that climbs if I leave the probe there.

Something is amiss :dunno:
 
Ok So I went back to square one. Looked at the Power board and there was a leaky Zener. D329. replaced that and got better voltages on TR343.

But back to the Equalizer board. Still Blowing TR107. Most voltages look good there. Not sure if maybe there is abad transistor that only acts up on turn on and then frying TR107 then settles down after?

I also changed out the 9.1 genres and the regular diodes 1s1555 with the 1n4148's .

Might do the the 24 volt and 30 volt zener's there too but they seem ok.

Right now I have both TR 108 and 107 pulled and measured the voltages around the board and all seem close for each channel relative to each other.

So for example The gates of both TR107 and 108 with the part removed have the same voltages, as do the source.
And the voltage on both sides of R115 and R116.


-7.2x and -14.2x on both sides pretty close for each.

But one area where its a little off but the parts test well is C123 and C122. The negative sides don't match up as close.

C123 + side -24.42 - side -6.74

C124 + Side -24.41 - Side -8.35

Read more: http://repairalmostanything.com/thread/491/yamaha-960?page=12&scrollTo=8744#ixzz4QapaZqXj


Mac and I are stumped for now but think we are getting close.

I have all new transistors coming in for that board and plan to replace them all and go from there.

Nashou
 
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I don't have enough knowledge to contribute but I'm thoroughly impressed by your perseverance on this one.

Hope you get it going again pretty soon, sounds like it's close.
 
I don't have enough knowledge to contribute but I'm thoroughly impressed by your perseverance on this one.

Hope you get it going again pretty soon, sounds like it's close.


Yeah me too. Its not the value of the amp of why I keep on chugging along its the learning experience.
I'm no tech but have taught myself alot over the last 10 years working on CRT projectors and other things here and there.

Its all about the knowledge gained here and on other sites.

I wish more knowledgable Yamaha techs would chime in her like Avionic and Clinic audio. But like I said this is one of those amps thats nothing special so maybe thats why. ;) lol

Athanasios
 
I have to agree with Zaibatsu. I've been following this thread with interest but don't have the knowledge to make a useful contribution. But I can salute you Nashou for sticking with it and working so hard. It gives me inspiration to go back to my CX-600 and take another shot at it.

Cheers,
James
 
One thing I noticed just now is that C127 and C128 on the board are 22uf 50v but the schematic shows 4.7uf 50v .

Also those two transistors TR119 and TR120 are really really hot !!! 82c !!!!

Here you can see the burnt board under TR119

30360465273_bb73ee6b38_b.jpg


Nashou
 
Also just found that D104 was bad. Not sure how I missed it or if it went bad while futzing around ;)

Athanasios
 
Quite interesting. What value does the SM / parts list give for those two caps?
Do they / their solder joints look like factory specced items?
Wonder if something fishy is going on there.

82C is really hot. I've been poking around in some of my amps with a thermal cam and the hottest parts are reaching about 60-70C max (generally only with Class-A bias).

Not to hijack your thread, but do you know if clip-on or stick-on heatsinks exist for that type of transistor? I've looked around on ebay and the like, but only found them to suit flat transistors.
 
Quite interesting. What value does the SM / parts list give for those two caps?
Do they / their solder joints look like factory specced items?
Wonder if something fishy is going on there.

82C is really hot. I've been poking around in some of my amps with a thermal cam and the hottest parts are reaching about 60-70C max (generally only with Class-A bias).

Not to hijack your thread, but do you know if clip-on or stick-on heatsinks exist for that type of transistor? I've looked around on ebay and the like, but only found them to suit flat transistors.

Just looked at the parts part and it does say 22uf . I removed them before when diagnosing but forgot if they looked factory instal.

The transistors may begetting hot due to something else on the board not being right. I got some better numbers now but on TR109 i am not getting the right numbers.

B=24.69

C=11.51

E=24.91

So I need to check some resistor values that Mac told me to look at.
R149 and 151. both were ok. So the next step is this

Mac Said,
"Well I think we are down to TR109,TR111,TR113,R143,R145,R147,R139. If you value test these resistors and they are ok then I guess you will have to wait until you get the replacement transistors you ordered."


Read more: http://repairalmostanything.com/thread/491/yamaha-960?page=13#ixzz4Qk8IshbP


But at least now on TR107 I am getting better numbers, i have the +3.6 volts and the gates are in the mV . Just have to figure out where the other transistors and resistors stand . It got late and i went to bed to read those values Mac mentioned above. But all transistors read well in my tester , but one may be leaky.
 
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Ok, I'll catch this tread up from the one on Repairalmostanything.com.

We went and tested the voltages on TR111 I still have R137 out to protect TR107 so no voltage gets to the Base.

with R139 Lifted i got

B= 22.70

C= -23.29

E= 11.47

Mac said the - voltage should not be there. So that is where we decided to wait for the new TR's for the entire
channel.

I got the remainder of the parts today and installed all the TR's on that channel except TR120 and TR119.

With that done I got all + voltage on BCE of TR111 23.05,23.70,23.72 Only R137 is out all other parts in place oh and TR108 is out.

I think it might have been TR113 that was the issue if this does work. But all TR's tested well in my Testing device.



Waiting for Mac to let me know If I should Instal R137 and give it a shot. Might pull TR107 and try it that way.

Nashou
 
Well we are stumped.

I changed every diode , electrolytic cap and all transistors and during the testing phase we some how have the same issue on
the right channel too now. with R137 and R138 installed it blows the one half or Tr107 and 108.

Something is causing the wrong voltages on a few of the TR's like TR115, 117,109, 111, 113. Tr 103 and 105 and 101 are fine.

So we thought maybe it was the servo amp section on that board. he explained that the servo amp section increases current to the circuit .

While looking at voltages I noticed after some time powered down and turned on I get the proper voltage on some TR's where they should be positive. Then after 5 minutes
the + voltage collapses to a similar voltage but negative !

So thinking about what Mac said about it increasing the current with the servo amp he gave this analogy.

"Think of this as a battle, a battle between positive and negative voltage. Both voltages are marching towards each other until they meet. And for now they meet at R139. Once they meet they stop moving. So when they are moving they are for this instance + and - 25 vdc. When they meet and stop it becomes zero volts, or in this case + and - 1.1vdc as shown in the schematic. R139 is a buffer to ram into.
What you have is the - voltage is overpowering the + voltage and pushing it back as far as between TR109 and TR111 as indicated by the 11v instead of 23.3v. So we are trying to figure out why the + voltage is loosing strength."

Read more: http://repairalmostanything.com/thread/491/yamaha-960?page=20#ixzz4S4AWJepN

Now I went and measured the current with my DMM and found that the + voltage has about 1 mA lower than the - voltage. as I switched the ampere setting
to volts It has to pass the AC current. now there should be no AC on the DC line right? Well there was !! About 2mA AC , so I decided to check for AC voltage and it would fluctuate rapidly from 35 to 4.9 to 0 then cycle over again.

So I unplugged the Equalizer board from the main amp board( +/- B line) and measured the AC current there and voltage it has a higher current and the voltage fluctuates from 29 to 0 rapidly.

its hard to catch it on the scope but it is there compared to the - line.

So I ask here and await Macs answer on the other forum. Can an AC component on DC cause the issue of the fluctuating voltage like described above and
in Macs analogy?

@avionic ?

or anyone else?




Athanasios
 
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Well, we decided to use an outboard Power Supply for the + side to see if the Amps internal PS is not working properly.

I set the + voltage to match the - side from the Amps PS.

Then in places where the - voltage was " out powering" the + voltage we now have the + voltage being too much with the external PS running at a
low ampere setting of about .05 amps to .15 amps, For some reason the amps change when the C.C. changes to C.V on my HP 1611 power supply I have.

BK PRECISION 1611 Instruction

SO not sure where we go from here. This is still with R137 and 138 out , which protects the J-Fet's .

One thing I noticed that , looking at the board layout, the outer halves of TR 108 and 107 gates are different than before the right half of the EQ amp got damaged as well. Tr 107 has a mV reading there on its Gate but TR 108's Gate for that half went up to -5 volts.

I need to know exactly what the ampere is for the +/- B line for that board. Doesn't seem to say it anywhere in the
Service manual.

Athanasios
 
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