Odd failure in MX-1000

[Ray Gianelli]

Recently picked up an MX-1000 that was sold as non-working. Popped it up on the bench and nothing when powered up on the DBT. Opening it I found both 10 amp power fuses blown (!). The 7 amp fuses on the power supply board were fine.

The only failures I could find were a shorted NPN output on each channel. It's uncommon to find both channels bad IME, but to make it stranger was the fact that this is all I could find; emitter resistors were all good, base resistors as well. In fact I can't find anything else that checks bad.

Am I missing something here? I'm used to major carnage from failures in direct coupled amps. Hell, the last M-80 I had on the bench had one channel that was toast from the outputs damn near to the input. That said, I don't specialize in any particular brand. So I'm posting here in the Yamaha forum in the hopes that someone can chime in. I'm still replacing all the outputs, but I'm not doing so until I'm absolutely sure there's nothing else wrong.

Thanks!

Oh, and I found lots of cracked solder joints on the main board and fixed them. Also replaced C517/518 100uF 6.3V caps, as one read nothing and the other had an ESR of 13 ohms.

 

[rottalpha]

Recently picked up an MX-1000 that was sold as non-working. Popped it up on the bench and nothing when powered up on the DBT. Opening it I found both 10 amp power fuses blown (!). The 7 amp fuses on the power supply board were fine.

The only failures I could find were a shorted NPN output on each channel. It's uncommon to find both channels bad IME, but to make it stranger was the fact that this is all I could find; emitter resistors were all good, base resistors as well. In fact I can't find anything else that checks bad.

Am I missing something here? I'm used to major carnage from failures in direct coupled amps. Hell, the last M-80 I had on the bench had one channel that was toast from the outputs damn near to the input. That said, I don't specialize in any particular brand. So I'm posting here in the Yamaha forum in the hopes that someone can chime in. I'm still replacing all the outputs, but I'm not doing so until I'm absolutely sure there's nothing else wrong.

Thanks!

Oh, and I found lots of cracked solder joints on the main board and fixed them. Also replaced C517/518 100uF 6.3V caps, as one read nothing and the other had an ESR of 13 ohms.

hi Ray,
in terms of restauration it all depends how deep you want to go. At the minimum, I would replace all lytics with the exception of the large cans and I would refloat as many solder joints as you can. This particular unit has a nasty reputation for cold joints. I found them in every board and I found bad joints on every MX I worked on and I worked on quite a few.

another thing to watch for is the dissasembly of the bub-boards. The traces and those pads will come off if overheated.


as far as why both channels going bad at once, that is strange indeed and I would not call the rest of the unit safe yet. Look at all small resistors on the main board!

re:

The only failures I could find were a shorted NPN output on each channel.

are you sure they were output and not power transistors and did you test out of circuit? I know you know what you are doing, but just checking....since I am not sure how familiar you are with this amp.

Reason I am asking is that out of the 5 pairs of the big toshibas on each channel, three pairs are output and two pairs are power transistors.
The power transistors in that amp would test shorted in circuit, but might be good otherwise....as a matter of fact, they are rarely bad. When the output transistors are bad, then a bunch of other components usually go bad, albeit I did have one instance when I had ony one output transistor bad and only on one channel.

 

[Ray Gianelli]

hi Ray,
in terms of restauration it all depends how deep you want to go. At the minimum, I would replace all lytics with the exception of the large cans and I would refloat as many solder joints as you can. This particular unit has a nasty reputation for cold joints. I found them in every board and I found bad joints on every MX I worked on and I worked on quite a few.

another thing to watch for is the dissasembly of the bub-boards. The traces and those pads will come off if overheated.


as far as why both channels going bad at once, that is strange indeed and I would not call the rest of the unit safe yet. Look at all small resistors on the main board!

re:


are you sure they were output and not power transistors and did you test out of circuit? I know you know what you are doing, but just checking....since I am not sure how familiar you are with this amp.

Reason I am asking is that out of the 5 pairs of the big toshibas on each channel, three pairs are output and two pairs are power transistors.
The power transistors in that amp would test shorted in circuit, but might be good otherwise....as a matter of fact, they are rarely bad. When the output transistors are bad, then a bunch of other components usually go bad, albeit I did have one instance when I had ony one output transistor bad and only on one channel.

I've got it in 3 pieces. Pulled channel boards, and thanks to the quality of Hakko soldering/desoldering tools no pads were lifted.

When I say outputs they were definitely outputs according to the schematic (the I-Amp section)... without the schematic I wouldn't have known the difference!

Like I said, odd. Thanks for weighing in. Previous posts show you know your way around this particular beast!

Edit: Yes, I did test them out of circuit. Pretty much have to in order to determine which ones are good, even though it's best to replace them all.

 

[rottalpha]

best to replace them all.

on-semi would work but have higher cob. These would be your best option: https://www.mouser.com/ds/2/408/TTA1943_datasheet_en_20131101-1144127.pdf
I would replace everything on that heatsink as in the two pairs power trannies for the A.P.S. the I-Amp drivers, if you have replacements. PM me if you need driver replacements, the pres can be replaced with ksc1845/ksa992. They are usually available in 400+ HFE and should be working well replacing the 2sc2240/2sa970 pair.
I am only suggesting you take a look at them since they have been prolly stressed to s**t and because you already have the beast with the belly open.

 

[Ray Gianelli]

Well, the parts came in Friday and I finished changing all the outputs and power transistors. Works fine, nothing else was wrong except what was previously noted. Thanks for the input Rottaplpha!

 

[rottalpha]

Well, the parts came in Friday and I finished changing all the outputs and power transistors. Works fine, nothing else was wrong except what was previously noted. Thanks for the input Rottaplpha!

did you go for the Toshibas?
Happy house shakin'

 

[Ray Gianelli]

did you go for the Toshibas?
Happy house shakin'

No, went with the On Semi's. Always had good results with them in the past.

 

[iliyanG]

Hello, I bought the Yamaha MX 1000 with a faulty transistor (stairs). But it turned out that there is a problem in the main board. From two days I am looking for repair schemes, but I find only the MX 1000U. In this scheme some elements are different. I'm interested in some resistors on the main board.
If necessary, I can provide pictures.
Thanks in advance.

 

[iliyanG]

 

[rottalpha]

View attachment 1146168View attachment 1146168 View attachment 1146169

welcome to AK

Your questin is unclear....if the resistores you circle are toast, you need to replace a few more components than those resistors

the reson you do not find the resistors in the schematic is because your unit is a newer serial number. There are a few changes in the MX-1K that are not captured in the schematic. Scematic can be found here (you will need to register) https://www.hifiengine.com/manual_library/yamaha/mx-1000.shtml

The resistors you circled in the two pictures on the left will not be in the schematic. All four are 1R resistors.
The ones on the picture to the right I would need to have a closer view or a unit near me to be able to see what you are reffering to. There are resistors underneath the power resistors you have circled.

You might want to start your own thread....

 

[iliyanG]

Thank you for the reply. The four resistors were burned and the color code was not visible. In the memento I have solved 4 resistors at 24.4 ohms. The others, which are respectively 583 and number 584, respectively, are not in this circuit, which I mentioned above . Those in the picture are new that I have set and are 0.450 ohms.

 

[iliyanG]

I do not know if everything I write is understood because I use a Google translator.

 

[rottalpha]

Those in the picture are new that I have set and are 0.450 ohms.

I see, so someone tried to repair the unit by replaceing the resistors and failed.....
I can tell you with certainty that there are no 0.450 ohms resistors in this unit. You are testing them in circuit.

Please open a separate thread

 

[rottalpha]

I do not know if everything I write is understood because I use a Google translator.

this does not help...

your unit is severely damaged. What you see on the main board is the tip of the iceberg. One of your driver boards is going to have a bunch of componentes that need replacement. Is going to be difficult to try to navigate you via google translator

 

[iliyanG]

Thank you for your time. I will open a new thread. Somebody tried to repair what was visible. It was black around the four resistors. I put four resistors at 1 ohm, but the transistors Q513 and Q514 and the large resistors heated up without the other two boards.
Thank you again. Good day!