MX-1000 LV rail issues. Please Help!

Thanks for sharing amr2,
I could not check the power supply this way because I do not have an oscilloscope.
I will use the load resistors when they arrive and keep on checking for faulty components until I find what is wrong with it. That is the best I could do for now.
 
I am still deciding on replacements for the drivers in this amplifier (2SA1306B / 2SC3298B)

Based on what I read so far, 2SA1837/2SC4793 as well as MJE15033/MJE15032 are being used as replacement.

The MJE's have a different casing from what I see. ...and I believe that could be a good thing, but are they sufficiently similar to the originals to where the performance is not affected?

While looking at these replacements' characteristics, I noted that they vary from the originals quite significantly, especially in terms of frequency.

How does this affect the performance of the amplifier?

So far I have seen three MX-1000's that are not working. In all of them the drivers are physically blown. See some of the above photos. I am curious to know if there is a reason why these transistors are attached to the same heatsink with the output transistors. They do not look capable to handle heat the same way. Could they be isolated?
 
MJE -> Needs to be isolated from the heatsink, be careful.
I would go for the Toshibas. Yamaha lists them as the substitutes.
 
Ok, that I understand the nead for isolation. The only reason I would consider the MJE is because they have a higher heat disipation (50w vs 20w)
But still, ...Why are the the drivers on the same heat sink as the output trannies?

Heat is the enemy of most electrical components, and from what I see, the excessive damage these drivers are sustaining, is most likely from intensive heat.

Other components burn out, and that is that. These predrivers are cracked in two. The heat also damages the board in most cases.
This I saw in three different amplifiers so far and with different board revisions / versions.
I am just trying to understand if there was a reason in terms of electrical circuitry of why the divers are located on the same heatsinh with the outputs.
 
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Those drivers need to be attached to a heatsink. They dissipate much less heat than the output transistors, but they need it. So, why not use one heatsink for all?

IMHO when a driver is blown, is due in most cases to a shorted output transistor.
 
Those drivers need to be attached to a heatsink. They dissipate much less heat than the output transistors, but they need it. So, why not use one heatsink for all?

IMHO when a driver is blown, is due in most cases to a shorted output transistor.

Thank You!
I have already ordered the Toshiba's. I greatly appreciate your help.
 
You are welcome.

Some tips when changing the output transistors (be careful, I have never repaired a MX-1000!)

  • Don't install them at first power up. Get 2x470 ohm 1-2w resistors and solder one across the base and emitter pads of a missing PNP and a NPN output.
  • Adjust the bias to class B operation.
  • When you fire it up check for sane voltages on the output transistor pads. The absolute collector voltages should be the same on the NPN and PNP sides (Beware that if the power supply hasn’t the outputs load, LP-RP could be higher than expected. You can wire a 1K/5W resistor between the collector and the emitter of the power transistor pads, to simulate the bias current).
  • Base voltage should be about < 700mv.
  • If everything is OK, you can install the power transistor and adjust bias.
 
Thank you for the tip with the resistors.
As far as the bias goes....I believe that this amp is slightly different then the M-80/M-85.

This amp does not have the Class A and Class B option as your amp does.

The only adjustment that can be made is for the idling current.
I believe that the hyperbolic conversion suppose to continuously adjust the bias (dynamic biasing...at least this is how much I understand). I need to do more research on that.

But in any case, I will utilize the resistors you suggest for the first fire-up.
 
As far as the resistors,
You mean I install the resistors across the base and emitter of any of the output transistors pairs, and not install the other pairs, right.
This thing has three output pairs. Does it matter across which one of the pairs I solder the resistors to?
 
As far as the resistors,
You mean I install the resistors across the base and emitter of any of the output transistors pairs, and not install the other pairs, right.
This thing has three output pairs. Does it matter across which one of the pairs I solder the resistors to?

All outputs..
 
Thank you!
Avionic, is there a way to test rail voltages on the main board of this thing without the sub-circuit boards attached.
 
As far as the bias goes....I believe that this amp is slightly different then the M-80/M-85.

Yes, but with no signal all you have to do is adjust the idling current to zero before doing the test with the resistors.
 
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Thank you!
Avionic, is there a way to test rail voltages on the main board of this thing without the sub-circuit boards attached.

Should be able to. Voltages may read a little high without the load. The last 1000 I repaired. I fabricated a fixture to hold the APS's off to the side of the amplifier with umbilicals tacked to the main board.
Its good to have a complete machine shop at my personal disposal.:thmbsp:
 
Should be able to. Voltages may read a little high without the load. The last 1000 I repaired. I fabricated a fixture to hold the APS's off to the side of the amplifier with umbilicals tacked to the main board.
Its good to have a complete machine shop at my personal disposal.:thmbsp:

Nice
 
Hello

.....
On the other hand, power supply needs to be loaded to work properly. You can use 1K/5W resistor to ground

Hi amr2,

I finally got the 1k/5w resistors today and I am planning on working on the amp over the weekend. If I understand correctly, I can load the amp by using a 1k/5w resistor to ground for each of the voltage rails, total of 4 resistors for LB-/LB+; HB-/HB+. Is that correct?
 
Hello

Sorry for not replying sooner... I used those resistors to load the LP rails and check the power supply voltages before install the output transistors (with the resistors across the base and emitter pads)
 
Hello

Sorry for not replying sooner... I used those resistors to load the LP rails and check the power supply voltages before install the output transistors (with the resistors across the base and emitter pads)

Thank you for replying. I will post updates soon.
 
Problem solved!

Once loaded, both voltage rails behave as they should. It took a while to sort it all out but I got the beastly amp running again. Next I will be working on my donor boards to have them as spares.

A couple of notes for other newbies such as myself:

When testing the main board separate from the rest of the amplifier, the LB voltage will be higher..50V+. This is pushing the limits of the 40000uf main caps.

When checking the eight power transistors in circuit, they will appear to be shorted out. Must be checked out of circuit for proper diagnosis.

Be aware that there are a few board and component revisions for these amps and most available schematic diagrams do not show these revisions.
http://www.audiokarma.org/forums/showthread.php?t=568138&highlight=sonavor

Improvising a test rig (followed Avionic's advise) will make diagnosing the sub-boards much easier and it is highly recommended.

I want to thank everyone for the help and advise. My knowledge of electronics is limited and I would have never succeeded without your help.
 
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