MX-1000 rebuild

were you able to test the main board without the subs installed?

No, but I think the problem is the RC networks present at the VAS output. Without them, and two dummy resistors to load the feedback network, I think it should work

Do you mean the output transistors or power transistors?

Output transistors

Also, I am assuming you are doing one channel at the time.

Yes

You can work one channel at the time and the main board voltages should be good (given that nothing is wrong with the main board) you do not need both sub boards connected. One board is enough to load the PSU for both channels and the voltage on the main rails should not be wonky.

Yes, that's how I'm working.

on the sub-boards; when your pre-drivers an drivers got toasted last time, did you check that 1watt zenner right in front of them? That zenner most likely went kaput as well.

Yes, it's checked and it's no toasted. Anyway I put a new one. All components in the sub-board and the VAS section in the main board are checked. I'm overlooking something

Are you powering up with a DBT?

No
 
I was able to test the main board without de sub-boards. :)

1- You need to load the power suppy to protect the capacitors, put a resistor between +LB and -LB pads
2- Disconnect the RC networks at the output to prevent HF oscillation. There are two in the main board (L CH: C525+R545 and R CH: C526+R546) and another two in the speaker connector board (L CH: C555+R571 and R CH: C556+R572). i.e.,remove C525 and C526, and disconect the output cables (blue and red) in the main board
3- Connect the output of the VAS (labeled as IN in the sub-board) to the feedback network. You can use a small resistor between between IN and OUT pads.

Both channels seems to work fine:

IMG_0671.JPG

Here you can see the effect of the RC network. To do this test a resistor between the output of the VAS and the feedback network must be used (to limit the current):

IMG_0668.JPG

So the problem seems to be in the sub-board...
 
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Problem solved. If dummy resistors are used in place of output transistors, the two zobel networks makes the amp oscillate, and this blows the drivers. With dummy resistors but without RC networks, the amplifier works. I recommend to be very careful with this method.
 
amr2, have you pushed it a little bit to see how it behaves?


One my puppies was on the surgery table over the weekend and now has left to a better home with a more responsible owner:rolleyes:

but before it left, I got the itch to test its limits. Since it was freshly rejuvenated, it was a good candidate.
I strapped an El Cheapo set of dummy loads of 400W @ 8R and started cranking the dial...well, I have to tell you that my dummy loads, which were not receiving any serious cooling, were telling me that they are about to leave to resister heaven, but the little monster was not sweating. I had to stop when I saw the DMM readout at 57v ...that was over 400W if the loads stayed at 8R :D
....measured the dummy-loads at that temperature and were reading 5 ohms. you do the math....Dong, it would have sucked if the loads would have crapped out...the chances of taking out the output stage were not in my favor...so I stopped

This unit was on all original semis and was never repaired, but was otherwise "rejuvenated"

Now, I am curious if anyone here has pushed one of these little monsters to see their limits????

When I make it to home town in a few weeks, I plan on doing just that, but I first need a set of serious loads with a large heat-sink and active cooling.
I plan on documenting the limits of one of these little monsters (or the Chernobyl effect of one) :D

sorry for hijacking....


but just for kicks,

MX-1000 reservoir cap next to a B-2 reservoir cap:
IMG_20170205_232103177_HDR.jpg

The herd is thinning out, but here are some of the remaining test subjects:IMG_20170210_232453220.jpg
 
Now it's time to repair this mess. The first thing was to remove the 10mm leds and start cleaning.

IMG_0784.JPG

See the difference. The bottom one is not cleaned:

IMG_0787.JPG

The problem to hold the new leds is that the plastic is broken, so I made 3 plastic holders.

IMG_0754.JPG IMG_0755.JPG IMG_0756.JPG IMG_0796.JPG IMG_0797.JPG

It's a little better now :D:

IMG_0793.JPG
 
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Now it's time to repair this mess. The first thing was to remove the 10mm leds and start cleaning.

View attachment 868673

See the difference. The bottom one is not cleaned:

View attachment 868675

The problem to hold the new leds is that the plastic is broken, so I made 3 plastic holders.

View attachment 868676 View attachment 868677 View attachment 868678 View attachment 868679 View attachment 868680

It's a little better now :D:

View attachment 868681
It looks as if that plastic was deformed by heat from the old bulb and the mess was only made worse when the LED's were added...

yours looks awesome mow.
on the one I worked over the weekend, I still have to tweak the resister on the straw-hat LEDs I used, to lower their intensity.
 
If you have the APS boards removed, you can do an easy modification to have DC instead of AC on the connector that goes to the lamps. In this way you will only need a resistor in series with the LED (s)
 
If you have the APS boards removed, you can do an easy modification to have DC instead of AC on the connector that goes to the lamps. In this way you will only need a resistor in series with the LED (s)
clever...I will consider that for the rest of the patients..
With this one, I had everything back together when I decided to change the bulbs....which I partially regret, because the bulbs were all still good and because I did not have enough time to tweak the intensity of the LEDs to where I wanted. The reason I wanted to switch to LED's in the first place, was to not have to go back under the hood of this MX, but I still do, so mission not accomplished...
Yes, a "global" solution for the feed to LEDs, certainly makes sense. I will also want to explore a few options other than the straw-hats I had on-hand.
Looking only at the schematic, I am assuming you are referring at placing a rectifier diode and higher value resister in place of each of R554 and R555 ?? Am I about right?
 
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Looking only at the schematic, I am assuming you are referring at placing a rectifier diode and higher value resister in place of each of R554 and R555 ?? Am I about right?

I didn't like to use R554 or 555 as current limiting resistor, it was more difficult to adjust once APS boards was mounted. I ended up using an external resistor attached to the led, it was easier to do tests.

This is how I did it. This should give you about 15VDC.

IMG_0781.jpg
 
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