MX-1000 LV rail issues. Please Help!

rottalpha

Yamaholic
Subscriber
Hello Everyone,

I am relatively new to the forum and have electronics basic knowledge only. I enjoy this as a hobby and love older analog amplifiers.

I am trying to resurrect an old MX-1000 that has seen significant abuse in its time. I have some spare parts from a donor amp and need to get additional components to complete the job. So far I was able to replace all bad components on the main board with original parts from the donor, and I am trying to test it before I move on to the Sub Circuit 1 and 2 / amp boards.

The problem I am having at the moment with the main board, is that the LB rail is to high. It is 50-51v and it is suppose to be around 42v. The high voltage rail is right on the money at 91-92v.

Two transistors, both 2SC3416 and four 10K 2watt resistors are getting pretty hot.

Both transformers check OK, with roughly 68.9v 30.2v 0 30.2v 68.9v

I am trying to individually repair all boards in the amplifier. The test was done with the main board connected to power only. No sub circuit boards connected to it.

Additionally, I have connected the donor board with a different set of capacitors to power and got the same result on the LB rail (except no hot transistors which the donor board no longer has).

What could be the reason for the higher voltage on the LB rail?. Am I testing this wrong without the amp boards connected? The voltage across the 40,000uf capacitors which are rated ar 50v is 50-51v so the risk of losing the main capacitors is there.....

Please help!
 
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No one has any experience with these?

I am aware that the M-80 presented a similar issue to at least one other AK member. How similar is the M-80 to the MX-1000 as far as the main board?
 
No, it is not regulated.
I have checked the voltage after the rectifier without the capacitors and the voltage on the LB is normal. but no voltage on the HB rail. Once I connect the caps, the voltage on the HB is normal and the voltage on the LB goes up by about 10V. I did try this with two different boards and two different sets of capacitors..actually different values capacitors as well.
I may be doing something wrong by testing the board without the sub circuit boards.
 
you need to test voltages under the normal load conditions as they will likely be high without ..
if there is a resistor in the circuit to limit current you can work out expected current draw by ohms law .
i suppose you could load the circuit with an appropriate resistor .
is the wall voltage correct for the unit ?
 
I see what you are saying about the load. Under load the voltage should be lower but why is the HB correct and only the LB high when not under load?

Wall voltage is correct, voltage after transformer is correct....voltage after the rectifiers is correct as long as no capacitors are attached.

I guess I should have rephrased my question:

Could I test the main board of the MX-1000 without the sub-circuit boards attached?

The main board contains the power supply and the V-amp. Most of the damaged parts were on the V-amp side.
 
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I could not attached the schematic in PDF format since it was too large.
Please see the attached partials. I appreciate your effort.
 

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The schematic I have is from the guys at HIFI Engine, but I believe that the AK database also has a full schematic for this amp. I realize that the attachments may be to small, to be of any use.
 
right so they are all raw unregulated rails ..
to be on the safe side you could pull the fuses for the lower voltages if that part supply is going to be unloaded .
trouble is though its going to throw the other voltages off having them disconnected so will limit your testing somewhat .
i would be testing with power off first to see what i can find out of tolerance / shorted /open etc .then replacing bad parts then powering up on a variac or dbt and measuring voltages then if stuff aint burning up .
 
Well I already tested it with the fuses on. I stopped when I saw the voltage on the LB is 51v, higher than the capacitors rating. I switched the capacitors out with a higher rating ones and turned it back on. The higher voltage does not appear to have damaged anything yet, except that Q513, 514 and R527,529 R528,530 are getting excessively hot.

Now, I also have the donor board from which I removed most of the components from the V-Amp section...(including the above named transistors)
I tested the donor board voltage with a different set of rectifiers and different set of capacitors, and I am getting the same 51v, high voltage on the LB. No transistors are getting hot on the donor since I have removed them.

There is an imbalance and I can not explain if it has to do with one of the components (although at this point I could not imagine what that component may be) or with the fact that I am testing the main board separated from the sub circuit boards.
 
Hello

My M80 had the same problem. Check the semiconductors in the power supply board, especially drivers and diodes. They can be out of specs.

On the other hand, power supply needs to be loaded to work properly. You can use 1K/5W resistor to ground
 
I checked all semiconductors after the first test and when I realized that there is a problem with the voltage. They all seem fine. I will definitely try the resistors next. Thank your your input. I will post updates soon.
BTW, do you remember what exactly was wrong with your unit?
 
BTW, do you remember what exactly was wrong with your unit?

Yes, on PSU: A pre-driver transistor (2SA1145/2SC2705) (although it passed the test with the multimeter). Later a 12V zener broke down, so I ended up changing all diodes (8 per channel) and the 4 pre-drivers. No problem so far...
 
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Thanks again. I have not made it yet to repair the sub circuit boards...although I already know that I will have to replace all the drivers...most of them are bad.
Did you find anything wrong on the main board...particularly on the PSU or the protection section?
 
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