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gde3 10th April 2013 05:57 PM

Help repairing Pioneer M3
 
Hi, firstly let me introduce myself, my name is Giulio and I'm 27 years old. I'm new to this forum and I have little experience with audio equipment. I just builded a couple of tube amps and two DACs and I really enjoyed doing it.
For my next project I would like to repair my dad's old amp a Pioneer M3. For me is a chance to get some experience plus it could be a good birthday present for my dad.
Unfortunately I have no informations on what parts are broke, the only thing I know is that it was sended to pioneer for repairing it but they wanted too much money. At that time the amp wasn't a priority and it ended up in the basement :(

I managed to open the case, this is a first look at the inside
http://i45.tinypic.com/154k2fa.jpg

The first thing I noticed is the bad condition of the transformer, so I decided to remove it. During this operation I discovered that the transformer wasn't actually connected and all the cables are cutted

http://i46.tinypic.com/51616h.jpg

Looking at the bottom conditions I don't think they did a good job at pioneer, as you can see 3 out of 4 "foot" are lost and the metal case is really damaged. The only good thing is that luckily I found where all the transformer cables should be connected, so fixing this part shouldn't be difficult. You can see some of the cables in the next photo

http://i49.tinypic.com/egpim1.jpg

In the above picture you can also see 3 fuses on the top right, I check them and they are all fine, hopefully it could be a good news.

Now my first question, how can I verify if the power transformer work? Should I just connect it to the 220 and check with a multimeter if the right volts come out from the secondaries (60, 47, 7.5) or there's something else I should do?
In the case the transformer is damaged and I need a replacement, how should I determine the VA required? (the only thing I know is that the amp is 340W)

The other parts that could be broke are the transistors, so I removed the left channel amp in order to check.
Front:
http://i45.tinypic.com/24qn2uc.jpg

Back:

http://i45.tinypic.com/2j2ygau.jpg

Amp pcb:

http://i48.tinypic.com/ehhq44.jpg

I checked with a multimeter all the resistors and diodes, everything is fine but I'm lost with the transistors... B, C, E, are evidentiated on the back, I tried to connect one probe to B and the other to C and E, then I tried the inverse. The problem is that I receive no value in all cases, the multimeter just "spark" and then go to zero after one second. What I'm doing wrong? Is it possible that all the transistors are broke?

Thanks in advance for the help and sorry for my bad english and my too basic electronic knowledge.

Mooly 10th April 2013 06:04 PM

If the meter sparks then it sounds like you are trying to test them with the amp on (which it must not be) or that there is residual charge on the rails from the PSU caps being charged.

This is an ambitious project to start with :) you have to realise that.

Power transistors almost always fail short circuit C to E

I would also advise against plugging in to the mains without some sort of current limiter. Search the forums for "bulb tester" :)

To even attempt this you need the full circuit diagram.

Mooly 10th April 2013 06:12 PM

Also... those power transistors in the picture look as though they may not be originals. The device numbers seems an odd mix for a Japanese amp.

jaycee 10th April 2013 06:21 PM

Someone has worked on that amp, poorly. The output transistors are all different types, suggesting that someone just put in whatever they had to hand. The original power transistors should be Toshiba. Whoever did that probably damaged the transformer too.

You need a schematic to get anywhere, fortunately a quick Google brings up this:
Pioneer M3 Exclusive Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

As for the transformer, the problem here is that the original has multiple windings. You're not going to find that "off the shelf", so you would have to have a custom transformer made.

gde3 10th April 2013 06:29 PM

Quote:

Originally Posted by Mooly (Post 3448895)
If the meter sparks then it sounds like you are trying to test them with the amp on (which it must not be) or that there is residual charge on the rails from the PSU caps being charged.

This is an ambitious project to start with :) you have to realise that.

Power transistors almost always fail short circuit C to E

I would also advise against plugging in to the mains without some sort of current limiter. Search the forums for "bulb tester" :)

To even attempt this you need the full circuit diagram.

I have the diagram but I can't post it since the file is too big.

Sorry probably spark isn't the righ word, what I meaned is that when I try to test the multimeter go to a number like 0.654 and then after a second go back to 0.000, so I could not determine if the transistor is working or not. By the way as you can see from the 5th picture I completely removed the trasistors from the amp for testing

I know is an hard project but the amp don't work and nobody except me really care about fixing it. So in the end even if I make some mistakes in the worse case I will just do some damages on something broke

Mooly 10th April 2013 06:40 PM

If any "normal" transistor reads 0.00 or anything less than 0.5 ish between any pins then its faulty. Your DVM is actually passing a small current when testing devices and the reading is the volt drop at the meter probes. So 0.654 is actually 654 millivolts and spot on for a silicon junction.

Be under no illusions about working on something like this. You would also need a source of replacement or suitable equivalents for any faulty semiconductors.

Mooly 10th April 2013 06:47 PM

1 Attachment(s)
Is this the one.

If you are really intent on having a go then we can come up with a plan to test and work on it (but not today :))

gde3 10th April 2013 06:48 PM

Quote:

Originally Posted by jaycee (Post 3448917)
Someone has worked on that amp, poorly. The output transistors are all different types, suggesting that someone just put in whatever they had to hand. The original power transistors should be Toshiba. Whoever did that probably damaged the transformer too.

You need a schematic to get anywhere, fortunately a quick Google brings up this:
Pioneer M3 Exclusive Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

As for the transformer, the problem here is that the original has multiple windings. You're not going to find that "off the shelf", so you would have to have a custom transformer made.

Thanks for the link, I know the custom transformer is the only solution and luckily there's a company close to where I live who can make it. But before I contact them I would like to test if this transformer is working. In any case I know the secondaries but what about the other characteristics of the transformer? should I look for a toroidal?

About the differents transistors, in my first post I skipped some details. The amp was sended to pioneer to be fixed (around half 80') they substitutes the transistors. After a month that it come back the amp broke again, It was sended back to pioneer for a second time but since they wanted more money to fix it, we didn't do it.
I can probably found in the diagram what the original transistors should be but there are chances to find a replacement nowadays?

Mooly 10th April 2013 06:54 PM

Your first step has to be to confirm or otherwise that the transformer is OK. If not then I suspect the project is doomed tbh

You can safely test the tranny by powering it via a 60/100 watt filament bulb. If excess current is flowing the bulb lights with no dramas.

gde3 10th April 2013 07:06 PM

Quote:

Originally Posted by Mooly (Post 3448954)
Is this the one.

If you are really intent on having a go then we can come up with a plan to test and work on it (but not today :))

One thing is sure, I'm not in a hurry and I'm really intentioned to do this project :)
I know is difficult at that I can achieve poor results, but this don't scare me at all. Plus if everything go right I could end up with a complete vintage hifi, the pioneer C3 preamp is working right and I recently fixed the speakers (and I love them) :D


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