Help repairing Pioneer M3

[gde3]

Unfortunately I don't have a 22ohm resistor available (I need to buy one) the closest one I have is 10ohm and the rest is all in the K zone

 

[Mooly]

You could fit a 10 ohm to test the amp (it would need to be 0.5 watt at least). That would be fine as a temporary measure.

Did the transistor read short circuit ?

 

[gde3]

I inserted the 10ohm resistor and substituted Q18 Q19, but now the channel don't work and the bulb stay on

 

[Mooly]

Good job your using the bulb.

Do you remember how we shorted the vbe multiplier out before, to force the bias to zero while you worked on it. I think we need to do that again by linking it out like this.

Have you replaced Q20 and Q21 yet ?

 

[gde3]

Yes I remember but I'm not sure about the connection I will check previous posts, should I also short R1?

I didn't substitute Q20-21 yet, should I also change Q8 and Q7?

 

[Mooly]

No need to short R1, just short the vbe multiplier out linking Q8 and Q10 collectors as shown in the post above. One step at a time, no need to change anything else yet apart from Q20 and Q21.

 

[gde3]

After substituting Q20-21 and shorting the vbe multiplier the bulb stay off.... what should I check now?

 

[Mooly]

Fantastic. Next check the DC offset at the output is zero (no voltage across the speaker terminals) If that's OK then you can test with a speaker at low volume. Keep the bulb in for now.

 

[gde3]

Offset with trim at min 55mv... after a careful adjustment +/-0
Tested with the speaker it work fine, little distortion but it could be created by the not so good connections (I'm using alligator clip cables to connect the board to the rest of the amp)

 

[gde3]

Bias is obviously zero, should I remove the short on the vbe multiplier and set bias?

 

[Mooly]

All these alligator clips sound very scary but it all sounds promising up to now.

What you have to do next is check that the output stages biases up OK and you do that initially with the bulb still in place and NO speakers attached.

So look at VR2 (thats the bias pot that goes to Q11) and set it to minimum resistance (so its like a short). Unsolder the link wire you put in and switch on and check the bias sets OK (thats the small voltage across the 0.5 ohms). The bulb will glow more as you turn up the bias. That proves its all OK. Turn the bias back down again.

If you want to test with a speaker again then increasing the bias even a tiny amount should remove all the distortion, If it does then I think we test on full mains and set it all up as we did originally.

 

[Mooly]

Another thought... thats all OK for testing but you really need at some point to fit the correct 22 ohm resistor and when you do you should go through the bias set up again.

And we need to think why it all failed in the first place. But lets get it all working first.

 

[gde3]

I need to feed them regularly or they bite me lol

I removed the short, Bias with trim at min is 0, but when I try to adjust it it goes only negative. I probably inverted the connections (positive pin 13, negative pin 9) since if I measure it on the 0.5 resistor bias will set fine and smooth

Now I will set bias accordingly to the other channel, then I will reassemble the amp and test it

 

[gde3]

I need to wait until monday for the resistor, in my town I can find only cheap Chinese parts. I will probably use some 2W kiwame

I think the fail is related to the MJE340/350 high temperature, the other predrivers have an heatsink while I didn't use it on the MJE.
Do the MJE340/350 need insulation? I can attach them two aluminum washers and see if it work

 

[gde3]

I called victory too early... The amp is now shorted (bulb on) and some smoke come out from R65

 

[Mooly]

Your not having much luck with this at the moment. Have any of the output transistors failed short circuit, and are all the 0.5 ohms OK ?

Yes, the MJE's need insulation. The collector is connected to the metal tab.

How did it come to short out ? What happened.

 

[gde3]

What happened.... No idea really

I regulated bias to 1mv and offset to zero on both channels, then I removed the alligator connections and reassembled the amp with original connections pin(3-4- 6-7-13-14-IN-E)
Turn on again and it don't work, I checked all connections but I didn't made any mistakes so honestly I don't have a clue

 

[Mooly]

This is where its difficult without actually seeing it all. See if any output transistors read short etc and just check for obvious physical shorts such as a transistor that's twisted or its legs are touching each other.

 

[gde3]

all the 0.5 ohm are fine, the only resistor who's clearly damaged is R65 22ohm. I clearly saw some smoke come out of it plus now it's color is a lot darker than before.

 

[Mooly]

Do any of the output transistors read short circuit ? Is it possible the collector of Q20 touched something ? Make sure Q20 collector (the metal tab) isn't shorting to the chassis.