Mackie CR4 amplifier no sound

[budm]

Do you have any small through hole 3-leg NPN Transistor such as 2N2222, 2N3904 around?

[UserXP]

Originally Posted by budm

Do you have any small through hole 3-leg NPN Transistor such as 2N2222, 2N3904 around?

I doubt that, but there is an electronics store relatively near by my place, I can go purchase one tomorrow (it's almost midnght here now ). Should I go for the ones you listed above or buy the 2N5551 one?

[budm]

You can use the through hole Transistor so it will be easy to solder for testing, the 2N2222 or 2N3904 are very generic Transistors so should be easy to get, just remember that pin out of the through hole is not the same as the SMD.

[petehall347]

any updates ?

[UserXP]

Originally Posted by petehall347

any updates ?

Sorry, I had a crazy week. Due to a sudden increase in the number of new corona cases in my country, there have been changes and new measures at my work being put into effect, so the whole week was in rescheduling and re-organizing activities and work from home, so I didn't have much free time.
I will post back in a couple of days for sure. Thank you so much for helping me with this and for having the patience.

[UserXP]

Hello, people.
Sorry for this delay, hopefully you are all OK and safe.

OK, here's the update: Today I bought four 2N5551 transistors for testing purposes, but I couldn't find the SMD version, so I though these would do for testing as budm suggested. Now, he suggested an open pathway between Q2 and the ground... I did some more testing...

Now, I did some more measurements prior to deciding to unsolder the SMD Q2. I again tested its forward voltages and they are the same like in the previous measurements, around 0.600 and seemes OK. However, I decided to check the components again. Now, here is some interested news... I tested all the through-hole resistors in this area and refered to the schematics. The unit was off during these measurements:

R87=4.7kOhm
R88=4.7kOhm
R91=5.6kOhm

Now, R22 is strange and I think it didn't behave like this before. Sometimes the multimeter displays 00.00kOhm, other times it starts at 4.2kOhm and will not stop, it continues to show the increasing number of kOhms. If I move the probes and touch again, it continues to count where it left off, as if something is "charging" the resistance. I also got a positive continuity result when touched this specific resistor with the probes a couple of times, but not always. This happens only with R22.

When I turn on the unit, the voltage across that resistor is 000.0mV and the resistance test shows 0. According to the specs, R22 should be around 10kOhms. Can a resistor become faulty and work intermitently, or are these readings due to in-circuit measurements.
Does this tell us anything, is it normal?

[UserXP]

R22 is the first of the two resistor above Q2:

-R22-
-R91-
-Q2-

On the other unit it reads 10kOhm like it's supposed to be.

[UserXP]

Unsoldered R22 from the PCB and measured, now it reads 9.9~10kOhm.

[petehall347]

now measure the board where the resistor came from

[UserXP]

Originally Posted by petehall347

now measure the board where the resistor came from

The same, it starts from 4.2kOhm and then slowly increases to 5, 6, 7...

[petehall347]

try swapping meter leads

[UserXP]

Originally Posted by petehall347

will be c39 charging up .. swap meter leads

Swapped, no reading now. It stays at ".OL" on display.

[petehall347]

put it back and try the new transistor ..be sure of transistor pin out

[UserXP]

I see that one contact of this R22 is connected to Z2 coming from the positive rail. The other is in contact with the ground through a joint near Q1.
On the working unit, when probes are placed on this resistor, it reads 9.9~10kOhm.

[UserXP]

Originally Posted by petehall347

put it back and try the new transistor ..be sure of transistor pin out

I barely managed to put this R22 back because it was very unapproachable to access its holes and keep the resistor in place for sodering. It fell out too many times.
I will attempt to desolder Q2 without damaging it, but it is so tiny i am afraid the soldering iron is going to melt it. If it turns out Q2 is not faulty, I will have no replacement. My local stores do not have the SMD type of these amplifier transistors in stock.

[petehall347]

you said no volts across R22 i would expect there should be .

[momaka]

Originally Posted by UserXP

I will attempt to desolder Q2 without damaging it, but it is so tiny i am afraid the soldering iron is going to melt it.

You won't melt it unless your iron is super-hot and you really keep the iron on that transistor for too long (i.e. more than 20-30 seconds at once.) By super-hot iron, typically those would be 40+ Watts irons that plug directly into the wall (i.e. no temperature regulation.) If it's a temperature-controlled station, things should be fine.

The way to remove those transistors with just a soldering iron...
Put flux on all leads on the SMD transistor and then add big solder blobs that stick up and above the case. Next, add more flux to the leads again. After this, melt a blob of solder on your soldering iron's tip that's big enough to cover the entire SMD transistor form the top... but just watch out not to make the blob too big and flood/bridge nearby SMD components too. Then put it over the transistor so the blob of solder on your iron contacts the solder blobs on the SMD transistor's leads. The transistor should either stick to the solder blob on your iron when you move it around a bit, or you may need to use tweezers to dislodge and pickup the transistor while you're heating it with the soldering iron (it just depends if the SMD components on the board were glued or not during manufacturing.)

It will also help if you use a fairly wide tip on your iron - so either chisel or bevel type. I suggest avoiding conical tips. If all you have is conical tips... if you have spare one (or preferably one that looks ugly and looks worn out), you can grind it into a chisel tip. Better yet, grind it into a "spoon" shape (i.e. concave like a cup.) This shape can hold a lot of solder and is what I use on my 30 Watt soldering iron for removing SOT-23 transistors all the time.

After you remove the SMD transistor, you may have to put flux on its leads and touch each one with the soldering iron to clear any excess solder or solder bridges. I do this while holding the transistor with tweezers.

Originally Posted by UserXP

My local stores do not have the SMD type of these amplifier transistors in stock.

If you damage that transistor, you don't necessarily need to replace it with an SMD one. Through-hole will work fine too. Just may not look as pretty with all the leads soldered to the board, but who cares as long as it works as intended.

[UserXP]

Originally Posted by momaka

If you damage that transistor, you don't necessarily need to replace it with an SMD one. Through-hole will work fine too. Just may not look as pretty with all the leads soldered to the board, but who cares as long as it works as intended.

Thanks, the unsoldering tips sound really useful and I will try that. I have a 30W soldering iron, but with a pointed tip. I can try to use the wick and slightly separate all legs from their pads, one leg at a time.

Now, the through-hole transistor I bought will be for testing purposes only. If after soldering it in place the unit starts to operate properly, I'll know it was the faulty transistor and will order a SMD type. I don't think I could leave the through-hole protruding from the board like that as it would probably hit the inner wall of the speaker housing as the board comes really close to it. But, if its the transistor, at least that's something and I won't mind ordering the SMD version to make a full reapir.
I will post back with the results.

[momaka]

Originally Posted by UserXP

I have a 30W soldering iron, but with a pointed tip.

That should work fine if you have a better tip for it or if you sand it into a chisel/bevel tip.

Originally Posted by UserXP

I can try to use the wick and slightly separate all legs from their pads, one leg at a time.

No, don't do that. You will most certainly destroy the transistor that way.
Those small SMD devices are not designed to flex their leads at all. If you try it, most likely you'll pull a lead out of that transistor, and then you would need to replace it for sure.

Leave the wick for cleaning through holes (or BGA work, if you ever do any of that.) For plain SMD work, wick has no place here.

Originally Posted by UserXP

I don't think I could leave the through-hole protruding from the board like that as it would probably hit the inner wall of the speaker housing as the board comes really close to it.

It also leaves the possibility of the transistor solder joints breaking away from the vibrations in the woofer cabinet... that is, unless you glue it down well to the PCB.

[petehall347]

dont force it in any way or it will rip the pads off the board .been there done that . hot air is best if your back can take it unlike mine .