Mackie CR4 amplifier no sound

[budm]

Re: Mackie CR4 amplifier no sound

Hi, and thanks for your patience with me, I am doing my best to understand all this, sorry for the late reply.
OK, I must kindly ask of you to provide some instructions or mark in the picture what exactly I should do so I don't understand you wrongly and do something that might damage the unit even more.

OK, when you say "check emitter to ground if it is 5.6k", I am to turn the unit off and use the DMM resistance dial? I put the negative probe on the E contact and the positive on the traces that go towards the ground? I see that E of Q2 is connected to the Z1 diode which is connected to the ground very close by (near the edge of the board, over the Q1).

Please be more specific, every idea or observation you make gives me a bit of hope like "OK, that might be the golden nugget" but then I don't understand what I am supposed to exactly and scatch my head. Sorry for the noobness.

https://www.badcaps.net/forum/attach...9&d=1603197623

Please look at the schematic and the spec sheet of the Transistor top get correct pinout of the Transistor.
As you can see, the BASE of Q2 is connected tot he Cathode of Z1, so I do not understand why you say E of Q2 is connected to Z1. I think we are getting wrong DATA due your not able to read schematic correctly and also wrong Transistor pin out. Please study the spec sheet and the schematic and see where the mistake is.

[UserXP]

Re: Mackie CR4 amplifier no sound

if q2 is 5.6k to ground best i can suggest is a new q2 and see what happens .
or firstly shorting Q2 C to E . then see what happens . well after verifying the 5.6k . that is

Hi, and thanks for your patience with me, I am doing my best to understand all this, sorry for the late reply.
OK, I must kindly ask of you to provide some instructions or mark in the picture what exactly I should do so I don't understand you wrongly and do something that might damage the unit even more.

OK, when you say "check emitter to ground if it is 5.6k", I am to turn the unit off and use the DMM resistance dial? I put the negative probe on the E contact and the positive on the traces that go towards the ground?

Please be more specific, every idea or observation you make gives me a bit of hope like "OK, that might be the golden nugget" but then I don't understand what I am supposed to exactly and scatch my head. Sorry for the noobness.

[petehall347]

Re: Mackie CR4 amplifier no sound

if q2 is 5.6k to ground best i can suggest is a new q2 and see what happens .
or firstly shorting Q2 C to E . then see what happens . well after verifying the 5.6k . that is

[budm]

Re: Mackie CR4 amplifier no sound

What gets me is the readings of the working unit as reported:
The voltages on Q1 and Q2 are "almost" the same (the orientation is as their contacts appear on the image above):

Q1:
Left leg: 000.0v Right leg:-1.9v
Base: +2.9v

Q2
Left leg: 000.5 Right leg: +2.9v
Base: +13.8v

The R88 also shows this (position from picture above):
Upper leg: (+12v)
|S|
|S|
Lower leg: (+2.9v)

Q2 junction shows E-B of 2.9V - 0.5V = 2.4V which cannot be right either.

[petehall347]

Re: Mackie CR4 amplifier no sound

measure Q2 emitter to ground ..expect 5.6k

[budm]

Re: Mackie CR4 amplifier no sound

So what does that actually mean in terms of further steps? Open path to ground is then caused by a short?

That means you need to check the path of Emitter pin all the way to circuit ground, if you cannot figure that out then take it to repair shop. You have the working unit that you can use for comparison so that should not be hard to figure out.

[UserXP]

Re: Mackie CR4 amplifier no sound

So what does that actually mean in terms of further steps? Open path to ground is then caused by a short?

[budm]

Re: Mackie CR4 amplifier no sound

When tested the NPN forward voltage of Q2 in power off state, I get:
B to E reads 0.638V
B to C reads 0.606V

While power on:
B to E reads 0.000mV
B to C reads -20mV

I find this transistor's contacts confusing. I thought the middle leg is always the Base, but I followed the 2N5551 datasheet. If it wouldn't be a problem, I could attach the image again without the marked voltages and I would kindly ask if you to circle the contacts you'd like me to test/measure and what with which probe, and then report it back. What do you say?

1) Transistor pinout always goes by the spec sheet.

2)Your report of of Base Voltage of over 2V with ref to ground, but E-B Voltage is 0V, that means the Emitter has open path to circuit ground via Emitter resistor, that is why Q2 is not on right now.

[UserXP]

Re: Mackie CR4 amplifier no sound

follow the schematic to be sure .. base goes to Q1 collector .collector to pin 5 of the ic . emitter to R91

Yes, that is correct and in accordance with the datasheet contacts layout. Q2 collector goes to TDA's Pin 5.

[petehall347]

Re: Mackie CR4 amplifier no sound

follow the schematic to be sure .. base goes to Q1 collector .collector to pin 5 of the ic . emitter to R91

[UserXP]

Re: Mackie CR4 amplifier no sound

The main key is that the forward Voltage of E-B junction should not be showing >1V, right now your readings show the Voltage difference between E and B is way more than one Volt.

When tested the NPN forward voltage of Q2 in power off state, I get:
B to E reads 0.638V
B to C reads 0.606V

While power on:
B to E reads 0.000mV
B to C reads -20mV

I find this transistor's contacts confusing. I thought the middle leg is always the Base, but I followed the 2N5551 datasheet. If it wouldn't be a problem, I could attach the image again without the marked voltages and I would kindly ask if you to circle the contacts you'd like me to test/measure and what with which probe, and then report it back. What do you say?

[UserXP]

Re: Mackie CR4 amplifier no sound

then you should try C to E both ways .

what budm is saying the transistors is way above it max rating going off the voltages .
to verify the voltages you measure voltage powered on between B and E .this is called the voltage drop measurement .

in case you want to learn about transistors . http://www.falstad.com/circuit/e-pnp.html

Thanks. I am not sure I understood what exactly to measure.
OK, I powered on the unit. Placed the probes on the C and E contacts. Mind, E and C as per the datasheet (B is not the middle contact). One way it neasures +20V, when I switch the probes it shows -20V between E and C.
Is B, C and E contacts layout correct?

[petehall347]

Re: Mackie CR4 amplifier no sound

If you refer to NPN testing, I placed the red probe on the B leg, and the negative one on E and C.

then you should try C to E both ways .

what budm is saying the transistors is way above it max rating going off the voltages .
to verify the voltages you measure voltage powered on between B and E .this is called the voltage drop measurement .

in case you want to learn about transistors . http://www.falstad.com/circuit/e-pnp.html

[UserXP]

Re: Mackie CR4 amplifier no sound

Do you mean voltages while testing the NPN with a diode function while the amplifier is unpowered, or the flowing voltages while it is on? If latter, those are already on the image. When I measured those voltages, I always put the black probe on the ground rail and then used the positive probe on the contacts.

If you refer to NPN testing, I placed the red probe on the B leg, and the negative one on E and C.

[budm]

Re: Mackie CR4 amplifier no sound

budm, I was looking at this datasheet:


The CR schematics of that area says those two transistors are 2N5551. The datasheet I looked into shows different pad layout in terms of B, C and E for 2N5551 moar-mounted version (unless they made a mistake in labelling the contacts). That's why I "drew" the layout in one of my previous posts like this:

E---B
...C...

Left Leg --- Right Leg
.......Lower Leg........

The bias also confirm this. It is a NPN transistor, so when I put the positive probe on the B pad designated by the datasheet, I can measure the forward voltage on E and C with the negative probe. Did I use the wrong document or did I do something wrong?

The main key is that the forward Voltage of E-B junction should not be showing >1V, right now your readings show the Voltage difference between E and B is way more than one Volt.
When you make the reading, what did you use for ground point?
What Voltage does it show if you put the RED meter probe on B and BLACK probe on E of Q2?
BTW, you can also check the E-B junction Voltage of other working Transistors, if they do not come out as they should be then it is measurement error.

[UserXP]

Re: Mackie CR4 amplifier no sound

it should be ok so long as you dont short anything else and you short the correct points .
you could wait to see what budm says in case i am missing something

Ok, thanks, let's do that.

[petehall347]

Re: Mackie CR4 amplifier no sound

it should be ok so long as you dont short anything else and you short the correct points .
you could wait to see what budm says in case i am missing something

[UserXP]

Re: Mackie CR4 amplifier no sound

Q2 is the prime suspect .
i suppose you could try a jumper between C and E of Q2 and see if it plays sound then .

Would that safe to do? I mean, I wouldn't want to damage anything else in the circuit.

[petehall347]

Re: Mackie CR4 amplifier no sound

Q2 is the prime suspect .
i suppose you could try a jumper between C and E of Q2 and see if it plays sound then .

[UserXP]

Re: Mackie CR4 amplifier no sound

how about R91 5.6k ?

Yes, it reads 5.66kohm. The one above (R22) reads 9.9-10kohm.