Fender squier 15 DC voltage on the output

[prulup]

Hi evebody, I have a problem with Fender Squier 15 reverb solid state amplifier. There is a negative voltage on the output after the repair (it was completely converted into a different amplifier by previous owner). Could you advise please?

schematics here:

http://www.amparchives.com/Amp%20Arc...0Schematic.gif

or here:

http://www.amparchives.com/Amp%20Arc...0Schematic.gif

The voltages (E C B) are measured just after powering on, because the DC on the output raises. All the transistors and diodes are new. I don't know why is there only -16V on the negative PWR supply, there was -22V without the transistors.

Q8
-16
-8
-15

Q7
-7,5
20
-8,5

Q6
-8,5
-15
-9,5

Q5
-8
20
19,5

Q4
20,5
-8
19,5

Q3
-8
20
-8

Q2
-8
20
-7



THANKS!

[Jazz P Bass]

I do not understand the Base voltage on Q5.(stated as +19.5Vdc)

Q5's base should connect directly to Q4's Collector (stated as -8Vdc)

It may be that you have an open trace.

[prulup]

I do not understand the Base voltage on Q5.(stated as +19.5Vdc)

Q5's base should connect directly to Q4's Collector (stated as -8Vdc)

It may be that you have an open trace.

My fault, I swap Q4 and Q5 when writing the thread. The correct voltages were:

Q4
E -9
C 22,7
B -8,5

Q5
E 22,7
C -8
B 22


The problem is that the voltages on the transistors (not the +- pwr supply) are rising after the power on... (for example the E on Q4), so it is not exact measuring. Thank You

[Jazz P Bass]

Those voltages are worse than the others.

Q4 is a PNP transistor.
(a high + voltage on the Emitter & the Base means the Collector should basically be OFF)
( A lower + voltage on the Base will turn the transistor ON (more voltage at the Collector)
(that is what the 4.7K resistor is for)

The Emitter (the arrow) should be a the high voltage rail. (+22Vdc)
The Base, being 4.7K away from the Emitter, should read slightly lower than the Emitter.
The Collector is the bias string. It connects directly to Q5 base.
This voltage should be around +0.6 Vdc.

I would suggest that you insure that the bias string diodes are good.
Regardless of the pin assignments, something is wrong that you have ANY negative voltage on Q4 & Q5.

Once you can properly identify the transistor leads, post Q4 & Q5 again. It would help if you also posted the lower transistor voltages.

It is possible that Q2 and/ or Q3 is bad & they are leaking a negative voltage.

[prulup]

Those voltages are worse than the others.

Q4 is a PNP transistor.
(a high + voltage on the Emitter & the Base means the Collector should basically be OFF)
( A lower + voltage on the Base will turn the transistor ON (more voltage at the Collector)
(that is what the 4.7K resistor is for)

The Emitter (the arrow) should be a the high voltage rail. (+22Vdc)
The Base, being 4.7K away from the Emitter, should read slightly lower than the Emitter.
The Collector is the bias string. It connects directly to Q5 base.
This voltage should be around +0.6 Vdc.

I would suggest that you insure that the bias string diodes are good.
Regardless of the pin assignments, something is wrong that you have ANY negative voltage on Q4 & Q5.

Once you can properly identify the transistor leads, post Q4 & Q5 again. It would help if you also posted the lower transistor voltages.

It is possible that Q2 and/ or Q3 is bad & they are leaking a negative voltage.

Jazz P Bass, you are absolutely right and I am a complete moron, first I wrote the numbers wrong and second I wrote everything wrong. The first thread was nearly OK written but one voltage on Q5. Here are the right values measured with no speaker:

Q8 NPN
E -23,3
C -12 (raises quickly up to -23)
B -23

Q7
E -12,5 (raises quickly up to -23)
C 23,3
B -12 (raises quickly up to -23)

Q6 PNP
E -9 (raises up to -23)
C -20
B -10 (raises up to -23)

Q5 NPN
E -9 (raises up to -23)
C 20
B -8 (raises up to -23)

Q4 PNP
E 20
C -9 (raises up to -23)
B 19,4

Q3 NPN
E -9 (raises up to -23)
C 20
B -8 (raises up to -23)

Q2 NPN
E-9 (when I touch the emiter with multimeter it goes both E and B up to -23)
C 20
B -8 (when I touch the BASE with multimeter it goes both E and B to ZERO)


I checked every small transistor /unsolder, check and resloder to PCB/, all OK. Such a simple amplifier and I am stucked! I re-checked Q2 and Q3 after the measurement again.

[prulup]

I took out the PWR transistors and the voltages are the same as I wrote...

[prulup]

Anyone..? :-(

[Dave H]

I checked every small transistor /unsolder, check and resloder to PCB/, all OK. Such a simple amplifier and I am stucked! I re-checked Q2 and Q3 after the measurement again.

Check the path from Q3 base through R24 to ground is not open circuit. Try bridging R24 with a 10k resistor.

[Jazz P Bass]

What I see is way too many 'negative' voltages.

Especially at the base of Q5.

Check Q4.

[DrGonz78]

Q4 is a PNP transistor.
(a high + voltage on the Emitter & the Base means the Collector should basically be OFF)
( A lower + voltage on the Base will turn the transistor ON (more voltage at the Collector)
(that is what the 4.7K resistor is for)

The Emitter (the arrow) should be a the high voltage rail. (+22Vdc)
The Base, being 4.7K away from the Emitter, should read slightly lower than the Emitter.
The Collector is the bias string. It connects directly to Q5 base.
This voltage should be around +0.6 Vdc.

I would suggest that you insure that the bias string diodes are good.
Regardless of the pin assignments, something is wrong that you have ANY negative voltage on Q4 & Q5.

Once you can properly identify the transistor leads, post Q4 & Q5 again. It would help if you also posted the lower transistor voltages.

It is possible that Q2 and/ or Q3 is bad & they are leaking a negative voltage.

Yeah Q4's collector should be around 0.6vdc and that is very important for setting the bias. Now please confirm whether D3 or D4 are good. Question for Jazz (and others)... Can Q5 be temporarily removed from cicuit to see if the Q4's collector goes to the correct voltage or it won't work that way? -9v at the Emitters of Q2 and Q3 are very suspect too. I mean what could possibly put -8v on the base of Q2?

Check the path from Q3 base through R24 to ground is not open circuit. Try bridging R24 with a 10k resistor.

Are we referencing the Squier 15 schematic? Seems that "Q3 base through R24 to ground" does not really make sense to me as on the Squier 15 schematic R24 connects to the collectors of Q2 & Q3. On the Sidekick 15 schematic R24 is in parallel to C16 going to ground from the Q2 base. Wouldn't the fact that there is 20v on the collector of Q2 tell us that R24 (referencing the Squier 15 schematic) is not an issue? Just curious to understand better what was being tested.

I have attached a cropped image of the amp schematic with voltages posted. Just helps me look at all voltages reported better.

[prulup]

Thanks guys, I will check it tommorrow! Actualy it is Squier reverb 15, so there is one transistor added, but that should not be a problem. I could not find a schematics for this one. It is from 1986, quite rare model.

[prulup]

Check the path from Q3 base through R24 to ground is not open circuit. Try bridging R24 with a 10k resistor.

Dave, I don't know if we have the same schematics, there is R24 on Q2 collector. I have this amp:

http://www.amparchives.com/Amp%20Arc...0Schematic.gif

[prulup]

What I see is way too many 'negative' voltages.

Especially at the base of Q5.

Check Q4.

I checked Q4, it works. After PWR on, the voltage on Q5 base goes quickly from zero to -6V and then slowly up to -21V, like there is capacitor charging.

[SpareRibs]

Hello, I have the exact same amp, as the schematic you posted. Looking at the schematic it does not show any reverb.

[Jazz P Bass]

I checked Q4, it works. After PWR on, the voltage on Q5 base goes quickly from zero to -6V and then slowly up to -21V, like there is capacitor charging.

Change out C21.