preamp section not working on LABSERIES L3

[ifys]

hello there,

i d like to thanks all the people on this thread
http://music-electronics-forum.com/t24847/#post211757

i own a labseries l3 and preamp section doesn t seems to work
the only sound i get from the amp is by plugging a jack half way into the EXT PROC input
i tried to plug an instrument cable all the way into the power section of another amp but nothing came out.

By the way the knobs have no effect whatsoever even the MASTER
on the other thread i m refering BILL says that it has CMOS preamp section maybe that what it is ?


thank in advance for your help

[Jazz P Bass]

Lab Series L3 schematic

The preamp runs on -15Vdc.
It derives that from a raw -19.3Vdc.
Schematic Page 2, top middle of print you will see a designation for the -19.3 V.
Check across header pin 9 & 8 to see that it is there.
If it is then jump to lower left of page 1 schematic & check that you have -19 v across C7.
If so, then check the output pin 1 of A1A iC.
It should read -15 Vdc.

[ifys]

The preamp runs on -15Vdc.
It derives that from a raw -19.3Vdc.
Schematic Page 2, top middle of print you will see a designation for the -19.3 V.
Check across header pin 9 & 8 to see that it is there.
If it is then jump to lower left of page 1 schematic & check that you have -19 v across C7.
If so, then check the output pin 1 of A1A iC.
It should read -15 Vdc.

thanks Jazz P Bass
i was able check everything you say and so far so good
everything is OK
what s next ??

[Jazz P Bass]

Send a signal through & see where it's failing.

[ifys]

i tried but not sure it s the proper way
it seems to me that RESISTOR 18 fails

by the way i did check J4 jack as BILL says in the previous thread
i desoldered it and put it back and it s not defective.

[Jazz P Bass]

WTH is a resistor 18.
I am not going through each schematic looking for it.
Rant over:

In which way did you try to signal check the amp?
Do you have a signal source?
An o scope?
A DVM will do.
You know the power amp works.
So 'Divide to Conquer' the problem.
Is the signal at the jack input?
Is the signal at J4?
Start & end point.
If not at J4, use the schematic & divide the preamp in half & test that point.
See where this goes?
Keep dividing until you have the circuit (if not the component) at fault.

[ifys]

well i meant r18 82k on SCH1 on the top corner left

when you asked me to check the value
it's right close to it
i plugged my guitar into the HIGH input and R7 and R8 ,on the bottom still on SCH1,and it works the next resistor on the board is r18 and it fails there
i don t own a o scope but i have a DVM

i can hear my guitar thru the amp using j4 input with jack half way plug
i should start there then instead of j1 and j2

[Jazz P Bass]

If you want to use your DVM to signal trace the preamp, set it to read Volts ac.
Connect the black lead to the chassis & use the red lead as your probe.

The signal is failing between the input jacks & J4.
To get a feel for the expected readings clip the red lead on the left side of R2 & play a chord. (input on J1)
That is the guitar output voltage. (50 mv's)
Now work your way down the circuit.
The first thing that will kill the signal is Q1.
Do you have signal on the gate pin?
Is there a negative dc voltage on the gate?
Is the signal on the drain & source pins?
If that is good put all the tone controls full on (CW) & see if the signal is on each wiper leg (middle pin)

[52 Bill]

You can work either way from the power amp forward or from the input jacks backwards. The end result should be the same, you find out where the signal stops.

You know that the power amp is working, and that the -15vdc power supply is working, correct?

If you turn up the master volume control and turn up the reverb control and then shake the reverb tank do you hear anything from the speaker? Do you hear any hiss and noise through the speaker? If you do hear hiss, does it sound different when you change the tone control settings?

[Steve L]

L3 Service Manual

Here is a little more information that might help.

Lab Series L-3 Service Manual.pdf

Steve

[Jazz P Bass]

The reason for using a DVM set to read Vac is the simple fact that the guitar signal IS an ac signal.
The amplifier 'amplifies' & shapes that ac signal.
You use Vdc setting to check the quiescent state of individual components in a circuit.
As to replacing all of those parts, can you stand back & look at that statement?
What are the chances of all of those parts being faulty?
Or is your test procedure at fault?

[Jazz P Bass]

Nice manual.
Thanks for the post.

[g1]

Resistors measured in circuit will often measure low. It does not mean they are bad. To get an exact reading at least one end must be disconnected from the circuit.
However, statistically speaking, the problem is not likely a bad resistor, solid state components (transistors, IC's) are much more likely to fail.