Chasm Reverb, Bypass Drain

[italianguy63]

Hey thanks Samhay--

Have determined that the bias voltage is too low overall.  There is not enough voltage to switch the JFETs.

I have another good working pedal here-- I will need to map out all the voltages and I will post.  Need to determine what is drawing the voltage down.  I have checked all the component values, for solder bridges, and the switch is operating correctly.

I have an event today.. So, I will probably get this posted Sunday.  Thanks again!

MC

[duck_arse]

an unhelpful post - to amuse samhay, perhaps. what is the purpose of the 1k resistors parallel to the 100k//diode in the footswitching section? should not the small value resistor get the diode, and the large value go undioded to allow it to both-way?

[samhay]

^I can't remember how this arrangement came to pass.
I suggest things, Rob takes them for a spin around orbit and tweaks a few things, and we rinse and repeat.
There were some fairly exhaustive tweaks of the switching to get it quiet and this may have been the first thing that worked. It probably isn't the best or only option.

[italianguy63]

Not sure this is entirely helpful-- as I don't think it will isolate the problem.  But, it is a place to start.  Need some suggestions as to how to proceed.  Voltages taken with effect (on), controls at midpoints, osc-switch (off).

       Working Pedal     Non-Working Pedal
Bat          9.0V                    9.0V
Va           5.03                    5.03
Vb           4.49                    3.37
Vc           9.00                    6.73
Q1 - S      4.49                    3.37
Q1 - G      4.49                    3.34
Q1 - D      0.20                   -0.5 approx
Q2 - S       4.49                   3.37
Q2 - G       4.0 approx           3.2 approx
Q2 - D      -0.5 approx         -0.5 approx
Q3 - S       9.00                   6.73
Q3 - G       4.49                   3.34
Q3 - D       3.1                     2.0
FS1           9.0                     9.0
FS2           0.2                     0.3   

I don't know if this shows anything other than there is a large load bringing the source voltage down... How to isolate?

I tried pulling both IC's-- the voltage remained low.

R22 is confirmed to be 33R.  Even tested the value with a VOM.

I have double checked all resistor values, and see no solder bridges.

MC

[italianguy63]

If this helps... I currently have IC1, IC2, Q1, and Q2 pulled off the PCB.  The voltage at Vc is still 6.7, and Vb at 3.4... so, that may help make the map smaller.

MC

[italianguy63]

I think I have found it.. probably.

By pulling all the components... I have been able to ohm out continuity across the board and rule things out.

It is not anything on Vb.  Therefore C14, C17, R23, R24, R2, R3, R4, R13, R14, and the related circuits are all good.

Va goes straight to the brick.. so maybe the brick is a brick.

Vc is untested.  I will I guess desolder and socket Q3 next to rule it out.

MC

[italianguy63]

The brick is a brick...

[italianguy63]

The brick is a brick...

And verified... reverb module is bad.

MC

[duck_arse]

too small for a boat anchor?

[deadastronaut]

oh shit!!!!!....thats bad news...

The brick is a brick...

And verified... reverb module is bad.

MC

[italianguy63]

Boo!   

[italianguy63]

VERY UNCOOL update:

Got the new reverb brick in... SAME ISSUE.  (bias voltage drops to about 3.3V)

As soon as the brick is put in the circuit, the voltage gets pulled way down.  WTF?  Any ideas?  I doubt seriously 2 bricks were faulty....

Maybe the voltage reg is dodgy?

MC

[italianguy63]

The issue seems to be when Va is put into play.  With the brick out, everything reads correctly.

I have also checked around R7, R8, R9, R10-- and they all ohm out.  Also checked for any shorts to ground around IC2.  I don't find anything amiss.

I'm seriously wondering if the voltage Reg1 is weak.

[samhay]

When the 5V drops, does the supply (Vc) stay at ~9V, or does it drop to <6-ish V?

[italianguy63]

6 ish..

3.3 ish on the bias/switching

and 4.5 out of the reg.

[duck_arse]

is there any chance that the reg is a) not what you think - or b) in backwards?

[italianguy63]

is there any chance that the reg is a) not what you think - or b) in backwards?

Nope it's right.  And it came from Mouser... and it is in right.  I will replace it and see what happens.  Man, I have been all through this thing....

[italianguy63]

Swapped out the regulator-- with a different batch.  Still the same.  I am out of ideas now.

[samhay]

The regulator won't work (hold 5V) if it is fed with <7-ish V.
Are you sure that R22 = 33R? You can try a quick test by jumpering this resistor with wire, tweezers, etc.

[italianguy63]

The regulator won't work (hold 5V) if it is fed with <7-ish V.
Are you sure that R22 = 33R? You can try a quick test by jumpering this resistor with wire, tweezers, etc.

Absolutely sure... one of the 1st things I checked.