Neovibe LFO doesn't work - please help

[yeeshkul]

I have a populated Neovibe GGG board. Everything is standard, i just omitted the power source part, because i am building my own supply.
I decided to check out just the LFO itself, because the power supply is not ready yet, i used DC power supply 19v/600mA.
I adjusted the bulb driver trim pot to max as well as Speed control to the middle and Depth the way that the joint H and F are shorted.

Then i connected the circuit ground to 0 and Q11,Q12 collector path to +19V".
Nothing happened the bulb was off, so i measured some voltages, where these are highly strange:

The Darlington pair (all transistors in LFO are 2n3904)

Q11
c: 19V
b: 1.06V (!)
e: 0.64V (!)

Q12
c: 19V
b: 0.64V (!)
e:  0.12V (!)

other voltages
joint of R39 and R40: 11.3V

Bulb driver Q
c: 19V
b: 3.85V
e: 3.25V (trimmer 500 ohm)


transistors are inserted the way they should be, all 1uF caps are film caps (no polarity), i used mag. glas for possible hairlines as well .. nothing like that
by the way the voltages all around the signal path (used the same supply, just 15V to the signal Q's collectors) seem to be correct

[yeeshkul]

here are some pictures

[plankspank]

Double check all the resistor values  in the LFO section. looks like the biasing isn't right, something is saturated..

[R.G.]

Q11
c: 19V
b: 1.06V (!)
e: 0.64V (!)

Q12
c: 19V
b: 0.64V (!)
e:  0.12V (!)

other voltages joint of R39 and R40: 11.3V

You have already solved your own problem. You just don't realize it.

The important bits of evidence in this mystery story are:
(1) The LFO does not work. [therefore the problem is most likely in the LFO or power supply]
(2) The voltages in the LFO darlington transistors are way off. [therefore there is likely some problem with the power supply or the bias]
(3) You have checked for board problems, soldering problems, wiring problems, parts orientation problems. [therefore the problem is either that you can't see the soldering/wiring issue, or that there is a part problem]
(4) There is a normal voltage on one end of R41 (the voltage at R39/R40) and an abnormal voltage on the other end. [therefore R41 is not conducting voltage/current correctly]

Put those together and you have that either R41 is open or dramatically too high a value, or that something is dragging down the transistor-base end of R41.

It is VERY difficult to measure the voltage at the base of Q11 correctly because of the high impedance there. However, it is easy to measure the voltage at Q11 and Q12 emitters as the transistors buffer the base voltage they are fed. So the measured voltage on the base and emitter of Q12 and the emitter voltage of Q11 are probably correct, and the base of Q11 is probably being dragged down by something.

Look there.

[yeeshkul]

Surprisingly R41 seems to work well - i've measured the connections (leg from one side/circuit from the other) and it also shows 4.5k right on the board (influence of the rest in parallel a suppose). Can this behaviour say something about one of the Darlington trannies is damaged? But the base collector drops are there ... so strange.

[yeeshkul]

I re-heated up all the joints, covered all the paths with tin just to be sure and nothing happened .

Well, despite the strange voltages on Q11 and Q12, shouldn't be the bulb glowing anyway? Even without pulses?
(by the way the bulb is allright - i just checked it with a quick 9V battery peak)

[yeeshkul]

no that's nonsense, sorry, the bulb can't glow because the bulb driver is off due to the voltage on Q12 emitter ...

[R.G.]

Surprisingly R41 seems to work well - i've measured the connections (leg from one side/circuit from the other) and it also shows 4.5k right on the board (influence of the rest in parallel a suppose). Can this behaviour say something about one of the Darlington trannies is damaged? But the base collector drops are there ... so strange.

R41 can't be correctly measured while both ends are still soldered into the board. It's 2.2M as I remember. You'll need to unsolder one end to see if it's correct. While it's pulled up, measure the resistance between Q11 base and ground with your meter.

It is possible that one of Q11/Q12 is damaged, but the voltage drops don't give much evidence of it.

[yeeshkul]

hmm... i unsoldered R41 (4k7 to the ground) and it is all right. The voltage of the R40/R41 jumped to 19V while the divider wasn't there. Also Q11 base voltage went up a bit (0.98V -> 1.2V and so did the emittor)I am gonna change both transistors as i have no clue what is going on at the moment.

[yeeshkul]

i changed both trannies(Q11,Q12), nothing has happened

[yeeshkul]

Finally!! i think i've got something. I got absolutely desperate and connected two 9V batteries together just to exclude the power supply from my suspection. And there we go, i measured 8-10V on the emitter

What can be wrong with the source though? Too soft? It was originally a source for a toy railway . It says 12V/0.6A but i measured 19.8V on it (and on the collectors of LFO). When the load was connected, there was still 19.8V on the supply lugs.