Weird Vox V847 wah problem

[TheVoiceOfSaturng]

I'm trying to fix my buddy's V847 wah (and likely trying to convince him to do some upgrades).  At the moment it is stock, but doesn't work properly. When the effect is engaged it seems as though the frequency sweep is stuck, that is, i can definitely hear some resonance, or Q, so the filter is engaged, but moving the pot does not change the frequency of the filter. I desoldered the pot and checked it with a meter, and even tried another 100k pot just to make sure but had the same results. I also verified that the switch worked and that the jacks were wired properly. I'm thinking the inductor is bad, or maybe the transistors.  Can anyone give me some clues?

Thanks,
Travis

[DiscoVlad]

Read this:

Then read this, and post accordingly, i.e. transistor voltages.

[TheVoiceOfSaturng]

Right, here's the voltage specs on the circuit:

Vox v847
Batt 9.14
nominal circuit voltage without battery connection: 0
Circuit: TJE 020V-0

Q1
C 4.22
B 0.705
E 0.11

Q2
C 8.8
B 3.84
E 3.385

L .705 on all
+ SIDE OF C2 0.705
- SIDE OF C2 0

interestingly, if you plug the instrument into the amp jack and take the output from the inst. jack you do hear some filtering but its nothing like the full signal one would expect. I still suspect the inductor, though I got an audio transformer from radio shack and tried to sub it as a coil to see if i could get some change in filtering and still no effect. It just sounds like it is stuck at a certain frequency.

[DiscoVlad]

If the voltage across the inductor is the same on both sides, it's probably ok - at least, it hasn't gone open circuit.

A quick spice sim using the component values** from RG's "Basic Wah Circuit" schematic, and 9.14V battery gives these voltages:
** Using LTspice's BC337-40 transistor model. Wah circuit only without an input buffer.

Battery
V(vcc):    9.14    voltage

Q1
V(vc1):    4.58814    voltage
V(vb1):    0.648558    voltage
V(ve1):    0.0931284    voltage

Q2
V(vc2):    8.77856    voltage
V(vb2):    4.19361    voltage
V(ve2):    3.62278    voltage

Inductor:
V(vl2):    0.649253    voltage
V(vl1):    0.649253    voltage

These are close to your measured voltages, so the board is probably ok

I would check the state of connections to, and the value of the potentiometer - check that the resistance changes (on both sides of the wiper) as the pot is turned.

edit: oh, you've checked the pot... but have you checked the other end of the wiring harness where it connects to the board?

[TheVoiceOfSaturng]

Thanks for the quick reply.  When I get back to my workbench tomorrow I'm going to go over the pot again and test the rest of the wiring.  The pot itself doesn't have the pins labeled and they are arranged symmetrically, so I tested each combination and perhaps missed something. Is there a way to orient these pots to be sure of what pin is what, or do I just figure it out with my meter?

Thanks for all your help.

[DiscoVlad]

There are several ways of figuring out which pin is which...

Assuming the wiring is correct and unaltered (and the same colours as a Crybaby... I haven't got a Vox) - the wiring harness should read left to right: black, red, black, green, yellow, blue, purple, EMPTY.

The wires on the Pot are:
Blue wire is Pin 3
Yellow wire is Pin 2 (wiper)
Black wire is ground, on Pin 1

a Dunlop Hotpotz II is labelled CCW, SL and CW for pin 1, 2 and 3 respectively.
The blue Bourns pot - or older "open" style potentiometers, are pins 1-3 going left to right looking down onto the shaft with the terminals along the bottom.

Using an Ohmmeter - and with the potentiometer set around halfway, measure the resistance between each pair of pins:
One pair will give you a reading around 100k - these are the ends of the track, meaning the other pin is the wiper.
Turn the knob fully clockwise and measure between the wiper, and each pin. Pin 1 will read 100k, pin 3 will read 0 (or close to it).

If these are drastically different, the pot is probably bad.
If it appears ok, and measuring the resistance at the wiper smoothly changes from 0 to 100k as the knob is turned, the pot is ok.

In this case check the switch. Off the top of my head and with the standard SPDT switch, the green wire is coming from the input jack/buffer (with board mounted jacks, if they're off-board, it's easier to trace) - i.e. clean signal, and the purple wire goes to the output jack (this could be the other way around... CHECK!) The output jack is connected to the centre pole of the switch, with the blue wire coming from Pin 3 of the pot. check with an ohmmeter that the resistance is 0 ohm between the output jack, and the other two switch poles as the switch is toggled.

Edit:

Oh... One more thing!
You haven't said if the output volume is different, and I haven't asked between the effect, and bypass signal. They should be pretty close.

If the volume is the same... Maybe there's a bad joint around the 22nF coupling capacitors (C4, C5). In spice simulation, removing C5 (this could also be a bad joint at the pot wiper, or the yellow wire going to it is broken) will stop the pot from sweeping the filter.

[TheVoiceOfSaturng]

Okay, so I checked again and the pot is good.  tested it with my meter completely removed from the circuit. I also check the harness and the jacks - no problem.  There's no signal loss when the effect is switched on, its just like the filter is stuck at a certain frequency.  FYI i'm testing this with a vco from my modular synth, the level of which i can control (so as not to saturate the circuit too much). not sure if that makes a difference.  I also went over the whole circuit board and heated up all the solder joints.  I also bypassed the stomp switch and directly wired the effect on so as to eliminate that.  Next I guess I should start replacing caps? I tried the electrolytic and replaced it with a 1uf and no change (not the right value, i know but its what i had on hand the other day).  I also have one of the standard radioshack audio transformers (the little guys) that i read can be used as a coil. Would that be a 1:1 test to plug into where the inductor is or would it require component value changing to see if there's any effect?

[joegagan]

did you test continuity oin the pot leads?

[DiscoVlad]

If you're feeding a wah with a fixed frequency (presuming the output of your oscillator is a sine wave), you'll only get a volume change as the pot is turned.

[TheVoiceOfSaturng]

I did check continuity on the pot leads and they had good connections to the board or jack depending on the lead. I have been sweeping the oscillator (square or ramp) with an LFO just to be sure that it wasn't a fixed frequency issue.

[DiscoVlad]

Well that's me out of ideas... If you've got the components: breadboard a v847 up and compare?

[Carlos Best]

Hey ThevoiceofSaturn
Did you managed to fix this problem?
I'm in the same situation here.
Replaced the 4.7uf cap, measured the pot disconected from the pcb and everythings seems to work fine.
But I only have the wha fixed, more like it is always at the heel position.
Let me know if I can try something I didn't.
regards!

[TheVoiceOfSaturng]

Hey Carlos - your problem sounds more like something in the circuit itself - perhaps the coil has gone bad?