Multivox CB1 / Boss CE-1 chorus HELP

[Electron Tornado]

I'm trying to repair an old Multivox CB1 chorus/vibrato. From researching, it turns out that the CB1 was a clone of the Boss CE-1. Here is a copy of the schematic:  http://www.tremolo.pl/Efekty/Chorus/BossCE-1v2.gif . Here is a photo of the CB1:  http://www.modezero.com/multivox-chorus-box.htm

Here is what's going on. The pedal works correctly in vibrato mode. Vibrato depth and speed work, and the rate LED flashes. When switching to chorus mode, however, the chorus intensity does not work, the rate control no longer works, and the rate LED stops flashing.

The schematic shows wave forms for several points in the circuit. Checking points J and K/L on IC4, I get the correct wave forms in vibrato mode, but they both die when switching to chorus mode.

The chorus intensity control and the switch for selecting vibrato or chorus mode both seem to be wired correctly. I also checked to chorus intensity pot out of the circuit and it checks OK.

The big question is what is the difference between what is supposed to happen when switching to chorus mode, and what actually happens. Can anyone suggest something to check?

[Seljer]

I may be just guessing, but I'd check the middle lug on the chorus intensity potentiometer. That and the switch itself. If either those were open it'd stop the LFO from LFOing

[Electron Tornado]

I may be just guessing, but I'd check the middle lug on the chorus intensity potentiometer. That and the switch itself. If either those were open it'd stop the LFO from LFOing

I checked the wiring on the intensity pot and everything is correct. I'll check the effect select switch itself again to be sure. One strange thing I noticed is that turning off the Effect On/Off switch will cause the rate LED to stay on steady bright.

[armdnrdy]

Check all of the resistors associated with the chorus intensity pot and switch.

R57, 65, and 82

[Seljer]

I may be just guessing, but I'd check the middle lug on the chorus intensity potentiometer. That and the switch itself. If either those were open it'd stop the LFO from LFOing

I checked the wiring on the intensity pot and everything is correct. I'll check the effect select switch itself again to be sure. One strange thing I noticed is that turning off the Effect On/Off switch will cause the rate LED to stay on steady bright.

I was referring to the wiper in the pot itself and not just it's wiring. Check the resistance between the terminals of the chorus intensity pot with a multimeter (if you short 1 - 2 or 2 - 3 together you may be able to get it oscillating if the wiper is not making contact) as well as the continuity through the switch.

[armdnrdy]

I also checked to chorus intensity pot out of the circuit and it checks OK.

[Seljer]

ah, missed that.

How about some DC voltage on the two opamp sections? Might help in figuring out why its stuck

[armdnrdy]

How about some DC voltage on the two opamp sections? Might help in figuring out why its stuck

I thought about that...thinking maybe one half of the op amp is toast but...there's this:

Checking points J and K/L on IC4, I get the correct wave forms in vibrato mode, but they both die when switching to chorus mode.

If IC4A was bad, the OP wouldn't have the correct waveform in vibrato mode. There would be no output.

[Electron Tornado]

If I get some time this evening, here's something I think I'll try.

I know that the LFO is working OK in vibrato mode, but it stops oscillating when switching to chorus. That switch is a DPDT, so if I disconnect the wires on the center lugs, I can switch each pole individually and see if that isolates the problem to the part of the circuit connected to that part of the switch.

Regarding the chorus intensity pot - the owner of the unit thought the pot itself was bad, so I took it out of the circuit and tested it and it seems to test good.

[armdnrdy]

As Seljer suggested above, check the switch. You don't have to disconnect any wire to do so. Use your meter to check continuity when switching between poles.

If the switch checks out OK.....check the resistors that I listed. Check continuity from the trace side to the component side to be sure that each resistor leg has a good solder joint. It is very common with older effect to have solder joints fail in time.

Also check resistance for each resistor. (If you lift one leg of R82, it should isolate all of the resistors for accurate measurements)

[Electron Tornado]

I checked the switch for continuity and it checked OK.

Check all of the resistors associated with the chorus intensity pot and switch.

R57, 65, and 82

Actually, I had already checked R81 and R82 by lifting one leg, checking the resistance of each, and returning them to the PCB. I had also already checked continuity from the switch, and each resistor, to the intensity pot, R65 to ground, and R81 and R82 continuity with IC4.

Additionally, the schematic shows the vibe depth pot being 50k, while the unit uses a 100k pot, which looks to be original. Again, that pot works correctly in vibrato mode. Bridging it with another 100k resistor has no effect in chorus mode. (That pot is wired to be a fixed resistance in chorus mode.)

[Seljer]

I'd still switch it to chorus mode and take some DC voltage measurements all around those two opamps. If you know the voltages, combined with the resistances you can quickly figure out where current is (not) flowing when it should be.

The depth pot doesn't really have much influence on the oscillation of the LFO itself.

[armdnrdy]

I'd still switch it to chorus mode and take some DC voltage measurements all around those two opamps. If you know the voltages, combined with the resistances you can quickly figure out where current is (not) flowing when it should be.

The depth pot doesn't really have much influence on the oscillation of the LFO itself.

At this point I agree!

Even though you had "good" readings at the test points, I think you have to start looking for the non obvious.

[Electron Tornado]

I'd still switch it to chorus mode and take some DC voltage measurements all around those two opamps. If you know the voltages, combined with the resistances you can quickly figure out where current is (not) flowing when it should be.

I checked some voltages on IC4. The power supply pins, 4 and 8, were at -14vdc and +14vdc, respectively.

On R76, coming off pin 6, in vibrato mode, the voltage pulsed between ~4.6v - 5.2v. Swtching to chorus mode, however, the pulsing stopped, and the voltage would slowly rise to a steady +13vdc.

Common point between C34 and C35 - 13vdc in both vibrator and chorus modes.

Points I rechecked:
        - chorus intensity pot lugs to PCB. All checked good
        - Point 15 on PCB (one of the end lugs on chorus intensity) to ground - 1.6k ohms
        - Wiper lug on chorus intensity pot to "chorus" pole of switch (right next to chorus pot in schematic) 100k ohms
        - Continuity in chorus/vibrato switch checked good.

[duck_arse]

I see a note about R80 and R81 on the diagram. have you checked the value of R80?

[Electron Tornado]

I did some fiddling this morning and found the problem. An intermittent connection where the wire from one of the lugs of the chorus intensity pot connected to the PCB. What is strange is that I had checked that point more than once for continuity. What happened is that the pressure of a probe on that point when checking it was enough to make the connection good. Remove the probe, and the connection was broken. Re-soldered the wire there, and checked continuity between R82 and the chorus control.

Problem solved, but man, that'll mess with your head.

Thanks to everyone who replied with help and suggestions.

[armdnrdy]

It is very common with older effect to have solder joints fail in time.

Good news! I was out of ideas. 

There are only so many connections and components to check in that chorus switching section.