EH 7850 Troubleshooting Tips

TROUBLESHOOTING TIPS

-Always check the power supply section first. This circuit has an unusual -15V supply.

-Check those pot legs! The long PC mount legs will often break. Unusual behavior is often a broken pot.

-Check those electrolytic caps. Check for unusual dc offsets. A leaky cap is usually to blame.

-High pitched whine? Check notes in BBD CLOCK section.

-No effect? Trying to tell if BBD chip is bad? Verify BBD clock is working, then make sure BBD chip is seating in socket good, then probe the IC PINS (NOT the socket pins) to verify that the chip is receiving 2 antiphase CLOCK signals and POWER (-15V, 0V on appropriate pins). Next, scope the outputs. The clock signal bleeds into the output and appears as a block of noise if your horz. sweep is set to audio freq. If you have full on clock signal, that means a MOSFET has shorted and the chip is dead. If you have nothing on the output pin(s), then something is open and the chip is dead. You could also have no BIAS on the input. Read the theory below for more info.

BBD Theory Light

The BBD chip is really just a LOT of MOSFETS lined up in a row. They are arranged just like any FET audio switch (see how a BOSS bypass works, for instance) where the audio passes from drain to source when the gate is ON and the audio is blocked when gate is OFF.

In addition to the FET switches, there are capacitors between each FET to “sample and hold” the audio voltage. So it’s not just MOSFETS. Updating the first statement, now we have: the BBD chip is a LOT of sample/hold circuits lined up in a row.

In order for this train of S/H circuits to reproduce audio, the sampling has to be really fast. It must be sampling faster than audio (~20kHz is the fastest audio) or else it isn’t going to work correctly (see the CLOCK check section). All BBDs need some kind of clock. The clock is attached to the gates of the MOSFETs, providing the necessary ON/OFF signals.

Early BBD chips only had one clock. You may have noticed that all the BBDs you encounter have 2 clocks, and if you’re really observant, the 2 clocks are 180 deg. out of phase. Why did BBD manufacturers end up with the 2 clock system? They were unable to prevent the clock from getting mixed up with the audio output. Even though it is ultrasonic, it still distorts the audio and requires heavy low pass filtering, which ends up reducing the quality of the delayed signal. The best solution was to use 2 out-of-phase clocks, and 2 in-phase signals, and then sum the outputs together at the end. In theory, the summed in-phase signals should get a little boost, and the summed out-of-phase clocks should perfectly cancel out. In reality, you can only cancel the clocks so much, and it’s rarely great, but often “good enough.”

The cancel effect is usually achieved with a trimpot. The CW and CCW ends of the pot are connected to OUT 1 and OUT 2, and the summed output is the trim’s wiper. Any time you see this trim (easy to identify), you can always just observe the wiper terminal and adjust for maximum cancellation. But this adjust is often the last adjustment, so if it doesn’t seem to be right you may need further adjustments “upstream.”

The other required trim is the BIAS trimmers. The BIAS trimmers can be identified as coming from the power supply and delivering a “bias voltage” to the inputs of the BBD chips. There is often 1 bias trim per input, although multiple inputs can share bias voltages.

The BIAS trim is adjusted by monitoring the output of the input you are adjusting. Adjust for best looking signal, i.e. good/max amplitude without distortion, and double check this throughout the sweep of the DELAY TIME panel control. There is usually only a very fine section of the trim pot that will allow the BBD to function. Moving too far CW or CCW will turn off the BBD entirely. Sometimes a “dead” chip simply has a wrong or missing bias voltage on the input.

IDENTIFYING TRIM POTS

When trying to identify the trim pots, the easiest ones to spot are the output summing trims. Next, try to identify bias pots which will have power supply and BBD input connections.

Beyond those types, there will be simple audio gain trims in the audio path. These will usually control the gain of an op amp (look in the negative feedback path) or attenuate a signal somewhere (CCW will be grounded, CW and wiper will be input and output, doesn’t matter which.)

FEEDBACK ADJUSTMENTS

The FEEDBACK control of any DELAY pedal should provide a smooth transition from 1 echo (full CCW) to a moderate amount of echoes at 50% to self oscillation at CW. I like self oscillation to begin around 75%, and I will usually adjust the circuit if oscillation comes on unusually early, late, or never at all.

Outside of the panel control, there is rarely a FEEDBACK trim, although some pedals will have this. Start will the FEEDBACK panel control, and trace it from output of delay circuit to input of delay circuit. Along the way, if no trim is found, you will almost certainly find a series resistor that you can remove and replace with a lower value for more/quicker-onset oscillation, or a larger value for less/slower-onset oscillation.

CHORUS/VIBRATO CIRCUIT TIPS

If a delay pedal has chorus or vibrato, this is just a LFO (low frequency oscillator) modulating the CLOCK signal. The CLOCK is ultrasonic in that it is always faster than audio (above 20kHz). The LFO is subsonic (below 20Hz). LFOs in general are usually less than 10Hz down to around 1Hz (1 second per modulation). LFOs can go beyond this, with periods of many seconds or minutes, up to audio range - although neither of those are too popular. Most delays will use the typical 1-10Hz range.

The modulation waveform is most often a triangle wave, and I will look for a 2 op amp triangle function generator. This circuit uses 1 op amp as a comparator, and another as an integrator. Comparator tell-tale sign is a bit of positive feedback, negative terminal grounded, and the integrator will have a capacitor in the negative feedback loop. The triangle wave comes from the integrators output. A square wave output of the same frequency is available at the output of the comparator, but will generally not be used for delay modulation.

The integrator capacitor is the most obvious thing to change if you want to adjust the range of your triangle wave.

The actual panel mounted “CHORUS” or “MOD DEPTH” control will most often just be a simple attenuator that feeds the LFO circuit to the CLOCK circuit. If troubleshooting a “no chorus” problem, check for the existence of the LFO at the actual oscillator first, restore that if necessary, and then proceed to the panel control which is probably just an attenuator (“volume control”) for the LFO, and finally check out where the LFO and the CLOCK are mixed.

The EH Memory Man CHORUS/VIBRATO selector switch just chooses two different rates (SLOW/FAST) for the LFO to run at, since there is no dedicated RATE control. To add such a control, check out the triangle function generators in PHASER pedals (try Phase 90), as these almost always use the 2 op amp circuit with a variable rate control, usually a 500k or 1M reverse audio pot.