EHX Frequency Analyzer (Reissue)

[pringe]

Hi all, I'm trying to repair an Electro Harmonix Frequency Analyzer (the more recent model) that has suddenly stopped ring modding. It is the version that has an LM13700, a TL072, 3 x 4558 and an AD633. I haven't managed to find a schematic for this version anywhere (is there one floating around?) but have been able to roughly trace the signal. When engaged the pedal will only pass dry signal, and turning the blend fully wet only lowers the input signal. Scoping the AD633 I can't see a carrier signal coming in anywhere, and also the supply voltages to this chip seem missing (although are all present on the op amps). Does anyone know what rails the AD633 should be operating on? Also am I right in thinking the LM13700 is being used here as a dual oscillator? I don't have much experience working with OTAs or multiplier ICs so am having problems troubleshooting this, not to mention the lack of a schematic! Any advice would be greatly received, thanks.

Steve

[Scruffie]

Hold up, an LM13700, not an ICL8038? I take it this is an XO model then? They must have changed the design if that's the case and no, there's no schematic for that.

In which case, the LM13700 and at least one of the 4558 must be making up the Sine Wave VCO.

The AD633 should be running on 30V single supply.

[pringe]

Thanks - I'm not sure of the model, I've attached images of the case and both sides of the PCB.

Voltages to the AD633 are:

pin 6 V+ 16.63
pin 3 V- 16.44

with the chip removed the voltages present are similar in relation but approx 200mV higher (i.e. ~16.8v / ~16.6v)

the input signal is present on pin 1.

am I right in thinking that the carrier should be present on pin 7, and the multiplied voltage output on pin 5?

[Scruffie]

That's a substantial redesign from the big box reissue.

What's U7?

Those voltages would be fine on the AD633... except those pins are the supply pins for the SOIC model, not the DIP.

Voltages on the op amps would probably be helpful.

[pringe]

U7 is an LM317.

Ahh that makes more sense...Mouser pointed me towards the datasheet and I didn't think to check if it specified the PDIP version. Just found the correct datasheet and the voltages make much more sense! All seems to be OK with that chip, it's getting a 30v single supply and all other pins biased to approx 16.7v.

I'll take a note of the op amp voltages and report back, pretty sure all the supply voltages checked out OK and I was able to trace the input signal through both op amps of U1, IIRC it left the second op amp with a gain of approx 16-17x and from there looked like it went to the AD633.

[Scruffie]

From your description I'm guessing the VCO isn't working, from the chips available, I guess it's a design similar to this


If you could trace that section out (or post some detailed macro photo shots of the area) it'd be great to see what they've used.

[pringe]

Yeah that's what I'm thinking, I can't see the carrier anywhere either on the AD633 or on any opamps.

Thanks for the schematic - in the ehx ring mod circuit I know it is supposed to generate two tones of equal distance away from the source, so does it take a single sine wave carrier for one and use an inverted copy of the carrier for the other? (this is a bit of a guess as I haven't delved a huge amount into the maths behind ring mods)

I'll see what I can trace out from the circuit!

[Scruffie]

Guitar signal to one input, sine wave to the other, sum & difference out.

[pringe]

Ah so it's just a standard ring mod circuit? EHX's description of it having a tone underneath and a tone above the inputted signal made me think it generated two modulation versions of the dry signal. So sum & difference results in this sound/psychoacoustic effect?

[bartimaeus]

Sum is a higher frequency. Difference is a lower frequency.

[pringe]

Ah OK thanks!