DOD FX45 Reverb problem

[The Groke]

Hi everyone,

I have a problem with my DOD FX45 "reverb" pedal - or at least I think it is a problem (maybe they just sound this way).

I have posted a copy of the schematic, appended with some of my notes regarding the function (as far as I can tell) of different sections, to help inform your replies (should there be any)

http://www-personal.umich.edu/~damont/DODFX45SchemNotes.pdf

The pedal is some sort of mutli tap delay based around two MN3011 ICs. The signal goes through an anti-aliasing (I think) filter then goes into the bucket brigades (3011) which are driven by a clock (MN3101 in the top right). The output is filtered and then sent back in a loop to the BBD (correct me if I am wrong on any of this).

Anyways, the problem is that I get a low drone over any note I play into this thing.

Two points:

1) The presence of the drone is dependent on the audio signal. It is only there when there is tonal audio input.

2) The tone of this drone is independent of the pitch of my input signal. I can modulate the drone pitch by turning the Room Size knob. A smaller Room Size gives a higher drone; a longer Room Size gives a lower drone.

I have recorded a short sample of the problematic drone here:

http://www-personal.umich.edu/~damont/FX45Issue.mp3

The sample is a Juno 60 arpeggiating a few short decay notes into the effect. Over the arpeggiating notes, which are constant, you will hear a rising and falling tone/drone. This is the problematic drone.

You can hear evidence of the first point (1) when I shorten the decay to almost nothing. As the arpeggiating notes lose tonality, the drone also starts to sputter out, and then goes away.

You can also hear evidence of (2) in that I can modulate the pitch of the drone without affecting the arpegiating notes (and the reverse holds true as well).

Any thoughts on what might be wrong or where I should start to debug this? I have checked and the BBDs are outputing taps at all 12 points (between the two).  This is a mint, NOS pedal from the 80s. Really shouldn't be anything wrong. There is also some possibility that this pedal just does this... not sure. they are pretty rare and I have nothing to compare it to except one demo on youtube.

I supose the most suspect element would be the low pass filter after the BBD... but does this drone actually sound like clock noise?  Would it make sense that this could be a problem with aliasing (and thus the filter before the BBDs)?

I should also mention that the transistors in the post-BBD LPF are marked 608 P N 100A (not 2N5088 as on the schematic). I have no idea what these are... I am assuming that they are something comparable.  I tried to do a diode test on them in circuit, but it wasn't really working out (I got V readings between collector and emitter on all of them).

Thanks for reading this... I hope someone can help...

best - DT

[ashcat_lt]

Aliasing would be dependent on input frequency.  It's a lot like ring modulation.

From your description I thought it would be that thing which happens with very short delays.  The beginning of each repeat is a lot like a zero crossing in a waveform.  If these come at a frequency that is within the audible spectrum...  I always think of it as the "20 Mike Mike" effect because the old GIJoe comics used to describe the way a 20mm machine gun fires so rapidly as to sound like a constant tone, rather than individual shots.

Course, listening to it makes me question that assessment some, cause it doesn't really sound exactly like the pitched ringing/stuck flanger sound which I expected.  Even if that is the issue, I couldn't tell you how to fix it.

[PRR]

> modulate the drone pitch by turning the Room Size

I would look around the feedback loop, including the low-pass filters. Start with finding bad solder joints, then bas lo-pass caps.

[The Groke]

Thank you both for your replies. It is useful to have confirmed that aliasing is probably not the issue.  I will start with the LPF that comes after the BBD and will check the feedback loop. I'll be sure to post what I find.

Thanks!  DT

[The Groke]

All solder joints seem fine in the loop. I replaced two electrolytics with off-values (a 330uF and the 47uF). I also replaced the two LPF transistors with 5088s.  Neither of these measures affected the drone in the least bit. I have not tested the smaller valued ceramic and bypass caps in the LPF.. once I get some, I will just replace them, since I need to take them out to test anyways....

However... I was just told by someone else who has one of these that this drone is normal for the pedal. I still find this somehow hard to believe, as it basically makes it useless (adds a dissonant note to everything I play). Does anyone else out there have one who can confirm this?

Or maybe it really is just a low point in DOD's history...

Any other suggestions as to what this noise might be caused from?

Thanks - DT