Got a vintage Electric Mistress that distorts. Easy fix, or a return?

[Delicieuxz]

I just got a vintage Electric Mistress, a 9v version (V6), so I could compare it to my 18v version (V5) and see which I like more. The seller said the 9v version was basically working all good. But when I use it, I get distortion from pretty much all settings.

Here are a few videos showing the issue. The first video, from further back in the room, doesn't pick the distortion up the best. But it's a lot more audible in the 2nd and 3rd videos, where I placed the camera directly in front of the speaker.

In your assessment, do the pedal's trimpots just needs a calibration with an oscilloscope, does it need some caps replaced, something else... and should I take it to a tech or return it? I got a good deal on it. So, if it's a guaranteed cheap fix, it's probably best that I don't return it. I'm capable of replacing caps and could play with the trimpots by ear.


The Mistress Mystery Page has detailed instructions on how to troubleshoot and calibrate an EM. But I don't have an oscilloscope to do the calibration myself, and I'm not a very experienced troubleshooter and tech, either.

http://www.metzgerralf.de/elekt/stomp/mistress/align.shtml#3
http://www.metzgerralf.de/elekt/stomp/mistress/repair.shtml#ts6

[Delicieuxz]

I've found that the 9v (V6) also passes a weaker and less bassy signal, both while engaged and bypassed, than the 18v (V5).

[anotherjim]

There's a lot you can do without a 'scope and I wouldn't just go changing parts or wiggling trimmers without doing a voltage check first.
There's this schematic...

I'm not sure I trust it 100%. 2k7 in series with the battery is something odd.
Anyway, I start by checking the supply voltages are ok as there are a few things that could have been damaged with the wrong DC power adapter in its past life.
If you don't know what the voltages should be you can measure every pin on the IC's and post the results.
See the Debugging thread sticky.

[Cybercow]

In my experience with those old SAD1024 and other EHX Mistresses, that actually sound quite normal. Have you much experience with the Electric Mistress or the Deluxe version? Please describe to what the 'distortion' is attributed.

[idy]

What the Cow said. They are not modern hi-fi effects. How hot a signal? These things sound better to me in a blend loop.

[Delicieuxz]

In my experience with those old SAD1024 and other EHX Mistresses, that actually sound quite normal. Have you much experience with the Electric Mistress or the Deluxe version? Please describe to what the 'distortion' is attributed.

I have a V5 18v Electric Mistress, which I've had for maybe 3 years. It doesn't distort at all. It also passes a louder signal both in bypass and engaged mode, compared to my newly-purchased V6 9v EM.

My friend has a V2 18v EM and a V6 9v EM, and they don't distort. That friend also uses the same guitar pickups that I do.

I've tweaked my amp settings (lowered gain, presence, and treble) and severely compressed the sound following the EM to ensure the distortion isn't a product of the amp, and in doing so have isolated its source to the V6 9v EM. The distortion comes easily while using the Red Lace Sensor bridge pickup in my Strat, or when playing my Les Paul which has humbuckers.

[DrAlx]

The lower the supply voltage on a BBD, the lower the headroom.
So the BBD in the V6 circuit (that has BBD supply of 9V) will have less headroom than the BBD in the V5 circuit (which has BBD supply of around 12V-13V).
If you look at the 9V circuit schematic you see a pair of 130k resistors at the input op-amp.
http://www.metzgerralf.de/elekt/stomp/mistress/images/1981-electric-mistress-v6-schematic.gif

That means the input signal voltage is scaled by 130/260 = 50% at the input, and that is because the BBD has reduced headroom in that circuit.  So signal is scaled down deliberately in the V6 to keep it within the headroom of the BBD.

In the V5 schematic, the resistors near the input opamp are 5k6 and 100k, so the signal voltage is scaled by only  100/105.6  = 95% at the input.  So we are not losing half the signal as in the V6 circuit.
http://www.metzgerralf.de/elekt/stomp/mistress/images/1980-electric-mistress-v5-schematic.gif

This explains why the V6 lets through less signal than the V5.  There are other changes in capacitor values elsewhere that means the V6 will let through more bass and low-mids, and that partly compensates for the signal loss.

Your problem is most probably down to the BBD bias not being set correctly.  You can adjust it by ear.

1) Put in filter matrix mode.
2) Put color pot at minimum
3) Put range pot at maximum
4) Adjust bias trimpot.  It should only need **small** fraction of a turn.  If bias is set too low or too high, you will first get clipping (signal is hitting either top or bottom headroom limit), and then nothing at all through the BBD.  The BBD only lets signals through for a small range of input voltages, and you are trying to set the bias voltage near the middle of that range.

Before doing any of this, you should measure the supply voltage of the circuit first to confirm it is at 9V.  If you are running from a battery or a supply that is giving less than 9V to the circuit, then the BBD bias may not be correct (remember correct bias depends on supply voltage) but does not necessarily scale with it.  For example, bias required for 9V BBD supply is not necessarily 3/4 of the bias requried for a 12V BBD supply.