View Full Version : Anybody know what the trimpots inside an Electric Mistress are for?
08-24-2005, 03:20 PM
I got an original Electric Mistress filter/flanger a few days ago ... I love the damn thing, but when I opened it up to put fresh batteries in it I noticed two trim pots. Anybody know what these are for?
Don't touch it it works fine.
The one to the left is bias control ( if this is advandced internal regulator that gives 12 V's to the circuit from 18 V may havoc and this is major fault in this circuit- there is hope. Johanna, you just replace with a better regulator.
The one to the right is feedback limit and this may cause unit to self oscillate and in conjunction with the first control you have a syntheziser just prior to the death of the circuit.
You would notice that this the right control sets the limit of how far the feedback control can travel.
I tell you this as I have restored several EH electric mistresses' after having had their trimmers tweaked.
At your service
"Don't do the crime if you can't do the time, don't do it" the Baretta theme
08-25-2005, 10:01 AM
Thanks man. A couple more questions, if you don't mind, since you know the circuit ... I tried running it with an isolated, regulated 9VDC adaptor (ADL Juicebox) and it worked fine. However I'm confused since it takes two 9V batteries ... should I use a different voltage adapter? Am I missing something?
Also, is there anything that can be done about the volume drop when you turn on the circuit?
08-26-2005, 10:08 AM
Well, it may work from 9V@s but not quite aswell as it would at 12V.
EH put two 9V batteries in to give 18 volt when fresh and something like 15V when worned down. Now circuit requires 12V to work properly so they put in a regulator to derive 12 Volts from the varying supply as above and then batterylife is considered realistic.
The EH Electric Mistress utilizes a bucket brigade chip, of the first generation, to make slight delay through which the effect of flanging is realized. Now these chips, of the first generation, really require 12V but not much higher than 14V so power must be stable aswell and the answer to this as in many flangers and delays from that time is to use two batteries and a regulator so that circuit sees 12V more or less regardless of battery condition.
One of the effects of too low voltage is loss of amplification through the chip.
Measure voltage at pin 8 of the 4558 chip which is downstream from regulator- thiis point should be dead on 12V for the circuit to work optimally.
I don't know if the ADL has this but the VL PP-2 has an option for making some outputs 12,7 V.
You can change C7 from 47nF to 10uF and that will make up for bassloss in the direct signal part of circuit.
There should be no other causes for loss of power in the circuit.
In the standard issue unit is not truebypass and inputof circuit is tied to input and thus about 100K is draped across inputline which may cause losses if there is no buffer ahead.
At your service
08-26-2005, 04:16 PM
Thanks Bjorn! I'll probably have the switch changed for true bypass one of these days ... the only buffer ahead of the EM is from a Boss DD-3.
The ADL Juicebox has one isolated output that can be set @ 13.5V ... would that be better or worse than to run it @9V?
Would using a Y cable from a 9V output, and connecting each end to one battery connection on the EM using those clip adaptors be an option?
Hope you have a chance to reply.
btw, I've yet to try one of your pedals but both the Sea Blue EQ and KOB seem like they could be sometime in my future.
Well, try 13,5V first.
Supply limit to the internal CD4013 is 15V and the SAD1024 17V, both work fine at 12V. Regulator should handle 13,5 and make it close to 12V.
If possible measure the voltage at pin 8 of RC4558 and if there is close to 12V all is OK.
The C7 cap is not all that critical if you only use it for guitar, though it is noticable.
The BOSS should be sufficient buffer and would drive down to 10K without problems.
If you have the inverted batteryclips that sometimes come with supplies like this and the outputs are isolated each output could mimic a 9V battery and allow series hook up as it is in the EH mistress.
The internal 12V regulator also provides a floating midpoint and that is regulated too at 6 Volts so if regulator sees much less than 12V midpoint will be asymmetrical which causes loss of power.
Ah, well you should get the KOB first as distortion is always fun- EQ is for refinement and making distortion right.
08-27-2005, 08:01 PM
08-27-2005, 08:09 PM
i have a new electric mistress, they dont take batteries, do they?
i never opened mine up.
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