vintage EHX Electric Mistress very strange issue

[beatnik]

Hi everybody

I am having a very weird problem with my Electro Harmonix Electric Mistress (vintage SAD1024 version)

The pedal is not working properly, only the clean guitar signal can be heard, with a slight distortion and lfo ticking in the background. If i disconnect the input jack and reconnect it (this activates the power supply from the batteries) the effect sound briefly comes on, but fades away after a few seconds

The strange thing is, if I connect the pedal to a signal generator, it works fine with absolutely no issues !

I am really baffled with this issue. I have never experienced anything like this

Obviously I have checked all the usual suspect. I have tried fresh batteries, and also an external power supply, double checked all connections. Not long ago I have replaced the electrolytic capacitors

I am wondering this has something to do with the input load impedance. The signal generator has a 600ohm impedance so that is the only culprit I have

Maybe the input opamp 4558 has some issues and it won't work properly with the guitar load impedance ?

I am very keen to hear any possible explanation of this weird issue, and hopefully getting some suggestions on what to repair

This is a schematic of the pedal

http://www.metzgerralf.de/elekt/stomp/mistress/images/1978-electric-mistress-v3-schematic.gif

[Mark Hammer]

ALL bucket-brigade chips require a DC bias voltage for the signal to ride in on.  The bias voltage is derived from the supply voltage, and is a divided-down version of V+.  If the bias is way off, whether high or low, the BBD fails to pass signal.  If the bias is just a bit off, it will pass signal, but not as cleanly.

The fact that you get audible ticking means the LFO is fine.  The fact that you initially get delay signal means the 4013 flip-flop is working fine, as is the BBD itself.  However, the fact that yo only get delay signal when power is initially applied suggests something amiss with the biasing arrangement.

My first hunch is that one of the two 10uf caps you see in the lower left-hand corner of the schematic are somehow off spec, such that it initially charges up but bleeds/leaks; an initial attainment of proper bias voltage is reached but then fails.

If your initial power-up delay signal sounds fine, make sure you don't monkey with VR1.

[beatnik]

thanks for your response Mark

I have a strong effect signal with no distortion when I connect the pedal to an audio measurement system

signal generator --> guitar pedal --> level meter + monitor speaker

Also the fx signal when the guitar jack is plugged in is strong and sounds good

I won't touch VR1 as I believe the bias should be in the right setting

I will replace again the two 10uF caps and see if that makes a difference. I also noticed that the 10uF cap in parallel with the 6,8V zener diode is actually marked as a tantalum type on the schematic. I am sure that on my unit that was originally an electrolytic cap, but I am wondering if this can actually be a problem ? I will put in a tantalum just to be sure

[beatnik]

Actually I have re adjusted the bias setting and everything works fine

Thanks

[Mark Hammer]

Excellent!  Thanks for the feedback.