EH Deluxe Electric Mistress Repair [Help Identifying Mods]

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PurpleOhmAudio
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Post by PurpleOhmAudio »

Hi Guys, I've got an old EH Deluxe Electric Mistress ( Rev EH 5150-C )

I am about to overhaul and clean up this old rusty unit, it smells, its full of spiders, a few 1980's hairs (my favourite) and loads of sponge/black tape for some reason.....

Needs DPDT Filter Matrix switch for the rear, still thankfully has its Reticon SAD and looks to be all there, so hopefully it will live again

There are a few caps added on the PCB and a few jumpered connections, also a cut trace, can anybody point me in the right direction to a site about EH modding, or suggest what these changes could be doing? I don't want to fit the new switch and turn it on if its going to damage anything!

The schematic is available at Ralf Metzger's Electric Mistress Site
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DELUXEMISTRESSPCBMODS.jpg

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Fender3D
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Post by Fender3D »

I think you'd better check and trace yourself what mods have been made there, instead of wasting time looking for "tutorials" that any idiot with an iron think it's worth to do to a pedal...
Ralf's site is gold: follow his schematic and write down the mods made.
BTW
EH used to tack parts on bottom PCB side...

Good luck with the pedal :horsey:

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PurpleOhmAudio
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Post by PurpleOhmAudio »

Good point, I was going to try and trace it when I get a chance to sit down with it, then mark the mods onto a schematic, it'll help me learn a load :P

The ceramic cap tacked on the bottom does look similar to others from that time, and the soldering looks original,

The other 2 caps look as if they've been soldered on afterwards, dont look quite right, kind of badly tacked on there

I'll get to the bottom of it eventually!

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Scruffie
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Post by Scruffie »

I checked it for you, the additional parts are 1) Extra filtering of the LFO wave (Electro) 2) Small amount of filtering on the input buffer (Ceramic) 3) Small additional filtering on the feedback path (Poly Cap).

The trim pot jumper up top is common to these, the other ones, they could have been factory changes (the circuit had a few revisions) or not... they're not huge changes whatever the case so try them in and out and see how you like it.

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PurpleOhmAudio
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Post by PurpleOhmAudio »

Fender3D wrote:I think you'd better check and trace yourself what mods have been made there,
Scruffie wrote:I checked it for you, the additional parts are 1) Extra filtering of the LFO wave (Electro) 2) Small amount of filtering on the input buffer (Ceramic) 3) Small additional filtering on the feedback path (Poly Cap).
Thanks for that guys :thumbsup When you say filtering, you do mean power filtering/decoupling, not something to affect the tone/audio signal?

I had a good check over it last night and gave it a clean up, the switches turned up in the post today, so the filter matrix switch is sorted, got it turned on and its working :P

Sounds as it should, so I don't think anyone's moved the other trimpots, the gain when switching effect on seems to be balanced nicely, no volume drop

So:-

The gain trim pot has been removed and bridged with a 7k5 Resistor

I'm so slow tracing through a circuit.... but it seems correct, I've tried to mark on the schematic whats been added/removed/jumpered [apologies for the poor editing, only got a basic annotating program here, the red lines are cuts and arrows for connections...]

The 10k Resistor across the Filter Matrix Switch has been removed [seems the corresponding DPDT switch poles have been jumpered to bridge the now missing resistor]

Also the 220nf cap around the 'Range' Pot has been removed

Found another pic online of a modded one, same revision, with the same brand of electro and ceramic caps on there, so maybe modded up the same person?
EH-Deluxe Mistress Mods.gif
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modded mistress-pcbside.jpg

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Scruffie
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Post by Scruffie »

PurpleOhmAudio wrote:
Fender3D wrote:I think you'd better check and trace yourself what mods have been made there,
Scruffie wrote:I checked it for you, the additional parts are 1) Extra filtering of the LFO wave (Electro) 2) Small amount of filtering on the input buffer (Ceramic) 3) Small additional filtering on the feedback path (Poly Cap).
Found another pic online of a modded one, same revision, with the same brand of electro and ceramic caps on there, so maybe modded up the same person?
EH-Deluxe Mistress Mods.gif
Not a likely situation, I'm 100% sure those are factory mods now, I was pretty sure originally but EHX didn't often use the style of poly cap in yours so I wasn't certain but they did often use the film variety in that other photo.

The LFO wave filtering just reduces depth at higher rates, it'd tick without (it's taken the place of the 220nF and does the same job but doesn't vary with range) the ceramic rolls off some extreme highs on the input buffer and with those older chips helps prevent oscillation and the feedback path filtering rolls off some of the regeneration high frequencies and will stop it being too harsh or oscillating.

What's the value of the cap after the 741 by the way? 33n or 68n?

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PurpleOhmAudio
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Post by PurpleOhmAudio »

Interesting, that will give me some reading to do

The cap after the 741 is a 68n,

I had a go on one of these originals nearly 20 years ago recording with a friend who loved Andy Summers and flangers, cant remember quite how it sounded, but I think this is clearer, has a lovely sweep and the manual filter mode is great, different to my others, its quality vintage flange

Great to get it out of the scrap pile and back on the road!

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