1980s Stereo Memory Man Chorus Issue

[GuitarFreak302]

I have an Electro Harmonix Stereo Memory Man from the 80s and it seems to have an issue with the chorus. This is the early version without the LED. The delay/echo works totally fine on this but for some reason the chorus sounds weird. It has a warble to it like you can hear the oscillator in the background. I thought this was normal at first. I didnt have use for this so I sold it but the buyer said it wasn't working right so he returned it to me. I replaced all of the electrolytic caps, some film caps and I also replaced the LFO IC. I am completely stumped right now as to why the delay would work properly and the chorus doesn't. It seems to me the only difference is the chorus bypasses the delay pot and might add a component or two. I am attaching the schematic so if I am wrong in my theory I can be corrected by someone. This schematic isn't the same version as mine but it is very close except the LED. Your help would be appreciated.

Thanks,

Ryan

[Govmnt_Lacky]

Test the Chorus/Echo switch. Make sure that BOTH sides of the DPDT are switching correctly.

[GuitarFreak302]

I think I checked that at some point but I will check it again.

[GuitarFreak302]

I checked the switch and it is fine.

[Scruffie]

I don't quite follow your description of the problem, are we talking a tick? a lopsided wave? whine?

I would take voltages of everything first though.

[GuitarFreak302]

It would be easier to explain if I could post a sound clip but I don't know if that would be possible on this forum. I will get the voltages when I get a chance.

[GuitarFreak302]

Here are the voltages on the 4558 ICs which seem wrong to me since this works great in echo mode. I am measuring these from the ground of the voltage regulator to the pin on the IC.


IC 1: 1) 0.001
        2).006
        3).006
        4)-14.95
        5)0
        6)0
        7)-.001
        8)14.79

IC 2: 1).014
        2).008
        3).007
        4)-14.96
        5).026
        6).03
        7).04
        8)14.79

IC 3: 1)0
        2)-0.001
        3)0
        4)-14.97
        5)0
        6).001
        7).001
        8)14.79

IC 4: 1)6-8v oscillating
        2)7.38
        3)7.38
        4)0
        5)6-8v oscillating
        6)7.38
        7)flashing OL on meter
        8)14.71

[Scruffie]

They look fine to me, what looks wrong to you?

[GuitarFreak302]

If you think they are fine than I am good with that. I am used to seeing voltages higher than that on other pedals so I thought it was odd but I never measured voltages on one of these before so that makes sense.

[Scruffie]

It's a bipolar supply for everything but the LFO opamp so they're getting referenced to ground rather than half supply, hence the low voltage readings.

This might be a tricky one to debug without a scope, you know your LFO is running and the chorus is working so an odd sound is hard to diagnose.

Could you upload a sound clip to soundcloud or whatever your preferred host might be?

[GuitarFreak302]

I actually have a scope so I would be able to diagnose it that way but I am not sure what the output should look like.

[Scruffie]

Well to start, triangle wave on the LFO output (pin 1) and square wave on the 4047 pin 10 & 11.

[ElectricDruid]

They really were cutting it fine on that 16V 10uF cap on a 15V supply. That's a smaller margin than I'd ever allow.

Sorry for the interruption - carry on.

[Scruffie]

Pretty common in vintage EHX stuff, I guess back then a higher voltage rated cap of larger value was probably quite a bit pricier than it is now.

Still, a lot of them still work even with the small margin.

[thermionix]

Check the Fender Musicmaster Bass Amp schematic.  300V filter cap for specified 295V B+.  Fender 5E5 schematic, 25V bypass cap on the power tubes, specifies 26V across the resistor.  Obviously they were banking on rather conservative voltage ratings, which seemed to work out for them with the old American-made caps.

[GuitarFreak302]

Here is a link to a sound clip. Let me know if it doesn't work. This was recorded with the switch in the chorus position. The delay knob is all the way CCW, the feedback knob and the blend knob are both at the black star setting which is supposed to be the best chorus setting according to EHX.

https://vocaroo.com/i/s0vzvxEMdwxd

Thanks

[Scruffie]

That sounds like the delay time is possibly a bit long but definitely that there's too much feedback.

The excess feedback might just be a gain trim setting, to set that I expect you would want equal level at pin 7 and 14 & 15 of the NE570/571 (whichever one you have in there, they were interchanged) but first off, when you set the feedback to zero in the normal delay setting do you only get a single repeat as expected?

[GuitarFreak302]

Yes everything works fine in the normal delay setting.

[GuitarFreak302]

I checked the 4047 and the LFO with my scope. The 4047 seems to be working properly. The LFO at pin 1 doesn't even display on the scope, the scope trace just blinks on and off.

[flo]

Do you perhaps have some gut-shots of your Stereo Memory Man PCB, top and bottom?

That would be awesome!
It will help me fix my old EH7811 Stereo Memory Man!

Thanks already for the posted schematic which helps a lot and the effort.