Moog Music Forum

Greetings all,

I'm working on a project to restore an old Moog modular system. (And when I mean old, I mean modules with serial numbers in the low 1000s and manufacturing dates as far back as 1968.)

I've got a reverb unit (beautiful old thing!) that I'm trying to troubleshoot. YouTube link to present behavior is here: https://www.youtube.com/watch?v=1gt5yhT7iTA

Anyone have any idea what's going on? I suspect a bad pot, wanted to get your thoughts.

Best regards,

~Rezin

Yes, I'd say you need to change that dual pot. I did on mine - in fact mine was also wired wrong, dry and wet were being turned up and down at the same time... not in opposite directions as it should be.

By the way the schematic in the service manual is full of errors. I can tell you where it's wrong if you need.

Would there be any chance of some photos of yours... the insides and some rear shots...?

Mine isn't like the production versions... it is on perf-board and quite messily put together. I have a feeling mine is a 'prototype'. No serial number and also there is no 'C' frame as such - just a bent piece of aluminium section...!

The hum you are getting is normal to a degree. You really need to get it as far away from the power supply as you can. Also replacing the electrolytic caps might improve it some...!?

Cheers
Tom

Thanks for responding! (No, really, I mean that- I appreciate all the help I can get )

A running photo log is here: https://www.flickr.com/photos/136277463@N07/

Some photos are reproduced below, more on Flickr if you're interested:

Moog 905 reverb unit by Los Wubbenhorsts, on Flickr\

Moog 905 reverb unit by Los Wubbenhorsts, on Flickr

Moog 905 reverb unit by Los Wubbenhorsts, on Flickr

The "corrected" schematics I have can be found here: http://modularsynthesis.com/moog/905/90 ... ected).pdf

Will definitely look into getting pots replaced.

Thanks,

~Rezin

Thanks for the photos.
So yours is a production model, with rear connector and serial number...!

Here are a few photos of mine...

905_1 by noddyspuncture, on Flickr
905_2 by noddyspuncture, on Flickr
905_3 by noddyspuncture, on Flickr
905_4 by noddyspuncture, on Flickr

I would recommend cleaning the stock dual pot before replacing. That schematic for the 905 also claims the unit ships with straps to hold the spring unit from bouncing around. Those black straps should be removed and the reverb tank supported only by the suspension springs. My 1969 Moog also had the original straps never removed. Sounded much better with those shipping straps removed.
Sounds just like my 905 here and is sensitive to hum if placed near any transformer.

Wish I could get my photos to show... what's the "secret" to it...?

@noddyspuncture, I posted links to photos on Flickr. I don't actually know how to upload photos that can be viewed within a post, but if you can get a link you can share it using the 'IMG' option (above the text box) when you're composing a post.

@CZ Rider, thanks for the advice- are the straps below what you're referring to?

Moog 905 reverb unit by Los Wubbenhorsts, on Flickr

[quote="Rezin"]@noddyspuncture, I posted links to photos on Flickr. I don't actually know how to upload photos that can be viewed within a post, but if you can get a link you can share it using the 'IMG' option (above the text box) when you're composing a post.

Yes I have also uploaded my images to Flickr - and am using the 'IMG' option either end of the photo's link... but they all come up as you see in my post. I had a look at your successfully attached link and yours seems to have not only the 'IMG' but also the 'URL' options used.

I cannot fathom what I should be doing. I have the links to Flikr but they just don't seem to want to appear in my post..!

Any idea what I might be doing wrong...?

Here the actual URL link to one of them...
https://www.flickr.com/photos/noddyspun ... 990476784/

Cheers,
Tom

Tom you didn't include the .jpg at the end and you left the s at the end of http.

But you have to click the share arrow, use the BBC code and I use medium 800 or so. And then it will give you a long link with some words (picture shared by noddy on flickr. Just cut out the actual link with the picture number and .jpg at the end.

EricK wrote:Tom you didn't include the .jpg at the end and you left the s at the end of http.

But you have to click the share arrow, use the BBC code and I use medium 800 or so. And then it will give you a long link with some words (picture shared by noddy on flickr. Just cut out the actual link with the picture number and .jpg at the end.

Thanks Eric... that worked a treat..!
If you go look at my previous post I edited it and the photos are now there...

Cheers,
Tom

noddyspuncture wrote:Thanks for the photos.
So yours is a production model, with rear connector and serial number...!

Here are a few photos of mine...

Wow Tom - those pics of your pre-production are
I wonder how many were produced like that?
Imagine the outrage if a modern module was delivered like that! - Still, I guess you could prove that it was authentic


---

M00G wrote:

noddyspuncture wrote:Thanks for the photos.
So yours is a production model, with rear connector and serial number...!

Here are a few photos of mine...

Wow Tom - those pics of your pre-production are
I wonder how many were produced like that?
Imagine the outrage if a modern module was delivered like that! - Still, I guess you could prove that it was authentic
---

I strongly believe that this reverb unit is some kind of prototype... now knowing exactly where most of my modules came from.
They were used by David Van Koevering and racked for use on the "Isle Of Electronicus" series of seminars..!

And I also have those rare 701's and 702's with a headphone amplifier, custom modules like an Octaver (built into a very early VCA case) the "mystery module" which we still don't know what it was supposed to do, a delays mixer module from the CEMS system and also this very roughly assembled 905 reverb. By the way - looking at that perf-board circuit you'd be forgiven for thinking it's not the correct circuit... but I can assure you it is. I followed it and drew it out and it is exactly what is in the Modular service manual.. minus those errors of course...

Cheers,
Tom

Hi Rezin:
Thanks for the great pics of the reverb module!
This certainly helped clear up some of the confusion surrounding this module and the errors in the published schematics, but also raised a few other questions for me.

Is there any chance that you could post a closer, more detailed picture of the component side of the circuit board from a few angles so I could get a better look at all of the components?
It would be greatly appreciated. Your module seems to be in very good shape, and you just don't see many of these around (especially with clear pictures).

I'm hoping to create a useful front/back overlay guide that might help answer a few nagging questions.
I'd be happy to share any unusual findings with the group here.

I'm particularly interested in the lower left hand section of the board, where Q1 (Bottom middle of the board), the 10uf, 220 ohm and .2 uf cap live. I think that there are some components (33k?) and jumpers that are not well visible in the current photos, as well as various wires connected to the board in this area.

Is the green wire coming down to the 100k resistor from the back power connector connected to pin 3 (-6v?) or pin 2 (ground?) Also, is the upper trace with the connection lug soldered to it connected to chassis ground or -6v? (Pin3)
It's hard to see these connections clearly without some additional photo angles.

I'm also interested in how Q1 is installed...are the leads installed straight to the pads, or are the leads crossed in some manner?
I think that Q1 is the only 2N2925, and Q2-Q4 are 2N3707/2N2926.Is that what's on your module?

I realize that these are a lot of questions, so thanks in advance for any help you are able to provide!

synthguy wrote: I think that Q1 is the only 2N2925, and Q2-Q4 are 2N3707/2N2926.Is that what's on your module?

That's correct... although the schematic specifies 2N3707 for Q2. So I suppose Q3 & Q4 can be either 2N3707 0r 2N2926.

If I were to build a 905 circuit from scratch I'd use veroboard. It's not a complicated circuit at all...

Cheers,
Tom

Hello,

@synthguy, no problem at all- just uploaded some more detailed photos at my Flickr page (Moog forums won't do them justice I'm afraid): https://www.flickr.com/cameraroll

(1) Seems that green wire is connected to pin 2

(2) Neither, I'm afraid- I think it's the output of the spring reverb itself. (Why Moog didn't just use an RCA cable out for greater modularity is beyond me. )

(3) I think the only transistors I'm seeing are 2N3391As... Straight into the pad, as far as I can tell.

I will keep you all updated on status of repair- I'm working to get some more components, hopefully I'll have those by the end of the week.

~Rezin