Troubleshooting Old 905 Reverb Unit

The Moog Forum Modular Forum
1

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

2

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

3

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

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/905/905%20Reverberation%20Unit%20(corrected).pdf

Will definitely look into getting pots replaced.

Thanks,

~Rezin

4

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

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

5

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.

6

Wish I could get my photos to show… what’s the “secret” to it…?

:confused:

7

@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

8

[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/noddyspuncture/24712814902/in/album-72157661990476784/

Cheers,
Tom

9

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.

10

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

Cheers,
Tom

11

Wow Tom - those pics of your pre-production are :open_mouth:
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 :laughing:

\

12

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… :smiley:

Cheers,
Tom

13

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!

14

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… :smiley:

Cheers,
Tom

15

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. :smiley: )

(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

16

Thanks a million! These newer pictures are a big help, and I really appreciate you taking the time to help us here at the forum!

A couple more questions, if I may…

  1. At the bottom left quadrant, there’s a 10uf blue cap, a 220 ohm resistor, then a resistor that I think is a 33k, but is hidden from view by the big brown .22uf cap next to it.
    Could you move that cap over to the right a bit and get a shot of whatever is stuffed there?

  2. Also, below that 100k resistor a bit to the right is a bunch of wires connecting out to the various jacks and pot connections. Buried under that bundle of wires, I believe that there is a bare wire jumper installed on top that connects two close by pads (below the 100k, and to the right of the 15k).
    I think I can see it a bit in your current picture, but could you verify that its really there?

  3. If you have an ohmmeter, could you ohm between that upper trace that the screw and big solder lug are connected to, and see if it is connected to pin2, or to pin 3 on the power connector.
    I believe that this is connected to pin 3 (-6v). Lots of reverb springs have the return jack grounded to the metal chassis of the unit for shielding, while the input jacks are completely floating.
    However, in your pictures, there appears to be nothing connected to the pin 3 trace, at least not on the top where we can see. There may be something soldered on the back (trace) side which I can’t see.
    There appears to be two wires (a green and a black one) soldered to the chassis of the reverb springs near the RCA output jack. Where do these go?
    My thoughts are that there’s a wire connected from pin 3 to the hole that the solder lug is soldered to, and in turn to the chassis of the reverb springs. This would supply the -6v to the reverb return “ground” shown in the schematics. There’s also the chance that the green wire is the one we see coming down the connector side to connect to what I think is the ground trace at the top of the 100k. Perhaps the -6V is not used at all?
    Is any of this close? :open_mouth: This is probably the biggest question about this so far…

The 2N3391 makes sense, as Moog used lots of these in later builds, and from what I saw of the way the transistor pads are laid out, they all have the same pinout. If the transistor shown in the schematic for Q1 were used, it would have to have the leads rearranged to properly work with this layout.

Thanks again for your help! :smiley:

17

Can you actually see the new photos…?
When I click the link it takes me to my own Flickr page..!

18

Yeah, me too!
I used the original link to his flicker account, listed in an earlier post, and there they were! :mrgreen:

19

Hi Folks,

I also posted this in the other 905 thread here in the Modular Forum…!
I have now extracted my 905 reverb from my cabinet and have had a look at those points for you.

I can confirm that my -6volts is indeed used and does indeed go to the output of the reverb, and also the 75uF/2.2K - 10uF - 100ohm and 82K components as shown in the schematic…!

Also the mystery “un-named resistor”… on my unit it is a small preset trimmer… and needs setting quite accurately for the recovery amplifier to actually work without being too noisey… :smiley:

Here are some more photos of mine…
Cheers,
Tom

[url=https://flic.kr/p/E6nKMZ]905_6 by noddyspuncture, on Flickr[/url]

[url=https://flic.kr/p/DDgi2s]905_7 by noddyspuncture, on Flickr[/url]

[url=https://flic.kr/p/D8DKeN]905_8 by noddyspuncture, on Flickr[/url]

[url=https://flic.kr/p/D8Z5TD]905_9 by noddyspuncture, on Flickr[/url]

[url=https://flic.kr/p/DUWVj1]905_10 by noddyspuncture, on Flickr[/url]

[url=https://flic.kr/p/DDgh33]905_11 by noddyspuncture, on Flickr[/url]

[url=https://flic.kr/p/DUWUDy]905_12 by noddyspuncture, on Flickr[/url]


905_13 by noddyspuncture, on Flickr

[url=https://flic.kr/p/DwTza8]905_14 by noddyspuncture, on Flickr[/url]

[url=https://flic.kr/p/E6nHyF]905_15 by noddyspuncture, on Flickr[/url]

[url=https://flic.kr/p/DX8B7V]905_16 by noddyspuncture, on Flickr[/url]

[url=https://flic.kr/p/D8Z3Ea]905_17 by noddyspuncture, on Flickr[/url]

20

Thanks for the great pictures, Tom!

So, what’s your 100k trimmer hooked to?

The 100k on the production unit is hooked between the output and ground…adjusting it low enough would eventually lower the output, but wouldn’t really affect the noise.

Yours might be the Q2/Q3 feedback?

Still hoping for a few more pics of Rezin’s production unit.

He’s mentioned that the green wire was hooked to Pin 2 (ground) which means that the 2.2k and 75uf cap are also hooked to ground in the production units. It appears likely that the upper trace in my previous picture is tied to -6V supply, although I still haven’t see the proper photo angle to confirm this.