904b schematic verify

rarecomponent · Post by **rarecomponent** » Fri Aug 08, 2014 11:18 am

I'm looking at C19 in the 904b schematics...it has one side to C20 but the other side to -6v rail. I'm not sure about this ...surely to earth?

MC · Post by MC » Fri Aug 08, 2014 11:51 am

Are you looking at RAMoog schematics or at Moog Music schematics? The Moog Music 904B schematic is known to have errors.

thealien666 · Post by **thealien666** » Fri Aug 08, 2014 1:07 pm

I could be wrong, but my guess would be, in looking at the other stages of the filter, that C19 would be connected to the + side of C10 (like C14 is) and that the small line between C19 and R21 (going to the connection node of R21 to the -6V rail) might be a mistake on the schematics ?

rarecomponent · Post by **rarecomponent** » Fri Aug 08, 2014 1:23 pm

Thanks for replies...yes I have Norlins schematics but all the others I've looked over say the same thing...so far I'm having a bad time with this clone project.

CZ Rider · Post by **CZ Rider** » Fri Aug 08, 2014 3:09 pm

The original R.A.Moog 904B used a different switching method than on the schematic listed as 904B's with serials above 1194. Have a May 1970 made 904B serial 118X that uses different cap values. Guessing the change may have been due to availabilty or cost. But those early ones switched the two caps in and out, where the newer revision adds on to the other cap in a parallel configuration. So in this earlier revision there are large 1.0 uF caps for the low setting and 0.39 uF for the high setting. There are four each .005 uF ceramic disk caps tied to ground. None of the ceramic disks look to be connected to the -6 rail in that part of the circuit on the original.
Can barely make it out on the blurry copy someone uploaded of the original shcematic here:

The switching on the bottom of that schematic looks cleaner than the later one. The values aren't legible but the bottom four sets of switched caps are the 1.0 uF (low), the ones above the 0.39 uF (high), and the .005uF ceramic disk all tied to ground. The four pole double throw selects either the 1uF or .39uF cap, and is wired between each stage.
Photo of the earlier revision caps:

And another view:(Large yellow 1.0 uF, orange top .39 uF Tropical fish type, and brown round .005 ceramic disk, four sets of eact type.)

It is always easier to clone/copy with good photos and the schematic. The photos above though do not match that later schematic and could cause confusion with that early switching scheme. I would try those values and switching, and would forget the later revision. Not sure if those ceramic disk connections are correct on the latter revision as I do not have a later 904B to look at to compare. Doesn't seem right though?
Right now I am trying to clone the original 1964 904 bandpass filter from just a few photos. (no schematic) It works but does not sound anything like the ladder filter. Still any Moog filter can have it's uses.
The original Moog 904 filter work in progress:

The original 904 voltage controlled filter:

rarecomponent · Post by **rarecomponent** » Sat Aug 09, 2014 6:27 am

As always CZ thanks for comprehensive info and superb detailed photos. I appreciate the time this takes .

I made a rough "breadboard" version years ago and having no 2N3392 '_s I used BC109's and BC214L for the 2N4058's...this prototype actually worked all be it with some faults. But then , after faults corrected and transistors replaced ....its all come to a dead stop. I get audio through but each note is accompanied by a heavy thump and there's no voltage control present.
Frustrating thing is I cannot get back to my original circuit.
So ...maybe the transistor frequencies are critical in this circuit?
The lack of 2N4058's a big problem.

CZ Rider · Post by **CZ Rider** » Sun Aug 10, 2014 1:21 pm

Did find a few photos of that later revision 904B board. Looks like those .022uF caps in question are the smaller brown "chicklet" type on either side of the four cap sets. (No ceramic disks like the older revision.)

This design is a more efficent use of the filter caps as the blue top .68uF caps are added to the orange top .39uF for the low range setting. The older design uses a more expensive 1.0uF cap and swaps in the different values of 1.0uF and .39uF.
Another view, but neither are clear enough to determine where the brown "chicklets" connect.

Someone noted on the synth DIY list that the adder section on that Norlin schematic had errors compared to the earlier R.A.Moog schematic.
Here are the errors. (There may be more)

I do get thumping when the 904B is way out of range by sending an envelope and fixed voltage at +6. It is far past where the filter stops passing audio and too much voltage puts it in a state where it begins to thump.

rarecomponent · Post by **rarecomponent** » Mon Aug 11, 2014 7:10 am

Thanks CZ. Again great photos of the 904b .

Studying the faulty schematic diagram in your post I altered my circuit accordingly . Now my control circuit should be correct....but alas still no effect on the filter .
There are no mistakes on my clone..I've checked and rechecked again and again. It simply is a dud as far as I can tell.

Going back to my original working clone I'm thinking that once I replaced the BC109's with 2N3392's it stopped working and also I must have made a lucky wrong connection or two on the controller but alas I cannot recreate the circuit.
I know the BC109's have a greater frequency range.

Carsten has kindly given me support with his working version in which he's substituted BC550 /BC560's...but I must say I'm underwhelmed to go and build a 4th clone as I've lost confidence in this particular circuit, the only one that has beaten me. In fact I've contacted Sebastion at Mos Lab and also COTKN for one of their modules as a last resort.
Like you say, detailed photos of original Moog 904b ie copper track and components is the key to this.

rarecomponent · Post by **rarecomponent** » Sat Aug 16, 2014 7:00 pm

Pleased to say I got my latest 904b clone attempt to work correctly.

The main fault being the adder section as mentioned. Once corrected and 2N3392 and BC214 L installed plus the odd 2N3391 the circuit works ...however...C19 does not go from C20 connection to ground.This stops the circuit in its tracks. Going to -6 as on the schematic the circuit works but I'm not too sure about the effect so for time being am leaving C19 off the circuit.

Still not sure about set up but it is possible to eliminate "thump" with the pre sets as per 902's.

So Norlin schematic (apart from adder section ) certainly not a dud !

CZ Rider · Post by **CZ Rider** » Thu Sep 11, 2014 10:24 pm

Next best thing would be diagram of the parts layout on the 904B board. Looks like a few schematics and drawings were posted by the BMF recently. One of them was the layout for the one 904B board. Sure enough C19 does go the -6 rail. Was the brown chicklet from that pic of the later revision 904B. I highlighted the -6 rail and C19 in green. From the drawing date I guess these were the revision from mid 1970 on.

rarecomponent · Post by **rarecomponent** » Fri Sep 12, 2014 4:55 am

CZ....pure gold !

Where on earth do you get this info ? Your the Indiana Jones of the lost Moog Temple !

But really ...great stuff, thanks for posting I wish I could have used this layout for my 904b...maybe in time .?

carkifelek · Post by **carkifelek** » Mon Feb 02, 2015 7:27 pm

CZ Rider, you’ve somehow stumbled upon a treasure of information on 904b and by sharing all this information, you’ve made us equally rich. Cheers for sharing the information. This one thing I’ve noticed which reviewing these schematics is that these circuits have been heavily protected from power failure issues and shorting etc. Great stuff, thank you.

noddyspuncture · Post by **noddyspuncture** » Mon Mar 19, 2018 6:01 pm

Terry,

I've researched both the 904-b schematic you mentioned and shown us here in this thread... and also the layout diagram you shared, and it appears to me that the 'errors' aren't actually errors in the schematic as such - not like the 'revised' or 'corrected' part would have us believe.

It looks to me - cross referencing the two - that all it is, is that Q3 & Q4 are mislabelled on the schematic (3 should be 4 and 4 should be 3)... on the layout they look to be otherwise correct...!?

Could anyone confirm my findings here please...?

Cheers,
Tom

noddyspuncture · Post by **noddyspuncture** » Thu Dec 13, 2018 11:48 am

rarecomponent wrote:I'm looking at C19 in the 904b schematics...it has one side to C20 but the other side to -6v rail. I'm not sure about this ...surely to earth?

Hi folks,

Having build two of my own 904B's and having used them for some time, I was looking into them again because I wasn't completely happy. I was getting 'thumping' on the output and found that the units were passing audio via their control inputs..! Both were behaving exactly the same way.

The first thing I found was my own error - because reading the instructions on the schematic I matched the four npn and also the four pnp transistors in "sets of four" - as per the note... but of course they need matching npn/pnp for each cell! After doing that it still wasn't right.

The next thing I looked at was this C19 .022 capacitor. Moving it to ground as opposed to -6v did indeed cure my problem -on both modules.

Cheers
Tom

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