Moog Music Forum

Hello all,

So, a few weeks ago, I had two 901-B oscillators running ("running" being taken to mean "making some kind of noise). Video here (https://www.youtube.com/watch?v=e0c7Rg4uEAg) for anyone interested.

However, this no longer seems to be the case for reasons that are rather mysterious to me. (Video here: https://www.youtube.com/watch?v=hHTSmnDhIqk)

Tested my speaker with a MicroKorg, so I know that works. I suspect some internal weirdness with the wiring, but have no idea why that would be the case.

Anyone have a clue to as what's going on? I find it strange that both modules stopped working at the same time. I did pull them of the unit to inspect them, but I don't know why that wouldn't have anything to do with the issue at hand.

Best,

~Rezin

First thing would be to make sure power is getting to the modules...?
I say this because neither your 901 or noise generator had any output. What about the 901b's that were working earlier...?

I cannot understand why, in the video, you seem to think (duh moment..!?) it might be a problem with the monitor..? There is a logical progression you should follow here otherwise you'll never get to the bottom of it and will be going round in circles like, forever...!?

Yes, I think I'll take a multimeter to it today or tomorrow.

You're right about that- I was running on about four hours of sleep when I made that video. It's a wonder I was making any sense at all...

Best,

~Rezin

Rezin wrote:Yes, I think I'll take a multimeter to it today or tomorrow.

You're right about that- I was running on about four hours of sleep when I made that video. It's a wonder I was making any sense at all...

Best,

~Rezin

I'd have used the multimeter before I made the video...

...so, as it happens, there doesn't seem to be ANY power going to any of the modules. I'm not sure why this is, or what would have brought it on, but if anyone has any ideas as to what's going on, I would certainly be interested in hearing them.

Best,

~Rezin

Rezin wrote:...so, as it happens, there doesn't seem to be ANY power going to any of the modules. I'm not sure why this is, or what would have brought it on, but if anyone has any ideas as to what's going on, I would certainly be interested in hearing them.

Best,

~Rezin

I cannot believe that I am going to ask you - if you have checked the fuses...?!
Have you...?

Rezin,

First of all, your pictures look awesome, especially the label with the RA Moog writing on it and Herbie Hancock haha.

But have you considered having an engineer look at your synth? You have an incredibly valuable and rare instrument there and if you get to prodding around, you can end up making things worse.

noddyspuncture wrote:

Rezin wrote:...so, as it happens, there doesn't seem to be ANY power going to any of the modules. I'm not sure why this is, or what would have brought it on, but if anyone has any ideas as to what's going on, I would certainly be interested in hearing them.

Best,

~Rezin

I cannot believe that I am going to ask you - if you have checked the fuses...?!
Have you...?

Perfectly legitimate question- just bought all new fuses for the thing, and installed them just last Thursday (which restored the functionality of the CV mixers). The power source stopped working properly before that, though, so I know it's not due to that operation.

@EricK, the synth is at a university right now, so I have a pretty steady stream of faculty coming in to look at it (hopefully one this Friday to check out the power source). You're definitely right about it's rare status- I'm honored, humbled, and a little nervous about patching it up myself

Thank you all for your help!

Best,

~Rezin

Hi Rezin!

I still have a few questions about the 905 reverb module, which I think got lost on a few previous pages.

Your help would be greatly appreciated!

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 and hidden 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.
The current schematic indicates 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. This would connect the reverb spring metal chassis to -6 volts (perhaps)
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) coming from the chassis of the reverb springs near the RCA output jack. Where do these go? I think that the black wire is the hot output, and connects to the large input capacitor at the top of the board. The green looks to be connected to the reverb spring chassis, but then where does it go to?
Ground?
-6v?

Perhaps the -6V is not used at all? (Contrary to what we see in the schematic)

Thanks, and good luck with your project!

synthguy wrote: Perhaps the -6V is not used at all? (Contrary to what we see in the schematic)

The -6v is definitely used... as I confirmed in the "Troubleshooting" thread.
Also it would be a very silly design fault to put a power rail onto the chassis/casing of a reverb tank...!?

And I am certain that the redesigned schematic posted earlier, correcting those misprints in the original schematic, would also have corrected the -6 to gnd symbol...?

Cheers,
Tom

The reason I'm asking is because while looking at the current photos on Rezin's site, I noticed that there doesn't seem to be any connection at all to pin 3 (-6V) that I can see in this picture...

https://www.flickr.com/photos/cryptorose/24921427145/

If you enlarge the area at the connector, you can see the red wire connected to pin 1 (+12V), a green wire which Rezin has confirmed is soldered to pin 2 (kind of hidden in this picture though), and nothing that I can see connected to pin 3 here.

Here, if you enlarge the area at the connector, you can actually see the green wire soldered to pin 2, and that nothing is soldered to pin 3. Mysterious...

https://www.flickr.com/photos/cryptorose/24730957161/

So, if that topmost trace is indeed connected to -6V (as the schematic indicates) then I'm wondering just how its being accomplished. There's no good picture of this yet that I can see here. A few measurements with an ohmmeter here would be a huge help.

That's why I'm hoping that we can get a better look at this particular area of Rezin's production example of the 905, and hopefully clear up a few final questions about it. You don't get to see many of these nowadays!

And I agree that having -6V on the chassis of the tank would be a strange design choice, Tom!

On the August 1969 revision 905 here the -6V tab connects to nothing. The listed schematic in the Norlin era book is not the same revision as I have here. The transistors on this one is a 2N3392 for Q1 and 2n3391A for the other three Q2-Q4. Usually seeing those old transistor numbers is an indication of the time period the schematic came from. Those 2N2926 were used in the older modules, my guess early 1968 and before. Have to remember there were many revisions, and when you received a Moog system back in the day you would have a packet of your current revision blueprint schematics shipped with your system. None of the Norlin schematics match any of the modules from this third quarter 1969 system here. Some are minor revision changes, some are totally different. The Norlin schematics are from anywhere from 1965-1974, and many are old revisions while some are the most modern revisions. The 905, 984 and cp-3 schematics seem to be from 1968 or so. While the 904B is a 1971 and later revision. If someone was to construct a Moog modular clone from those schematics, it would be a mish-mosh of years and revisions ranging anywhere from early modules to Norlin era modules.

CZ Rider wrote:On the August 1969 revision 905 here the -6V tab connects to nothing. The listed schematic in the Norlin era book is not the same revision as I have here. The transistors on this one is a 2N3392 for Q1 and 2n3391A for the other three Q2-Q4. Usually seeing those old transistor numbers is an indication of the time period the schematic came from. Those 2N2926 were used in the older modules, my guess early 1968 and before. Have to remember there were many revisions, and when you received a Moog system back in the day you would have a packet of your current revision blueprint schematics shipped with your system. None of the Norlin schematics match any of the modules from this third quarter 1969 system here. Some are minor revision changes, some are totally different. The Norlin schematics are from anywhere from 1965-1974, and many are old revisions while some are the most modern revisions. The 905, 984 and cp-3 schematics seem to be from 1968 or so. While the 904B is a 1971 and later revision. If someone was to construct a Moog modular clone from those schematics, it would be a mish-mosh of years and revisions ranging anywhere from early modules to Norlin era modules.

This is most interesting Terry... in fact it's fascinating...!
So the most common schematic for the 905 reverb - the one in the service manual - is an early one, for which it seems then that mine was a "prototype". My peg-board 905 exactly matches that schematic and uses the -6v supply..!

Cheers,
Tom

I've just published some musings on the 905 in my blog.
Synchronicity!