Major Progress

I'm happy to report some major progress over the weekend with the M3!

First, I got the AO-29 amplifier ready for servicing by disconnecting the three wires on the left front of the chassis that supply 120V AC power to the motors in the tone generator. I didn't want the tone generator running while working on the amplifier. This also removed the "run" switch from the circuit, so I made my own on/off switch for the amplifier using a common household light switch, a plastic electrical work box, a switch plate cover, and a few feet of 2 wire lamp cord. These were all materials I had stored away in the workshop from other projects around the house. I hooked up one end of the lamp cord to the terminals on the light switch and the other to the "blue" and "black" wire terminals on the amp chassis. Now, I could turn the amp on and off right where is was working, rather than go around front to the "run" switch.



Next step was to remove the bolts holding the amp down to the cabinet, and I disconnected the linkage for the expression pedal. I also had to remove the green and black wires that go the the switch on the expression pedal. I was then able to lift the chassis up and rotate it 90 degrees onto it's side, with the underside facing me. A couple blocks of 2x4 wood help provide some slack for the rest of the wiring still connected to the amplifier:



I powered the amp up with no tubes, and checked the AC voltages coming out of the power transformer. Filament voltages looked good, and the secondary measured about 760V AC, which agreed nicely with the schematic (380V from winding to center-tap). I turned the power off and inserted the 5U4GB rectifier tube; turned AC back on and started to measure DC voltage points. First thing I noticed was all the B+ voltages were 80-90V higher than the schematic indicated (more on that later). Nothing smoked, and so I decided to go ahead and put the remaining tubes I had available. These 4 tubes were enough to get the manuals, vibrato, and intermediate preamplifier sections working.

I remeasured DC voltages and found them all still 80-90V high. I decided to press on anyway. I disconnected my home-made on/off switch and reconnected the power supply wires for the tone generator motors. Next, I tapped into the intermediate preamplifier with a 0.01uF disk ceramic capacitor at R40, between V4b and V3b (sorry, not the best picture):



The wire connected to the cap goes to a RCA phono plug, plugged into the line input on an old Carvin bass combo amp I own. The ground side of the wire goes to a screw on the AO-29 chassis. With this all set, I started up the M3, and turned on the bass amp. Pulling out a few drawbars, I turned up the input volume on the bass amp and hit a key on the organ....

SOUND! YooHoo!! After a bit more fiddling with levels and the disconnected expression pedal control, I got a pretty decent sound level coming out of the amp, with no sign of hum or other bad sounds. I was then able to verify that the vibrato was working perfectly for both manuals, and all but one key (the highest one on the lower manual) were working fine, too!

Here is a really bad video of my oldest soon noodling around on the organ - basically shot this to prove that it was now making some sounds 8-):



Remember those voltages that were too high? I posted a question about it to the "Hammond Zone" forum on YahooGroups and almost immediately got the answer that it was likely the missing power amplifier tubes that was responsible. Of course! Without the tubes drawing current, and with no regulation to speak of in these old tube amp power supplies, the DC voltage will rise much higher than with the power tubes in place.

Next step is to get some 6V6 power tubes, because the voltage without them is too close to the working limit of alot of the capacitors in this old amp. I will also pick-up tubes for the percussion and bass pedal sections of the amplifier at the same time and see if I can get the rest of the preamp working.

Vacation

Things have been quiet on "Bringing Back Baby" because I was on vacation in Washington DC all week... we're back now and I'll be back to work on the Hammond this weekend.

We also managed to miss the 7 inches of snow here in Vermont while we were gone...

Tubes!

I mentioned in a previous post that the amplifier in my M3 didn't have any tubes in it. The amplifier used in this organ, the model AO-29, takes 11 vacuum tubes (or "valves" for our British friends), including two 6V6 power tubes that are used in the power amplifier section to deliver 10-12W of output power.

I'm not too interested at this point in restoring the power amplifier section (I will do so at a later date, when everything else is working), so I'm concentrating on getting the pre-amplifier section working. The preamp will boost the tiny signals coming from the tone generator, vibrato, percussion, or bass pedals into a signal large enough to drive a power amplifier. In my case, I'm going to tap into this signal somewhere near the end of the preamp chain and bring that signal out to the outside world.

I started looking into purchasing some tubes from online suppliers like Antique Electronic Supply. I wanted to purchase enough tubes to get the power supply, vibrato, and manuals working. Unfortunately some of the tubes, like the 12AU7, can be relatively expensive - running into the $15 to $20 per tube range. I also really just wanted something to get the amp working; I don't need high grade tubes at this point of the restoration.

In my ham radio hobby, I used to have a friend that bought and sold tubes on the side, but he got out of that business several years ago. Then I remembered my friend Ed, who is a Collins radio enthusiast (Collins was a high-end US radio manufacturer from the 1950's to the early 1980's). I suspected Ed might have a few tubes, or know someone locally that did. Well, bless his heart, not only did he have the tubes I was looking for, but he tested them for me to make sure they were OK, and didn't ask anything for them!

So a big THANK-YOU goes out to Ed W1OKH!! Here's the cache of tubes:


Left to right: (3) 6AU6, (2) 12AU7, (2) 12AX7, (1) 5U4

The 5U4 is the rectifier tube for the power supply. The rest are used in the preamplifier section. I needed 2 6AU6's and one each of the 12AU7 and 12AX7, so I have some spares now, too.

BUT, before I put any tubes in this amplifier, I need to pull the chassis off the organ and get a look at the physical condition of the components inside the amp. I have a strong suspicion I will need to replace the electrolytic capacitors before I do anything else... in fact, I may just do a preemptive replacement of the electrolytics. Better safe than sorry!

Building an Organ Dollie

So I have this 250 pound organ sitting in my already somewhat cluttered garage. I was thinking it would be great to be able to move it around myself if needed; also it would be easier to work on if it was raised up a bit higher off the concrete floor. I decided to build what I'll call a platform dollie to put the M3 on - a bit of 2x4 lumber framing, with a 3/4" plywood deck and 4 locking casters at each corner. Here's the completed dollie:


It's 48 inches long by 29 inches wide, just big enough to comfortably hold the M3. Materials were (2) 2x4x8's and a couple 2ft by 4ft "handy panels" from the home store, 3/4" thick, sanded on one side. A few 3 inch drywall screws for the framing, and some 1.5 inch screws for the decking hold it all together. On the bottom I mounted (4) 2 inch lockable casters. For the frame, I cut one of the 8-foot 2x4's in half for the sides, then cut (3) 26-inch pieces from the other 8-footer for the ends and a brace in the middle.

Here's a picture of the organ on the dollie:


The only thing I think I might have done differently was to use larger casters, like 3-inch or maybe even 4-inch, which would make it even easier to roll around. Note the toothbrush in the picture - starting to work on the amplifier. There's quite an accumulation of dust and "stuff" on it and all around the wiring, so I was starting to clean that up.

Welcome to "Bringing Back Baby"

Welcome to "Bringing Back Baby", a blog about restoring a Hammond M3 spinet organ (also known as the "Baby B3") to operation. My name is Frank, I live in the state of Vermont in the USA, and I'm an electric bass player, not a keyboard player. So what the heck is a bass player doing with a Hammond organ? Read on....

I've always loved the sound of the Hammond organ, probably due to my growing up in the 70's listening to a lot of rock bands using the organs in their music. My first love is the bass, but I have had a moderate interest in learning to play keys for a number of years. Also, for reasons I can't totally explain, I've had a fascination with the idea of owning some vintage instruments, especially keys (besides the Hammond organ, think Wurlitzer/Rhodes electric pianos).

So just a few weeks ago I was innocently scanning Craigslist and there it was, in the "free" section: "Free Hammond M3 organ - Doesn't work". Wow! A free Hammond organ! I already knew about the M3 - a smaller version of the famous B3 organ, with the electro-mechanical tonewheel tone generation system. The pictures attached to the post showed a M3 spinet that looked to be complete and in decent shape, except there were no tubes in the amplifier, and no speaker. The owner did mention in the post that it had been converted to 1/4" preamp output. While I was somewhat concerned about the condition of the amplifier, I have a degree in electronics (and my other hobby is ham radio) so I felt pretty confident I could fix whatever might be wrong.

I have gained enough wisdom over a number of years of marriage to know I needed to check this with my wife... small radios, bass guitars, etc. are pretty easy to sneak into the house, but not an organ! Her response was "if you're really going to fix it and use it, then OK... I don't want a project sitting around for years though!". Yes indeed, my wife knows me well... so with that tacit approval in hand, I sent an email to the organ's owner and a meet to look at the organ was quickly set up.

I met up with the owner in his garage/practice room and found out that he had bought the organ off someone in the mid-Hudson valley, and it did not have a speaker in it at that time. A friend of his converted the amp to 1/4" preamp output and he successfully used the organ for some time... until it suddenly "died" on him. He pulled the tubes to use as spares for his other Hammond, a nice A-100. We powered the M3 up, and the tonewheel generator seemed to spin up nicely. Everything else, other than the amp, seemed solid and I had no reason to doubt that it was working as he claimed until the amplifier died. After long and careful deliberation (about 1 minute) I said "I"ll take it!". It took a few more days to round up a friend with a pickup truck and some extra help to move it to my garage.

Here's a few pics of "Baby" right after she arrived at my place:




So now on to the rebuild and hopefully a new life for this "Baby B3". Why blog about it? Well to be honest, I think putting my effort out in public will spur me on to complete the job. Hopefully along the way, you'll find this blog to be informative, maybe a little entertaining, and possibly will inspire someone else to tackle bringing their old organ back to life!