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KAPOW!! Recapped Hammond preamp mistake (BANG!)
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author KAPOW!! Recapped Hammond preamp mistake (BANG!)
Rex Coil 7
How did this happen? Read on ....

My 1962 Hammond A102 needed the preamp's power caps replaced pretty badly. Intolerable hum, as well as telltale wax drippings on one of the main transformers that had leaked from old wax caps. Since these preamps hang upside down when installed in the organ, the leaking wax runs down and all over the transformers, sometimes enough of it even leaves a small puddle of wax in the bottom of the organ.


The job on the A100 series requires complete removal of the entire preamp to get it done. So, I figured "while I'm in there" may as well do up the whole thing. This job can be done using High Bux Vishay replacement caps (Millions ... no ... BILLIONS of dollars .... y'know .. cuz Hammond, cuz Vintage, cuz reasons). Erm ... Nope.

Or if you're resourceful you can use Nichicon capacitors that sound every bit as good and last just as long, and the entire list of required caps cost less than thirty bucks (Mouser or Digikey). The only hitch being they won't fit in the stock "cans", so some careful planning and even more careful execution is a must.

Here's the preamp, removed from the organ. Those cans have capacitor "clusters" inside of them, those caps needed to be replaced as well as others within the preamp.


Precaution Number 1 ..... I prefer to be able to actually see with my own eyebones that the instrument is disconnected from mains.

Two things I do when working inside of power amps or preamps. One, is I hold some non-conductive thing in my weak hand. We humans have a tendency to try to use both hands when doing things. You really do not want both hands inside of a power amp with high voltage capacitors inside of it until you have proven several times that the caps have been bled off. So, I hold this old stick with a grip on it in my weak hand (which keeps it occupied and reminding me to not use it) while I'm discharging the caps with the other hand. This prevent me from completing a circuit with my body, and getting a 400volt ZAP. Yup, four hundred (plus!) volts hanging around in those caps until you've made ABSOLUTELY CERTAIN that they've been bled off. All it takes is a bad bleed down resistor permitting the caps to retain their charge ... even for years. Another variation of the "stick in hand" method is to keep one hand in your pants pocket at all times while futzing around inside of the amp until you've bled off the caps. ONE HAND ONLY RULE.

So I made up this cap bleeder kit with the stick, a high current resistor, a clamp to short the resistor to ground, and a banana jack to connect an old meter lead that has an insulator shield rated for 1k-volts ....

Here's the kit, demonstrating it's use prior to bleeding off the innards of the preamp. You simply touch the test lead to each cap's terminals, hold it there for a few seconds, and continue this until you've hit them all.

Ok, so safety lecture overwith ..... you kids get off my lawn

Here's what the new Nichicons look like compared to the space the stock ones fit in ... obviously this won't work so I had to build up cap clusters that fit inside of the preamp instead. You can see how the new replacement capacitors simply will not fit within the stock can. I had to make up some new cap clusters to replace unavailable "can caps". As you can see, it just wasn't going to happen.

So here's how I bundled up the new caps to fit inside of the preamp chassis .....

I bonded them together with a bit of "Blue Pooky" (RTV silicone ... high temp stuff) ......

(This is becoming long, continued next post ......)
Rex Coil 7
(continued from previous post)

It took a while to correctly map out everything. I took a lot of pictures of the stock wiring prior to digging in, I also took a lot of notes, and wrote up an order of assembly for the job. I also removed all of the tubes, carefully marking them to match the schematic, and labeling them in bags to keep things organized....

A note about Hammond tubes. Many times folks will hear hum and noise, and immediately blame the tubes for the issue. And all too many "Vintage Hammond Parts Vendors" are all too happy to take your money for a set of new tubes that you more than likely DO NOT NEED. Hammond tubes run at voltages far below their rated maximums. The will live for many decades simply because of their under-stressed use. Don't waste your cash. It's far more likely that you need to recap your preamp. Which also costs far less than retubing the preamp as well. And don't be tempted into buying some pre-assembled "cap kits" either .... you can easily chase down the cap values and piecemeal what you need from a parts supplier such as Digikey or Mouser. Movin' on ....

Here is the installation of one of the newly made-up cap clusters .... keep in mind I had to design an all new diagram prior to installing things. It pays to get this right, because one mistake can cause a serious problem. At the very least it can necessitate removal of the preamp and redoing some things (as you will see shortly!).

The silver Sharpie markings numbering wire bundles were coordinated with the new wiring diagram I made up... one, two, and three little marks designated wire destinations per the new diagram.

Here's what it looked like before I installed the new cluster ... the old cap bank (in the cans) was connected to the internal wiring via that "square" of terminals. That was removed and replaced with the new cluster.

From this .....

To this ...

So, that was HALF the job. I also had to install several other new caps inside of the preamp. Here's the other cluster .. stock.

And replaced with new caps .....

And there's where I screwed up!!! See that little blue cap on the right, next to the larger black ones? Yea ... I connected it per the factory wiring schematic ... which proved to be INCORRECTLY DRAWN! This was later proven that the particular drawing was incorrectly drawn and using it to wire up the caps proved to be a serious problem. The small cap had a voltage rating of something like 30-odd volts, the way it was wired up it was being hit with over 250 volts. The poor little thing couldn't handle much of that. With all of my careful picture taking, notes, diagrams, double and triple checking my work against the published schematics ... I went all "human" on this and improperly wired that little one. The consequences were very DRAMATIC indeed ... read on.

After initially finishing the job, I cleaned everything up, reinstalling/rewiring the preamp in to place, I anxiously fired the organ up. AH! SUCCESS! NO MORE HUM!! So there I was, happily playing my wonderful 1962 Hammond A102, rather loudly (hey ... gotta rock, y'know?). And quite suddenly, with no warning ....

FRIGGIN KUH-BANG-O!!!!!!!!!!!!!

Holy SHIT! It was as loud as a rifle shot. No kidding, it actually sounded as if someone had fired a high caliber rifle inside of my living room. My ears actually hurt. I instantly shut the organ down, and sulked. What the absolute HELL happened?

Here's what happened .... Dead Banana



This is the cover plate for the preamp, the cap puked it's guts and splattered on the cover as well!

So ... there ya have it, folks. How to blow the CRAP out of a preamp cap job in several easy steps. After I discovered what I had done wrong, I just installed another capacitor in that location ... PROPERLY THIS TIME ... and pensively fired the organ up again. It's been running just fine ever since.

I also made annotations and corrections on the factory schematic to prevent this from happening again should I find myself doing this job on another Hammond console that uses the same AO-28 preamp.

The sound of that ~rifle shot~ eminating from my poor Leslie and nearly splittin' my noggin open with the sheer volume of an audio wave has never left me. These days, as careful as I used to be when working on amplifiers .. I'm TEN TIMES more cautious today than yesterday. Well, perhaps not "ten times" ... but you get the drift.

Posted here, for your entertainment. thumbs up
Nice (?) read, interesting for sure smile
Thanks for sharing
Thanks for the cool story & pics, and well done on surviving the explosion!
Rex Coil 7
ArboriaAuralist wrote:
Thanks for the cool story & pics, and well done on surviving the explosion!
Thank you.

Heheh ... yea, I chalk it up to one the less intelligent things I've done. it was just so loud, and ~sharp~ of a sound. I'm impressed about how robust the Leslie speaker is (actually, it's an Ampeg 15 inch woofer removed from a 1960s B15 bass amp that I used to substitute the stock Leslie woofer ... it's a ~tonal preference~ thing).

At the end of the day I should have compared the factory diagram specs on the caps used to the actual cap specs that are mechanically imprinted on the cap ~cans~ themselves. If you look at the cans, especially the larger one, you can see the cap cluster specs right there on the side of the can. Had I used that information to either confirm or impeach the specs on the diagram I would have called the diagram sheets into question, at the very least.

I could have avoided this ....................... Dead Banana

We live, we learn. At least, that's the idea anyhow!
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