SJ12T schematic

Another In A Series of "Close Cover Before Striking" Moments

This is a Weston Model 7145 bench digital multimeter which I obtained as sort of an homage to The Old Man, who'd worked for Weston about thirty years or so before the new owners, Schlumberger,  decided they didn't need him, or anyone else, or the factory and research labs in Newark New Jersey. He never got over it.

The picture, from 1906, didn't look all that different except that some of the slant roof glassed in assembly halls were removed to make way for a new office building in the 1940s I think.

But I digress. This was part of a line of digital test equipment that Weston introduced shortly before it's demise, and when it popped up new in the box on fleabay I had to have it. It was mine for about thirty dollars.

I powered it up the other day and saw in the display what you see-looking like missing parts of numerals. So today I decided to clean up the edge contacts on the LCD display. I took it apart enough to get access to the innards, popped the display out, squirted it full of juice and reinstalled it. It looked exactly the same. Then, some deep seated brain cell inside my head which had been waiting for this moment for years unleashed its voltage load to the mainframe and I stopped. A few minutes was spent looking for the manual-it's in my shop somewhere. At a loss, I turned the device over and there, as you see, are the display notes and rather than being 0009 with missing segments it reads ORNG-for range.

Good thing I was by myself and didn't have to answer to anyone for wasting an hour in another close cover before striking moment.

Straight Outta Yellville-The Ampeg Jet Reissues UPDATED

I've recently acquired two, and disposed of one, of the Diamond Blue series of Ampeg amps produced by St. Louis Music in the mid nineties which, we are told, are part of the URI a/k/a Updated Reissue series-reissues that have the look of older Ampeg amps but have all new circuitry.

There are four models that I know of-the J12R and J12T, and the SJ12R and the SJ12T. The J12R has the standard volume/treble/bass/reverb layout, but the J12T has volume/tone/speed/strength/reverb control setup.The -R series amps cater to the overdrive types but do not interest me much.

It's tube modulated vibrato, so why anyone would not want that is beyond me. I snagged a J12T off of fleabay for a good price, upgraded the reverb tank with a Mod tank from AES, and then passed it on to a happy person to use in church. It's cathode biased, 15w, uses a pair of EL84s and is a pleasant companion to pass the afternoon with.

I decided I had to have its big brother the SJ12T -alleged to be 50w with a couple of EL34s but more like 30-and soon had one here with a few quirks. The  rivet holding the terminal block on the speaker had broken so it was a small job to get rid of  the remains and install a new pop rivet. It's a nice big Ampeg labeled Eminence speaker by the way.

More seriously, the reverb sounded stinky and thin, and the vibrato was an on again, off again proposition. In addition there were random crackles and spits and pops.

The cabinet is wide enough to accommodate a 17 inch reverb pan so in one went, and things got better.

Replacing the plate load resistors is pretty easy because they're all on the tube board and easy to get at. That helped with the spitting but the vibrato was still dodgy, and then it quit altogether.

Studying the schematic led to the indictment of the J112 JFET in the vibrato circuit, after I'd replaced everything else because it was open and I had the parts. Installing the JFET cured most of the vibrato problems but it still crackled when the vibrato strength was turned up. This seemed to be related to the first preamp tube which is also the vibrato oscillator. This morning I tried a 6072 preamp tube which sounded fine but the vibrato failed to function. As I'd just tried a JJ and a Sovtek 12AX7 and still had the crackling in time with the vibrato, I dug into the archives and found an oldish Bugle Boy which had come from my brown Concert.

It was just what the amp doctor ordered. Now, I can get all the vibrato I want without crackling.

UPDATE: This did not solve the problem. In a day or so it was back with intermittent operation and crackling when the depth was turned up past a quarter. What to do?

I pulled off the back cover and removed the molex connector that links the foot switch board from the chassis. I still had the problem but found that monkeying around with the cable could shut the vibrato off completely or induce random crackles. If the foot switch board and foot switch were connected the LEDs on the foot switch could be made to flicker or shut off entirely.

So today I had a few spare moments to dig into it and the molex connector looked like a likely suspect. I removed the main board and resoldered the molex connector to the board. I then gave the board a lookover and resoldered all the joints that looked dodgy-there were a few, particularlyu on the ribbon cables.

I powered it up and all seems to be well, so far.

Internally the amps are simple and easy to work on, the traces on the board seem pretty healthy, and they've got the look. There's no secret sauce, channel switching, high gain doodads and so on. At 400 volts on the plates they'll last forever, and the tone is nice and clean, not buzzy and fizzy, and enough volume for smallish clubs. I got mine for $300 in minty condition, and the Jet II came for $250. The price is right for a nice medium weight amp with a classic look.

One thing I do not like is the small blue pilot light so I figure a regular jewel style pilot light with a green lens will do the Ampeg thing just fine.

And if they ever explode there's plenty of room to build something more traditional.

If you have one and you're wondering where to get a schematic, if you email Loud Technologies-the current owner of the Ampeg name-and give them your serial number and let them know you own the amp, they will send you a schematic.

Some Needed Clarification

Other than the name, the proprietor of this blog has no connection with the original Judybox debacle or any of the people involved. If you think it does it's a case of mistaken identity. I do have six circuit boards and a power transformer from the Judybox garage sale but that's it. There were rumors of a notebook full of technical type stuff but that trail petered out. Anyway the Revival is a very simple amp and the Hooligan is about the same, only it has EL34s.

Originally Posted by Blue Strat

Prairie Dawg "is" Judybox? I remember hearing about Judybox. Weren't there some TGP threads?

Just so everyone's clear, that's an adopted name.

The suspects from THAT debacle (which I have no connection to) have disappeared down a rathole although I'd be glad to help anyone with what little I know about the amps.

Some of my early posts are on the subject of the Judybox meltdown. It piqued my professional interest because it is a story that seems to be repeated over and over and over again around here. Someone builds a few amps, people like them, maybe they show up at NAMM, they take a whole bunch or orders and of course they don't have the dough to build them or never intended to, so they take people's front money and then they crash and burn and all but a lucky few get nothing. Pyramid scheme? It lends itself to it.


I got a Judybox amp because my wife is named Judy and I thought it would be a good gag for her to come home from work and see her box in the living room.

I liked the name, found it wasn't copyrighted, and staked it out. Since then it's a vehicle for my rants.

The  other point of information if you've read this far is this. Anything you do with any of the information presented here is strictly your own responsibility.

On fixing stuff

One of my associates has taken me to task (at least it looks like it but I could be wrong-it may have been some good natured chiding) about my slightly flippant observations on a site she linked to which offers instructions in how to repair things, particularly electronic doodads and gizmos.

It may be because this person has a sneaking suspicion that we males are possessed of some secret fund of tribal knowledge passed down around the campfire that we're not sharing with the distaff side.

I am flippant by the way. It is a habit that I can't seem to shake. But nevermind.

So here goes.

Fixing stuff.....I must say that first of all, no male role models in my family shared any of the male centric secret knowledge everyone seems to think we as the (slightly) minority half of the species possess.

My father was an engineer all right but he never worked on anything at home and if he did he didn't tell me about it. My first journey to Planet Fixit was when I was 18. I had bought a 1957 Chevrolet for $25 and of course it broke down. I had a date and I asked my father if I could use his car and he took a swig of his Schaefers and a long drag on his Camel, looked over his wire rimmed glasses at me and said "Have a nice time on the bus."

Believe you me, I started learning about the drum brakes on my old Chevrolet that very afternoon. My next adventure came when I decided I had to have a 1956 Ford. I found one in a guy's back yard. dragged it out with a chain, hauled it over to Dixon's gas station and started swapping parts with a wrecked 1956 that was sitting behind the place. Eventually I had a working car, even though I left the lug nuts loose on one wheel and lost it on Vineyard Road in Edison, New Jersey.

This is how you learn-from what doesn't work, what you screw up, and what blows up in your face or bloodies your knuckles.

If there was anything I wanted to be as a kid growing up, it was a really good mechanic, because they knew all the secrets of what I was interested in, and they did not-repeat NOT-share their secrets.

Later on I worked myself into some knowledge by working for Mickey Spitzer for two dollars an hour in his garage. I could use the lift at times though and the Motors Manual was on the shelf, and there was a phone where I could talk to other equally clueless codefendants.

The entire first half of my life was bound up in shitty cars and stuff that didn't work right because I was too busted to afford new or slightly used. it is said that yankee ingenuity is born out of sheer desperation, and I tend to think that's true.

Then, I got some technical training, but it was on the other side of the country, and I had to get there in a $75 Ford Falcon. If you don't think that was a test of my nascent mechanical chops, you aren't thinking. The training morphed into a job, and that led to some tinkering with obsolete electronics like what this blog's about.

See, it's not some kind of secret lore that's passed down only to men. The knowledge is out there and you have to go and find it. There's no shortcut to it. Along the way I've learned a few things.

I've learned that unless you have some basic tactile understanding-hard to explain but you learn when something doesn't feel right (or smell or look right for that matter)-an instruction sheet is going to be just so many hieroglyphs. Unless you have a basic understanding of systems you're adrift in a sea of parts .

But these things are knowable. It takes a while to get there. Being a good technician is a matter of study and application and a quest for understanding-kinda like being a Buddhist monk on a journey.

Having said that, there are some useful published works out there. I'm not going to tell you what they are, because if you hunt them out that will get you to internalize what's in there. merely being told what to do is not a learning experience.

Handy Troubleshooting Stuff

Sometimes when you're working on an amp you get a sudden volume drop as you advance the volume past a certain point. It could be oscillation above the auditory range, but how do you sort this out?

The answer is the garden variety AM transistor radio, an example of which is depicted here. Oscillation splatters frequencies all over the place, including the AM range of 535 khz to 1700 khz more or less, and the amplitude modulation radio system is more sensitive to interference.

Simply tune the radio to a place between 600 and 700 khz where there are no stations broadcasting, turn the volume up, and then wave your magic wand over the chassis you have advanced in volume until it oscillates. You'll hear a distinct hiss, and this can also be used to track down other sources of RF noise in the house like noisy computer power supplies. Some of the boys and girls who make cheap power supplies probably figure that nobody listens to am radio anymore, which is an attitude common among people who think the world is their anime oyster and nothing else matters but their own navels and their own disorder.

After all, if you don't like truth make up your own, right? You'd be surprised at how many of these folks think that Germany won world war 2. But I digress.

You've got to be careful here because oscillation imposes crazy loads on the circuit and you can find yourself in red plate country. If you're doing this it's wise to have some method of measuring bias current connected because if it spikes that's also a clue that you're into oscillation above the audible range.

Thanks to irishsammy, where ever he may be for the image that follows.

Here's a red plated tube in case you were wondering.

Danelectro DS100 Schematic

Folks, here's something that I found in the back of the Neptune Bound book by Tulloch. Most of us get by with the schematic for the Silvertone 1485 amp which is similar but not identical.

The DS100 uses a split chassis instead of having everything on one hunk of metal.This schematic will prove useful if you are working on one and trying to figure out where the two octal connectors-the one between the chasses and the one to the six speakers-go to. Aside from the fact that they call the same circuit a tremolo on one amp and a vibrato on the other, the DS100 and the 1485 are pretty similar.

This picture is of the very amp I'm working on, and it came from Springfield, Illinois. You can see more pictures here . I actually got a phone call from this fellow when my friend Justin arrived on my doorstep with this amp, which looks an awful lot like an Easter Island stone head.

The excavation is going to begin shortly, and I can't wait to see what's inside.