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Author Topic: EMC "Zodiac" Amplifiers - Seeking Any and All Information  (Read 2053 times)

polaris26

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EMC "Zodiac" Amplifiers - Seeking Any and All Information
« on: November 21, 2017, 11:59:20 AM »
Hello everyone,
I hope this is ok to post here - I am trying to find any information about the solid state music instrument & PA amplifiers made by an obscure company called EMC (Electronic Music Corp?) from Ohio, USA in the late 60's/early 70's.  I have one of their Sagittarius guitar amp heads that I've been slowly restoring/modding, and any information like schematics, layout drawings, etc. would be helpful.  I was able to find a small sales brochure for that product line on eBay but it doesn't have any deep technical data.  Also I am missing some knobs and it seems like a total long shot to try to match them (molded silver plastic type).   Again any and all info would be helpful.
I do know that there are sites selling the info for the later models like the G300, etc but mine is an earlier design and I'm not sure how much overlap there is to pay for a schematic that may not be relevant.

Regards,
Dave 

Enzo

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Re: EMC "Zodiac" Amplifiers - Seeking Any and All Information
« Reply #1 on: November 21, 2017, 08:17:38 PM »
EMC, makers of weird stuff.   Information is sketchy.

GO over to MEF and ask there as well:
http://music-electronics-forum.com/forum.php


And it is always good to tell us WHAT information you really need, beyond "just want to have it".

Jazz P Bass

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Re: EMC "Zodiac" Amplifiers - Seeking Any and All Information
« Reply #2 on: November 21, 2017, 08:32:17 PM »
Yeah, there is not too much if anything 'out there'.

Layout drawing?

Hmmph.

galaxiex

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Re: EMC "Zodiac" Amplifiers - Seeking Any and All Information
« Reply #3 on: November 21, 2017, 11:41:35 PM »
Got pics of the amp?  :)

Is the circuit board so complex that it would be difficult to trace/draw schematic?
If it ain’t broke I’ll fix it until it is.

polaris26

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Re: EMC "Zodiac" Amplifiers - Seeking Any and All Information
« Reply #4 on: November 22, 2017, 12:46:19 AM »
This amp has three boards in it - one is for the "normal" channel, one for the vibrato/trem channel, and one is a power supply & power amp driver board (tethered to the output devices, which are mounted to a section of aluminum L-channel that bolts to the rear of the chassis).  The power amp is quasi-complementary - 4 pieces of RCA 40636 (hometaxial 3055s?) driven by two pairs of TIP29C/TIP30C.  I have maybe 85% of the power amp section sussed out, although a few things here and there are baffling. 

Apparently EMC liked to use what are now totally obscure devices, like JFETS called CFE13049, a GE 42C196 being used as the VAS in the power amp, etc.  Most of the devices are of what I call the 'space ship' package (small cylindrical body with a dome on top).  The boards are complex enough that flipping around back and forth to trace all the traces and figure out the entire circuitry would be time consuming.  I'm at the point where the amp works alright, but it doesn't sound great and is still a bit noisy/hissy. 

I made a few minor mods - the bias pot was not doing much even at the top end of the sweep, so I changed a resistance slightly in the bias spreader to give me just a few mV across the emitter resistors since it was sounding a bit cold (basically there was no bias current).  Also I lowered the gain of the 'mixer' JFET that goes in the front of the PA section to lower noise.  This is a clean amp - no overdrive or distortion of any kind is featured, nor would you want any, I suppose. I put a pair of fairly closely matched low-noise transistors in the PA input differential stage to replace the 2N4249's, which greatly improved the DC offset (there is no adjustment) and I think cleaned up the sound a little.
 
The tone stack I think could be improved; its like there is no sweet spot on the treble knobs, you either get a dull rounded jazzy tone, or you slightly dial in a little more treble and you're right into harsh earpick territory. 

I would also like to play around with multi-mode or totally current feedback since the amp has voltage feedback and I associate that with one cause of the hardness of early SS amps, but please correct me if I'm wrong (my experience up to now has been mostly with repairing and building tube guitar amps). 

If I could get rid of some hiss that would be great too.  I have a lot of low noise devices here, including JFETS, but I'm not wanting to go replacing parts shotgun style.  I'm not sure if the JFETs are hand selected, and there are quite a few in the pre-amp boards.

I realize the amp is kinda rare but at the same time not really worth much in terms of cash value, so I thought it would be fun to see how far I could go with it.  I changed the non-polarized two-prong power cord over to three prong and grounded the chassis (previously the amp had a polarity switch with "death cap" being switched either side of power cord to ground)  I installed a new neon indicator (the old one was blinking) and a new fuse holder.   I changed the electrolytic caps on the PA driver board with ones I had on hand. I reformed the large main filter caps with a variac over a 12 hour period.  I deoxited the controls and jacks.

I guess I could keep going the reverse-engineering way, assuming I have time.  If I end up tracing the entire circuit I will be sure to post it in case anyone else is interested.

Regards,
Dave

Enzo

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Re: EMC "Zodiac" Amplifiers - Seeking Any and All Information
« Reply #5 on: November 22, 2017, 02:35:58 AM »
I can see through the board to the traces on the other side from here.  Shine a light behind it, you should be able to trace it from one side.

Hissy noise is most likely a bad semiconductor.

galaxiex

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Re: EMC "Zodiac" Amplifiers - Seeking Any and All Information
« Reply #6 on: November 22, 2017, 09:19:14 PM »
Cool amp!  8)

Agree with Enzo, those boards would be super easy to trace.

I remove the board from the amp and scan the copper side.
Then manipulate the image in a graphics program, (I use paint.net).
Print it off so the traces are seen as if looking xray view from the top (component side) of the board
and then hand draw the components right on the page.
Now I have a layout and an easy picture to draw a schematic.

Also, I've had 70's vintage SS amps that were hissy.
Usually changing preamp transistors to modern low(er) noise types helped greatly.
Seems like the front end is where most of the hiss usually originates.

I've seen knobs like that somewhere.... let me check... I'll be back...

Hmmm, faulty memory on the knobs...

I thought this guy had some   http://www.surplussales.com/shafthardware/Knobs/Index.html

but I don't see them in his list.
Still, you may want to scan through what he has, or maybe shoot him an email.
Maybe he can help?

I've bought stuff from him before, no issues, good vendor.
« Last Edit: November 22, 2017, 09:29:26 PM by galaxiex »
If it ain’t broke I’ll fix it until it is.

polaris26

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Re: EMC "Zodiac" Amplifiers - Seeking Any and All Information
« Reply #7 on: November 23, 2017, 09:30:50 PM »
I made some progress today in that I was able to trace the main PA board to a schematic, and even work out a plan for implementing mixed-mode feedback.  I will enclose a jpg of the LTSPICE schematic (is there a way to attach an .asc file?)

The added current-mode feedback has lowered the gain and noise, and given the amp a lot better feel.  I am really starting to like this thing, even though it's still kinda junky, really. 

Note that there is no zobel (RC) network on the output, nor flyback diodes, nor an output inductor.  Only a crude VI limiter, and some DC current feedback loops.  What could possibly go wrong?

Maybe if I feel ambitious I'll start in on the preamp boards!

Regards,
Dave

Enzo

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Re: EMC "Zodiac" Amplifiers - Seeking Any and All Information
« Reply #8 on: November 24, 2017, 03:12:06 AM »
My first EMC experience was with their STEREO 8 channel powered mixer.  It was amazing.  The first four mixer channels went to the left amp, and the remaining four to the right amp.  I guess that makes stereo.

Your description of the bare bones circuit is consistent with their engineering.

polaris26

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Re: EMC "Zodiac" Amplifiers - Seeking Any and All Information
« Reply #9 on: November 26, 2017, 08:34:27 PM »
Thanks to galaxiex for the idea of inverting the PCB photos - that worked out very well, as I eventually traced out the entire amp (3 boards).
 
I just wanted to post a quick n dirty demo video of the amp as it stands now - definitely better than I ever remember -

https://www.youtube.com/watch?time_continue=288&v=AgJ1ot5Df8M

I may yet put together an upload file for the 'schematics' section, if anyone is interested.

Still looking for those silver plastic knobs!


Regards,
Dave

galaxiex

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Re: EMC "Zodiac" Amplifiers - Seeking Any and All Information
« Reply #10 on: November 26, 2017, 09:30:29 PM »
Glad it was some help.  :)

Amp sound quite good.  8)

Nice Strat! I like natural finish on those. Was it always natural or did someone strip it?
Great playing/demo too.   :tu:

Oh ya, we'd be interested in seeing the schematics.  :dbtu:

Edit; I'll keep an eye out for those knobs.
I'm often browsing for vintage 70's amp stuff.
« Last Edit: November 26, 2017, 09:32:17 PM by galaxiex »
If it ain’t broke I’ll fix it until it is.

polaris26

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Re: EMC "Zodiac" Amplifiers - Seeking Any and All Information
« Reply #11 on: November 26, 2017, 09:43:58 PM »
Thanks!  It's a Squier that I painstakingly (and ignorantly) refinished - came out rather well I must say!
I will try to get my docs together for a schematic post soon!


Regards,
Dave