Cloning without a schematic...?

This DIY disease is progressing... ;D

It started with a pair of SCA N72's, then moved onto cloning an internal gain boost/preamp for guitar (including etching my own PCB), then most recently a RevD 1176.

My next conquest is an old Lee Jackson guitar preamp that has been out of production since 1988. I have searched far and wide, and unfortunately, I can't find the schematic anywhere. Now, I have no problem copying the layout exactly, etching my own board, and stuffing it with my original as a reference like a chimpanzee with no knowledge of electronics. But I'm sure that most of these components have become obsolete over the past 20 years.

There are a few components in particular that are stumping me. The main one is the blue thing on the lower left (ALCO HG2C-6S); it's just behind the DPDT push/pull pot for distortion. I have no idea what it is, or if any equivalent exists today. Nothing comes up in searches, so this may have become obsolete long before the days of the internet.

Also, there is a gold-colored component near the upper left of the board, which is what I believe to be a capacitor. I've never seen any like that, so hopefully someone can tell me what the composition is. The value and brand may be marked on the underside, so I may have to remove it to find that out.

http://dl.dropbox.com/u/324863/GP1000.JPG

Everything else is fairly straightforward and wouldn't pose too much of a challenge, at least it would seem that way considering how simple the design is. I guess a project like this would serve to satisfy my curiosity and get me some experience in reverse-engineering and schematic drawing. Design is still way out of my league though...

Many thanks to anyone who can help out.  

The blue thing - it wouldn't happen to have 8 pins on the underside, would it?  I suspect it might be a relay, especially based on the apparent routing going to it on the underside of the board.  We'll need you to at the very least take a pic of the backside of the board to tell you for sure.

That blue part is definitely a relay. Most likely a very standard layout DPDT relay, quite apparent from the traces going to and from around it. Hundreds of equal parts around, just find out the coil turn-on voltage with a multimeter.

The gold/silver/glass small part is a styrofoam cap. Quite widely available even today.

Thanks guys!

Here's the the PCB underside, complete with a little "afterthought" cable in the interest of grounding. And the many empty pads reveal the true Frankenstein nature of this design - definitely one of the goofiest layouts I've ever seen.

http://dl.dropbox.com/u/324863/GP1000_PCB_underside.JPG

It looks like only part of the relay is being used.

Ben J said:

And the many empty pads reveal the true Frankenstein nature of this design.

It looks like only part of the relay is being used.

Click to expand...

The empty pads are likely so multiple size relays could be used, or so it could be configured in a hardwired mode using standard .1" jumpers/header blocks.

As far as only part of it being used - pretty common.  They probably only need to switch 1 thing, hence only needing 1 of the 2 poles in the relay.  If they already were using the same DPDT in other designs, there would be no sense in adding another part to thier inventory.

In the end though, who really know......

Lee Jackson huh..didnt he design the Metaltronix amps around that time?...I know I've seen schematics for those online..possibly quite similar..?..
J

Yes, he designed the Metaltronix amps, and the M1000 is similar, but not quite the same.
http://www.metaltronix.net/manuals/m1000preamp_schem.pdf

I may have to put this project on hold for now, as I'm not sure how to go about the transformer - as far as I know they were custom made for Lee Jackson, and I'd probably have to make some sort of modification to get something off-the-shelf to work. I'm getting ~180VDC at the red and yellow secondaries, but nothing at blue and gray ones...

Bottom line is, I'm not as well equipped with knowledge of electronics as I should be to be working with dangerous voltages like this.

BTW, I couldn't seem to get any voltage readings from that relay to find its rating, no matter where I put the probes...  ???

Ben J said:

I couldn't seem to get any voltage readings from that relay to find its rating, no matter where I put the probes...  ???

Click to expand...

The relay does not MAKE voltage.

Meter an unconnected 120 Volt light bulb.... there's no 120V anywhere on it.

I bet, up to 3V across the coil, nothing happens; over 10V, smoke comes out then nothing happens. But 5V to 7V makes the relay clack, contacts move. At least that's _my_ guess: relays are made different ways _and_ in different voltages, "HG2C" is probably the construction, "6" may be the voltage. I found mention of HG2C-12S and HG2C-24S on surplus parts sites.

I'm getting ~180VDC at the red and yellow secondaries, but nothing at blue and gray ones...

Click to expand...

In an interview on this, Lee Jackson said it had 400v on the plates, so this should have some high voltages from the transformer. You might be able to find one that would work at Antec. But this is a challenging DIY project due to the dangerous voltages.
You should see ~6 volts for the heaters from the transformer also, which is possibly rectified and used for the relay.

Ben J said:

I may have to put this project on hold for now, as I'm not sure how to go about the transformer - as far as I know they were custom made for Lee Jackson, and I'd probably have to make some sort of modification to get something off-the-shelf to work. I'm getting ~180VDC at the red and yellow secondaries, but nothing at blue and gray ones...

Click to expand...

hi ben,
i'm not sure how you measured the xfrmr voltages as the secondaries from the PSU xfrmr should be putting out VAC (not VDC - unless you measured after the rectifier bridge).  what do you measure for VAC between the red and yellow and VAC between the grey, blue and brown (CT heater secondaries most likely) - my best guess from the images and wiring as to the HT & LT leads.

also look at the tubes and use a data sheet to find out more.  

kind regards,
grant

PLEASE BE VERY CAREFUL AS THIS PREAMP CONTAINS HIGH VOLTAGE.  KEEP ONE HAND IN A POCKET WHEN PROBING INSIDE ANY CHASSIS CONTAINING HIGH VOLTAGE!