Hey everybody - I'm a total newbie; I've had some experience with circuit boards and op amps, but it's been since college which is over a decade now...yikes.
Anyway - my bassist brought a little practice amp from the 50s called a Mighty Atom into our rehearsal space, and I actually love the way the little guy sounds, and I'd like to replicate the circuit. Using a multimeter, I've been able to identify all the resistors in the circuit, and the axial caps are obvious (their farad rating and voltages are printed right on them, so that's easy), but I'm having trouble with 6 littler capacitors that aren't so obvious, and I haven't been able to find similar looking ones online anywhere. They're little, thin, square and terra cotta colored, and I'm wondering what the modern equivalent of them would be. I also don't really know how to measure them; my multimeter claims to be able to measure capacitance, and I was able to get accurate reads on the axials, but I can't really tell if these little guys are just too...well, little, or what. There's also a little diode in the circuit with "CPMST101" printed on the side; from above, it's fully circular (not a half-circle like some other diodes).
Any ideas? I'm attaching a picture of the circuit board here, with a couple of the caps I can't identify circled. I think once I figure out what these little buggers are, I'll be able to reverse-engineer the circuit, and I'll certainly share the schematic once I do!
Thanks so much for any help!
br00klynbear
Those brown square ones are ceramics.
They *must* have some kind of code printed on one side.
Copy it here.
I don't see the diode ypu mentiom.
The "1.1K" are probably 1K (isn't the 2nd band black?) and "320K" probably 330K (2nd band orange).
Big problem will be obsolete TBA810.
Lift the circuit anyway, if it has some interesting details it can be adapted using TDA20xx or something .
My mistake; the diode is actually a transistor, and at first glance it appears mostly circular, but has a very small flat edge to it.
Here are pictures of the caps (which I believe now are probably just old versions of ceramic equivalents today.
I'm also including the back of the PCB and a pic with the PCB reversed/superimposed with a few other notes.
Thanks!
Well, we have found your first problem: reversed battery !!!
Your *RED* (hint hint) cable labelled "9V negative" is obviously *positive*.
Besides, it goes to the positive leg of the vertical blue electrolytic.
And the 9V negative goes to the jack ring.
The circuit works as is with the polarities as they are...why is that?
Please recheck. :)
EDIT: meaning, I'm sure "it works"; not so sure you have correctly identified battery terminals or something like that .
What do you measure from that red wire pad to ground?
What from TBA810 pin1 to ground?
From Pin1 to Pin10?
Ah, it seems you're totally right; my mistake...again, haha. Looking at the 9V battery adaptor, it looked as though the red lead was connected to (-), but after cracking one open, I saw that it doubled back onto the (+) lead.
At this point, it seems I've been able to identify all the caps, but I still need a little help with the transistor, as well as finding a modern equivalent replacement for the IC. I did find a datasheet for the TBA810, attached here.
Any thoughts?
Thanks for the taking the time, I really appreciate it!
Since you will not get a TBA810, that PCB design is useless for you.
I'd lift the "preamp" schematic, since you like the sound, and after that adapt it to some TDA2030 or similar.
If you can't read some value, draw the part anyway, sometimes its value can be deduced depending on what it's connected to.
Quote from: J M Fahey on September 24, 2012, 12:58:24 PM
Since you will not get a TBA810, that PCB design is useless for you.
I'd lift the "preamp" schematic, since you like the sound, and after that adapt it to some TDA2030 or similar.
If you can't read some value, draw the part anyway, sometimes its value can be deduced depending on what it's connected to.
Actually, I found them on eBay for as cheap as $1.10 each. They may well be obsolete, but they are still available if you look.
Also you can get them as part of a $20 kit: http://electronickits.com/kit/complete/ampl/CANUK153.htm
So, I did a schematic today, but I'm still not sure how the transistor is functioning (e.g. PNP, NPN, FET...). Can any of you infer from the rest of the schematic? I'm also still unsure of the value of the larger green resistor; I have it as 220k, 10% right now, but there's another resistor with that value that looks way different...any thoughts?
I'm attaching the schematic (hand-drawn, so kind of sloppy, sorry!) and the key here.
Cool :tu:
Thanks for posting !!
On first sight looks very reasonable, will check with a little more available time.
NPN transistor preamp, Volume control with bright cap, somewhat similar to Big Muff tone control, straight into the chipamp.
I'm mainly interested in the gain and EQ ; so it can be reproduced either as is or with a modern chipamp.
What is the actual wall wart voltage?
Because many of these portable amps claimed 9V (which actually was the minimum) but were happy with, say, 12 or 16 V. :dbtu:
EDIT: Hey!! , look what the cat brought !!!
Not a 100% clone but, what, 95% ???
http://www.schematicx.com/schematic/vox-escort-mainsbattery-amplifier--late-1970s-schematic/
Whoa, that's interesting. Very similar indeed, give or take a few values. Interesting that pin 7 goes through a cap to ground; on mine, 7 is deaded. It also seems that pins 4 and 12 have been swapped.
I think the amp originally used 2 6V batteries in series; they were strangely-shaped terminals, like for a 9V but farther apart. I haven't seen batteries like it. There's no input for AC/wall wart.
Then those were 2 x 9V batteries, not the small pedal type ones but some *much* larger which were favored in the UK.
Must have cost a small fortune in their day.
Old ceramic caps; 220p = 220pF, 470p = 470pF, 2K2 = 2200pF, 4k7 = 4700pF.
I suspect that the two resistors marked "1.1k" are actually 1.0k, the second band is meant to be black; similarly the one marked "320k" will be a 330k and the big green one marked "220k 10%" looks like red-red-green a 2.2Meg to me, the "50ohm" is marked green-blue-black or 56 ohms.
If I recall, the batteries JM is talking about were roughly 2" x 3" x 4" high, a "layer battery" with a couple of snap terminals on the top that were very similar to a modern 9V battery but bigger, about 1/2 inch, and wider spaced. These were used in the largish early transistor radios. They must have co$t, but they ran an 8-inch speaker for quite while.
Agree and add: Roly's corrections are fine, but I would go even one step ahead: given that the Atom is *clearly* a Vox Escort knockoff, I would go straight to the source and build the original Vox one.
Or, worst case , copy the Atom PCB (it will save you some time) but use Vox values.
Even better, I would adapt it for a TDA2030 or 2050 (same pinout).
In fact I'll probably draw a PCB and make a few commercially ;)
Only problem is getting a cheap and good transformer, because winding thousands of turns of very fine wire is not cost effective to me.
I *much* prefer winding a 100W transformer than a 7W one.
Yep, copy the preamp; modernise the power amp; use a couple of dry 6 volt "lantern cells" or a 12 volt gel cell for portable, and 12-18V plugpack for home/recharge (with suitable recharge cct).
Dear Roly, thanks for posting and simulating, but please correct your schematic (and re-simulate) by making the bright input cap 2200pF (written as 2.2N in the original schematic). (yes, it looks like a capital N ; they should have used small caps)
Yea, I know, Vox had a peculiar way of writing parts values inside the (too small) box and in confusing column way.
Oh well.
Vox Escort preamp related stuff...
Rev 1.1 (thanks JM)
I too have one of these little gems . I believe that mine says Rochester N.Y. on the front sticker/label . I sounded great and yes it has/had 2 of the large 9 volt batteries . one volume and one tone . to power it up you just put the guitar cord into the jack . there also was a speaker out on the back . I was hoping to get mine fixed as it had incredible tone. I have been reading thought the slew of messeges but havent finished them all . I was hoping somebody came up with a schematic that I can bring to a repair guy as I really am not one . Is anybody confident in what has been submitted here? The only reason I found you guys was that everyonce in a while I try a search for Mighty Atom on yahoo. Alas I have found hope!
This is all amazing, you guys! Thanks for the schematics and tests; it all looks pretty promising. I can't believe posting this thing on here got such a thorough and careful analysis. You guys are truly champions, and I really appreciate the response!
My main question at this point is how to integrate the preamp circuit with a TDA2030 or 2050. Would it really be as simple as replacing the original opamp and use the same pinouts? Is it still acceptable to use 9/18V power? And will using a newer amp chip alter the tone in any way? I'm assuming not, as the "preamp" stage has already been resolved, but I'm just curious. Again, not as thorough an electronics engineering type as y'all, but I'm trying to learn! :duh
Thanks for the thanks.
Quote from: fursteak
Is anybody confident in what has been submitted here?
Well looking at the Atom pix and the Vox circuit I agree with JM that they are
very similar if not actually identical. I didn't examine the chipamp portion since it appears to be one that is no longer current, and the OP related to the preamp section.
Am I confident about the preamp? Well since JM spotted my component value error (now corrected) and a one transistor preamp is hardly rocket surgery, I think I can answer "yes".
While any repairer likes to have a circuit any tech worth their salt should have no problems at all with this amp without one, and while I can't speak for JM or Enzo I'll bet that like me the vast majority of gear they have fixed over the years has been without the luxury of a circuit. Given parts, any tech who can't knock this over in an hour or two isn't worthy of the title.
Quote from: fursteak
Would it really be as simple as replacing the original opamp and use the same pinouts?
If a dead TBA810 is your problem, and you can obtain a replacement, yes, otherwise no. JM suggested the TDA2030 which is functionally similar with a higher power capacity, but the pinout is entirely different.
Note that while this chipamp has a higher power capacity that
doesn't mean that you will get more power, only that it is much more robust in this application, the actual power output being set by the supply voltage and speaker impedance, both of which will be the same.
Quote from: fursteak
Is it still acceptable to use 9/18V power?
Yes; the TDA2030 will work on supplies from 6 to 18 volts.
Quote from: fursteak
And will using a newer amp chip alter the tone in any way?
Very unlikely. The dominant factor in a small amp like this is the speaker and the cabinet it is in. In fact one way to give any small amp like this "legs" is to hook it up to a much bigger speaker cabinet; it may seem a little louder, but it will certainly sound "fuller".
The other main factor is the rather low input impedance of the preamp which will tend to make any
passive guitar sound a bit dull. While the tone control, very similar to that used in the "Big Muff", has a good range and can compensate to some extent the damping effect of the input on the guitar means some tonality is lost and cannot be recovered. One way around this is to use a high impedance buffer such as the AMZ FET buffer, however if you have an
active guitar (one with a battery in it) then it already has an inbuilt buffer and it shouldn't be an issue.
HTH