Colorsound Mighty Atom 7W battery powered practice amp - help with caps & diodes

[br00klynbear]

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

[J M Fahey]

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 .

[br00klynbear]

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!

[J M Fahey]

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.

[br00klynbear]

The circuit works as is with the polarities as they are...why is that?

[J M Fahey]

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?

[br00klynbear]

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!

[J M Fahey]

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.

[joecool85]

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

[br00klynbear]

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.

[J M Fahey]

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/

[br00klynbear]

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.

[J M Fahey]

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.

[Roly]

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.

[J M Fahey]

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.