If you're going to think about projects like this then you might want to get more familiar with a soldering iron. 😉

http://www.youtube.com/watch?v=r4f0Fi_fCyE

An approach that should work is a variable AC adapter and some alligator clips to attach to the end-point positive and negative contacts in the battery compartment, but be careful to match the voltage and current (amperage).

A non-DIY solution is Eneloop AA batteries mounted in C "spacers", but the power capacity isn't great compared to C cells.

If you have a multimeter measure how much current flows. If you don't have one an approximate running time and using the Ah rating you can work out the current. Then you can pick a suitable AC DC adapter.

The power requirements are often printed on a sticker on the back. If not, you'll have to measure the current like Phil said. Voltage is easy: if all batteries are connected in series just multiply the number of batteries by 1.5V.

Then you basically have four options: either connect the adapter leads straight to the battery terminals, drill a hole in the case and add a DC power plug, make a dummy battery pack, or take the adapter apart and mount it inside the battery compartment. I personally like the last option since you'll only have an AC lead coming out, without a wall wart taking up three sockets 😀

Keep in mind though, that the circuit might not be designed for AC adapter use. Batteries have a very steady voltage, slowly dropping over time. Voltage from an adapter may be more fluctuating or just downright noisy. Audio or video equipment can be especially sensitive to this.
Luckily, with a C-cell battery compartment, you'll have more than enough space to add extra capacitors if this turns out to be an issue 😀

jwt27 wrote:

Keep in mind though, that the circuit might not be designed for AC adapter use. Batteries have a very steady voltage, slowly dropping over time. Voltage from an adapter may be more fluctuating or just downright noisy.

That's actually something I didn't consider... Maybe I should first see just how well the thing will last on batteries and then worry about modding it if it eats through batteries like candy. I've read reviews which suggest either's possible, meaning it might run badly on rechargeables but perfectly fine on non-rechargeable.

Don't want to say what it is yet because I might actually make a video about it. :B

I had a small audio mixer that when operated with batteries it was good, when I used a power adapter I was having noisy output.
I think it was jwt27 😊 that offered me advice on how to fix it, IIRC I just put a 1800uf electrolytic and a 100nf ceramic caps in the power input and the problem went away... I am pretty sure that doing the same and using a quality adapter you'll be fine.

Connections clipped to the battery terminals, or make up some dummy batteries to take the connections and pack the space.

Use a regulated PSU of the appropriate voltage (1.5V/cell, so if it's 4 cells, 6V) - avoid unregulated ones, as the off-load / low load voltage can rise considerably - the current rating needed would be a bit of a guess.

It might need a notch cut in the battery cover to take the wire out through.

You just need a good power supply that's appropriate for the load. It's easy to mod these things. Keep updating so we can help you.

I wouldn't just go attaching a regulated power supply to the battery terminals willy nilly. Especially not on a bit of retro gear.

Remember that batteries are not a regulated supply, their voltage drops as load increases (separately from the voltage drop due to the battery being depleted). Some old devices relied on this effect (those Casio portable TVs come to mind), and were susceptible to damage if using batteries with different internal resistance (NiCd or NiMH cells).

YMMV. Tread carefully.

Wow, never heard of that before. That's one of the dumbest things I've heard in a while, to make your circuit design rely on some flaw in an external factor, that's not guaranteed to be constant, and in theory shouldn't even exist in the first place! 😳
Wikipedia tells us:

The internal resistance of a battery is dependent on the specific battery's size, chemical properties, age, temperature and the discharge current. It has an electronic component due to the resistivity of the battery's component materials and an ionic component due to electrochemical factors such as electrolyte conductivity, ion mobility, and electrode surface area. Measurement of the internal resistance of a battery is a guide to its condition, but may not apply at other than the test conditions.
[...]
Internal resistance depends upon temperature; for example, a fresh Energizer E91 AA alkaline primary battery drops from about 0.9 ohms at -40 °C, where the low temperature reduces ion mobility, to about 0.15 ohms at room temperature and about 0.1 ohms at 40 °C.

Anyway, if this turns out to be a problem, you could simulate the internal resistance with a simple resistor in series, right? (not exactly of course, but close enough to simulate a full battery that never wears out) I guess 0.15 ohm multiplied by the number of batteries should do it.

If the product is using digital logic it's going to need a 5V supply or what not. It could just be a 7805 circuit being fed 4 C-cells.
I have better access to low noise switched mode power supplies than most people I guess, but that's what I use on this kind of stuff. Definitely don't use a random 1990's, wall wart, made in China, found at a yard sale unit.

jwt27 wrote:

Wow, never heard of that before. That's one of the dumbest things I've heard in a while, to make your circuit design rely on some flaw in an external factor, that's not guaranteed to be constant, and in theory shouldn't even exist in the first place! 😳

It isn't a flaw that "shouldn't even exist", it's a fact, because there's no such thing as an ideal voltage source.

And I guess I could have worded it better. The device doesn't "rely" as-such on the effect to function, but it is very much designed with the effect in mind: that the battery pack can only deliver a certain amount of current. Rechargeable batteries can deliver more current, which could kill certain sensitive components.

You're right that it does vary, but the difference in internal resistance between primary cells and rechargeables is quite large, compared to the difference between alkaline batteries of different brands etc.