In the thread "Maximum Impedance" I am writing about microphone building experience in the middle of which my Radio Shack 32-2056 4-channel Mic Mixer stopped working.
Replacing a defective diode wasn't enough to get it going and while I continue to repair it I was going to purchase another one, but the gods are making it difficult.
The Radio Shack website twice went into total freeze and locked my browser, so I looked elsewhere and have reason to believe that this particular mixer is no longer available.
Who else makes a simple 4-channel microphone mixer?
Shure M67....tried and true!!!:)
I've used several over the years....
Check FleaBay......Good Luck!
True Oldie919 the Shure mixers are excellent.
As far as repairing this Radio Shack 32-2056 it would be nice to have a schematic. Does anyone know if Radio Shack schematics can be found somewhere?
The manual does not contain one.
Since the last post I've found a mixer stashed in our microphone closet... a Realistic 32-1105. It's 4-channel mono or stereo with XLR connectors, battery power only. The XLRs show surface rust and should be replaced. A "battery test" button is for some reason removed. This baby will be worth fixing up.
Also I found this website for Radio Shack product documentation, manuals, spec sheets, whatever was provided with the products.
Radio Shucks Product Literature
During our big commercial days we used a Panasonic Video Audio Mixer AVE-5 and that's still in storage, but takes up a fairly large footprint which would not fit well in the present control room.
In fact, this control room is out of control.
This August Saturday will be spent reverse-engineering both of these Radio Shack mixers so we have the benefit of schematics.
All other projects are on hold for now.
I take time-out every few moments to report on this activity here on the website as an act of slacking, which is highly recommended as a way of making projects last longer.
The old Realistic 32-1105 Audio Mixer is a real time-machine to the 90s when it played a role in our AV business.
As if I was a different person all together I discovered that I rebuilt the mixer way back then for a project at that time... I removed the unbalanced (RCA?) jacks and installed XLR inputs and output connectors.
Two inputs have Beyer Mic Matching Transformers installed by me so we could use our professional mics.
The output is still unbalanced, despite the addition of XLR connectors, so we had to be sure to use the right matching cables or it might not have worked.
Partially restored memory recalls that the mixer was plugged into the mic input of a Sony Hi8 Video Camera.
I'd enjoy meeting myself back then, I was very smart on short notice, a trait that's gone by the wayside.
I don't mean slam down your phone. No, I mean I am presently back to working on the Radio Shack 32-0256 Mixer, acting on the hunch that the voltage supply section was injured... this is where the unit switches between battery or wall wart... I am hung up checking two transistors in the circuit.
All six transistors in the mixer are C945 P57E, the kind with a flat edge on one side.
A part search tells us this part is obsolete, and the alternative is 2SC945, data sheet here:
What's got me stumped is the pin-arrangement shown... it shows the collector in the center... normally the collector is on one end. When I test it I would prefer to know which pin is which. Do our veterans of electronics know about non-standard pin line-ups in transistors?
Poking around on the Search Button I found what passes as an answer about transistor pins
M. Bla, 2SC945, that's an extremely popular transistor in so many products starting 20-30+ years ago, they were and are in everything, you should have no trouble finding that part.
I remember it well from youthful experimenting, when I had a 1 transistor FM transmitter. Can you believe you can make a transmitter with one transistor and a handful of parts that will sound high-fi on a an FM radio, though in mono?
No one would believe it today with the ICs that are out, it was as simple as a one transistor radio receiver, about 10 parts you could get from Radio Shack, and a coil wound around a Bic pen.
This was after the AM kit I used with my friends. There was a preamp-modulator, but I fed speaker audio from a tape deck right to the base of the final, the B lead on the transistor, I think it was fed through a 5 UF cap going to the base.
The cool thing about that simple circuit was they weren't limited in power like today's ICs. I could crank up the power by putting 3- 9 volts together and it got out all over the place! I kept blowing out the oscillator, so that's why I knew where to get transistors, he he. I think the first was a C710, but I found C945 worked great too. I'd just clip the leads on the old tansistor and solder the new one right on top, it was just hanging there.
It had a tiny frequency trimmer cap that got staticy cause I keept turning it, then the screw part of it just fell off, and I replaced it with a a disk cap, like a 24 pf with a black dot, NPO, and used a stick to bend the coil spring to tune it in to my frequency, and that was hard to do.
I built it from the schematic from another kit, along with the trashed kit, and I rebuilt it. I didn't have friends broadcasting with me on FM, too many breakdowns, and the engineering wasn't up to snuff, too experimental.
I can write about this now, because the Statute of Executions is probably expired 🙂
Good answer, good answer, and I like that guy's page, nice and clean layout, and no ads, good for him! Thank god for Radio Free Web!
It's true. The two transistors in the battery/power section of the Radio Shack Mixer are defective.
While not having extra 2SC945 transistors nearby, we do have several PN2222A transistors.
The 2SC945 is a very low voltage device, the PN2222A capable of higher voltages, and the pin arrangements are different, but other than that, why can't I just twirl the legs of the PN2222A transistors and install them? They're all in the NPN family.
Carl, your transisters are crossing their legs.
They're shy.
While I patiently await a reply to questions raised in recent postings, I am preparing to select two NPN transistors from my old stock of obsolete parts and install them in the Radio Shack Mixer to see if they can get us back in operation.
The worst thing that could happen is that it won't work.
And the worst thing that could happen is that the transistors will explode.
Two worst things.
I won't do it right away.... we'll set the deadline as 5 PM CDT but haven't said which day.
There exist many pinouts for transistors such as you have discovered with the 2SC945. Using an ohmmeter one can identify the base but not differentiate between the collector and emitter. If you have a meter which measures the hfe then you can switch the C and E leads and the one which gives the higher hfe is likely the way it is labelled on the meter socket.
Many times transistors are specified for a design based on cost rather than operating parameters. For switching and audio amplification the 2N3904 is pretty much a universal replacement for NPN transistors. Its cousin, the 2N3906 is the PNP device of choice for repair and prototyping here.
A good design is one in which the biasing and gain are set by the circuit and not by the transistor. This allows wide latitude in substitution.
Two specs need be watched though. The VCEO specification needs to be high enough to work in the circuit and the fT (cutoff frequency) needs to exceed the circuit operating frequency. Other than that, the transistors mentioned above will satisfy most replacement requirements.
Neil
Excellent, Neil.
Found nine BC 109B transistors from an audio pre-amp project in the 1980s, confirmed NPN, pin arrangement known, and I'm still cross-checking the other characteristics.
Because the BC 109B operates at higher voltages than the transistors being replaced there is no obstacle expected, and we may be back in the audio business "before you know it" which is a very speedy rate of time.
Don't go anywhere until I come back.
Yes, it looks like there might be some weather building up in the sky.
Meanwhile, from earth, a "Hunch Report" is now issued as to the probable reason this Radio Shack Mixer blew some parts.
The power from the wall-wart connects to the mixer by an L-shaped female power connector with DC+ on the outer ring and DC- on the center connector.
When the power connector is pushed into the power hole on the rear of the mixer it breaks a circuit by pushing aside a blade which disconnects a battery if one is installed.
One day not long ago the mixer was pulled forward on the table during a clean-up process. The L-shaped power connector snagged on the back edge of the desk and partially pulled out, causing intermittency between external and internal power, which perhaps caused a string of parts to burn out, including a diode and two transistors.
The manual page 6 states "Always connect the AC adapter to the mixer before you connect it to AC power". It is reasonable to believe that the reason for publishing that advisory is precisely to avoid the breakdown we experienced.
As far as I know so far the damaged circuit plays a role in turning on an LED indicating a low battery. Perhaps I should check the LED?
One odd thing about this mixer... it was sold with no power whatsoever... both a 9-Volt battery and the AC adapter were optional accessories. A mixer that can't be used as purchased is not a mixer, it's only a potential mixer.