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Author Topic: Bozak CMA-481  (Read 1579 times)
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W3SLK
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« on: May 29, 2023, 10:51:24 AM »

I have a Bozak CMA-10-1 mixer that is populated with 8 CMA-481 input transformers. Unfortunately, the unit does not specify. I contacted Bozak, (now in the UK), but they suggested either sacrificing one and have a winding analysis done or look on Ebay for descriptions. Does anyone know what the input impedance is of these? I know they also made a -482, so I suspect there are different input impedances. Help as always is greatly appreciated.
Mike(y)/W3SLK
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Mike(y)/W3SLK
Invisible airwaves crackle with life, bright antenna bristle with the energy. Emotional feedback, on timeless wavelength, bearing a gift beyond lights, almost free.... Spirit of Radio/Rush
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« Reply #1 on: May 31, 2023, 09:49:30 AM »

I have a Bozak CMA-10-1 mixer that is populated with 8 CMA-481 input transformers. Unfortunately, the unit does not specify. I contacted Bozak, (now in the UK), but they suggested either sacrificing one and have a winding analysis done or look on Ebay for descriptions. Does anyone know what the input impedance is of these? I know they also made a -482, so I suspect there are different input impedances. Help as always is greatly appreciated.
Mike(y)/W3SLK


generally if balanced input then is low impedance:  50 to 600 ohms with 200 being typical

I would establish turns ratio by applying small ac  voltage to secondary and measure voltage induced on primary.  Impedance is then the square of the turns ratio.
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K8DI
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« Reply #2 on: May 31, 2023, 11:07:37 AM »

Seems like an opportunity to rationalize buying a LCRZ meter…. Turn it on, hook the meter to the input and measure input Z. Take one out of circuit set up a sig gen and measure the voltage/turns ratio. Calculate output Z from ratio and input Z. Done.

https://www.amazon.com/Precision-879B-Display-Handheld-Universal/dp/B004O8Z8E2/ref=mp_s_a_1_3?adgrpid=56791795595&hvadid=580720828947&hvdev=m&hvlocphy=9017487&hvnetw=g&hvqmt=b&hvrand=134052140927091153&hvtargid=kwd-300864203413&hydadcr=17061_13422678&keywords=bk+879b&qid=1685545603&sr=8-3

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« Reply #3 on: May 31, 2023, 11:11:28 AM »

Find some Triad A-67J transformers.  150/600 to 150/600, hi-fi ratings.

eBay is a good place to buy them, less then $100 per.

Don't make this any more complex than necessary.

73DG
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« Reply #4 on: May 31, 2023, 06:57:09 PM »

Like I said, I have 8 of them. But I'm going to remember those Triad transformers if I need some. Frank, WA1GFZ sent me this via PM: Mike,
Actually pretty easy to measure input Z. Take a variable resistor and put it in series with a function generator output. Set the generator to say 1kHz with 1 volt output. Monitor both sides of the resistor with a scope. Adjust the value of the resistor until you see 1/2 the function generator output voltage across the resistor. This means the reflected impedance of the transformer is equal to the resistor value. Then you can sweep the transformer
to see the freqquency response.
It is better to do it in circuit so you get the reflected Z rather than the open loop winding reactance that
should be much higher.
I do this all the time.

I'm going to try this just for the shear exercise. Maybe someone else has a use for it too. Right now, 'alternate forces' have dragged me away from the bench for the time being!
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Mike(y)/W3SLK
Invisible airwaves crackle with life, bright antenna bristle with the energy. Emotional feedback, on timeless wavelength, bearing a gift beyond lights, almost free.... Spirit of Radio/Rush
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« Reply #5 on: June 02, 2023, 04:56:11 PM »

So, someone over on the Antique Radio Forum had this little gem and posted it to me. It pretty much confirms what I suspected but I wanted to make sure.

CMA-10-2 Stereo Mixer
All -silicon solid-state ten-in/two-out stereo mixer;
each input has individual level control, speech/music switch, and 10 -dB input attenuator in mic
mode, switchable at front panel to left or right output or both; modular design accepts variety of plug-in low- and high-level circuit cards. Output channels have separate bass and treble controls with 10
dB boost or cut, VU meter, and independent range
switch; master gain control. Gain 78 dB max. with
A -1002E card and CMA-481 transformer; frequency response 20-20,000 Hz -0.25 dB; power
output -24 dBm; input impedance 200 ohms (with
mic transformer), 100k ohms (low-level direct), 47k
ohms (magnetic phono), 50k ohms (high level);
load impedance 600 ohms balanced (with CMA-558 transformer) or unbalanced; dist. 0.25% at
24 dBm; noise -125 dBm; 7" H 19" W x 12" D $1250
CMA-10-1. Similar to CMA-10-2 except has mono
output; gain 90 dB with A -1002E card and CMA-481 transformer; output hum and noise 70 dB;
H x 19" W x D $825
A-1002E. Microphone preamp card $25
A-1017A. High-level preamp card. $6
CMA-481 . Microphone input transformer $43
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Mike(y)/W3SLK
Invisible airwaves crackle with life, bright antenna bristle with the energy. Emotional feedback, on timeless wavelength, bearing a gift beyond lights, almost free.... Spirit of Radio/Rush
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