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Old 3rd Dec 2022, 4:56 pm   #1
rv2p800
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Default Redifon MCU6B

I used this receiver multicoupler (1-30MHz?) for several years until it was stowed away with the rest of my "shack", now it will be put into use again - pending repair.
This coupler has two separate amplifiers with 5 outputs each and should actually be quite stright forward, not so. One channel is ok, one 30 dB down. It seems that there is quite a advanved level-feedbak circuitry that conrols the amplifying chain, the output amplifying transistors seems ok so the fault will be somewhere else.
So then, the usual question is, does anyone have any knowledge or schematics to help me out with this unit ?

Erik/
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Old 3rd Dec 2022, 9:11 pm   #2
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Default Re: Redifon MCU6B

I've no information on that unit, but can offer some general thoughts.

Low intermodulation distortion is important in a multicoupler, and this has been understood well for many years before that unit was built. It seems to use a push-pull pair of transistors in a transformer-coupled arrangement. Earlier units used power valves to get reasonable linearity. Those are probably 1-2W class TO-5 transistors. 2N5109 or 2N3866 for example. I'd have expected clip-on heatsinks on them.

An AGC arrangement, looking at the peak RF output voltage and controlling some variable gain element earlier in the chain wouldn't surprise me. The trouble is in getting low distortion in the variable gain section. In their transmitters, Redifon used to use long chains of diodes as variable impedance elements, but the linearity is not likely to be good enough for this application.

That large wound inductive thing seems out of place. Lots of inductance, but a low self-resonance frequency which doesn't suit 1-30MHz. I wonder if it is a saturable reactor or transformer where a ferrite core is driven into saturation to act as an attenuator, where the gain control sets the DC coil current?

David
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Old 3rd Dec 2022, 9:26 pm   #3
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Default Re: Redifon MCU6B

The Redifon MCU6 was announced at the same time as the R499 HF receiver, see the attached WW 1986 June item.

There is probably some chance that the MCU6 used the same, or similar RF amplifiers to that of the R499, of the class A push-pull feedback type using RCA 2N3866 devices. So absent an MCU6 manual, perhaps the R499 (for which the manual is available on-line) might serve as a rough guide.


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Old 4th Dec 2022, 11:39 am   #4
Jon_G4MDC
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Default Re: Redifon MCU6B

AGC is not used in Receiver Splitter Amplifiers in my experience.
Fixed attenuators yes, AGC no.

The aim is to provide just enough gain to dominate the splitter losses and so determine the overall noise figure.
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Old 5th Dec 2022, 3:16 pm   #5
G8BBZ
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Default Re: Redifon MCU6B

Hi Erik,
Synchrodyne is quite correct in thinking that the amplifier in your MCU6 is similar to that in the R499. In fact it is the same 20dB amplifier but with a splitter on the output rather than a direct feed to the mixer and without the tuning elements of the R499 amplifier.
There is an AGC loop but this should not come into effect until the input signal is about 500mV. The controlling device is a diode attenuator which is the sandwich unit in the middle of the board.
If you have any problem finding a circuit diagram let me know. I have a diagram but will need to scan it.
Good Luck
Peter G8BBZ
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Old 5th Dec 2022, 7:44 pm   #6
rv2p800
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Default Re: Redifon MCU6B

Hello Peter, I would very much like to get my hands on any information on the unit if You can help me ut
If this had been an ususal stright forward amplifier , what would all the other components on the board do?
The 100mA fuse on the board could be a supply-fuse; Nope, it is in the signal path protecting the input. From there the signal goes to the "sandwich". Before desoldering the unit, it consists as far as I can see 2 diodes, so the idea of feedback still makes a bit of sense. If the "variable attenuator" circuitry serves any purpose i my envionment is another story.
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Old 5th Dec 2022, 10:03 pm   #7
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Default Re: Redifon MCU6B

Ah, it's a diode set. From the photo it looked to me like a wound assembly of some sort. I seem to have been fooled by a brass/copper screen.

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Old 7th Dec 2022, 3:44 pm   #8
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Default Re: Redifon MCU6B

Hi Erik,
On closer inspection I realise that the RF Amp in the MCU6 is more similar to that in the R551 than the amp in the R499. The R499 amp has two balanced stages of amplification versus one in the R551 and MCU6.
I attach the circuit of the R551 front end in case you don't have it available.
The 100mA fuse in the MCU6 has been changed to a 250mA in the R551 but located on the primary side of the transformer. There is a 30MHz LPF on the output of the R551 amp to stop any stray 38MHz getting through to the first IF.
Apart from that I think the amplifiers are very similar.
Hope this helps
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Old 10th Dec 2022, 4:21 pm   #9
rv2p800
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Default Re: Redifon MCU6B

Problem solved, one of the output transistors (2n3866) had turned into a diode and after a quick change the coupler section works normally The transistors are getting quite warm so i added a cooling star on each, it can't hurt. I used the occation to solder ut the mysterius "attenuator", as the drawing shows, consisting og 12 diodes and 3 caps, clamping down the input voltage to about 4V !
Thank you for all help
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Old 10th Dec 2022, 4:22 pm   #10
rv2p800
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Default Re: Redifon MCU6B

The attchment
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