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Old 15th Nov 2018, 1:46 am   #41
Bill
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Default Re: ZN414 and TA7642 radios

Quote:
Originally Posted by nutteronthebus View Post
My Dad worked at Ferranti's and got me one of the pre production ones in the early 1970's ( I was still at school) I built a matchbox radio with it
Co-incidence there then! I was a supervisor in the pre-production assembly area at Gem Mill in those days. We assembled a load of ZN414s for the "Evaluation" department to test. I remember being amazed at the claimed performance of the one chip radio. We did a lot of other stuff then, including various ULAs and street light controller/timers that would work from the mains with just a series resistor to drop the voltage.
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Old 15th Nov 2018, 6:25 pm   #42
Neil Purling
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Default Re: ZN414 and TA7642 radios

Andrew: In post #38.............. What does that mean in terms of the input level & the output?
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Old 15th Nov 2018, 7:17 pm   #43
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Default Re: ZN414 and TA7642 radios

It's no good, Neil. I'll have to lash one up and do the checks again, my memory has failed...
There's nowt on the telly at the moment so I'll do it now. Do you want the levels in mV rather than dBm?
I'll also attach a photo of the output waveform to show the distortion.
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Old 15th Nov 2018, 8:04 pm   #44
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Default Re: ZN414 and TA7642 radios

OK here we are, Neil.
Test cct consisted of TA7642 with output load res (pin3) of 1.2k, supply 1.5v.
AGC bias res (pin 3 to pin 2) of 120k.
Input from sig gen via 0.1uf to pin 2.
nothing was properly terminated, so levels may be somewhat questionable.

Useful audio (around 100mV p-p) output appears with 70uv p-p input. There is very little AGC action at this point.
AGC starts to cut back the gain at 200uV input, distortion appears on output audio, which is now at around 200mV p-p.
With 400uV p-p input the audio out is slightly over 200 mV p-p and the distortion is worse. Screenshot attached.
I thought the distortion was perhaps caused by audio getting back onto the input pin via the bias resistor, but splitting the 120k into two 56k's and decoupling the junction made no difference.
Hope this helps. It's still all lashed up so let me know if you want any more tests before we start on the wine & crisps!
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Old 15th Nov 2018, 8:16 pm   #45
Neil Purling
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Default Re: ZN414 and TA7642 radios

I was feeding the signal into a Mullard 3-3, which was designed for 100-200mV on the input & it has given a decent output with the volume half way using just the ferrite rod coil oriented for best signal.
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Old 19th Nov 2018, 12:08 am   #46
Neil Purling
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Default Re: ZN414 and TA7642 radios

Andrew2's test result matches my results, for the output of the chip with no amplification.

I have tested the Thai Future Kit (FK709). The 32 ohm speaker I was getting turned out to be the same diameter as an English 1p coin! Like that's going to sound powerful, so I soldered a jack to the output terminals and used some Sony headphones instead.
For power I was using a pair of re-chargeable AA's.
The external aerial wire was very short & I wound it around a screwdriver shaft to make a spiral. If you wanted a longer external aerial insert a wire into the coil , making a loose coupler.
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Old 19th Nov 2018, 12:36 am   #47
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Default Re: ZN414 and TA7642 radios

How is the sensitivity and selectivity on the Thai kit, bearing in mind that it does not use a ferrite rod/ bar antenna ? Would mounting it in a small box with a couple of turns of wire around the frame as an antenna lead to better reception and make it directional ?

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Old 23rd Nov 2018, 10:42 am   #48
Neil Purling
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Default Re: ZN414 and TA7642 radios

I really do not like those Polyvaricon tuning capacitors I don't think the tuning range is sufficient to cover the whole AM broadcast band. So then you have to fiddle with the ferrite slug of the coil.
I might remove the Polyvaricon varable capacitor & use one from a scrap radio. At least I could add a proper reduction drive & get the spindles & pulleys from a scrap radio.
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