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Old 15th Nov 2018, 1:46 am   #41
Bill
Tetrode
 
Join Date: Dec 2005
Location: Denton, Manchester, UK.
Posts: 74
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

Dave L
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
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,890
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
Andrew2
Octode
 
Join Date: Mar 2010
Location: Dukinfield, Cheshire, UK.
Posts: 1,330
Default Re: ZN414 and TA7642 radios

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Originally Posted by Neil Purling View Post
Andrew: In post #38.............. What does that mean in terms of the input level & the output?
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
Andrew2
Octode
 
Join Date: Mar 2010
Location: Dukinfield, Cheshire, UK.
Posts: 1,330
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
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,890
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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