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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 22nd Sep 2023, 8:49 pm   #1
FERNSEH
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Default The Transeven transistor radio.

Construction details of the "Transeven" radio appeared in the March 1959 Radio Constructor magazine.
In the text we are informed that because of the shortcomings of certain HF transistors the receiver will not tune to 200 meters. If so, no Radio Luxembourg with this set. The components list shows TR1 is an F.C. junction or white spot. In 1959 hadn't the OC44 been available to home constructors since 1957?
The IF amplifier transistors are either OC71s or red spot types. The transistors operate in common emitter mode. The IF is 315Kc/s.
The volume control should also be the second detector load resistor.
I'm sure this part of the circuit diagram is incorrect? The slider should go to the base of TR4.
The push-pull output transistors can be either OC72s or OC71, no mention of the red spot type for this application.
Two advertisements from 1957 offering red spot transistors for ten shillings each. Also, RF transistors good for 1.5Mc/s available for fifteen shillings.

DFWB.
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Old 22nd Sep 2023, 9:29 pm   #2
Philips210
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Default Re: The Transeven transistor radio.

That circuit looks unusual David. The IF transistors being in a common base form. There's no AGC action which would normally be applied via the tapped IFTs to the base of the IF transistors. For those you'd have thought they would have specified white spot types or OC45s.

Regards,
Symon
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Old 23rd Sep 2023, 9:52 am   #3
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Default Re: The Transeven transistor radio.

Odd by today's standards, but common base is a good method for getting the best high-frequency gain out of a transistor and back when transistors were rather limited at higher frequencies, RF types were extra expensive. So the use of common base in the IF allowed cheaper types there, or got extra gain out of the IF stages.. The self-oscillating mixer either needed something expensive, or was low gain, needing gain to be made up for in the IF.

So that design was a sensible compromise in its era. By 1959 people were still learning how to make better transistors and how to squeeze the best out of what they did have.

Transistors were so expensive that it's unlikely that the people designing he early portable radios had cut their teeth on transistor radios, they would have started with them in fields where cost was far less of an object. In Radio Constructor, you could always wonder what the designer's day-job was.

David
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Old 23rd Sep 2023, 10:59 am   #4
FERNSEH
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Default Re: The Transeven transistor radio.

From the circuit description: "The transistors are Mullard OC71 types, or alternatively red spot equivalents may be used. Both transistors are in the earthed base mode - thereby proving more stable operation at the intermediate frequency of 315Kc/s than would be in the earthed emitter mode."
I guess common base operation obviates the need for neutralising components thus simplifying assembly and final adjustments.
My memories of those red spot transistors are that some were rather noisy. Mullard transistors were much better but more much expensive.

DFWB.

Last edited by FERNSEH; 23rd Sep 2023 at 11:14 am.
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