ВЭФ 12 (Astrad VEF 12 Project) - including a transistor tutorial

[Murphyv310]

Excellent Pun Lawrence.

A turret is a good idea for a multiband radio like so many VEF, Riga and other CCCP radios.
The turret wasn't just used in Soviet radios, the vast majority of mid fifties and early sixties 405 line sets has 13 channel turrets, the interchangeable coils were commonly known as "Biscuits".

[Nowhere-Man]

I am a bit suspicuous of a P43 transistor on the radio. Very suspicious. It is used in RF and IF stages. Said transistor has been known to require substitution. Given my goal is to learn from this project, there's no harm in carefully checking the suspect transistor against the schematic. So I will zero in on this exact transistor. It is late now so I'll call it a day.
As to comments on the turret I agree it's a good idea. I would just like to add the more complex engineering became, the shorter the life-span. This was made in 69 as the Beatles were splitting. Possibly it ceased to work in the eighties? Yet I have simple chassis-mounted tranny's made in 59 that still works.

[Pat Pending]

I've worked on many of these sets down the years, most arrived on my bench via skips and dumps, and had very minor faults, most had suffered with tarnished contact fingers on that turret, even if they look clean they often have a coating that may be tobacco tar(?) As Mike says, switch cleaner, (careful of the plastics), then coat the surface sparingly with petroleum jelly (Vaseline). The little transistor sockets used in these sets can also give trouble.



Andy.

[Murphyv310]

The bowler hat CCCP germanium transistors were unreliable too but I used AF127's as a replacement with great success in the mixer and IF stages. AC126/128/127 in the audio .

[Nowhere-Man]

I thought I'd add the schematic to this VEF 12. One of the P423 transistors could be suspect. Apart from Mark's postings on AF117's, I found an old post by Miguel where he successfully substituted 1T308 transistors.
As you can see, the schematic below is a bit more like it and there's a service book being sold too.
I suppose you could argue whether a 1969 radio is either classed as "vintage radio" or "classic radio". By that time, radio engineering was getting much more sophisticated with far more on offer than just LW, MW and SW.

[Nowhere-Man]

In my spare time, I'll be investigating sources possibly to buy transistors from the Russian Federation, possibly even Latvia. I found a Russian site here with technical information. It's in Russian but the graphs may make some sense. Problem is always post and packaging. If the price is 8 roubles with the exchange at 0.017 US Dollar = 1 rouble, sounds to me like a good deal.
http://tec.org.ru/board/p423/46-1-0-2975

[Nowhere-Man]

If I manage to obtain supplies of these transistors maybe I'll have the odd one to spare. They seem cheap to buy.
I've been racking my brains on the basic transistor theory. After some observation it seems to me the P423 transistors in the schematic are common emitter. I'm not at all used to non valve circuits but my ARRL book seems to show these as PNP with grounded emitter?

[Terry]

If I understand you correctly you seem to be saying that the transistor type governs the configuration. Not so - it is the surrounding circuit design that determines the configuration.

It might help you to compare the transistor to a triode valve. Both have three electrodes and both can be configured with any one of them common.

Thus common emitter ≡ common cathode, common base ≡ common grid (more commonly known as grounded base or grid) and common collector ≡ common anode (more commonly known as emitter or cathode follower because that better represents the circuit function).

[Nowhere-Man]

I was saying the circuit schematic looks like grounded emitter. I'm trying to outline the basics first where it's new territory. Ideally a book exclusively on transistor radio circuits would be the norm but I have a few pages in my 1960 HAM book.
Here it's like a war zone. Very heavy wind. Even sirens going off. My friend phoned to say her garden swing is damaged and her bins blown away. Hoping it will die off soon imagine lots of damage resulted.

[Nowhere-Man]

Spent a few minutes today learning how transistors amplify current. The ARRL is a bit dated but there is a graph that struck me. It shows base current in microamps and collector current in milliamps at given voltages. It stresses the "dreaded holes" are the principal charge carriers. Not everything I understood that was covered but it's OK for an old book.