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 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Apr Fri 06, 2018 11:39 pm 
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Looks a good book. I had one by F J Camm from 1930s. It was all battery tube TRF sets with updates on superhets. I really like the ARRL.but it is a constructors book and in 1960 people didn't know transistors would displace tubes. Anyway the VEF is all PNP.Germanium BJT. Cosmetically the transistors are impressive as they resemble top hats. This was the post Krushchev era and science and space inspired radio design. I guess the rotating barrel was intended to reflect technology. What I did find was the contact points got grubby or distorted.


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 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Apr Mon 09, 2018 12:19 am 
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Turns out the radio has a diode. Really the schematic is beyond my current level. I can.understand tube radios fine but here I am not.as sharp. I started by trying to locate the oscillator. The IF stages are reasonably clear. All in all though pretty much most vintage tube radios in England are much simpler. Let's face it by 1969 we were almost in the era of modern radio. This one even has a tape recorder socket to plug in.


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 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Apr Mon 09, 2018 4:31 pm 
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All the details of the VEF are here:
https://kot-ivan76.livejournal.com/307493.html
This includes servicing and alignment. Google translate for Russian is pretty good due to a lot of interest in the language. Also, Yandex (dot com) seems to have incorporated Google and I use yandex all the time.
For example from the text:
"ВЭФ-12», как и его предшественник, собран на 10 транзисторах и двух полупроводниковых диодах"

Google translate gives: "VEF-12", like its predecessor, is assembled on 10 transistors and two semiconductor diodes".

Obviously the "on" here means "on the basis of" but clearly the main meaning is clear enough.

I just reprinted and enlarged the schematic as I've been struggling to make out a different one I had (text too small).


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 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Apr Tue 10, 2018 7:48 pm 
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The schematic gives РИС 3 which gives the transistor voltages. It also shows the RF connections to T1 but here I am not clear why the diode Д101 is pointed the way it is by T2
Although I've seen tube ratio detectors with cathode rectification. Also I assume L30 should be the LO. On this schematic you can see the vague screens of all the RF cans which match the 8 on the set itself
It looks to me like T3 is the mixer. However I must say I am so used to seeing triode hexodes followed by IF amplifier. Not only is this a transistor circuit but also foreign. I will need to try and clarify roughly how the circuit works. It is actually very clear but just need a bit of time.


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 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Apr Tue 10, 2018 9:42 pm 
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T1 is the local oscillator, and the coils L1-x, L3-x, L26 and L28 are the oscillator coils for the various bands.
The coils to the left of those are the antenna coils for the various bands.

T2 is a current source, where D1 is b1ased in the forward direction and with R9 sets the current through T2.
The stabilized current is used by the oscillator stage to keep it stable as the battery voltage goes down.
T3 is an RF-amp. L3 and C49 forms a series resonant circuit for 465kHz and is a "trap" for the IF-frequencies. The mixer is T4, the LO-signal is injected at the emitter through C51.

You picked a hard circuit to understand transistorized radios! You should start with something simpler, like the "The All Japanese Six Radio", plenty of good explanations there:
http://www.angelfire.com/planet/funwithtransistors/AJ6-1.html

As for the hard to read the schematics because of its print quality, as mentioned before, the circuit is the same as of the VEF-206 radios', and there are plenty good quality prints out there on those.

Hope this helps!

Peter


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 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Apr Tue 10, 2018 10:53 pm 
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It helps, thanks. Just very different. Even German FM tube sets are very easy to see the oscillator coils. American sets usually show the LO apart with perhaps cathode injection. I will just have to march forwards
Yes T1 was referred to as the гетеродин or mixer linked to R7 and lots of information is supplied. I am reading the text but have been thrown by the mixer circuit in this. You are right it's a jump but the radio just came my way so I will see what I can do


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 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Apr Wed 11, 2018 1:18 am 
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Maybe a good idea to compare my East German 1964 tube radio schematic with the ВЭФ. The oscillator is as usual a triode and the windings very simply shorted out or switched in as required. The mixer then produces a heterodyne and detection is a double diode. As noted, though, the ВЭФ has lots of bands. How come you get the oscillator winding L1 to L10 and then it jumps to L16? I correct my last post T1 as Peter stated is the LO called the ГЕТЕРОДИН in Russian. Maybe it will clear up for me as I study the texts.


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 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Apr Wed 11, 2018 8:04 pm 
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Just been comparing a more modern tube oscillator circuit to the even more modern transistor system. This German pcb constructed tube radio uses an ECH81 with its triode section as the oscillator for AM LW. The FM uses a different oscillator. The oscillator coils are small windings mounted on a board and fed to the ECH81 triode. A 455 khz heterodyne is mixed in the heptode and from there directed to the IF amplifier stage. And that's it!! There are only a small number of IF cans and 7 tubes of which one is a rectifier and another a magic eye. Sets like these were still built in the East up to the seventies on large pcbs. Simpler but higher voltage.


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 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Apr Sat 14, 2018 12:00 am 
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orbanp wrote:
T1 is the local oscillator, and the coils L1-x, L3-x, L26 and L28 are the oscillator coils for the various bands.
The coils to the left of those are the antenna coils for the various bands.

T2 is a current source, where D1 is b1ased in the forward direction and with R9 sets the current through T2.
The stabilized current is used by the oscillator stage to keep it stable as the battery voltage goes down.
T3 is an RF-amp. L3 and C49 forms a series resonant circuit for 465kHz and is a "trap" for the IF-frequencies. The mixer is T4, the LO-signal is injected at the emitter through C51.

You picked a hard circuit to understand transistorized radios! You should start with something simpler, like the "The All Japanese Six Radio", plenty of good explanations there:
http://www.angelfire.com/planet/funwithtransistors/AJ6-1.html

As for the hard to read the schematics because of its print quality, as mentioned before, the circuit is the same as of the VEF-206 radios', and there are plenty good quality prints out there on those.

Hope this helps!

Peter

T3 has had me puzzled but I can now see how the oscillator coils are selected and the connections to T1. I have the voltages for the RF and AF transistors. As I say, the design seems pretty different to a typical tube set. Likewise ARRL in the 1960 format tends to not focus overly on stuff like transistorised mixers. So, as I progress I think tracing the actual LO on the board and testing voltages should get me into the swing of it. I have other transistor radios that inclides a mid sixties Philips auto radio with MW LW. This has AF117 transistors and still works although mot very well. I also have a stone dead Radiomobile radio with 8 track. Also scores of very antiquated tube sets.


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 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Apr Sun 15, 2018 5:23 pm 
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I was testing some AF117 transistors to get into the swing. Between collector and base I got ohms readings both ways on 2K.Emitter base was only reading one way and open circuit the other way. My conclusion is the CB diode is leaky. I assume that is the logical conclusion. As to the VEF T1 tests well below volts for collector and (given sockets are installed) I figured I could repeat the test as above. That is if what I found is a reliable indication of defective transistors.


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 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Apr Mon 16, 2018 7:19 pm 
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Looks like the LO Т1 transistor is defective. I tested a modern transistor and that seemed OK but the П423 showed ohms pretty much between all leads, forward or back
Collector voltage too was way down. I am a bit weak on transistors but logic suggests there lies part of the problem. The job of re-inserting the transistor in the socket was fiddly. I did put it back for now but need to try finding a new oscillator BJT. Not sure if silicon is a no-go due to bias.


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 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Apr Tue 17, 2018 4:33 pm 
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As discussed before, T2 acts as a stabilizer for some of the front-end circuits in the radio, including the oscillator stage.
Because of that (and due to other circuit design details) changing T1 to a silicon device would barely change the bias current of the oscillator transistor, it changes from 1.1mA to 1.02mA, according to simulations.
So simply just try it!
I used a 2N3906 transistor in the simulation (it is as common as it gets), it has an Ft of 250MHz, plenty enough!
Be mindful of the pin-out though, the 2N3906 has E-B-C, when looking at the flat face.

Peter


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 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Apr Tue 17, 2018 5:12 pm 
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orbanp wrote:
As discussed before, T2 acts as a stabilizer for some of the front-end circuits in the radio, including the oscillator stage.
Because of that (and due to other circuit design details) changing T1 to a silicon device would barely change the bias current of the oscillator transistor, it changes from 1.1mA to 1.02mA, according to simulations.
So simply just try it!
I used a 2N3906 transistor in the simulation (it is as common as it gets), it has an Ft of 250MHz, plenty enough!
Be mindful of the pin-out though, the 2N3906 has E-B-C, when looking at the flat face.

Peter

Sure, I will try it as you suggest. I will let you know how things develop. I did spend some time going over the schematic. I'm so used to tube designed radio equipment this has been a bit different. As of yet, I may find there is more than one suspect transistor but I'll start with T1.


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 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Apr Tue 17, 2018 6:00 pm 
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Best way to find bad transistors is by checking the voltages on their terminals.
The documentation you pointed to does have a page with the transistor voltages.
Do check those!

Peter


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 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Apr Tue 17, 2018 6:56 pm 
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The transistor voltage panel fortunately allows to match the board to the panel so you know which transistor is which. On T1 the voltage was dead low. I have had a similar issue with triode oscillators, except in that case HT supply was absent. In the case above it does seem the P transistor is a dead duck. All diode junctions showed leaks whatever the direction. So too many AF117 sixties transistors tested suspect. Then again the AF117 was found to be prone to Internal contamination.


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 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Apr Tue 17, 2018 11:09 pm 
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As an exercise I had a go at working out the emitter current of Т1 based on the schematic. There's a site called Learning About Electronics where I learned from one example. First find the base voltage - you can find the example by googling "find emitter current" then the site name. What threw me was the tutorial must be using NPN whereas these very antiquated sets use PNP. Anyway I gave it a shot. It made me think as well. I assumed a Beta of about 50. Hopefully this doesn't make boring reading but years ago when we did sciences you would be given this kind of question
It if far less scary at home over a cup of coffee.


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 Post subject: Re: USSR 1969 RADIO PROJECT
PostPosted: Apr Tue 24, 2018 4:35 pm 
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Unfortunately I'm now forced to order transistors (as well as capacitors) due to closure of even more high-street stores. Years ago, there were electronics stores such as Tandy where you could walk in and buy any electronics components you required. They even did quite decent kits, such as the one I bought from there in the pic below. Anyway, I was forced to order so I am going to try the transistor suggested by Peter for the Local Oscillator (when the order arrives). Not that I'm expecting the radio to just burst into life straight away as there are probably more faults to locate. Of course, I will need to be very careful to insert the correct leads.

In the meantime, I figure I made some progress with the schematic and the ARRL book I already have does briefly cover RF amplifier stages, mixers and detectors. It includes at the back a list of the American transistors that were typically used around 1960. At that particular time, the HAM hobbyists were using transistors a lot to add 100 Kilocycle crystal calibrators to the tube sets. I found one example where the voltage dropped across the cathode of an output tube was then fed back to the transistor circuit (around 10 volts, I think). That is, as a simple voltage source.

I spent some of my free time this week revising oscillators as, for some reason, the numerous coils and switches always got me confused over that stage of a radio. I did notice that the transistorised oscillator circuits just seem a fair bit different from what you tend to get with a tube design. By the way, with regard to oscillators, I can say that when battery tube radio first kicked in around the 1930's, the detection (as many will know) was regenerative. Pretty much positive feedback from a winding. Unexpectedly what happened was it was found when people were tuning their receivers with the feedback dial, they accidentally pushed the winding into oscillation and generated signals on a different frequency. These then interfered with other radio sets and that was how the idea of an IF started (I think they call the Armstrong oscillator a tickler).


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