Hello from Cuba

[Nowhere-Man]

The "holes". Easier to think of 10 men in a row, each with a football. First man in the row chucks his ball into a basket, then turns to the second man and says, "I need your ball now". The second man gives up his ball but that leaves him empty handed. So he turns to the third guy in the row and says, "I need your ball!". The third man gives up his ball. That leaves him with nothing so now he has to ask the fourth guy in the row for his ball. So the balls move in one way and the empty handed appear in the opposite direction.
Information I have at home with regard to transistors is limited to a 1960 textbook. The terms used are a bit non-standard like "back biased". One superhet receiver shows a transistorised calibrator incorporated into the valve circuit. Not something I've noticed before.
Something else unusual too. Variable gain in the cathode circuit of an IF valve. This used variable resistance between chassis and cathode. I briefly looked up more information and someone stated it drives the cathode into more positive value and increases gain. The IF here, though, was 1700 KC.  

[pwdrive]

That IF valve might have been a variable mu valve, increasing the cathode voltage increases the grid bias voltage thus reducing the gain, manual gain controls were used quite often in comm's receivers as were calibrators which were usually xtal controlled for accuracy and stability, the harmonics of those could be tuned in through all the bands, quite often the tuning scale was moveable by a small amount so that it could be aligned exactly to a harmonic of the marker frequency, check out the Murphy B40d receiver for an example of that.

Lawrence.

[Nowhere-Man]

It was a 6BA6 variable mu pentode. Not sure how much they cost these days.

[Terry]

Variable gain in the cathode circuit of an IF valve. This used variable resistance between chassis and cathode. I briefly looked up more information and someone stated it drives the cathode into more positive value and increases gain. The IF here, though, was 1700 KC.

AGC is achieved by driving the grids of the controlled stages negative, the negative voltage being derived from the detected signal.

For a manual gain control it is easier to drive the cathode positive as it doesn't need a special negative supply. As Lawrence says, as far as the valve is concerned it sees the grid as going negative with respect to the cathode. We tend to measure with respect to earth so we see the grid voltage as negative (if it is a AGC controlled stage) and the cathode as positive but the valve has no direct connection with earth (ignoring any internal shield) so the cathode is its only point of reference.

The 1700kHz IF for a communications receiver is not unusual - it improves image rejection - and 1.6MHz is also common.

What the strange book you are using? 'Back biasing' is, presumably, what anybody else would call reverse biasing.

[pwdrive]

Back biasing is usually referred to as a method of obtaining a bias rail voltage from the -ve HT return, usually from across a resistor which is connected say between transformer secondary center tap and chassis, ok for Class A but not so good for Class B (poor regulation)

Lawrence.

[Nowhere-Man]

Just to refer back for a moment to Miguel's theme, I have just been using his diagram to see how it helps. What's curious is Miguel's schematic shows 16 spade tabs (you can see them above) and 13 is absent both on the schematic and on the radio.
I haven't worked much out yet but I need to get that Russian radio picking up stations.
I am working on more than one project at a time.

[Terry]

What's curious is Miguel's schematic shows 16 spade tabs (you can see them above) and 13 is absent both on the schematic and on the radio.

What's curious about that? If it isn't needed, why fit it? It might seem trivial to you but to the bean counters it is a cost saving which, together with other such economy measures that I'm sure you could find, when multiplied by the number of sets built will add up to a useful saving.

Obviously eastern bloc bean counters were no less scrupulous than their western counterparts when it came to shaving costs!

Anyway, you seem to have proved that Miguel's schematic matches the radio!

[Nowhere-Man]

To be honest some of this is new. I am used to the very old switch and coil system of much older valve radios. These as you know were ganged and the oscillator coils wound to produce a heterodyned IF frequency. Now a more modern Russian tranny has a lot more wavebands and the ferrite coils. As a novice on Russian transistor sets it seems I'm looking at some tappings as well as the windings. This is a more advanced design. My plan is to test the windings for continuity and trace the broken wires to their location. Plus clean the waveband contacts.

[Nowhere-Man]

OK I was gonna travel out to get some Servisol today but the store is open till 20.00 tomorrow and so I'll work on the Russian radio today.
My eyes are very poor but I noticed there are more than two disconnected ferrite winding leads. Just found another one. Four in total. Miguels schematic shows L14, L15, L11, L12 and L13 although visually I can only see three. Maybe the others are tapped.

[Nowhere-Man]

I will return to the cathode circuit discussion later. At the moment, I am starting to get the general idea as to how to fix the ferrite coils. L14 was a broken circuit and I was just able to spot a tiny thread disconnected. I used a blade to scrape the end and then got an ohms reading. All I have to do is solder it to the tag. With my eyesight that will be tough going.
Waveband connections are awful. You can see surface rust. If the Saalburg is that bad it will be tricky. With the Astrad I have full access so can clean away.