Soviet East German Radio

[Nowhere-Man]

Hey, Trevor, hope you get better. Years ago I nearly died of flu. Honey may help you recover or even garlic. I used to just chop garlic and swallow with orange juice to kill the taste.

[Nowhere-Man]

"Ref the Saalburg, have you figured out why a tapped primary audio output transformer is used yet, that system is used in many valved receivers."
Yes, I had to rewire it. Some HT wires had come off and it needed to be right. Besides that the wiring of the transformer was aged. And could you believe a resistor had bridged because the lugs weren't firm. So, yes, I'm aware of the centre tap. And that HT wire must contact R12. I suspect a mechanical fault with pushbutton 1. My feeling is R12 is bypassed on FM but switched into circuit for AM. I may be wrong but so far it seemed that way.
That output transformer once gave me a huge jolt with the set unplugged. Best left unsaid what I yelled out that day.

[pwdrive]

The transformers primary is tapped in order to reduce any hum in the transformer from the HT that might otherwise end up at the loudspeaker, the transformers primary has three connections, main HT in (1), the anode connection to the audio output valve (2) and HT out to supply the rest of the receiver (3) It's all designed for hum cancellation.

The HT hum voltage at connection 2 (the tap) branches two ways via winding 2-1 and 2-3, and in effect those two branches consist of winding 2-1 plus the valves internal resistance (ra) and winding 2-3 plus the resistance of R35 (1k), the value of that resistor in conjunction with the turns ratio of those two windings and the value of ra in effect helps to cancel out the hum that would otherwise be present in the secondary winding, cancellation won't be perfect but the arrangement goes a good way to reducing the hum, that's the hum on the HT, and not any hum that might be introduced via the valves grids etc.

Lawrence.

[Nowhere-Man]

OK, thanks for that. I'll be resuming late tonight. I'll add too that if my thread here is boring some of the others, I don't mind dropping my theory questions and posting more applied material instead. I understand emphasis is generally on repair projects. Here is a pic of a technique I took from the Russian radio manufacturers. You will see on the right the tiny green components were soldered the opposite side of the board by the Russian firm. Well, further left, I did the same because my capacitor has short legs and I figured to do it the same way. Although the lead looks like it might short across track, in actual fact there is no way it can as the body prevents movement. Plus the solder joints are strong as hell. And ohms tests fine for continuity. Given my eyes are so poor, I tend to mask adjacent tracks just in case.

[Mark Hennessy]

What's wrong with your eyes?

[Nowhere-Man]

Myopia.

[Mark Hennessy]

I thought that resulted in good close-up vision but poor distance?

Glasses no help?

[Nowhere-Man]

I have amazing hearing to compensate. I can hear conversations from big distances.

[Mark Hennessy]

But how does that help you repair radios?

I'm now at the age where I require glasses and sometimes extra magnification (and lots of light) at the bench. Having recently had to learn all this myself, I thought I might be able to help 

[ppppenguin]

I've always been myopic (about -3.5 dioptres) and used glasses. As I've got older I've used varifocals. For close work I always have the option of taking off my glasses, especially as I don't have much astigmatism. This is a benefit of myopia that you don't realise until you get older. If you have normal vision or are long sighted then you will definitely need glasses for close work as you get older. If you're very myopic, perhaps greater than -6 dioptre correction, then you'll probably need glasses for just about everything as your far point will be uncomfortably close to the end of your nose for looking at live kit or soldering.

Like Mark, I use an illuminated magnifier for some detail work.