A Day Spent Puzzled

[Nowhere-Man]

Now the weather finally decided to turn sour, it's time to roll up my sleeves and renew my radio project work. I'm in no rush and probably need to ease myself into gear. Other essential work on my boat really needed to be done but, as I said, I'm rained off.

Here is a puzzle: My HMV 1121 nearly has all the new capacitors soldered in but I have 3 more to do. I took one out today and it was a typical dirty, grubby and sorry-looking wax capacitor. I cleaned it up and could make out . 22 mF. Usually the point assumes a zero so we have 0.22 mF. I looked at my service manual capacitor list and couldn't find a 0.22 mF. There is one that's 0.022 I seem to recall but it didn't seem to be the right one. What does seem to fit, however, is the 0.015 mF listed as a part tone capacitor.

The thing is, the capacitor with .22 mF was removed between a lug direct to chassis and a lug direct to the IF can. So, that was the point I was racking my brains as I searched the schematic for a capacitor that would be shown from chassis to connection on the IF can. This is close to the Mixer and detector valves. I can see absolutely nothing else between the said connection between transformer and IF can. So far the only cap that fits is a 0.015 mF but, if that's the case, it's a fair bit out on spec.

I suppose you could say if I have a capacitor that has .22mF and I know exactly where I took it from, just go and buy the said cap and solder it in. I do like to try and trace circuits a bit more, though, these days. I guess I'll take another look later on. Any idea if engineers ever deviated from a schematic and just bunged in a very close equivalent?

I just posted this mainly as I'd like to point out the radio work will continue and that is what I was doing today - just tracing the circuit.

[Joe]

Any idea if engineers ever deviated from a schematic and just bunged in a very close equivalent?

Frequently; and sometimes not very close!

Also service information isn't immune from mis-prints and manufacturers can change designs after this was produced.

— Joe

[AlanBeckett]

It's probably just a Decoupler for the Mixer Supply.

'While we try to keep the Circuit up to date we reserve the right to change ..........'

Translate that as 'S**t, we've run out of .15s. Stick in .22s as we've got plenty.'

Or alternatively 'Test Dept. says the Mixer needs a bigger Decoupling Cap. Well stick a .22 in and we'll catch up with the Documentation in a bit.'

'twas ever thus

Alan

P.S. When you've spent as many days as I have being puzzled it won't worry you.

[ppppenguin]

A quick point on nomenclature.

When you write 0.22mF you almost certainly mean 0.22 microfarads. For lazy folk like me we usually write 0.22uF or even just 0.22u where there's no ambiguity. (Terry will slap me on the wrist for not using the "mu" symbol instead of "u" but as I siad, I'm lazy and there's no loss of meaning) Strictly speaking 0.22mF means 0.22 millifarads or 2200uF, a very different beast.

Old components and diagrams used a whole variety of ways of writing values. MFD or M.F.D being common for microfarads. Go back even further, to the early 1920s and further, and the units are even more wierd and wonderful.

Coming up to date modern engineers often write 220nF (or just 220n) meaning 220 nanofarads. Exactly the same as 0.22uF but without a decimal point to get lost. We even write 4n7 for 4700pF or 0.0047uF. Again saves losing a decimal point or interpreting lots of zeros.

Finally in the old days we often used 0.2uF or 0.5uF etc. These days we use the closest preferrred values 0.22uF and 0.47uF and this will be OK except in a very few cases.

And as Joe says, service data can be incorrect. Or the jolly bodger may have been there before you.

[Joe]

There's a special place in Hell for people who use nanoFarads.

I don't know why but they confuse the bejabers out of me.  There's no logic to it: we use Ohms, kilOhms, and MegOhms and nanoFarads make more sense than lots of picoFarads or zeroes in front of microFarads but I still end up mentally converting them.  The nanoFarad is also as near as damn it a Jar so has a certain historical appeal.

I've also seen microFarad written MF, which would strictly be megaFarad, a mind-boggling unit.

I looked the HMV 1121 up in R&TVS in case there was a mis-print on the Trader sheet but it's not in.

If the cap in question is part of the tone control circuit it may have been deliberately upped to change the frequency response to somebody's taste.

— Joe

[ppppenguin]

Specially for Joe:  10E-9 goats  = 1 nanogoat

In the same vein, 10E12 bulls = 1 terabull. So there

It's only relatively recently (1960s? 1970s?) that SI units and abbreviations were fully standardised. I like this site: http://www.unc.edu/~rowlett/units/index.html Don't blame me if you waste an hour or 2 browsing it.

[Terry]

There's something funny going on here!

This thread allegedly has two pages but page two is a one-page copy of posts 1 to 6 above ...!

Even stranger is that now I've posted my composition the duplicate has disappeared!

EDIT: I've now discovered the split thread! Obviously I innocently stumbled in here during the splitting process!

[Mark Hennessy]

Thread has been split - the special character posts are now here: http://golbornevintageradio.co.uk/forum/...p?tid=5046

[Nowhere-Man]

There's a special place in Hell for people who use nanoFarads.

I don't know why but they confuse the bejabers out of me.  There's no logic to it: we use Ohms, kilOhms, and MegOhms and nanoFarads make more sense than lots of picoFarads or zeroes in front of microFarads but I still end up mentally converting them.  The nanoFarad is also as near as damn it a Jar so has a certain historical appeal.

I've also seen microFarad written MF, which would strictly be megaFarad, a mind-boggling unit.

I looked the HMV 1121 up in R&TVS in case there was a mis-print on the Trader sheet but it's not in.

If the cap in question is part of the tone control circuit it may have been deliberately upped to change the frequency response to somebody's taste.

— Joe

I can work it out but it takes time. I have to break it down as follows: If there are 1000000 Picofarads in 1 Microfarad then 0.5 mF will be 500,000 Picofarads. That's a good halfway mark and point of sanity to begin some rational approach to the bewildering figure before you. Thus, 0.1 mF will be 100,000 Picofarads.

I do really go to Maplins and ask some poor sales-assistant, "Excuse me can I have a 100,000 picofarad capacitor that can handle 500 VAC?" In fact, I went today.

Now the dreaded Nanofarads. If there are only 1000 Nanofarads in 1 Microfarad, maybe just divide, say, the 500,000 PF by 1000?

I agree, though, I dislike Nanofarads. I've noticed in sixties American Ham they just say Micromicrofarads for Picofarads.

[ppppenguin]

Over in the USA they also used to say millimicrofarads for nanofarads.

Micro, nano and pico (and all the other multpliers) are simply ways to avoid having to write lots of zeros. They also conveniently go up and down in thousands.

So:
1000pF = 1nF
1000nF = 1uF
1000uF = 1mF (millfarad, rarely used in practice but it's why you shouldn't use mF to mean microfarads. Just recognise it was often done in the past)
1000mF = 1F (a truly huge capacitor)
1000F = 1kF (Kilo farad? Now we're getting silly)

Al these multipliers, pico, nano, kilo etc can be used for ANY units of measurement. Hence microseconds, kilohms, megawatts, kilometres etc.