20-11-2015, 10:56 PM
Yes, I'd second this 
It's surprising how far components can drift before problems occur. Obviously, it depends on the context, but most resistors in a domestic radio are very non-critical indeed. For a beginner, this is difficult. How are you to know what is critical or not? There's no simple answer to that, but suffice to say, it comes with experience. There's no short-cut to experience, but being on a forum is perhaps the nearest you can get to a short-cut. In other words, if in doubt, ask!
It's well-known that the most commonly encountered type of resistor used in valve radios ("carbon-composition") are notorious for drifting up in value. But if they are within 20%, definitely leave alone. If they exceed 30%, then perhaps it might be worth changing them. But remember, other components also drift. Sometimes, two or more resistors drifting together can actually still give reasonable results. It just depends...
Moreover, some designers just had "favourites". Sometimes you just need some resistance. Almost anything will do. Let's say that anything between 10k and 100k is needed. If there was no strong reason to hone in on a certain specific value, I might choose 47k because I like the colour of the bands! I'm not entirely joking
Alternatively, let's say the radio already has 4 or 5 resistors elsewhere that are 39k... In which case, I might choose 39k because that makes ordering the parts easier and potentially cheaper. But if it was a "one-of" for my use - as opposed to a "production" model - then I might choose 56k instead, on grounds that I have plenty, and it's nice to have some variety. But whatever I end up choosing, it's almost entirely on a whim. And that particular value could go well out of tolerance before the circuit starts to misbehave.
But against that, there will always be an exception. It just depends. On one of my products, I specify a 14.7k resistor. 15k will do the job, but 14.7k puts a preset trimmer dead in the centre of its mechanical travel (15k has it about 75 degrees off). As long as 14.7k resistors are no more expensive than 15k, I'll continue to use them to satisfy my OCD
Just had a thought. Here's some advice for your next set: get yourself a Hacker Mayflower.
Why?
Firstly, it's delightfully easy to work on. The chassis is lovely; the tag strips are nicely laid out; everything is very accessible. Secondly, when working, they are a wonderful thing to listen to. But, most importantly, I have restored dozens of them, so I (and many others, I'm sure) can give you all the advice you need. It will be an excellent learning experience for you, with a lot less frustration than this HMV. OK, you might not like the Mayflower's styling, or it might not be your usual collection interest, but you could always sell it on afterwards - good ones get a fair price these days.
Anyway, just a thought. If anyone else fancies doing a similar thing with a different set that they know really well, perhaps they could make a suggestion? It might work well, but entirely up to you...
But we do need to think about alternatives to your bench setup. Especially if valve sets are your thing. Would there be space for a folding table, for example? Quite apart from anything, I dread to think what this is doing to your back!
Oh, and I finally got around to looking up the HMV. John has a good picture of the chassis here: http://www.thevalvepage.com/radios/hmv/h...mv1124.htm
All the best,
Mark
It's surprising how far components can drift before problems occur. Obviously, it depends on the context, but most resistors in a domestic radio are very non-critical indeed. For a beginner, this is difficult. How are you to know what is critical or not? There's no simple answer to that, but suffice to say, it comes with experience. There's no short-cut to experience, but being on a forum is perhaps the nearest you can get to a short-cut. In other words, if in doubt, ask!
It's well-known that the most commonly encountered type of resistor used in valve radios ("carbon-composition") are notorious for drifting up in value. But if they are within 20%, definitely leave alone. If they exceed 30%, then perhaps it might be worth changing them. But remember, other components also drift. Sometimes, two or more resistors drifting together can actually still give reasonable results. It just depends...
Moreover, some designers just had "favourites". Sometimes you just need some resistance. Almost anything will do. Let's say that anything between 10k and 100k is needed. If there was no strong reason to hone in on a certain specific value, I might choose 47k because I like the colour of the bands! I'm not entirely joking
Alternatively, let's say the radio already has 4 or 5 resistors elsewhere that are 39k... In which case, I might choose 39k because that makes ordering the parts easier and potentially cheaper. But if it was a "one-of" for my use - as opposed to a "production" model - then I might choose 56k instead, on grounds that I have plenty, and it's nice to have some variety. But whatever I end up choosing, it's almost entirely on a whim. And that particular value could go well out of tolerance before the circuit starts to misbehave.But against that, there will always be an exception. It just depends. On one of my products, I specify a 14.7k resistor. 15k will do the job, but 14.7k puts a preset trimmer dead in the centre of its mechanical travel (15k has it about 75 degrees off). As long as 14.7k resistors are no more expensive than 15k, I'll continue to use them to satisfy my OCD
Just had a thought. Here's some advice for your next set: get yourself a Hacker Mayflower.
Why?
Firstly, it's delightfully easy to work on. The chassis is lovely; the tag strips are nicely laid out; everything is very accessible. Secondly, when working, they are a wonderful thing to listen to. But, most importantly, I have restored dozens of them, so I (and many others, I'm sure) can give you all the advice you need. It will be an excellent learning experience for you, with a lot less frustration than this HMV. OK, you might not like the Mayflower's styling, or it might not be your usual collection interest, but you could always sell it on afterwards - good ones get a fair price these days.
Anyway, just a thought. If anyone else fancies doing a similar thing with a different set that they know really well, perhaps they could make a suggestion? It might work well, but entirely up to you...
But we do need to think about alternatives to your bench setup. Especially if valve sets are your thing. Would there be space for a folding table, for example? Quite apart from anything, I dread to think what this is doing to your back!
Oh, and I finally got around to looking up the HMV. John has a good picture of the chassis here: http://www.thevalvepage.com/radios/hmv/h...mv1124.htm
All the best,
Mark
