HMV 1124 - Has Been Tested And........

[Nowhere-Man]

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

Mark, I think the point I was making with the HMV is it's definitely the most difficult radio I worked on so far. The GEC radio was probably as easy as the Hacker or at least not too far off. On the GEC, you can easily solder across tube pins. In fact, there isn't even any need to remove the chassis with that set since taking off the cover gives you full access. Then there was an Ultra I worked on which again wasn't overly challenging (apart from the output transformer fitting).

This HMV was tough due to recessed tube bases in the chassis and a lot of wiring that over-layered these connections. I think now possibly I could take a lot more of it apart and hopefully be able to re-assemble but not back when I started the work earlier in the year. I wasn't as familiar with the set design as I am now but, of course, the voltage tests helped me figure where I was at.

Anyway, here are the updates:

The voltage on the Local Oscillator is normal. So, it's fair to say both the I.F. Mixer anode and the L.O. voltage are fine.

Touching the grid with the meter probe squashes out the signal? I'm not quite happy yet I've made complete sense of the two resistors I tested around the Mixer tube so will look into it to be sure.

The audio stage is really quite strong, especially if the lights on my boat get accidentally left on and create a din of RF interference.

I was a bit curious over a 47 picofarad capacitor in the AVG line. The IF transformer has no capacitor in line to the detector tube but it does have one for the AVG diode. Guess I need to check the bias where I can.

Today I messed a bit with the aerial and this is definitely working. I seem to recall there may have been about 40 Ohms resistance, though, but not sure if I recall it so well. Anyway with the aerial disconnected from the one I have on the roof, there is no reception. Connected I can get signals.

I can get Radio 4 very well. I'm also now getting another local station on another band.

I feel the set maybe improving with use. In fact, I'm quite encouraged that what was once a pile of junk is now at least working a bit and I can even follow the news.

I'll keep doing what I'm doing, trying out any ideas thrown at me on the site and reading up. I have heard of cases actually where the smaller picofarad caps can give trouble if they're shot or badly connected and I'm still a bit concerned about the ganged waveband switch because most switches on this set have had very high resistance. The scale lamps for a start must have given up the ghost maybe 20 years ago.

Apologies if this may be now getting a bit boring and ongoing. I'm yearning to fix up the case and then move on to another project but I'd like to see this one through to the end.

P.S. I may see if I have a spare X78 just in case the tube itself is a bit worn.