Noisy Hackers!

[Joe]

I look forward to it.

- Joe

[pwdrive]

I look forward to it.

- Joe

Yes, but don't laugh at the solution to that particular fault that i will describe.

Lawrence.

[pwdrive]

Ok, will kick off this post in the members lounge.... as in the next 20 mins.

Lawrence.

[Mark Hennessy]

Just been playing with another one of these. The usual LF rumbling noise...

Twiddle RV3, just in case. No difference.

Change TR1, which makes it about 5dB quieter. But still too noisy...

Change TR2, which makes no difference. Often the way, but it's nicer to have a pair of BC108s rather than a BC108 and a BC148.

Tried R1, but it hardly made any difference. Refit the original. Likewise for R4. But a replacement R3 made things about 3dB quieter...

Noise is now -45dBqs (unweighted, quasi-peak responding rectifier). Better, but noise still audible through loudspeaker. Should be about 10dB less noise, so something must be going on...

As before, the noise is affected by the position of the bass control, so it must be related to TR1. We've already checked R1, R3 and R4, so...

...and it's probably not R5.

And this time it was indeed R5. Replacing it brought the noise down to -55dBqs, which is pretty much as quiet as I've seen from these. One day I must do a blanket replacement of all resistors with film types (along with replacing all transistors) to see what the original design was capable of.

This time, the noisy resistors were standard axial 10% carbon composition types. The resistors that were OK were the "box" types that caused me grief earlier on; neither type is more or less reliable in my experience. Of course, both were out of spec in resistance terms (6k4 and 1k12).

Still, these sets are worth the effort

[ppppenguin]

Just goes to show how noisy and nasty those old carbon compo resistors can be. No some wonder audiophools insist on using them

[Terry]

When I was at college we had a lab tech who was a dab-hand at repainting the colour bands on resistors when putting faults on sets for the practical exams.

How do you think he would have got on with modern colour coded resistors? The bands on some of them are so narrow and the colours so bland that it is well nigh possible to read the damn things before anybody attempts to tamper with them, anyway!

[Mark Hennessy]

These are nice old things. Dark brown body with vivid colours that are easily seen with the naked eye and are nigh-on impossible to mis-read. Unlike the modern metal-film types with the light-blue body. Whoever chose that colour needs to explain themselves! The Mullard dark greens were better, but only a bit. And of course, with the addition of the 5th band - invariably black, of course - it means that the multiplier has dropped by one, with the net result that circuit boards are a less colourful, less beautiful thing to behold. Let's not even consider SMT!

Silly, I know, but as a kid, I really loved the pictures in the Ladybird books. As I've said before, my favourites were the deep red/maroon body types from Piher. If you wish to indulge, see here: http://www.mds975.co.uk/Content/george_d...radio.html - page 39 is probably my favourite. The real book is better than the scans, naturally. Just lovely...

Back to the radio, another common problem with these is poor sensitivity on FM. Usually caused by Lockfit transistors, but I can't help wondering if the resistors could in-part be responsible. The noise seems to be the 1/f type - not that I've taken rigorous measurements so far - so perhaps at 10.7MHz it might not be an issue. It's fair to say that I must have serviced at least 50 of these over the last 5 years, and the spread of sensitivity is at least 30dB. Clearly, I can't be curing all the maladies on the worst of them, and I'm sure that the 2 or 3 that were exceptional were flukes, but there's clearly something going on. Supply voltage to the RF and IF stages is important, and often carbon-comps that have drifted high are part of that problem.

By comparison, the other FM sets from Hacker - the Sovereigns and VHF Heralds - are much more consistent and reliable. Clearly something in the design of this set was a bit marginal. Not a problem for most owners, but we're in a dreadful reception area for FM here. My bedside radio was Jeffrey's VHF Herald (one of my earliest restorations) until an accident resulted in a broken aerial at Christmas, so I'm using a Hunter instead at the moment. I'm forever having to tweak the position of the aerial to get half-decent reception. Anyone have a spare VHF Herald aerial?

[Mark Hennessy]

OT, but just realised that the attachments were missing from my earlier restoration story, so have just restored them

[Terry]

...  my favourites were the deep red/maroon body types from Piher. If you wish to indulge, see here: http://www.mds975.co.uk/Content/george_d...radio.html - page 39 is probably my favourite. The real book is better than the scans, naturally. Just lovely...

Interesting to note how the Piher carbon film types with their curved dog-bone shape are increasing diluted by the RS carbon composition ones with their more austere rectangular as the book progresses!

By the time you get to page 48 the Pihers are being overwhelmed by the RS types!

Interesting also the way that small capacitor values are expressed in ρF whilst the tuning cap is 0.0005µF ... I wonder how many kids visited their 'local radio dealer' for this item, as advised, and got the number of noughts wrong!

Come to that, how many local dealers would have stocked such items as solid dielectric variable caps and ferrite rods as late as 1972? By the time I left the trade in late '69 I'd built up an excellent range of components in my service department and a trade to match - woe betide my newsagent if my copy of the various magazines didn't land on my doorstep on the morning of publication so that I had some inkling of what unusual items the local schoolboys would be asking for in the afternoon* - or their mums the following morning! However, the demand for such capacitors, Repanco coils and the like had completely dried up by then and we were never asked for (or stocked) 'naked' ferrite rods!

* We stocked the full range of E12 resistor values in several ratings, of course, but E24 would have been too much, however, many of the magazine contributors obviously worked for the likes of Marconi where such values were often used. But I wished they could have used a bit of common sense in their designs. It was quite obvious that many values were based on quick mental arithmetic - collector load resistors in small signal circuits, for example, were invariably 10k but those that weren't were 5.1k. Why couldn't they have used 4.7k - after all, resistor component lists always stated ±20% tolerance!

[Mark Hennessy]

I believe that many shops made up kits of components for people wanting to build the radio. But I was -1 in 1972, so can't really comment. And although my parents brought me the book (along with Simple Electronics), they wouldn't buy me the components to allow me to actually build anything! This meant that I spent too obsessively long dreaming about building stuff, and that led to me taking apart things for components - things that were already working perfectly well! I had a soldering iron for my 5th birthday, so extracting components was easy for me, but of course, they didn't have the long leads needed to work with the bread-board, and attaching wires to extend was just ugly. Still, I guess it wasn't a bad start...

People have been selling kits in relatively recent times: http://www.ebay.co.uk/itm/Ladybird-Trans...0474963750 - naturally, a few substitutions have to be made, but that's not unrealistic in electronics at the best of times.