04-08-2020, 02:01 PM
I'd say that nearly all Lockfit failures in my experience have been noise. Probably 95% or more. Yes, complete failures happen, but I can't recall many. I've lost count of the number of transistor sets (mostly Hacker and Roberts) that I repaired for a local friend when he was selling sets, but it was high hundreds, perhaps more (he was very successful!), and if we assume that each of those radios had perhaps 10 or more Lockfits, that's a lot of meaningful data gathered. While it was good to gain that experience, it left me with no energy to restore any of my own collection - that's hopefully something for retirement, which is still a long way away. But that's why I was careful to point out that things might be different in other equipment, like TV sets, for example. My experience of transistor radio repair is extensive, but it is just that: my experience.
And as I said, when it comes to noise, it depends where they are in the circuit. In many cases, you just wouldn't know about it. And in other places, it's ruinous.
The classic location being the first transistor in Hacker amplifiers. It's so common that I wrote about it specifically here: https://www.markhennessy.co.uk/articles/...tm#lockfit - that includes a discussion (and typical Hacker schematic) about why circuit location matters.
I replace BC148s with BC108s in the Hacker amplifiers because that's what the service manual lists, but BC548s contain the same silicon as far as I can tell, and therefore are equally good electrically if you don't care about how things look. Obviously, there are many, many possible alternatives.
For many years now I've been promising myself I'll build a test jig to test for noise. Doesn't need to be anything complicated, and I have bread-boarded a couple of potential ideas that were really quite promising. It's just a case of getting around to it.
Next time I'm up in the attic I'll dig out an R600 to see what you mean by "very badly mounted".
Anyway, I'm not in a position to speculate whether it's a Lockfit problem or not based on what you've reported so far, so my reporting of my experience is just general, and intended to be useful to people less expert than yourself. For those folk, perhaps the most useful things to pick out of my previous post are 1. current limited bench power supply, and 2. start with 'scope or powered speaker at the volume control.
Oh, I've just remembered a reasonably common fault that happens with the Hacker amps, and perhaps could happen with this one too.
The VAS in the Hacker circuit - an AC128 (TR4) - can develop intermittent tin whiskers that cause this fading effect. Sometimes you hear cracking, which can be alarmingly loud, but often you don't. The fault changes the circuit conditions in a way that looks like heavy collector-base leakage (which can also be caused by C7 going leaky). What's happening is intermittent bursts of current leaking between collector and base via a high-resistance whisker (or not-so-high if you hear cracking), and this gradually shifts the DC conditions - the time constant being formed by the large capacitors in the circuit having a peak-hold effect. If present at all, the programme audio comes and goes while all this is going on.
Looking at the Roberts circuit (Trader 1852), the VAS (T3) is an AC128, so it could be behaving in a similar way. The equivalent to C7 is C22, but I'd suspect T3 over that.
Tin whiskers in TO1 packages seem to be much more frequent than they used to be. For years, folk on the vintage forums didn't believe me when I told them it happens, but it seems to be accepted now. I've also written about that in that article on my website.
Anyway, first/next step is to determine if it's before or after the volume control...
And as I said, when it comes to noise, it depends where they are in the circuit. In many cases, you just wouldn't know about it. And in other places, it's ruinous.
The classic location being the first transistor in Hacker amplifiers. It's so common that I wrote about it specifically here: https://www.markhennessy.co.uk/articles/...tm#lockfit - that includes a discussion (and typical Hacker schematic) about why circuit location matters.
I replace BC148s with BC108s in the Hacker amplifiers because that's what the service manual lists, but BC548s contain the same silicon as far as I can tell, and therefore are equally good electrically if you don't care about how things look. Obviously, there are many, many possible alternatives.
For many years now I've been promising myself I'll build a test jig to test for noise. Doesn't need to be anything complicated, and I have bread-boarded a couple of potential ideas that were really quite promising. It's just a case of getting around to it.
Next time I'm up in the attic I'll dig out an R600 to see what you mean by "very badly mounted".
Anyway, I'm not in a position to speculate whether it's a Lockfit problem or not based on what you've reported so far, so my reporting of my experience is just general, and intended to be useful to people less expert than yourself. For those folk, perhaps the most useful things to pick out of my previous post are 1. current limited bench power supply, and 2. start with 'scope or powered speaker at the volume control.
Oh, I've just remembered a reasonably common fault that happens with the Hacker amps, and perhaps could happen with this one too.
The VAS in the Hacker circuit - an AC128 (TR4) - can develop intermittent tin whiskers that cause this fading effect. Sometimes you hear cracking, which can be alarmingly loud, but often you don't. The fault changes the circuit conditions in a way that looks like heavy collector-base leakage (which can also be caused by C7 going leaky). What's happening is intermittent bursts of current leaking between collector and base via a high-resistance whisker (or not-so-high if you hear cracking), and this gradually shifts the DC conditions - the time constant being formed by the large capacitors in the circuit having a peak-hold effect. If present at all, the programme audio comes and goes while all this is going on.
Looking at the Roberts circuit (Trader 1852), the VAS (T3) is an AC128, so it could be behaving in a similar way. The equivalent to C7 is C22, but I'd suspect T3 over that.
Tin whiskers in TO1 packages seem to be much more frequent than they used to be. For years, folk on the vintage forums didn't believe me when I told them it happens, but it seems to be accepted now. I've also written about that in that article on my website.
Anyway, first/next step is to determine if it's before or after the volume control...
