15-05-2015, 11:35 AM
(15-05-2015, 09:47 AM)ducatiguzzi Wrote: I was considering taking the AF117's out and blasting them but due to the fact that the screen removal never helped the hiss an I correct in saying its probably not worth it?
With these, a tin whisker making contact with one of the internal electrodes is normally enough to stop the set functioning. But once that's happened, the whiskers don't suddenly stop growing. Their growth is not fully understood by any means, but we are reasonably sure that the presence or otherwise of an electric field is not a significant factor. So if your set has been dormant for 20 years - say - since it originally stopped working, then in that time, other whiskers might well have made contact with other electrodes.
In that scenario, you have a short circuit between 2 or 3 electrodes of the transistor, and cutting the screen lead will not help. Indeed, once someone has cut a screen lead in the past to get the set going, you can be sure that sooner or later, the thing will fail again. I always advise against cutting screen leads for that reason.
The only way to know if you need to "blast" a transistor is to remove it and measure the resistance between the case and each of the legs in turn. If there is any hint of leakage - even 100s of k - then blast it. Indeed, as you've removed the transistor, you might as well do it anyway. It does sometimes change things, inexplicably (perhaps the physical disturbance while removing it is enough to temporarily displace the whiskers?).
In case you haven't seen it, I did write about this problem a while back: http://www.markhennessy.co.uk/articles/v...istors.htm
To the specifics of the set, it sounds like it's working, but with too much background hiss?
The first thing to establish is where is that hiss coming from. Is it the audio amp, or is it the tuner? To do that, simply turn the volume to minimum, and assess how much hiss is left.
(If that sounds like a case of stating the obvious, it's actually a common problem with many Hacker sets that use a transistor in the audio amplifier that is prone to failing to noise, but some folk think it's a problem with the tuner and report "poor tuning" or similar)
Next, is the hiss actually local interference? My workshop is in my cellar, which is poor for AM reception generally, but added to that there is a computer or two (my LCD monitor wipes out LW), some high-frequency light fittings (which run at 100kHz typically), plus a small rack full of IT gear and a load of Cat5 cables running there. I have an Ethernet switch mounted on the rear of my bench, and I have to turn that off to line up a radio (it especially affects FM). And interestingly, different radios respond to this "electronic fog" differently - so it's not enough to assume that because radio A is OK, radio B must have a fault. So, try taking the radio somewhere "quiet" - perhaps outdoors?
Finally, if you're sure the front end is definitely unwell, that's when things get complicated! Start with the DC conditions (essential in any fault-finding), and be suspicious of leaking capacitors that might change them (e.g. the AGC capacitor). Check that the resistors are not wildly off their value (note that carbon composition resistors drift high). Then, consider aligning the IF cores - not an easy job for someone who is new to all this, but not impossible either. Resist the temptation to randomly fiddle, but see if any of them improve the signal. If you turn them by no more than a couple of turns each way, and return them to the original position if no improvement can be had, then you're probably OK. Not making things worse, at least. Be very careful when adjusting, as the ferrite core inside is very brittle. Ideally, use the correct plastic tool.
Another possibly obvious one: is the ferrite aerial OK? Has it cracked and been repaired in the past? Does it look like the coils have ever been disturbed (their exact position on the rod affects sensitivity). Are there any broken wires (easy to miss)? I've had the windings fail just where they enter the solder tag - if held in place by tape or glue, it can be hard to spot that one! Finally, clean the wavechange switches. If you've already done so, do it again!
Have you got another example to compare with? Some early AM radios simply weren't very good! That said, by the time they'd got to AF117s, sets were generally pretty lively.
Anyway, hopefully a few ideas in all that - good luck!
Mark
