09-11-2011, 09:00 PM
Well, finally made some progress...
I tried new diodes - no change. I tried a few other things, then it seemed to start working. I wasn't happy, because I wasn't sure about what had changed. I want to see cause and effect in action!
So, did the complete FM lineup - IF and RF, and find that things were fairly far off, but soon got it working reasonably well. But, it was insensitive, and the tuning meter only reaches about 25% deflection... Definitely not right. But then it started sulking again, and we were back to the start.
I checked/changed lots of components, but nothing really helped. So, that only left the IFT. And that was where the fault was. However, I can see why I was so confused...
After a careful visual examination under a x7 eyeglass failed to reveal anything obviously amiss, I checked the continuity of the various coils and found an O/C. By carefully scraping away enamel, I was able to prove that thankfully the O/C was at the joint, rather than within the coil itself (which I guessed would be unlikely - the wire wasn't all that fine). The bad joint was at pin 8, where the centre-tap of the secondary is picked up, and the O/C section was to the winding that terminated at either pin 4 or 7 (can't remember which). The joint looked fine, but I was able to unwind a small section of wire and tin that, just to be sure, although I'm sure that the good sweating I gave it would have burnt of the rogue enamel buried deep inside.
But, the reason this was confusing was that when in fault mode, there was a healthy sine wave present on both pins 4 and 7 (and a correspondingly good DC potential on C66 and C69). But of course, this was coming via C58, inside the can. At 10MHz, 68pF has a pretty low reactance (around 200 ohms). Of course, if I'd put a 'scope across pins 4 and 7 simultaneously, I would have seen that the sine waves were in phase - and annoyingly I did consider doing this, but ruled it out because it would have meant going upstairs to get a better 'scope; my DM63 is only 15MHz officially, and the triggering isn't brilliant at 10MHz.
Now, the set seems a little more sensitive, and the signal strength meter reaches 75% deflection with strong signals fed in from the FM dist amp. I guess that means that when it working before, it wasn't a zero-ohm connection.
As mentioned, my examples have all been worked on in the discriminator area - in fact one of them has had R53/54 shorted to increase the meter reading. I wonder if this is a common fault? I suppose that now they're all in pieces, I should spend some time fixing them before I forget everything I've leant about them...
Hope this helps someone in the future!
Mark
I tried new diodes - no change. I tried a few other things, then it seemed to start working. I wasn't happy, because I wasn't sure about what had changed. I want to see cause and effect in action!
So, did the complete FM lineup - IF and RF, and find that things were fairly far off, but soon got it working reasonably well. But, it was insensitive, and the tuning meter only reaches about 25% deflection... Definitely not right. But then it started sulking again, and we were back to the start.
I checked/changed lots of components, but nothing really helped. So, that only left the IFT. And that was where the fault was. However, I can see why I was so confused...
After a careful visual examination under a x7 eyeglass failed to reveal anything obviously amiss, I checked the continuity of the various coils and found an O/C. By carefully scraping away enamel, I was able to prove that thankfully the O/C was at the joint, rather than within the coil itself (which I guessed would be unlikely - the wire wasn't all that fine). The bad joint was at pin 8, where the centre-tap of the secondary is picked up, and the O/C section was to the winding that terminated at either pin 4 or 7 (can't remember which). The joint looked fine, but I was able to unwind a small section of wire and tin that, just to be sure, although I'm sure that the good sweating I gave it would have burnt of the rogue enamel buried deep inside.
But, the reason this was confusing was that when in fault mode, there was a healthy sine wave present on both pins 4 and 7 (and a correspondingly good DC potential on C66 and C69). But of course, this was coming via C58, inside the can. At 10MHz, 68pF has a pretty low reactance (around 200 ohms). Of course, if I'd put a 'scope across pins 4 and 7 simultaneously, I would have seen that the sine waves were in phase - and annoyingly I did consider doing this, but ruled it out because it would have meant going upstairs to get a better 'scope; my DM63 is only 15MHz officially, and the triggering isn't brilliant at 10MHz.
Now, the set seems a little more sensitive, and the signal strength meter reaches 75% deflection with strong signals fed in from the FM dist amp. I guess that means that when it working before, it wasn't a zero-ohm connection.
As mentioned, my examples have all been worked on in the discriminator area - in fact one of them has had R53/54 shorted to increase the meter reading. I wonder if this is a common fault? I suppose that now they're all in pieces, I should spend some time fixing them before I forget everything I've leant about them...
Hope this helps someone in the future!
Mark
