12-12-2016, 11:27 PM
Thanks for your suggestions. I spent a whole frustrating evening on it yesterday.
First of all, the 220K anode feed resistors. I took the chassis out of the cabinet (the resistors are underneath the chassis and otherwise inaccessible), turned it upside down to reveal the component side, then reconnected the speaker and aerial. I powered up the chassis in this position, but to my surprise the cracking noise did not appear, even when I sprayed the 220K resistors with freeze-it. So I put the radio back together and guess what, it started crackling again.
This time I tried to narrow the fault down to the tuner or the amplifier section. This radio has a phono socket for connection to a tape recorder. While the radio was crackling away, I plugged the tape output into an external amplifier and listened - no crackles! In other words, an amplifier fault.
Removed the chassis again. The next suggestion - the paxolin tagstrip. Looking at the strip with the aforementioned 220K resistors connected to the output valves, nothing seemed obviously burnt. Again, with the chassis upside down there was no crackling, but when I tapped some of the components on the strip with a screwdriver handle, I heard a brief crack from the speaker. Could it be a dry joint?
There didn't appear to be any obvious dry joints, but I decided to re-solder the whole tagstrip anyway. While doing this, I noticed a few of the tags were screwed to the chassis. This completes the connection from component to earth and HT negative. I unscrewed these tags, cleaned them and tightened the screws. Now when tapping the tagstrip there was no cracking noise, and after reassembling the radio, the crackling had stopped.
Finally I thought I had fixed it, but shortly after putting everything back together, the tuning cord snapped
That's as far as I've got now. I guess the crackling problem was most likely due to poor earth connection of the relevant tags on the strip. I'm rather surprised that Hacker simply used ordinary self-tapping screws without any "star" or shakeproof washers to connect the solder tags to chassis. Eventually they must have come loose causing crackling noises, but I suppose they held for over 50 years ...
Now I need to replace the tuning cord. Where to get it from, and what is it called? It's been years since I've replaced one (luckily!)
First of all, the 220K anode feed resistors. I took the chassis out of the cabinet (the resistors are underneath the chassis and otherwise inaccessible), turned it upside down to reveal the component side, then reconnected the speaker and aerial. I powered up the chassis in this position, but to my surprise the cracking noise did not appear, even when I sprayed the 220K resistors with freeze-it. So I put the radio back together and guess what, it started crackling again.
This time I tried to narrow the fault down to the tuner or the amplifier section. This radio has a phono socket for connection to a tape recorder. While the radio was crackling away, I plugged the tape output into an external amplifier and listened - no crackles! In other words, an amplifier fault.
Removed the chassis again. The next suggestion - the paxolin tagstrip. Looking at the strip with the aforementioned 220K resistors connected to the output valves, nothing seemed obviously burnt. Again, with the chassis upside down there was no crackling, but when I tapped some of the components on the strip with a screwdriver handle, I heard a brief crack from the speaker. Could it be a dry joint?
There didn't appear to be any obvious dry joints, but I decided to re-solder the whole tagstrip anyway. While doing this, I noticed a few of the tags were screwed to the chassis. This completes the connection from component to earth and HT negative. I unscrewed these tags, cleaned them and tightened the screws. Now when tapping the tagstrip there was no cracking noise, and after reassembling the radio, the crackling had stopped.
Finally I thought I had fixed it, but shortly after putting everything back together, the tuning cord snapped
That's as far as I've got now. I guess the crackling problem was most likely due to poor earth connection of the relevant tags on the strip. I'm rather surprised that Hacker simply used ordinary self-tapping screws without any "star" or shakeproof washers to connect the solder tags to chassis. Eventually they must have come loose causing crackling noises, but I suppose they held for over 50 years ...
Now I need to replace the tuning cord. Where to get it from, and what is it called? It's been years since I've replaced one (luckily!)
