Hi Edd,
Thanks for the homework, which I've diligently undertaken in my workshop. What I will say is that this Scott set is probably the best radio I've ever dealt with in terms of build quality (like a tank) and ease of finding parts (Resistors, Capacitors, Transformers being helpfully labelled).
As to the detail of the audio 'rumble' (a mild 'motorboating' perhaps?) I've tried the following, but without success:
i) I've replaced (temporarily), C148, and C156.
ii) I've tried to clean the AF switch, but it's actually impossible to get any cleaning solution inside; the switches appear to be covered with something similar to the 'tropical use' shellac used on a number of WW2 radios I've encountered. Anyhow, it's sealed shut. When I adjust the volume, it makes no difference to the 'ticking' (just makes it louder, but does not alter the speed or sequence of the noises.
iii) I found R121 (helpfully labelled on a bakelite board along with several others), and tried grounding as suggested. The audio signal disappeared and the dial light became brighter, showing a short to ground I guess.

Further indications:
i) the ticking starts to occur after the set begins to warm up. The station audio is heard, and then the ticking begins after about 20-30 seconds.
ii) the sound is less obvious on weaker signals. The more powerful the station, the more intrusive the noise.

Head Scratching:
i) Do you think this could be caused by a 'noisy tube'; one which tests OK (they all do), but has a fault which isn't shown on my tester. As someone used to say to me, 'your radio is your tube tester'...
ii) If it could be a tube with a short, which would be the likely villain? V107 (for example)?
iii) I've turned my eyes inside out looking for any loose wires, though the 'ticking' doesn't sound like that, and no moving the set around will change the nature of the 'ripple'.
iv) would there be any sense in changing out all the electrolytics just in case? I'll reconnect C148 and C156 as they seem to fine and 'if it ain't broke, don't fix it'.

Thanks for all your help, Sir Edd, and I hope I can get to the bottom of this gremlin before the Scott is put to gather dust at the back of my workshop as one of those sets requiring an exorcism...

Cheers,

Leslie

:Hi Edd,
:Thanks for the homework, which I've diligently undertaken in my workshop. What I will say is that this Scott set is probably the best radio I've ever dealt with in terms of build quality (like a tank) and ease of finding parts (Resistors, Capacitors, Transformers being helpfully labelled).
:As to the detail of the audio 'rumble' (a mild 'motorboating' perhaps?) I've tried the following, but without success:
:i) I've replaced (temporarily), C148, and C156.
:ii) I've tried to clean the AF switch, but it's actually impossible to get any cleaning solution inside; the switches appear to be covered with something similar to the 'tropical use' shellac used on a number of WW2 radios I've encountered. Anyhow, it's sealed shut. When I adjust the volume, it makes no difference to the 'ticking' (just makes it louder, but does not alter the speed or sequence of the noises.
:iii) I found R121 (helpfully labelled on a bakelite board along with several others), and tried grounding as suggested. The audio signal disappeared and the dial light became brighter, showing a short to ground I guess.
:
:Further indications:
:i) the ticking starts to occur after the set begins to warm up. The station audio is heard, and then the ticking begins after about 20-30 seconds.
:ii) the sound is less obvious on weaker signals. The more powerful the station, the more intrusive the noise.
:
:Head Scratching:
:i) Do you think this could be caused by a 'noisy tube'; one which tests OK (they all do), but has a fault which isn't shown on my tester. As someone used to say to me, 'your radio is your tube tester'...
:ii) If it could be a tube with a short, which would be the likely villain? V107 (for example)?
:iii) I've turned my eyes inside out looking for any loose wires, though the 'ticking' doesn't sound like that, and no moving the set around will change the nature of the 'ripple'.
:iv) would there be any sense in changing out all the electrolytics just in case? I'll reconnect C148 and C156 as they seem to fine and 'if it ain't broke, don't fix it'.
:
:Thanks for all your help, Sir Edd, and I hope I can get to the bottom of this gremlin before the Scott is put to gather dust at the back of my workshop as one of those sets requiring an exorcism...
:
:Cheers,
:
:Leslie
:
:
Hi Leslie, I would change the electrolytics for sure. Did you check them for tolerance? I pretty much always change them out and almost always change the wax/paper caps unless they look good and are in tolerance. Good Luck, George T

PS Hi Edd how ya bin doin?

Hi George,

I'll give it a try but this particular set is so beautifully built and (inside at least) in mint condition I'm reluctant to start a wholesale electrolytic purge. I'll try changing them out one at a time. It would be nice to change them in order of 'likely villains' though...
Cheers,
Leslie

:Hi George,
:
:I'll give it a try but this particular set is so beautifully built and (inside at least) in mint condition I'm reluctant to start a wholesale electrolytic purge. I'll try changing them out one at a time. It would be nice to change them in order of 'likely villains' though...
:Cheers,
:Leslie
:
:
Hi, that would be a good idea, do you have a tester? That might just filter out your rumble in the jungle. Best 2 U, George T

Hi,

I do have a tester, though I tend to use it just for resistors rather than capacitors (either electrolytic or non-electrolytic). I could have a go and see what happens, though when I've tried to use it for capacitors the values seem different each time apply the leads.

Can you test electrolytics while they're still connected to the circuit in the radio, or do you have to cut the wires and then test them?

Cheers,
Leslie
:
:

:Hi,
:
:I do have a tester, though I tend to use it just for resistors rather than capacitors (either electrolytic or non-electrolytic). I could have a go and see what happens, though when I've tried to use it for capacitors the values seem different each time apply the leads.
:
:Can you test electrolytics while they're still connected to the circuit in the radio, or do you have to cut the wires and then test them?
:
:Cheers,
:Leslie
::
::
:
:
Hi Leslie, most of the time you have to disconnect them to get an accurate reading. Sometimes when the tubes are out you can get accurate readings but it is rare. If the electrolytics have symbols on the side of the cans for the different values and are mounted on top of the chassis those marks are by the lugs under the chassis. Just found that out after 150+ radio repairs. The one the guys on this site helped me out with had a fiber board and the symbols were cut out right by the lugs, a square, and a triangle. I use to just trace out the schematic but I couldn't read the schematic for the set I was working on at the time. That little bit of information really made life much easier. I really think that should clear up your problems. Another thing I have used in the past is a Stethascope with just the rubber hose hooked up and you can hear the different sounds but I use that mostly on resistors, I generally change all of the electrolytics and wax/paper caps so the set is safer and less prone to failure. I do a lot of rebuilds for customers and don't want them to have any problems down the road. Good Luck, George T

Hello Leslie. Sorry, I'm not Edd either :>)

Have you tried moving the radio to another location? Or even taking it to a friend's house for a trial? I'm thinking your radio is sensitive, MAY be picking up external noises from electronic everything from light fixtures, alarm systems, fancy toasters, etc.

Hi John,

I haven't tried the set in other places, but I've got numerous radios, including some very sensitive military 'boatanchors' and in my workshop (attached to the loaded whip antenna I have on the roof) none of them have this noise (kind of like a random series of bass 'ticks', not scratchy like a loose wire or bad connection).

I was wondering if it could be a faulty audio amp tube (perhaps); all the tubes test fine on my vintage tester, but I know that sometimes a tube that tests ok can have shorts. In this case, the noise does not start until about 30 seconds after the set has warmed up and a broadcast signal can be heard.

Anyhow, thanks for the suggestion.

Cheers,
Leslie

PS: I've also found that LED lights are great RF noise generators, along with toasters et. al.