Rockola 449 - Rotational Thunder Sound

[kopon]

Hi all,
New here so please forgive any transgressions!
I've had my 449 for about 4 years, finally got it going last year and was very pleased just to hear it operating. However, recently I discovered that I was only getting sound from the right speakers. Swapping the input wires to the amp showed only output from the left speakers so I suspected the wiring at the tone arm but there was continuity at all three leads which lead me to target the stylus but still no output to the right speakers. That left me with just the cartridge to work on. To my horror, any replacement or refurb was around £300! So I eventually found a used cartridge at Jukebox Revival for under £100 and bought it. However, the mounting casing was different which I managed to overcome by swapping my own original casing and, sure enough, output from both speakers.
Now, I'm noticing what I can only describe as 'Rotational Thunder' from some records i.e. a low rumbling sound that increases and decreases with the rotation of the record, some worse than others. I initially thought it was the setting of the volume, base or treble switches but since it seems to be on a rotating pattern I have discounted that theory. Still, switching to mono seems to eliminate it as does switching the bass to medium or minimum but then, of course, the overall sound resembles a transistor radio! I'm currently wondering if the cartridge is picking up static somehow as on most records there seems to be a certain amount of undulation, or perhaps is the cartridge sitting too close to the record surface?
I'm reluctant to start modifying anything in case I'm off in the wrong direction completely. So I'm turning to you guys to see if you have any previous knowledge of this 'rotational thunder'.
Any help or suggestions would be greatly appreciated.

[jukeboxmarty]

You may be picking up a rumble from the turntable motor.

I would check the turntable mounting grommets to make sure they are in place and pliable.

[kopon]

Thanks for the reply.
Do you mean the turntable motor mounting grommets?
The turntable itself is mounted on a solid pin.
The sound appears to increase and fade at the same interval of the rotation of the record i.e. 45 rpm. So essentially I'm getting 45 'thunders' per minute. I imagine if the motor is the guilty party the rumble would be continuous?
Also. I seem to recall, when I had the issue with the output, looking at the underside of the cartridge I had found a gathering of dust on the bottom face, the one closest to the record, which I mistakenly thought at the time was the culprit for the dead speakers. Now, thinking back, I'm wondering if perhaps the cartridge is somehow too close to the record? But there doesn't appear to be any simple way to adjust this?

[jukeboxmarty]

Yes, the turntable motor mounting grommets.

Another possibility is that the 'idler wheel' has some sort of flat spot or defect.

I know you initially suspected the cartridge, and may not have had sound on both channels, but did you get this noise with the old cartridge? If not that may be a clue. And no, there is no simple way to adjust the cartridge mounting.

[kopon]

I really can't recall hearing the thunder sound with the old cartridge. But, considering that it was only producing 50% or maybe less then it probably would not have produced the thunder sound? I've discounted anything to do with the idler wheel as the gearing between it and the turntable is such that any issue there would not reproduce on regular rotations of the turntable. I have now resorted to stalling the record mid-play, removing the record, restarting the turntable, holding the stylus in close proximity to the turntable to test for any pick up but nothing. So I've also removed the turntable, restarted the motor and still nothing. Any contact with the stylus while the motor is running produces a thump at the speakers but I still can't find any feedback anywhere else. I'll keep searching and post any findings.

[kopon]

Oct 1, 2023 17:21:11 GMT jukeboxmarty said:

You may be picking up a rumble from the turntable motor.

I would check the turntable mounting grommets to make sure they are in place and pliable.

So, progress at last. Turns out you were right, Marty!
The turntable motor, although fitted with 3 new grommets, is missing the washers & 'E' rings and was instead supported by a small wooden block which held the motor tight to the underside of the chassis. The noise from the records was due to the motor being in contact with the chassis. As I don't have the necessary washers & 'E' rings, I've simply reduced the height of the supporting block and immediately eliminated about 90% of the noise. The tone arm spring is now set at it's shortest (heaviest) position where it achieves the least noise. There is still some small amount of unwanted feedback but very bearable as opposed to the previous. Now that I've traced the source and the path of the feedback I see that the turntable does not have a rubber mat which I suppose could be a further improvement.

Looking back now, I realise what was throwing me in the wrong direction was the rotational aspect which had me convinced that it was all to do with the turntable. That now seems weirdly to have been almost correct in that the noise was actually being transferred from the motor via the chassis, via the turntable, via the record to the stylus. The rotational aspect I now believe to have been the undulations of the records where they made contact with the turntable, obviously some more than others.

I'm now trying to source the missing bits but very pleased with the huge improvement so far.

Many thanks for all the support, suggestions and advice in helping me get this sorted.