Hum in sound only when lamp is on

[John Burgess]

Here’s a tricky one, I have a Ricoh St 8 mag sound machine projector, it has a valve (tube amp) and runs a 12 v 100 watt A231 lamp via a separate transformer which is situated within the projector body. The amp is separate ,but attached to the base of the projector. With amp on and motor running the sound is really good , clear and bags of gain.when lamp is switched on an annoying hum occurs. It is constant regardless of volume , it’s not that loud but really annoying. Have any members any ideas how I can eliminate this, would be a great early Christmas present!
thanks John

[Steve Klare]

Just a guess: does the lamp wiring or your lamp transformer sit close to the sound head?

Either of these create magnetic fields, and if they are close to the head it will pick up their variations in the field and blend it into the audio signal. The more power they are handing the stronger the field will become.

[Lee Mannering]

Lamp conversion probably causing it.

[John Burgess]

Steve, the tranny is at the other end of the projector well away from the sound head. When I bought the projector many years ago the lamp tranny was missing, I installed the 12v 100 w tranny in its place. The amp was shot and its only recently I have got it fixed and realised the problem. Thanks for your response.
John

[Joseph Banfield]

Could it be that the lamp transformer you substituted for the missing original does not have the same shielding?

[John Burgess]

Hi Joseph, good thinking I will check that out. Thanks !

[Maurice Leakey]

And perhaps, a good earth connection?

Maurice

[John Burgess]

Thanks Maurice ,will also check earth, if all fails I will probably substitute the tranny for a toroidal version , this will almost certainly solve the problem.
Thank you John