The accompanying note contained no fault report and asked only for two modest modifications:
- Replace and rewire speaker wire with heavier wire.
- Rewire to shorten power feed cable from tranny.
The speaker wires were soldered at both ends and just long enough to let the chassis rest on a bench. The AC cable referred to was internal and went from the chassis to a 230V to 115 V step-down fitted in the factory 50 years ago. The two wire cable had been wound neatly like a rope and rested on the bottom of the case next to the step-down.
The 240V lead was modern, 3 core and there was a wire linking the frame of the step-down to the chassis.
I connected the amp to my Variac and gradually powered it up to about
00V - just as I expected it let out an almighty hum, mostly 50Hz, through the speaker. The hum mostly disappeared if the volume pot was turned down.Of course bad electros were the cause of the trouble, but the situation was not so simple since as someone had been there before me, decades ago and substituted two pigtail electros for ones inside a triple electro that was mounted off the chassis on a clamp.
It soon became clear that the third electro in the triple had now expired - but that should cause only 100Hz hum, not 50Hz.
Then came the *shocker*, one of the pigtail electros was grounded to the AC heater supply instead of the chassis - this imposed 3.15 volts AC at 50Hz on a DC rail ( the screen supply) that should have had only a trace of 100Hz ripple.
Removing the dead triple electro, adding a new pigtail one and rewiring soon put things straight leaving only a minor amount of 100Hz buzz at full volume.
There was one pigtail electro hanging onto the 5Y3 socket, grounded to a lug on the frame. Soon as I shifted the ground point to the common chassis ground, even that hum vanished.
With the amp now working nicely, I modified the speaker and internal AC leads as requested in the customer's note.
Apparently the customer believed the AC lead tied like a rope was causing the hum.
Oh dear..........
... Phil