Yamaha M-70 / Light bulb tester - Question!

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Hello, I picked up an old Yamaha M-70 a while back and I JUST decided to clean it up and start replacing all the original electrolytics and old carbon resistors.

Before replacing ANYTHING, I plugged it in (using a mains light bulb tester). I used a 3 way bulb (25W; 60W; 100W).

When I powered it up, the lowest setting (25W) did not light whatsoever, The 60W was very dim and the 100W was a bit brighter! Then I turned the bulb (switch) in to the off position AND ran through the process again WITHOUT turning the amp off. This time, the first setting (25W) started off dim and progressively glowed brighter, the next setting (60W) was brighter yet and the final (100W) was Very bright.

I powered off the amp and repeated the process! The EXACT same thing happened.

Can someone PLEASE Explain what is happening?
why do the bulbs only glow slightly the first time through the sequence of wattages (25,60,100) and then actually light (and grow brighter) if I go through the process again WITHOUT POWERING OFF THE AMP?
Then, when I DO power off the amp, The bulbs lowest setting (25W) shows nothing???

I know that using a Light bulb tester is essential for plugging in any old electronics to check for possible problems BUT I would like to know what is actually happening in the process!

Thank you

How is the light bulb tester wired?
The light bulb must be in series with the amplifier. Your description indicates otherwise.

How so?

YES the bulb is wired in-series! It's an old extension cord. Clipped the (+) wire, placed the bulb socket between the "clipped" wire. The other wire (-) side is uninterrupted! Plug the ext cord in to the mains socket, plug amp in to ext plug!

I've used it before... For instance, on my small integrated amp, the bulb Lights Bright for a SPLIT second, then dims completely and then reaches a steady glow after about 15 seconds. But my integrated is up to spec (been re-capped) and only draws 1/5 of what the M-70 draws. The M-70 is old and likely needs complete re-capping.

Something different is happening with the Yamaha M-70!

I want to know how to interpret the ways in way the bulb lights (OR doesn't light) with respect to the amp powering up!

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