Fabricated my dim bulb tester, tested it, plugged the amp, fired it up and the bulb kept lighting bright.
Not really sure of what to expect I plugged a pair of Baby Advent II speakers and an mp3 player through the aux input and music came through.
Not even a minute playing and noticed that peculiar burning smell of electronic components so I turned it off in a hurry.
Heat came from one of the driver boards. A 470uf cap on one of the driver boards was warm to the touch. Very warm.
I installed the cap backwards
Removed the cap and the capacitance dropped from 470uf to 322uf.
Installed a new cap (proper polarity this time
) and checked every component on the board. Everything seemed fine so I proceeded to test again with the dim bulb.
This time the bulb was bright when the amp started then went to an orange low-dim state a couple of seconds later.
Connected the speakers again and sound came up. 2 minutes into playing there was only music and no burning smells or smoke
.
From prior readings I remembered that I should check the bias and offsets.
A google search gave me
this thread.
Measured offset was 12mV and 21mV. Bias was 35mV and 56mV.
Offset was adjusted to 1mV and bias to 20mV on both boards.
The 100 watt bulb had no glow whatsoever after adjusting.
2 hours playing and all seems good.
The sound is night and day compared to when I first got the amp. Very clear and... fast? (for lack of a better word).
Cool to see how the bulb lights up when playing heavy bass music.
This project has been a huge learning experience. Having the dim bulb tester while adjusting DC offset and bias has been very revealing :thmbsp:
As side note, I redid all the 10uf caps on the eq/phono and tone control boards with Nichicon KL (low leakage) trying to keep the recap as close to the original specs as possible.
Would like to thank all the people that helped me throughout this thread and also the "gurus" of the AK community that have posted so much useful information in other threads that were found through searching.
Next on the recap list is a Sansui 9090 receiver that has been playing in the garage for a year. Although I recently acquired a Pioneer SA-9500 II that keeps distracting me
It should be a good shootout between the two.