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Sansui 4000 LED re-lamp...just one issue

secondmouse

secondmouse

New Member
Finally getting around to re-lamping my 4000. Several lights are out. This unit suffered some kind of surge in the past, evidenced by a partly melted plug, but does work just fine in all respects. LED kit from Joe is on the way, but doesn't include the protection bulb, and finding an original 25v 90ma bulb is nearly impossible. I did locate a 28v 40ma E10 bulb (not LED) which seems to maybe be OK, but I've noticed the schematic shows just 8.4ma on that 25v line (??!!) feeding the original bulb. Why did Sansui spec a 90ma bulb?

Help please. I know some of you have re-lamped these receivers. Any insight appreciated. Joe's tech says not to try any LED replacement for this bulb as it may not trigger properly. I'm lost. The rest of the lamps are straighforward.
 
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The power to turn on TR901 and TR902 in the protect circuit comes through that lamp. The TR901 and TR902 circuit supplies the 24 volts to Q701 and Q702 (the first stage tone circuit). When protect is triggered SCR901 grounds the low side of the lamp, removing the forward drive to TR901 and TR902, which kills the audio signal due to no B+ to Q701 and Q702. If a lamp with too low of a current is used, it will not properly forward bias transistors Q901 and Q902, and cause the lamp to illuminate when not in protect.

Realize that the lamp impedance (resistance) when not lit is very low, not the expected 31 ohms for a 28 volt 90mA lamp (like a dim bulb tester lamp). It is likely about 10% of it's lit resistance, or about 3 or 4 ohms when not lit.

One of the weird things about that "protect" lamp is that if its filament fails, there is no audio. I also had one that was slightly intermittent that made the lamp "microphonic", and caused noise in the output when the lamp was tapped.
 
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Thank you Bob! I actually understood about 2/3 of what you said, but I'm not an audio engineer and I often struggle with the "big picture" in schematics. If I understand properly, the 90ma protection lamp illuminates when either SCR opens and the low side of the lamp goes to ground, resulting in TR901 and TR902 not passing voltage to the tone circuit. Even so, the 25v supply to the lamp is only 8.4ma (1/10 of bulb rate?). How is that enough current to even light the bulb when the low side is grounded? Maybe I need to go back to school...
I am more fuzzy about the "normal" condition, and the SCR are not grounding the low side of the lamp. There would be some voltage/current passing through the lamp, not enough to illuminate it but enough to drive TR901 and TR902? If that is correct then 90ma was probably a critical design choice for the lamp. Unfortunately, I can find no such bulb. (A 28v 100ma bulb MIGHT be had for $30 from the UK, or maybe a 60ma bulb). Seems that other Sansui 4000 owners would be painfully aware of this situation. If another bulb will work, even with a mod, I'm all ears. VR901 and 902 also have a roll in this, looks like.
Da
 
Is your protection bulb bad? The protection bulb doesn’t get used unless it’s in protection so it doesn’t really need replacing. I would leave it.
 
That bulb was lost, unfortunately. Trying to find a replacement, now. I have located a 28v bulb in 100ma, but expensive from the UK - like $30. A US supplier has 28v bulbs in 40ma or 60ma. Would these work as well? i.e. stay unlit unless the low side is driven to ground? I have stared at the circuit a lot but cannot discern how critical the amperage is. It shows 24v normally on the low side of the bulb, if not grounded by the SCRs, and 25v and only 8.4ma on the supply side. Thanks for any help choosing a bulb.
Dave
 
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