Sansui 990DB lights…..

[R&B Stereo]

Greetings AK friends,

Long time admirer of AK, first time posting (well, second time as I just posted a question in the Marantz forum). I recently fully restored a 990DB for a client (yes, included the DB board), but for the life of me can’t figure out why suddenly I have no “juice” to the lights. The unit plays sound and sounds great. Just not a single bulb lights up. And it was lit up just fine, before I started. It appears the lights are powered by a separate little power supply board, which I think also powers the FM board (aka TPS or tuner power supply). That TPS has been recapped. It also had a few suggested replacement of diode and transistors “per the supplied kit”. I eventually reverted back to the original small transistors and diode. Still no change. There is still the “TIP41” (per memory) installed instead of the original 313. Head scratcher is that the tuner is playing fine, though. Thoughts welcome, I have exhausted my brain power over this one.

 

[DougBrewster]

The lighting is supplied by a separate 6V winding on the main transformer and fed via fuse F3 or 003.

 

[R&B Stereo]

The lighting is supplied by a separate 6V winding on the main transformer and fed via fuse F3 or 003.

Thanks DougBrewster. The fuse is not blown, which is why I have my head scratching. The schematic appears to refer to a resistor on the illumination display board. Worth checking?

 

[AE Bob]

You absolutely need to remove the fuse to test it with a multimeter. When it is in circuit the path through the lamps and transformer winding will show continuity across the fuse even when it is blown.

 

[Sansuiman]

If the fuse checks out, most of the dial lamps (including the vernier needle) are controlled through the mode selection switch, so you may have an issue there. Of course, the meter lamps would still work even with fault in the rotary switch, I believe. If they are out also, I'd suspect a fuse issue.

 

[R&B Stereo]

You absolutely need to remove the fuse to test it with a multimeter. When it is in circuit the path through the lamps and transformer winding will show continuity across the fuse even when it is blown.

AE Bob.... you're the best! That was it. I shake my own head in disbelief that I was so ignorant and spent so much time trying to diagnose.... thanks again.

 

[R&B Stereo]

Problem Solved. Thank you all for your support and special thank you to AE Bob.

 

[LBPete]

If the fuse checks out, most of the dial lamps (including the vernier needle) are controlled through the mode selection switch, so you may have an issue there. Of course, the meter lamps would still work even with fault in the rotary switch, I believe. If they are out also, I'd suspect a fuse issue.

Not on the DB models. The dial lights are on all the time.

- Pete

 

[Sansuiman]

@LBPete, thanks for pointing that out. Dumb mistake on my part, I'd forgotten the non-DB models had the dial lights configured for "always on" operation.

 

[LBPete]

Lets try this one more time. The 8080, 890, 9090 and 990 switch the dial lights off when not in AM or FM mode. It's the DB models that the Dial lights are always on regardless of mode.

- Pete

 

[AE Bob]

Many of the 9090,990,890, 8080 (non DB) also had the dial lamps wired on always. The early units ran the lamps through the selector switch, but later ones were wired always on by the factory. Sansui engineers finally realized that the selector switch could easily carry the lit current of the dial and meter lamps, but was failing due to the LARGE current surge when they were first turned on. There are seven 300 mA lamps (5 in the dial and 2 in the meters), all wired in parallel. That calculates to 2.1 amps of running current (.3 X 7 = 2.1). That seems like a lot for the switch, but that is not the issue. Incandescent lamps have a much lower impedance when not lit com[paired to there lit impedance (around 1/10 the lit impedance). If you measure a 8 volt 300 mA fuse lamp that is not lit with an ohm meter, it measures about 2.7 ohms. 8 volts feeding 2.7 ohms would draw 2.96 amps for a single lamp. Multiplying that by 7 for all the lamps in parallel gives a result of 20,7 amps of initial current when the selector switch is initially closed. There is no way that the switch can continue to handle that sort of surge current long term. The factory eventually figured this out and changed the production. I wire all of the units that come through the shop for lamps on always.

 

[Sansuiman]

@AE Bob. Thanks for that informative clarification. I guess I wasn't entirely mistaken after all. The only non-DB models (one each 8080 and 9090) that I ever worked on had switched dial lamps, to the best of my recollection. I know the 8080 did, as I clearly remember the switch giving me problems on that one. No idea where the production breakpoint was for the change to "always on", and can't say I've ever seen it discussed here.

 

[LBPete]

@AE Bob, that's an important point. Many of the 8080/9090 models will have the function switch bypassed to keep the lights on in all modes. So it won't be uncommon to see the lights on many of them.

- Pete