NAD 7030 - Dim bulb issue if speakers selected at power up

[Byron Boldt]

When I power this up with a dim bulb and the speakers switched off the bulb goes dim and everything works. I can then switch the speakers on and the bulb stays dim and it sounds fine through the speakers. However, if I power it on with the speakers selected the bulb does not go dim after a few seconds so I shut it off. There doesn't appear to be any dim bulb issues if no speakers are connected. I just replaced the filter caps on this thing because they were in bad shape. I can't put the old ones back on because one has a broken lead. I did measure the DC offset which is around 90mV on both channels when the bulb is dim. It is a little high according to the service manual but sounds fine. When the dim bulb doesn't go dim, I measured about 4V for DC offset. There is no sound when the bulb is bright. I don't keep the power on very long when it is bright.

Any ideas why the bulb stays bright if the speakers are selected at startup but not if you select them after startup? It seems like it doesn't like driving a speaker load at startup but once it is powered up it isn't an issue.

Thanks

 

[p4spooky]

I have observed this in couple of repairs I have done. The workaround was to install a speaker protect circuit like THIS. These amps had blown outputs (and no speaker protect circuit built in) which after fixing started to behave exactly like OP has described. Looking forward to experts weighning in so that I can understand why this happens.

 

[mondialfan]

Strange startup anomalies can occur when using the dim bulb due to the current limiting that is going on which keeps the power supply in the gear under test from coming up fully. Due to the fact the amp will operate normally if you leave the speaker load disconnected on power up and then connect the load after the bulb has dimmed it sounds like this is what's going on. It's possible that an elevated DC offset shows up for a split second and this offset when the speaker is connected puts enough load on the power supply to keep the bulb from dimming and the power supply from coming up to a level high enough to overcome the DC offset.

What wattage bulb are you using? You may need a slightly higher wattage. Also, Try reducing the output stage bias to zero and try the test again. This will reduce the load on the bulb. I will always run the bias down to zero because this allows me to use a lower wattage bulb for initial testing.

I've seen a lot of weird things go on when using a dim bulb so I never expect that some strange behavior is automatically due to a failure.

 

[Byron Boldt]

Strange startup anomalies can occur when using the dim bulb due to the current limiting that is going on which keeps the power supply in the gear under test from coming up fully. Due to the fact the amp will operate normally if you leave the speaker load disconnected on power up and then connect the load after the bulb has dimmed it sounds like this is what's going on. It's possible that an elevated DC offset shows up for a split second and this offset when the speaker is connected puts enough load on the power supply to keep the bulb from dimming and the power supply from coming up to a level high enough to overcome the DC offset.

What wattage bulb are you using? You may need a slightly higher wattage. Also, Try reducing the output stage bias to zero and try the test again. This will reduce the load on the bulb. I will always run the bias down to zero because this allows me to use a lower wattage bulb for initial testing.

I've seen a lot of weird things go on when using a dim bulb so I never expect that some strange behavior is automatically due to a failure.

I have tried it with a 40 and 60 watt bulb so far. I have a 100 watt bulb but am leery to try it. I don't have any dummy load resistors at the moment to hook up and am scared to fry my test speakers. I haven't adjusted the DC bias yet because this thing doesn't have pots to adjust it. You have to solder specific sized resistors to the board to make adjustments. I am not sure I have the exact sizes they call for at this time. I may try the 100 watt bulb and shut it off pretty quickly if needed.

 

[Byron Boldt]

I went ahead and tried a 100 W bulb and that did the trick. The 100 W bulb didn't light up at all when powering up with no speakers selected. When the speakers were selected the bulb went very bright for a split second. So yes it appears as if there wasn't enough current with the smaller bulbs to get over the bump when powering up with a speaker load. Thanks.