Personally i would skip the LED altogether and use the space to install a bias measurment connector. The design
where a transistors "knee" wholly decides the bias level is to coarse and has already given you problems.
Bias check and adjustments needs on be done infrequently and is better done with precision instrument like a dvdm.
where a transistors "knee" wholly decides the bias level is to coarse and has already given you problems.
Bias check and adjustments needs on be done infrequently and is better done with precision instrument like a dvdm.
With a versatile DVM you can also measure the LED. Or use a simple battery with a series resistor to see if the LED is still OK.
Thanks as always guys. So, I have a fairly decent dmm. How would I go about testing the LED? I just don’t know the voltages there. I do not dislike the idea of putting a ground post there, but it would be more preferable to me to make the lights work as designed. There are many good ground spots available already. Also, I want to know that the unit is running as designed. I do not know the condition of these tubes either. Should I take the time to test the other components in the troubleshooting list or can we assume they are fine because the units play muisc? Thanks again
Can’t you test the LED in circuit, in other words without removing / desoldering anythinbg at all?
As long as bias measured on the cathodes is 0.5V you are all set.
The design with transistor + LED is much less accurate then your DVDM.
The design with transistor + LED is much less accurate then your DVDM.
Bias is checked between ground and the "biascheck" testpoint ( as shown in the schematic at page 5 )
If this testpoint is not at the front then my suggestion is to add one in the hole left by the LED.
As for the question in #26 : LEDs will light up at a specific current, the voltage across the LED varies depending on the LED model. 10mA will probably ligt up most LED's. The construction in the schematic used a transistor that starts to
conduct, when it does the current is limited by the resistor between LED and +
The current that is injected in the transistors base and causes the transistor to start conducting is very transistor dependent, thus i do not thing this is a good and portable design.
If this testpoint is not at the front then my suggestion is to add one in the hole left by the LED.
As for the question in #26 : LEDs will light up at a specific current, the voltage across the LED varies depending on the LED model. 10mA will probably ligt up most LED's. The construction in the schematic used a transistor that starts to
conduct, when it does the current is limited by the resistor between LED and +
The current that is injected in the transistors base and causes the transistor to start conducting is very transistor dependent, thus i do not thing this is a good and portable design.