I was almost right. The LED circuits are fed by their own tap from the power transformer, via an (unfiltered!!!) full wave bridge constructed from discrete 1N4004 diodes on the high side (B
2+), and the low side is connected to ground. The output voltage is not specified but we can do some homework:
The signal from the amp channel is applied to a voltage divider network. The divider consists of 5 resistors, two 820-ohm resistors in parallel, series connected to two 220-ohm in parallel, and a single 220-ohm that can be shorted out by the range selector switch. Effectively, this is (220 or 0) ohms in series with 110 ohms and 410 ohms.
The junction between the 220-ohm and 820-ohm resistors in that divider is then halfwave recified, and filtered by a 10 uF 30 VDC electrolytic capacitor. The filered signal is then applied to a chain of individual resistive voltage dividers which feed the bases of the LED switching transistors. The ratio of each pair of resistors sets the base voltage for each transistor (and with it, the LED connected to its collector), to turn on. The emitters of each transistor are connected to ground.
I replaced all of the LEDs in the PL 200 (original series) I was working on, because some were physically broken. I did not want an inconsistent appearance with newer, brighter and more power efficent LED mixed with older, dimmer, less efficient ones. The part I used was here:
https://www.vishay.com/docs/83494/tlhk46q1r2.pdf.
It has a 2.0 volt typical and 2.6 volt maximum forward voltage drop. The LEDs probably drop less than 2.6 volts when turned "ON".
It has a 20 mA typical and 30 mA maximum forward current.
Since the 1K resistors are in series, the drop across them is 20 volts typical to 30 volts maximum, from ohms law. The value can be less than this, if full brightness is not achieved. The minimum voltage is for the part is not specified.
What is left is the voltage across the transistors. These are TIS97s. the data sheet is here:
https://www.mouser.com/datasheet/2/308/TIS97-1125550.pdf
From the data sheet, it appears that about 5.0 V will appear between collector and emitter in the "ON" state.
A range of estimated total voltage is:
[ ?
(min) 20
(typ) 30
(max)] + [2.0
(typ) to 2.6
(max) ] + 5] volts.
Looks like 27 volts using the typical values.
Hopefully, I have neither bored you with my detective work and analysis nor led you astray with unfounded assumptions, defective logic and bad arithmetic!!!
What I got wrong in the prior post??? Surprisingly, there is no regulator and no filter capacitor on that LED anode bus!!! I think I now solved the mystery of the pulsating LEDs I observed when testing the one I repaired with a signal generator!!!!
Good luck in repairing your PL 200!!!!
