Quote:
Originally Posted by ms660
Yes, pin 14 of IC2 is connected to B+ as is pin 1 via the 100 ohm resistor.
|
I forgot to mention that pin 1 shows a slightly lower voltage than pin 14 on IC1, so the 100 ohm resistor is doing its job.
Quote:
Originally Posted by ms660
Pin 1013 and pin 607 should have zero ohms (or near as damn it) between them when S1002 is switched to on and the mains connector is plugged into the machine so far as I can make out.
|
I don't follow this. With S1002 open, the only connection between 1013 and 607 is the 2.2k R81. This is a new resistor, checked before putting in. With the mains out, I get 2.2k between 1013-607. With the mains in but the unit off, I get 0R. With the mains in and the unit on, I get 2.2k.
Quote:
Originally Posted by ms660
I would replace T10 and also the bulbs with the correct type or something of similar ratings, I wouldn't just replace the transistor and leave those replacement bulbs you got in circuit as they would draw too much current through T10.
|
The bulbs remain OOC. I suppose the reason they're incandescent is because white LEDs had not been invented. As the bulbs don't bias anything and just go to ground, replacing them with white LEDs and suitable current limiting resistors (or single resistor, as they all light simultaneously) would be easier? Finding a low-current 3V bulb appears impossible as the circuit specifies 30mA.
Quote:
Originally Posted by ms660
There should be 5.6 volts on the base WRT ground irrespective of S1002's position when on mains and ditto voltage when on battery with S1002 switched to on.
The Zener diode pegs T10's base voltage at 5.6 volts WRT ground, a typical base-emitter junction voltage drop for a silicon transistor would be 0.6 volts, that would make the emitter approx. 5 volts WRT ground, that would be a sensible supply voltage for the series connected 3 volt bulbs.
|
Very sensible and clearly explained. I was groping at the avalanche breakdown voltage explanation in my understanding, thinking that if it rose to 9.5V, the Zener would drop it down, which having read your link I see it does. So it's a simple form of regulation for that base of T10.
With T10 replaced (there are a number of BC548Bs on the spare board, so I can afford to lose another), Z131 plugged in, unit 'off' at volume control/on switch:
1013: 5.6V
1014: 900mV
607: 9.5V
So ~2mA passing the 2.2K resistor, the Zener doing its job. 900mV on 1010, the jumping off point for the panel lamps (OOC).
However, when switched on (but S1002 off):
1010: 9.1V
1013: 9.5V
1014: 9.5V
This would match the situation before replacing T10, when the lamps were lit with S1002 off.
EDIT: Zener is good tested for resistance both ways on the AVO...