If thats split .. physically split that is then whatever have you supplied this with?.
Not mains by any chance?..
If that really is split then change the cap next to it, the Zener diode will have suffered. I expect and I reckon the other ones will be OK.
Do a simple ohm meter check on them if you know how to and those transistors are nothing that special, most any NPN will do there and they do not have to be matched at all.
Also check the 1.8 K to see if it still is at that value..
two. Does a balanced pair just mean that the gain of both the BC238 transistors needs to be the same or is there more to it?
matched pair when I looked for the schematic on-line. (There was a paper copy stapled inside the keyboard)
power supply that was used.
Its acting a s a 7.6v regulator, and there's absolutely no protection against surges into the 100uF cap. I'd replace it with a 7806 with 2 diodes in its ground lead. 78s are self protecting.
NT
two. Does a balanced pair just mean that the gain of both the BC238 transistors needs to be the same or is there more to it?
matched pair when I looked for the schematic on-line. (There was a paper copy stapled inside the keyboard)
power supply that was used.
surges into the 100uF cap.
Actually it is current limited to beta*(Vin-8.2)/1800 taking gain beta as 300 and Vin as 12v that gives about 650mA worst case peak current (or
160mA off a 9v supply). But it won't occur for long enough to stress the transistor unless the capacitor is a dead short to ground.The original circuit is OK, but might benefit from a small capacitor in parallel with the zener to filter out noise at source.
There is a 7808 but it won't like a 9v battery. You need about 2v of headroom over the regulated output voltage.
And 78xx series are also inclined to oscillate wildly at RF if you don't put the appropriate decoupling capacitors close to the device.
against surges into the 100uF cap.
fair enough - but the 328 is specced as having beta 120 - 800, and an unregulated 12v psu would deliver about 19v, so its all a bit iffy
A 12v psu would be fine with a 7806 plus 2 diodes. It'd give much better psrr too.
NT
two. Does a balanced pair just mean that the gain of both the BC238 transistors needs to be the same or is there more to it?
matched pair when I looked for the schematic on-line. (There was a paper copy stapled inside the keyboard)
power supply that was used.
Ah. Its been connected to a much bigger battery probably.
Very simple and very poor circuit design all round.
That first BC550 has almost no temperature of component tolerance stability at all.
The second one does although its limited to 5:1 gain
I would be very include to fit a 7809 type regulator in place of that transistor and short the zener out and cut the resistor out as well. .
If you do decide to replace the transistor any bloody NPN silicon shit will do. Even a Sinclair branded transistor would work in that position.
Except in the first transistors case with has no feedback at all to speak of.
Really;! You do surprise me they rarely worked or stayed working in most all of his products;!...
Exactly. Thats how uncritical it is.
two. Does a balanced pair just mean that the gain of both the BC238 transistors needs to be the same or is there more to it?
I think the voltage regulator in my head needs a heatsink. I am slowly working my way through the posts and with wikipedia holding my hand I am getting there. There may be other questions, but I am enjoying where the original question is taking me. Thank you all.
"I diverted an electric piano from the skip the other day."
"Ten thousand holes in Blackburn, Lancashire"
Four thousand?
That was 35 years ago ...
(CBA to look up the precise year)
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