PHILIPS SCOPE REPAIRNow I've got infected1 by your long posts, so bear with me:
Well I am glad you got infected because I don't see how you could have conveyed that much help/detail with a one liner
Thanks for sharing that. I won't lie, I got only 90% of it, some stuff escapes me somewhat.... I only studied the basic single BJT common emitter amplifier at school. Amplifier-wise, anything any different or fancier gives me a hard time
I guess I need to read again my copy of the Art of Electronics...
Especially that cascode voltage amplification part, the middle transistor, oh well.
I would need to have you here at the bench to explain to me why current is going this way and this way rather than through here or there but... sadly we can't quite do it here. Maybe you should take a trip to France one day.
9V=Common emitter on V1419&1422; Oh, and V1419(BSX20) is NPN - schematics mistake, you should request refund
No, no refund ! Look twice... they did draw an NPN symbol... OK I had to look closely, zoom on it and stare at it for a little while but... it's definitely an NPN that they drew.
The arrow is quite blurry and not well defined, I will give you that !
I measured that 9V on the common emitters, and found 9.85V instead !
It's made with a simple voltage divider from 12V and a 1K and 3K resistors... precision resistors at that.
12V supply is spot on, as are the two resistors, so I should get 9.00V shouldn't I ?! Unless the diode is leaking in reverse ?
I thought I was on to something... but I decided to first test some more DC voltages with my DMM, and 15 minutes later the scope was fixed ! Believe it or not !!!
See schematic below.
I concentrated on the area I coloured in yellow.
This part seemed easier for me to understand, so I thought, if anything is off there, the diagnosis should be easier for me to establish with a good degree of confidence... I was not disappointed.
So if I am not mistaken it looks similar to the current generators for the amplifier ? A transistor with a fixed DC voltage at both its base and emitter. A resistor in the emitter, and that's it ? So we have two constant current sources here, that sink (12 - 8 - Vbe) / 3.16Kohms = about 1mA ?
So I measured the fixed -8V that's fed to both transistor bases... and I do get 8V spot on, great. -12V supply is spot on too.
At the emitter of the upper transistor V1406 I get -8.65V or something, as expected, good.
At the emitter of the lower transistor V1407 I get........ -12V !!! NOT good, eh ? Means there is zero current flowing through the emitter resistor R1403, which means... the BE junction of the transistor must be failed open-circuit ! For sure eh ?
Felt confident enough about that, that I desoldered that transistor. I was glad to see that despite it being soldered on both top and bottom layers... I was actually able to remove it very quickly and easily, zero damage to the board/traces, phew ! I worried for nothing then, earlier.
Transistor is a BC548C NPN.
Looked in my inventory spreadsheet. No 548C but I have a bunch of BC547B instead.
Datasheet says they are the same type/family. 547B has a higher voltage rating, doesn't hurt, but has much lower DC gain.
Still, I thought it had plenty enough gain to achieve that mere 1mA current it's trying to sink, so I gave it a try.
I was well rewarded, scope now works perfectly !!!
No bright dots at the ends of the trace any more (due to the clipping we had), and the trace now goes from edge to edge, and the X Shift control now behaves properly, it can shift the trace half a screen to the left, and half a screen to the right. The gain trimmer of the amplifier now works properly so I could adjust the accuracy of the main time base, it's now properly calibrated and the future owner will be able to rely on it to make his time measurements...
IT WORKS !!!!
Well, the X10 MAG does not any more, the switch is dead for good it seems... it doesn't click even once in a 100 attempts at pulling the knob. Sadly I guess it will remain that way, given how hard the pot on the front panel is to get to !
Mystery finally solved then, it works, I can button it back up, quick clean, a few pics and put it up for sale ! Hoping to get 50 Euros for it, with some luck !