PHILIPS SCOPE REPAIRYesterday I worked on the issue of the remaining issue, the trace that does not span the entire width of the screen.
I ran out of time / enegy /motivation to report about it yesterday, so I am doing it now. If you find this delay totally unacceptable, please feel free to log on to our website, click onthe "Contact Us" tab. There you will be able to fill in a form and send it to our customer service department.
So.. what I thought should be a simple 10 second "tweak that trimmer and that's it " job, to my dismay turns out to be a full blown troubleshooting exercise, believe it or not.
I tried to be logical and started upstream : checking the time base / sweep generator before the X amp.
I scoped the sawtooth to make sure it looks proper, it does, and that the trimmer named on the schematic "Sweep time" did work.
That's when I realized that when they "sweep time" they don't have in mind what you and me have in mind : these two vocables are NOT meant to refer to the trace length on the screen !
That's why I thought it "did not work" at first ! I was mislead...
Nope. It does what it says on the tin : "sweep time" refers to the actual sawtooth signal itself ! So yeah, I could see on the scope (no the DUT I mean), that playing with that trimmer did vary the duration / "time" of the ramp.... when I would have needed it to vary the amplitude instead ! Nope, amplitude is set in stone, at about 5Volts.
I read the theory of operation as well as the calibration procedures, chapters and nowhere does it tell you how to adjust the trace length !
The only time I ever saw this subject mentioned in the manual, was when they said : " Make sure the trace spans at least 10 division" .... but that's it ! They never say "if needed, adjust it by dong this or that ", no !
So at that point it all started to make sense and the conclusion was that :
1) the sweep signal is fine.
2) it normal if neither the sweep trimmer or gain trimmer in the X amplifier have zero effect on the trace length... tehy aren't meant to to begin with !
3) If the trace length is not wide/long enough, then A) the problem must lie in the X amplifier and B) the gain trimmer is not supposed to fix that, and instead it's a defective circuitry and I need to troubleshoot it.
So that's were my focus is now. Concentrating on the X amplfiier alone (schematic attached), and trying to figure out why the excursion of the plate voltages are not as wide as they should.
The amplifier is basically two identical amps, one for each plate. I drew roughly the "mirror" axis in dotted blue . Sorry looks crap I did it by hand. I don't know how to draw a dotted line with The Gimp just yet
The lower amp is fed with the sawtooth signals (in green), via transistor V1414.
The upper amp (in blue) is fed with the DC offset voltage from the " X Position" control on the front panel, via transistor V1413.
I checked power rails, +/- 180V they are good.
I checked all 6 Zener diodes (while powered up I mean), they all drop their expected voltage BUT for the two 5.1V ones.
They drop only 4.4V or so... both of them. Suspicious that they would be both bad, and by the same amount... so I looked at the schematic and figured it must be "normal" / acceptable : They are fed with 180V and a series resistor of 150k or so. So that gives a current of about 1+mA doesn't it. A little bit low I though, might not be enough to get past the "knee". So I checked the datasheet (they are BZX79C series) and indeed voltage is specified for a current of 5mA and even then with a 5% tolerance, they say that 4.8V is acceptable even with the proper 5mA current. It's not far fetched to think that 4.4V for only 1mA is... to be expected. So not a fault.
So that's where I am. Need to keep working on it to figure out what's wrong.
I would check all transistors but only two of them (in red) are socketed. Plus, testing them won't tel me much since they obviously "work". One of them maybe it "tired", if that's even a thing for a transistor, but how would I be able to tell anyway...
I don't have a proper desoldering gun, and they are all soldered on both sides of the PCB. It's a recipe for disaster. I don't want to damage the pad and tracks and make my life miserable, having to fix ruins.
I think I could maybe extract these transistor harmlessly for the PCB, if I sue my hot air station. I used it with success to removed DIP 16 packages (optocouplers) that were soldered on both sides. So I guess a mere TO92 3 legged trannystor should do it pretty easily. Still I run the risk of overheated the board or trannystor.
So my idea is to first do my best to try to narrow it down to a smaller subset f transistors.
My plan for that, well a starting point at least, is to look at the X Position circuitry (in yellow), which feeds the input transistor (V1413) for the upper amplifier, because when you use that knob it clearly misbehaves : there is a massive offset of the trace to the left of the screen. You need to turn the knob at least 3/4 CW to get the trace centered on the screen.
The schematic as you can see, is nice and tells you what voltages the circuitry is supposed to produce at the base of the input transistor : it should vary between -1.5V and +4.1V. I do get that ! So that's good. It means the X- position problem/offset is not due to the circuitry, but due to the amplifier itself. So maybe the input transistor itself, or associated transistors : V1406 on its left (don't know what that one does, manual doesn't explain), or the stuff on its right, i.e. the amplifier per se.
So I will concentrate on these transistors first, and leave the lower amplifier alone for now.
I guess I could check all resistances first, you never know I might get lucky and find a bad one ! Worth a shot.
Wish me luck...