throbscottle
Well-Known Member
I thought I'd better get my oscilloscope fixed (Cossor CDU 150). Been rather like chasing a poltergeist.
So far I've fixed:
constant very bright display which was due a resistor gone high connecting the blanking/bright-up circuits to the tube,
duff triggering due a resistor gone high in the trigger circuit,
the alt sweep which had a duff transistor
the channel 2 vertical position pot which had a broken wiper (being a an obscure dual-gang one I went hardcore and soldered the wiper back together)
Aforementioned pot being very stiff, I cleaned and greased the shaft. Knob wasn't fitting properly because of spindle being stiff it had worn grooves in it, some thread-lock fixed that,
The most bizarre fault was regular-ish transients in the "A" timebase, causing the display to flick off and on every few seconds. Trying to track it down I killed both transistors in the multivibrator which produces the flyback, eventually found the j-fet which receives the signal from that multivibrator has a jittery gate. Lucky I had an old j-fet - wrong type but works ok
Another bizarre fault in the timebase is what I'm calling the cheese triangles. When the larger value caps are engaged in the timing circuit, the flyback becomes a slope. So I'm getting cheese triangles instead of sawteeth. Nothing wrong with any components in the t/b afaict and nothing connected which can affect it. Leaving that one for now (and it did it before I swapped the fet so it's not that).
Whilst I was messing about with the "A" timebase, the "B" timebase stopped working. At least that was easy, ramp transistor gone. I will have to replace it another day.
So I'm left with faults consisting of:
Bright spot at start of display - t/b delay not being blanked I guess
Z-modulation - I expect to find a duff cap in the hv supply. Also now gets gradually dimmer as it warms up.
Stiff channel 1 position pot, getting like the ch2 one was
Stuff I don't know about yet but will doubtless turn up.
Still looking out for a suitable switch to replace the one for ch2 "invert". The original fell apart when I first did fault fixing on this 'scope. It's a slide switch but because of the pin spacing I've put in one of the sort normally found in gangs on older stereo systems. Be nice to find a slide switch with that pin-spacing, to replace it properly. Or maybe I could go hardcore again and mould a new slider for the old one, out of something...
So far I've fixed:
constant very bright display which was due a resistor gone high connecting the blanking/bright-up circuits to the tube,
duff triggering due a resistor gone high in the trigger circuit,
the alt sweep which had a duff transistor
the channel 2 vertical position pot which had a broken wiper (being a an obscure dual-gang one I went hardcore and soldered the wiper back together)
Aforementioned pot being very stiff, I cleaned and greased the shaft. Knob wasn't fitting properly because of spindle being stiff it had worn grooves in it, some thread-lock fixed that,
The most bizarre fault was regular-ish transients in the "A" timebase, causing the display to flick off and on every few seconds. Trying to track it down I killed both transistors in the multivibrator which produces the flyback, eventually found the j-fet which receives the signal from that multivibrator has a jittery gate. Lucky I had an old j-fet - wrong type but works ok
Another bizarre fault in the timebase is what I'm calling the cheese triangles. When the larger value caps are engaged in the timing circuit, the flyback becomes a slope. So I'm getting cheese triangles instead of sawteeth. Nothing wrong with any components in the t/b afaict and nothing connected which can affect it. Leaving that one for now (and it did it before I swapped the fet so it's not that).
Whilst I was messing about with the "A" timebase, the "B" timebase stopped working. At least that was easy, ramp transistor gone. I will have to replace it another day.
So I'm left with faults consisting of:
Bright spot at start of display - t/b delay not being blanked I guess
Z-modulation - I expect to find a duff cap in the hv supply. Also now gets gradually dimmer as it warms up.
Stiff channel 1 position pot, getting like the ch2 one was
Stuff I don't know about yet but will doubtless turn up.
Still looking out for a suitable switch to replace the one for ch2 "invert". The original fell apart when I first did fault fixing on this 'scope. It's a slide switch but because of the pin spacing I've put in one of the sort normally found in gangs on older stereo systems. Be nice to find a slide switch with that pin-spacing, to replace it properly. Or maybe I could go hardcore again and mould a new slider for the old one, out of something...