I have an old cheap oscilloscope that works perfectly in normal mode, but in digital storage mode, it adds noise to the signal - making it useless in storage mode
My knowledge goes only as far, as not getting myself killed probing this thing
I only have a multimeter to troubleshoot with. It is a Fluke 87 III, so i think the voltage readings should be pretty accurate.
Here im in normal mode, with no input. A nice straight line as expected.
Here im in digital storage mode, and as you can see there's some noise on the display. This might not look too bad actually, but send a sinewave through and its caos!
I read the service manual, and found something about y-amplification (page 27) - Probing the pins 7,8 on IC301 (LM733CN) reads 3.390V and 3.445V, which i guess is reasonable within spec. (3.5V). Pins 1 and 14 however is different. When i probe these in normal mode and storage mode, it adds extra noise. Is this expected? (check the attached ...mode-probe pictures) I have no idea if this is a clue to solve the problem, or if it's a no brainer, that probing these will add noise. However, the service manual says pins 1 and 14 should be +1V ±0.8V, and they both read just about -0.300V, only half a volt, but still 50% of its intended value of 1V.
Measuring the voltage on the test pins on the top board: +11V ± 0.5V (+10,65V), -11V ± 0.5V (-11,29V), +210V ±21V (+215,2V), +7V±0.4V - 0.6V (+6.84V), - 5V (-5.39V), so i guess the main power supply is working properly. And the Ramp test pin (not mentioned in the service manual?) reads +1.934V.
At this point im pretty much stuck. I have absolutely no clue where to go from here
I really like this analog scope, and with the service manual and (as far as i can tell) only jelly bean components, i really have high hopes for a repair.
Please share your wisdom, i will be forever grateful
I uploaded the service manual here:
http://stev.dk/gould_os1420_2x2mv,20mhz_oscilloscope_1982_sm.pdf