30 Jul
2019
30 Jul
'19
11:58 a.m.
On 2019-Jul-29, at 6:34 PM, Jim Brain via cctalk wrote:
I have an ill NLS MS-230 Miniscope. Is there anyone
on list that might be interested in getting it running for me? I'm willing pay for
the privilege. I'd like to see the unit working, but I have no experience with analog
scopes, and I'd rather just entrust it to someone who can see it to success. I did
replace the batteries and let it charge for quite a while. The red LED lights up on the
front when on, but no sign of a trace, even when fed a known good 1kHz wave. The CRT does
not appear to be on.
Anyone a fan of these units and might be interested in taking a look?
It's tempting but I'm across the border and some shipping distance away.
Looking at the schematic (readily available online), it's fairly straightforward.
If you're willing to spend a little time on it, you could do the basics and check
for the power supply voltages as listed on the schematic.
The power supply is essentially 3 stages:
1) line -> charging circuit for the batteries,
2) batt -> +5 regulator,
3) +5 -> simple switching supply for +/-7V, +80V, +100V, -720V, 0.5V heater, 12V for
U11.
With nothing on the CRT (esp with the intensity turned full up), suspicion may fall around
the little switching supply.
A key point to note with 'scopes like this is the cathode/heater runs at high negative
voltage relative to GND, rather than the TV/monitor convention of the anode running at
high positive voltage. This is done so the amplifiers to drive the electrostatic
deflection plates can be be operated near GND level rather than having to raise them way
above GND.
So according to the schematic the heater (acting as cathode; either pin) should measure
-720V relative to GND and there should be 0.6V across the two heater pins.
U11 should have 12V across pins 14 & 7, but like the heater, it to is floating at
-720V below GND.