Schematic Gould 3300B scope

[Mat Nieuwenhoven]

Hi,

I've got a bad Gould 3300B oscilloscope where the + and - 15V are not coming
out of the power supply. I'm looking for a schematic of that section (or
better, of the whole thing, or even better, tell me what's wrong with it
).

Thanks, Mat Nieuwenhoven

 

[Mat Nieuwenhoven]

On Wed, 27 Oct 2004 23:59:26 +0200 (CEST), Mat Nieuwenhoven wrote:

Hi,

I've got a bad Gould 3300B oscilloscope where the + and - 15V are not coming
out of the power supply. I'm looking for a schematic of that section (or
better, of the whole thing, or even better, tell me what's wrong with it
).

I haven't been able to locate a manual or schematic, but managed to get it to
work. In the interest of other owners of the same scope, I list what was
wrong.
- No +/- 15 V from power supply: 723 regulator was bad, determined by not
finding the reference voltage.
- No light, but high voltage was 12.1 kV and cathode voltage -1325: turning
up the 'cut-off' pot on the H defletion board produced light. It turned out
that a BC448 in the 'bright-up' circuit on the same board was bad. This is a
80V, hfe 70 minimum transistor. I had a 2N5400 handy (120V, hfe 40 minimum)
which seems to work fine (beware: pinouts reversed).
- Serious mains hum on the brightness and horizontal sweeps, but only
occasionally, not related to how long the unit had been on. Also measurable
by a 6V dip on the 120V capacitor at the back. It turned out the mains
voltage was just below the bottom range. Setting the scope to one lower range
fixed it.
Now I still have vertcal gain which is way off, and further adjustment, but
it is workable.
Disassembly tip: the whole backplate plus transformer and power supply can
come out as a whole, by loosening the four screws at the inner-scope bottom
of the transformer (2 of which also hold the timebase unit), the screw at the
back of the metal rod above the transformer, one standoff at the lower right
back near the power main connector, plus all the edge screws of the
backplate. You also have to temporarily remove the 2 screws of the CRT
holding ring at the back in order to slide the backplate out.

Mat Nieuwenhoven

BTW: 20 8.2 MOhm resistors with 2 x 82k to earth in the tip of an old
fibreglass fishing rod make a nice high-voltage probe for occasional use,
although one must keep in mind the maximum voltage allowed over the selected
resistor type.