Re: 485 No Trace


On 12/13/2012 1:55 PM, Robert wrote:
Picked up a 485 recently with the expectation of it being a parts scope.
However, when I had a chance to look at at, it seemed complete and then
Gasp, need air, getting sucked in, fix this scope... ( Y'all know how
that feels)

The scope turns on, that is the graticule and some front panel lights
come on. The main problem is no trace even with max intensity and no
beam finder, not any green glow at all.

I have now made a number of measurements, and read over the manual but
could have missed something. Mostly I have worked on 7603s and a couple
of 7904s( Got lucky on the 7904's, found some bad diodes and one burnt
part) This 485 is a bit more complex than the 7603's at least to me, so
I'm not sure what the measurements signify, as where do I go from here

Line voltage set to 115 ( I have 120)
Low Voltage (Spec/as measured)

Cath Reg (Where do I measure that?)


Ripple is low enough it is hard to measure using p6053B on a 465M except
for the 120 and 180 lines which are within spec

Nothing in LV jumps out to me as a problem that would prevent a beam, so
on the HV section. Here it gets interesting.

After about 30 secs from turn on, I measure about 2KV at the 3KV test
point, but it steadily increases to 3KV then starts jumping between 2.9
and 3.3 KV. DMM with HV adapter measured significant AC about 250V. Also
looked at it with a P6015 scope probe (no freon). At the 3KV test point
there is some kind of spike or comb like ac signal on the DC (See photo,
5V/div at 0.1 msec per div) initially 3 div vert measurement, then
settles to one div.

Also measured the CRT as follows:
Filament Pin 1,14 with socket removed 12.2 ohms.
Powered up measures 3323 volts either filament pin and stable. AC
measured 33V. Looking at the filament with the P6015 looks like 15V but
more like a square wave (see photo). Wave form likely makes the DMM AC
measurement invalid. Cap C1848 in the Filament feed Power Supply shows
4.5 ESR

Strange: When I touch the filament lines at the PCB connector I hear a
clicking sound like a relay cycling and get a green flash on the CRT
face! With help manged to capture this display (see photo). This effect
was repeatable yesterday. However today I see the front panel lights
flicker as the PS is cycling on and off. It's slightly worse using the
P6015 than the HV probe (40KV type).

I thought about checking CRT adjustments but will the HV issues interfere?

Also looked up "485 High Volt" in the archives. Per discussion checked
TP 1951 balance is -5.5 mv
TP 1958 HV is -0.47V
TP 1959 LIM is +14.8


Thanks, Bob
Check all the caps in the hv section for leakage with an ohmmeter.
I got lucky that way - had a cap that read about 5K ohms so it
was obviously bad. Replaced it - bingo! All ok after that. :-)


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