Re: Tektronix 475


Michael W. Lynch
 

On Thu, Aug 13, 2020 at 09:33 AM, Chuck Harris wrote:


Have you checked that VR1374 has 82V across it? That 82V puts the grid
bias control into the range where the grid is in the cutoff region for
the CRT.

High voltage zener's and the capacitors that parallel them often start
to break down at too low of a voltage... they become leaky... and that
would put your CRT grid into the blindingly bright range.
Chuck,

I know, this is an old post and an often discussed topic.

I have a similar problem. I am working on a 475. This one had a bad CRT a broken support rod inside the CRT and a tilted trace. I have located another CRT and have installed it. I have set the grid bias, which seems to agree with the manual. No more Tilted trace. The scope works fine when you first start it up, including the Z axis amp input on the back. However, after about 20-30 minutes, the CRT intensity goes "full Bright". Z-Axis input stops affecting the trace, The CRT Trace or dot is also erratic, with the intensity changing in a random fashion from off to dim to full on bright with lots of blooming (it is mostly bright during these times). Intensity control seems to effect the brightness, but will only make the trace brighter than it is at the minimum setting, so intensity control makes it either Bright or Brighter with blooming and loss of sharpness. Focus and Astig controls work as expected when cold and with a super bright trace.

I must admit, I do not fully understand the DC Restorer. I have been reading these and other posts on the subject.

Have you checked that VR1374 has 82V across it? That 82V puts the grid
bias control into the range where the grid is in the cutoff region for
the CRT.
YES! Testing across VR1374 I get about 66VDC. This seems at odds with your statement quoted above. This is with the + lead on the band side and the - lead on the opposite end of the diode. Reversing the leads gives a negative voltage of similar potential. So I have "Low" voltage and something like you are describing below, correct?

High voltage zener's and the capacitors that parallel them often start
to break down at too low of a voltage... they become leaky... and that
would put your CRT grid into the blindingly bright range.
I was expecting 82Volts but perhaps I am not measuring this correctly? From your statement, 60-66VDC would put me in the "blindingly bright range", correct?

Is it possible that this CRT has issues?

or

Would you say that the zener (VR1374) and related capacitor (C1373) could be breaking down?

CR1373 and VR1374 check "good" with a conventional diode check, using a DVM. Both show infinity one direction and .576 VDC in the other.

Like I stated previously, the scope works OK for the first 15-20 cold minutes of operation. After that it all goes nuts.

Your thoughts?


--
Michael Lynch
Dardanelle, AR

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