G8MNY > TECH 11.06.25 09:15z 113 Lines 4757 Bytes #172 (0) @ WW
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Subj: Strange Scope Hum Update
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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Mar 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
I have been working on an old Telequipment D61 scope. It had a few minor
problems which I have fixed OK, but one seems to baffle me...
It has a hummy Y1 trace on max sensitivity with the input open circuit (1Mê
impedance) WHEN HOT, it goes if you earth/terminate the input into lower than
10Kê. You can see the smooth 50Hz hum on the Y1 FET amp with a 2nd scope. There
is nothing nearby to cause it that I can see. The hum is much bigger than on
the circuits supply rails etc. Heating the PCB greatly increases the hum level.
The identical Y2 preamp is in the far corner of the PCB & does not suffer this
problem.
Removing components to isolate the circuit up, I found the hum is ON the PCB
tracks, in fact any track at high Z track has the hum. (plug in transistors use
in this kit make testing easier!) So I did some further testing, the 1kV AC for
the +2.5kV doubler & -1.2kV rails is also on the same PCB, but 5cm away & well
screened with a wall of large soothing caps.
What surprised me more is the 1kV AC is the reverse phase to the hum pickup!
PCB LAYOUT
--------------------------------------------------------------
+2.5kV* ²²²²²²²²²²²²² +EHT CAP
Diode Û
1 Û ÜÜÜÜÜ Diode
² 2
² AC Cap
²
* <--1kV AC
Diode Û EARTH POST
2 Û
/~~\/~~\ 3 LARGE SERIES
\__/\__//~~\ CAPS FOR -1.2kV
NPN -7.5V Reg D \__/
/\ /~\ Low Voltage
\/ \_/ Caps
() Y2 FET Y1 FET ()
PREAMP PREAMP
-------------------------------------------------------------
FURTHER TESTING
Removing the 1kV AC & hum goes looking on a 2nd scope. Cooling the PCB the hum
also goes. So I conclude the hum is travelling inside the PCB & the out of
phase pickup is some strange capacitance field effect see below.
FIXES?
1/ I tried an insulated earthed screening plate to no avail, as my scope probe
investigations already showed me there was little electric hum field in the
air around the FET preamp!
2/ I cleaned the PCB, both sides removing dirt from the high voltage area, but
there was no hum level change.
3/ I noted the maker had punched anti-tracking large holes in the PCB under
diodes 1 & 2, so I drilled another one under diode 3. Some hum reduction,
4/ Then I added 2 metal plates from the earth post to clamp the PCB & take away
some of the PCB electric field near the AC source. This does seem to have
fixed it.
WHAT HAPPENING?
I think the old PCB is slightly conductive, why it should be so heat sensitive
I don't know.
I think it electrically looks like this according to my computer model...
COLD Y1 input>ÄÂÄ100KÄ¿
³ ³
1kVÄÄÄÄÄ2GêÄÄÄÄÂÄÄ2GêÄÄÂÄÄ2GêÄÄÂÄÄ2GêÄÄÁÂÄÄ´ÃÄÁÄ>FET NO
AC ===15pF ===15pF ===15pF 1Mê 100pF HUM
ÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄ (< 1mV)
Caps made of PCB tracks & components
HOT Y1 input>ÄÂÄ100KÄ¿
³ ³
1kVÄÄÄÄÄ1GêÄÄÄÄÂÄÄ1GêÄÄÂÄÄ1GêÄÄÂÄÄ1GêÄÄÁÂÄÄ´ÃÄÁÄ>FET > 5mV OF
AC ===15pF ===15pF ===15pF 1Mê 100pF HUM 180ø
ÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄ out of phase
Caps made of PCB tracks & components
FIXED Y1 input>ÄÂÄ100KÄ¿
³ ³
1kVÄÄÄÄ1GêÄÂÄÄÄÂÄÄ1GêÄÄÂÄÄ1GêÄÄÂÄÄ1GêÄÄÁÂÄÄ´ÃÄÁÄ>FET NO
AC 200pF=== ===15pF ===15pF ===15pF 1Mê 100pF HUM
ÄÄÄÄÄÄÄÄÁÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄ (< 1mV)
New PCB cap Caps made of PCB tracks & components
from plates.
QUESTION
Could this be an aging/carbonising PCB problem? I can't imagine they left the
factory like this?
Any other thoughts on this phenomenon? Feedback welcome.
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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