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G8MNY  > TECH     15.10.18 10:30l 105 Lines 4398 Bytes #7 (0) @ WW
BID : 41862_GB7CIP
Subj: Strange Scope Hum Update
Path: ED1ZAC<ED1ZAC<GB7CIP
Sent: 181015/0813Z @:GB7CIP.#32.GBR.EURO #:41862 [Caterham Surrey GBR]
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 (1M
impedance) WHEN HOT, it goes if you earth/terminate the input into lower than
10K. 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*  +EHT  CAP
                         Diode 
                           1     Diode
                                         2
                                 AC Cap
                                
                                 * <--1kV AC
                          Diode             EARTH POST
                            2   
                              /~~\/~~\  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>100KĿ
                                             
1kV2G2G2G2GĴ>FET      NO
AC            ===15pF ===15pF ===15pF  1M 100pF          HUM
          (< 1mV)
              Caps made of PCB tracks & components

   HOT                       Y1 input>100KĿ
                                             
1kV1G1G1G1GĴ>FET    > 5mV OF
AC            ===15pF ===15pF ===15pF  1M 100pF        HUM 180
         out of phase
              Caps made of PCB tracks & components

  FIXED                      Y1 input>100KĿ
                                             
1kV1G1G1G1GĴ>FET      NO
AC   200pF=== ===15pF ===15pF ===15pF  1M 100pF          HUM
          (< 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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