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The output from the 1940's-era modulated oscillator I'm restoring is being fouled by a 50Hz signal and I haven't been able to find the cause of it - so I'm far from being able to eliminate it. It's a valve instrument - a Paton Electrical "MO" Modulated Oscillator.

Thinking the unwanted 'hum' likely to originate in the unrectified 6.3V filament circuit I put together a DC filament supply and nothing changed. All electrolytic and paper capacitors have been replaced.

With the instrument powered up, but the main switch in the 'OFF' position, the high voltage portion of the secondary side is open circuit, but the low voltage filament circuit is not - so the valve filaments are kept warm. Whether the filament circuit is AC or DC, and with the main switch OFF, this signal appears at the output of the instrument:

enter image description here

It is only about 700mVpp, but the maximum RMS amplitude of the output of the desired signal is 1V, so this constitutes pretty unacceptable interference. It has the effect of modulating the otherwise symmetrical sine wave output to the point of being useless (no waveform capture, sorry.)

Schematic (prepared by me, not an experienced hand:)

enter image description here

The high voltage circuit is full-wave rectified, so I assume that the interference originates elsewhere.

The form of the interfering signal reminds me of magnetising current in a saturating transformer - is this possibly the issue - and if yes, what next?

UPDATE 07 JULY 2022:

Well, the issue has been resolved, however I'm no closer to understanding what the issue was.

I put together separate DC supplies for the valve heaters [6.3V] and high voltage [280V]. These are in their own remote enclosures.

I removed the original power transformer and rectifier valve, and replaced all wiring. I'd already replaced all paper and electrolytic capacitors.

It's made the instrument much easier to work on - now that the enclosure seems empty compared to its previous state!

For interest, updated schematic follows. Thanks to all for your help.

enter image description here

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    \$\begingroup\$ Are C1 and C2 original 1940's era components? Are they electrolytic? If so, I would replace them, and probably any other electrolytic capacitor on the board. I don't believe they will have aged well. \$\endgroup\$
    – Math Keeps Me Busy
    Jun 17, 2022 at 22:47
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    \$\begingroup\$ Make sure it is not due to ground differences between the scope and the oscillator? Measure the ground of the output connector with the scope. \$\endgroup\$
    – Kevin White
    Jun 17, 2022 at 22:56
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    \$\begingroup\$ Is there any filtering at the mains input? \$\endgroup\$
    – Tim Williams
    Jun 18, 2022 at 2:36
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    \$\begingroup\$ The wave form looks to me like the garbage a high impedance input picks up from the household AC. Pin 1 of the 6SN7GT (the modulation input of the main oscillator) looks like a potential candidate. I'd first switch S2 so that the signal going to C6 is grounded - from the schematic it looks like it has a position for this function. If that doesn't help, I'd check the transformer in the modulation oscillator, SW2, C6, and the wiring. This is just guessing. I don't normally mess with tubes. \$\endgroup\$
    – JRE
    Jun 18, 2022 at 11:12
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    \$\begingroup\$ Oh. I didn't see that you mentioned the noise is present when the high voltage is off. What does the modulation output look like? Does it have the same garbage as the modulated high frequency output? \$\endgroup\$
    – JRE
    Jun 18, 2022 at 11:18

1 Answer 1

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Assuming you've already eliminated the possibility of measurement error (i.e. scope ground loop, etc), here's a hypothesis: a short between the power transformer secondary winding and the core or case.

With the power switch off, and the center-tap of the power transformer isolated from ground, there shouldn't be any B+ voltage, and therefore very little current flowing in the output stage (left half of the 6SN7, a cathode follower configuration) to provide. But with a short in the secondary, you'd still have B+. Depending how far from the center tap is the short, you'd get a B+ with a mix of 50 and 100 Hz out of the rectifier, looking like that trace.

Can you test the B+ with your scope with the power switch on and off? If the high voltage is an issue, instead just pull the AC plug, remove the full-wave rectifier tube, and ohm out the transformer secondary with the power switch off and on. Let us know what you find.

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  • \$\begingroup\$ Just measuring resistances I found that one half of the HV secondary is open. \$\endgroup\$
    – Simon Owen
    Jun 19, 2022 at 21:23
  • \$\begingroup\$ ...so it will be a little while before I can offer any substantive update - many thanks for the input. \$\endgroup\$
    – Simon Owen
    Jun 20, 2022 at 9:41
  • \$\begingroup\$ Having only one leg of the transformer working would cause huge 50 Hz ripple on the B+. You may be able to salvage it by using a solid state bridge rectifier on the working half. \$\endgroup\$
    – Mark Leavitt
    Jun 20, 2022 at 13:33
  • \$\begingroup\$ That was the first idea I had! Then I thought that the average current on that half might be asking too much. I've already built a separate 6.3V DC supply for the heaters, and I have a 240-240V 240VA shielded transformer salvaged from medical equipment so my plan is to build an external B+ supply. I can provide about 280VDC with less ripple than the original power supply so hopefully there won't be any noise going into the instrument. From my assessment, 280V is about what the original supply was giving / able to give. I will post new schematic too. \$\endgroup\$
    – Simon Owen
    Jun 24, 2022 at 1:35

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