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Triumph 830 Wobbulator / Oscillograph

September 8, 2011

Here is a fascinating relic from the past, the manual was copyrighted in 1943. It is a 884 thyratron based repetitive sweep wobbulator / oscilloscope (or as termed then, oscillograph) having identical single-ended 6SJ7 pentode X and Y amplifiers. The sweep generator was horribly non-linear as received. I surmised that leaky timing caps would be the cause of that so I rebuilt the timing switch assembly and was rewarded with a respectably (for the period)  linear sweep. Most of the capacitors required replacement and I padded critical carbon resistors back into tolerance discreetly using parallel resistors. I was not able to remove the shells of the paper-in-wax condensers (period terminology) and so I simply replaced them with Russian paper-in-oil capacitors. I was able to gut and re-stuff the main power supply electrolytic. It now works as well as it can, as an oscilloscope:

The term wobbulator refers to a signal generator that is frequency modulated or wobbled over a limited range either side of the set frequency. It is used to align tuned circuits such as superhet radio intermediate frequency stages. Some wobbulators used motor driven rotary capacitors and others, a voice coil / magnet linear pump arrangement to obtain the wobble. This one has neither, the wobble being generated from the 60Hz heater supply which is passed via a 6J5 control tube to modulate the 6K8 1MHz oscillator resulting in a frequency wobble around the 1MHz centre frequency, the wobble can be varied from zero to +/- 15kHz. The way the control tube works is that the 60Hz signal is arranged to modulate the Miller capacitance of the 6J5, the grid of the which is connected to the oscillator tuned circuit such that modulation of the Miller capacitance causes the oscillator frequency to wobble. (I would like to be able to claim that I figured this out for myself however, it was actually my friend Kenneth Kuhn who pointed out that the 60Hz is modulating the Miller capacitance for the 6J5. It was perplexing to see the take off to the RF oscillator from the grid of the 6J5.) Having understood this, it was now obvious to me that the reason it was not wobbulating was due to the cathode bypass capacitor on the 6J5 having developed high ESR and so the Miller capacitance variation would be minimal. Replacing it restored normal wobbulation service! In wobbulator mode, the oscilloscope part of this beast is used to display the wobbling frequency response against a 120Hz baseline that is synchronised to the 60Hz wobble resulting in a double peak display. If the centre frequency of the wobble is set at the centre frequency of the circuit undergoing adjustment, a single peak would be displayed:

If the setting of the tuned circuit is slightly off centre, a double peak would be displayed:

This instrument then, permits the technician to see the results of his adjustments (or fiddling if like me, you don’t know what you are doing) in real time. The 6K8 RF oscillator tube is a frequency changer or mixer type (as used in superhet radios to convert the RF signal to a fixed intermediate frequency); an input on the front allows a RF generator to be connected that can be set to mix with the wobbling 1MHz signal internally generated to produce a difference frequency at the centre frequency required, often around 465kHz for intermediate frequency stages of AM radios. This avoided the complexity and expense of having an accurate on-board variable RF generator. This is quite a cunning and well thought out piece of equipment.

Here is the schematic: (click on it for full size.)

Here is the left side showing left to right, 6X5 rectifier, 6J5 wobbulation tube and 6SJ7 X and Y amps:

Here is the right side showing left to right, timing switch (in can), RF oscillator coil, 6K8 RF oscillator / frequency changer and 884 sweep thyratron:

Top view dominated by 3AP1 CRT, you can clearly see the Russian replacement caps:

View showing shields re-installed, ugly!

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11 Comments
  1. John permalink

    Great post, thanks so much! Makes me want one of these.

  2. Jeff permalink

    I enjoyed this post. I have one of these units that needs to have an upgrade. It was my dad’s. He acquired it surplus after the war, and it was my introduction to oscilloscopes.

    Thanks for posting it. There doesn’t seem to be much information out there about these ‘scopes.

  3. Gerry permalink

    I have one of these also. Mine didn’t work when I got it and had service tags. Turned out one wire didn’t get connected at manufacture and now it works perfectly. Recently made an octopus tester for it

  4. This was my first oscilloscope. My father gave it to me when I was in high school in the mid-1960s. I think I completely gutted it and rewired it with more modern capacitors and it worked pretty well, but a few years later he gave me an HP-130A lab scope that had been underwater in a flood, but survived after I cleaned it out and dried it thoroughly. I still have the HP but I gave away the Triumph.

  5. David M. permalink

    I’d like to reproduce one or more of these images for a book I’m coediting. Could you please let me know how to be in touch with a more formal request? Thanks in advance. By the way, I enjoy learning about your research/passion.

  6. Tim permalink

    Any chance you have a list of the component values? It would greatly help in the restoration of my hacked-up Triumph 830! Thanks.

    • Hi Tim.
      I’m sorry to say that I cannot help. I have marked the value of some of the components. I would have though it is possible to read the values of the other components off the components themselves? It must be trashed if not:( For the X and Y amplifiers, perusal of a tube manual should give you workable values. Say 100k for the plate resistors then use a pot in the cathodes to get decent linearity. Do you have another scope you can use to see what the waveforms look like?

  7. Hello i have just purchased mine from a guy on offer-up for $5.00. i also found a new 3AP1 tube on EBAY as the tube arrived i was lucky enough to find the original owners manual. now having a more thorough exam of the contents, I regret to say your is a Cadillac to my Fiat. also did you fabricate the 3AP1 cover/shields or was your’s like that from the start? now I have the opportunity to purchase another but this one will cost close to $300. but they say it almost all works everything but the 3AP1 all other tubes are getting power and illuminated. in your opinion is this a sound investment? for resale purposes? because right now i am out $65. total but have one hell of a paper weight.

    • The original design includes the shield. $300 is an attempted rip off.
      Richard

      • Roger Brumley permalink

        hello again Richard i was wondering if you would be so kind to tell me the farads and volts of the metal capacitor located just below of the 3PA1 near the rear of the unit. its visible in one of your photos?

      • That looks to me like one of the -HV caps. 0.1u at 1000V or more should do it. It may be two caps in one can or there may be a second one hidden under the tube base platform (I can’t remember). However there are two caps, one at the anode of the 1V rectifier and one after a smoothing resistor. Ceramic replacements are fine and cheap. I would leave the metal caps physically in place and tuck the ceramics in discreetly.

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