Signal generator trouble - Printable Version

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Signal generator trouble - EdHolland - 09-04-2018

I managed to damage my "KKMoon" 25 MHz DDS signal generator while testing a chassis this weekend. Even through I was being careful, and using a decoupling cap, contact with the IF stage took out one channel's output. I'm annoyed with myself for this, but realistic about the low cost and subsequent fragility of the unit.

Anyway, should anyone else be in possession of one of these units, be warned that extra care is needed to prevent damage. I am actually thinking of making an R.F. isolating transformer for the output.

The unit is useful (and precise) enough, that I ordered another. I also scoured eBay, and picked out an old Heathkit signal generator. The Heathkit will probably want a bit of restoration, but I chose it because it allows for modulation (good for RF stage alignment) and is "Tough as old Boots" when dealing with vintage gear. And I like them

Cheers

Ed

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RE: Signal generator trouble - Eliot Ness - 09-05-2018

Good luck with the Heathkit Ed, it should work just fine with a few component replacements.

For anyone else who is wondering what the heck is a KKMoon 25 MHz DDS signal generator, below is a link:

https://www.amazon.com/KKmoon-Precision-Dual-channel-Generator-Arbitrary/dp/B071HJ31WN

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RE: Signal generator trouble - morzh - 09-05-2018

We discussed this here before, some years ago. SS generators used with tube equipment should be protected either with a zener or a double diode to limit the voltage it can see. BTW the transformer might not help: it will reflect the voltage spike if it is 1:1.

Depending on the amplitude of the signal I would either use two diodes (in reverse-parallel) in parallel to the output after the capacitor, or a symmetrical Zener, also in parallel.

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RE: Signal generator trouble - EdHolland - 09-05-2018

..Wish I'd been here years ago...

Thanks for the tips on protection. With two instruments, I should be in decent shape now, as the Heathkit can take on any direct connection duties. The local surplus place can furnish all the necessary parts. I reckon 5 V Zeners and a cap would do the trick.

It occurs to me now that even the transformer idea needs to be done with care, as a DC path to ground should still be avoided on the device test side, otherwise all sorts of interesting fireworks and blue smoke releases may occur. Plus Morz's point about it passing deadly spikes...

I do have one of those frequency counter displays that could be used in combination with the Heathkit, so the test setup should be robust and accurate.

The old SS generator still has one good O/P, which I might try as the "Exciter" for my low power shortwave beacon. It would be nice to try some different frequencies in the 22 m (ISM) band in the hope of a few more reports. It has limited tuning by pulling the crystal, but that doesn't get me far from the center band noise that predominates on 13,560 kHz.

Cheers and thanks again everyone,

Ed

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RE: Signal generator trouble - EdHolland - 09-07-2018

The Heathkit came in today. It looks to be in completely original condition, including the "Mighty Mite" 2 section electrolytic, and other reactive suspects, and what is probably a selenium rectifier.

Try it with a dim bulb, or have I another project before me? (not that a project is bad )

Cheers

Ed

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RE: Signal generator trouble - EdHolland - 09-16-2018

Today I dug into the Heathkit. I replaced the 20 + 20 uf electrolytic with two new caps. I might restuff the old one so kept it. A quick check through other parts suggested nothing in immediate need of attention, so power was applied.

Odd sizzling sounds! Power smartly removed!

Another quick chancer - I was being treated to a superb light show from inside the variable capacitor.

I had noticed a bit of dust inside the cabinet, and sure enough, there was all sorts of grime between the plates visible on closer inspection. I set my sights on a good clean. To get this thing out was a pain - had to loosen off all the front controls to get enough movement of the chassis from the front panel. Anyway, out it came for a bath in dish detergent and gentle scrubbing with an old toothbrush. After that, a thorough drying with the hot air gun, followed by a clean and regrease of the bearings. More fettling to get it back into the chassis.

Fortunately, this did the trick, and the instrument is working nicely again, with no more fireworks. Should be a handy bit of kit.

Cheers

Ed

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