Racal Dana 9008 Modulation Meter woes.

[kkyahoo]

Used 1.5k range and 3k filter, just assumed it was hf noise.

Kevin

[kkyahoo]

Looked at the AF output. When residual is reading high there is a noisy sine wave observed 100mV pk to pk and 34kHz with 145.1Megs at 100mV going to input. Sine wave disappears if the 3k filter switched on. Sine wave varies as input freq varies. Can clear it by either switching the mains on and off or removing the input then reconnecting it.

I am baffled!

Kevin

[G0HZU_JMR]

Do you have any kind of spectrum analyser that could look at the 430kHz IF output at the rear of the meter whilst all this is happening? If so, does the sideband noise appear on the 430kHz carrier at a 34kHz offset? Something like a cheapo RTL USB SDR if it can go this low or a RSP1(A).

Otherwise, you could use a scope FFT or mix it down to view it on a PC soundcard. But you want something fast with a fluid display.

Normally, once locked, the fast lock loop should be automatically switched out and the loop response (settling time) should then be very slow. This is obviously to prevent the meter fighting (removing!) any wanted low frequency FM modulation fed to the meter.

How does the 430kHz signal look when you do your on/off tricks? Is it stable and not twitching or wobbling? When the test sig gen is turned off and back on does the 430kHz IF output carrier show a fast lock to be roughly right and then slowly creep towards a nominal 430kHz as it should?

[kkyahoo]

IF output looks ok. Stable and approx the right frequency. No 34kHz visible when meter reading high on residual. AM and FM modes were both affected,

Kevin