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MPX-200 adapter significantly rolled off as opposed to my FM-200B

CJVx

CJVx

AK Subscriber
Subscriber
This MPX-200 adapter has PC packs for the de-emphasis circuit, and the FM-200B has the WX decoder with discrete components. Both are unmodified, but the MPX-200 is severely attenuated on the high end (not measured, just by listening) as opposed to the FM-200B, or even my other nicer tuners like Scott 310e and 433.

is there a known issue with this PC pack de-emphasis network, and/or a correction to bring it up to sounding like the aforementioned tuners?

thanks
 
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I've only had one MPX-100 come across my desk, and while I didn't measure it's response characteristics, neither was I drawn to any obvious difference in audible performance versus that of the countless sub-chassis adapters I've seen. Both versions of the MPX-100 and the MPX-200 use identical second order de-emphasis filters that operate in a dedicated, active stage for de-emphasis tailoring. Early MPX-100s use discrete components, while later MPX-100s and the MPX-200 use the same PEC component for the de-emphasis filter.

On the other hand, the sub-chassis de-emphasis networks are entirely passive, so to make up for the loss of the active element that the stand alone units employ, the sub-chassis units employ a notch filter to effectively eliminate the 38 Khz sub-carrier from the output. The response of these units I have measured, and while not perfect, they are darn good at what they do even though they are a passive design, thanks in large part to the notch filter they employ.

So to answer your question, I don't know of any issue with the de-emphasis networks in your MPX-200, but it wouldn't hurt to check as many component values within it that you can, and maybe even build out the filter in one channel with discrete components, and see how that differs from the other channel still using its PEC network.

I hope this helps!

Dave
 
Dave, appreciate the advice, turned out to be an issue with the PEC’s after all. It’s still hard for me to believe that both of them had failed identically, but the proof is in the pudding. I fabricated a pair of these using SMT components…some nice NP0 0805 caps and 1206 resistors. This MPX-200, paired with the R100 tuner it was accompanied with sounds spot on in its own unique way. So far, as compared to the tuner built into my Scott 350, there is no longer any hint of HF roll-off.

One mystery to me still in this MPX-200 is why would the left channel C20 coupling cap be a .02uF ceramic cap and the right channel counterpart C21 is a .022iuF Mylar cap? I would think the audio signals should be symmetrical, but who knows
 
That would invariably have come down to an inventory issue to keep the production line moving, since there would be no design issue requiring dis-similar caps in each channel. As well, in that day, if substituting a Mylar for a ceramic cap (or visa-versa) in that type of application, nobody would have even wasted the time to raise an eyebrow.......

Glad you got it resolved!

Dave
 
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