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Title: Replicate the slow decay effect of the FT101 S/Db. meter ? Post by: Superhet66 on April 30, 2010, 09:24:51 AM Has any one reproduced the FT101's "buffered" S/Db needle return effect in a different meter circuit?
I would like to try and replicate it in some other rcvrs here. Maybe with a small pot. for more control over the effect. Is it a diode/cap arrangement? I'm digging thru my paper piles for the Yaesu 101 road map right now. Definitely not a high priority but I'd like to take a stab at it. Tanks ;) D. Title: Re: Replicate the slow decay effect of the FT101 S/Db. meter ? Post by: w3jn on April 30, 2010, 11:04:02 AM If you are just concerned about the S-meter's operation, that's very easy. Just add a 1 -10 uF cap in parallel with the terminals. Play around with the actual value to get the effect you want.
In the case of most radios, however, the meter tracks the AGC time constant. SSB xcvrs usually have a fast attack/slow decay, which is why you see the slow decay on the S-meter. Messing with the AGC time constant can be tricky... doing so depends a lot on the radio and its AGC circuit design. Title: Re: Replicate the slow decay effect of the FT101 S/Db. meter ? Post by: Superhet66 on May 01, 2010, 01:39:35 AM John
I'm just looking to tame the meter movement (swang ;) ) a bit. The decay effect is pronounced in the 101's so I used it as an example. Thanks for the info. mucho danka D. Title: Re: Replicate the slow decay effect of the FT101 S/Db. meter ? Post by: KA2QFX on May 06, 2010, 12:54:30 PM Here’s some info for ya on the FT-101 S metering.
Thar’s a lot going on in this AGC circuit… But basically - the primary RF is filtered with a 100K feeding a .47uF and bled off through 100K to a darlington pair which drives the meter in emitter follower style. The meter load and is 147 ohms in parallel with a 500 ohm pot to ground. The time constant appears to be about 1 second, taking about 5 seconds to go from +60dB to S-0. The AGC gain tracks metering exactly since the collector of the Darlington pair drops voltage across a 2.7K which is also filtered with a 1uF to the AGC gate (G2) of the front end amp. The 2.7K and 1uF are quite a bit faster than the RF filter so I do not think they play any role in AGC speed. Given the slow AGC decay I figure the gain of the Darlington pair to be quite high in this case. Yaesu usually uses pretty “gainy” devices. Assuming you have the schematic you can see for yourself there’s some other components of lesser consequence in their but that’s basically what I found. I just happened to have a well working 101 on the bench at the moment. Hope that’s of some help to you. Title: Re: Replicate the slow decay effect of the FT101 S/Db. meter ? Post by: Superhet66 on May 10, 2010, 06:51:13 PM That is of help, Thank you. AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
My Yaesu has followed me around the country and lay idle for 20 years. I'm waking it up slowly and getting a bit nostalgic with it. Thanks again, Dylan |