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Post by mark4 on Feb 24, 2017 21:40:28 GMT -5
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Post by mark4 on Feb 24, 2017 21:42:58 GMT -5
I just picked up the NOS TRC-458 yesterday .
Also have a NOS President Grant I picked at a very good price. Couple months ago.
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Sandbagger
Administrator/The Boss
Posts: 6,245
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Post by Sandbagger on Feb 25, 2017 9:12:19 GMT -5
I just picked up the NOS TRC-458 yesterday I've always liked those radios. Lots of fun experimenting back in the day. I could stretch the most extra channels out of that chassis than any other.....
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Post by 2600 on Feb 27, 2017 0:58:11 GMT -5
With all the added channels you could get from it, might be the second-most modified model of all time after the D201s.
73
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Post by radioreddz on Mar 1, 2017 17:43:29 GMT -5
I have 3 of the 457's and 1 458 I will never part with them. I know the best radio I have ever heard CB or HF rig was a guy running a 458 one day on USB on the 27555 I don't know what he did but it had absolute studio quality audio. great radios you picked up there.
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Post by mark4 on Mar 2, 2017 22:24:01 GMT -5
There is a mod you can do to give the radio stellar tranmit audio. I use a Silver Eagle on mine. With a 20db pad. This radio doesn't need a power mic with the speech processor.
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Post by 2600 on Mar 2, 2017 23:43:46 GMT -5
Oddly enough, the mike audio input impedance on this radio is high enough to use a straight non-amplified D104. The mike audio feeds into a compressor chip that has a surprisingly high input impedance.
One of the very, very few solid-state radios that sound good with a straight D104.
73
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Sandbagger
Administrator/The Boss
Posts: 6,245
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Post by Sandbagger on Mar 3, 2017 7:47:13 GMT -5
There is a mod you can do to give the radio stellar tranmit audio. I use a Silver Eagle on mine. With a 20db pad. This radio doesn't need a power mic with the speech processor. This is true. The upC592H2 preamp was originally designed to be used in tape decks, and has a high impedance. But there are a few versions of this chassis, with minor differences, some of which have a 560 Ohm resistor across the mic input leads, which drastically lowers the input impedance. Removing that will allow even un-amplified D104's to sound good with that chassis. There is a lot of potential in the audio circuits. The only bad spot (for me anyway) is the diode clipper circuit, which follows the mic amp/compressor circuit. Due to the characteristics of clipping, it requires a low-pass filter downstream of the diodes to remove audio harmonics created by the clipper. This also has the effect of slightly distorting the audio. You can't really hear it, but it shows up on the scope. I usually bypass the entire clipper/LP filter circuit and adjust the internal mic gain so that the preamp's audio AGC holds the modulation near 100%. Far less distortion shows up on the scope, and allows for expansion of the fidelity of the modulation chain. IMHO, the radio is a little too "bright" and sounds better with an expanded low end response. But that brightness, is also what busts through the skip pileups and many people prefer that sound. Bottom line, there is something in that radio for everyone, no matter which way you want to go.
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Post by 2600 on Mar 4, 2017 1:03:53 GMT -5
The two diodes you mention are not meant to be clippers. They are configured as a series-resistance attenuator. The current-source resistor on some radios does not push enough current through the diodes to allow full audio. The AMC and ALC feed into it. Reducing the forward current through the diodes raises their resistance in series with the audio-signal path. Raising the forward-bias diode current reduces the attenuation. We got in the habit of changing the resistor that set the max diode current on some of these radios. Couldn't get full mike audio level without it.
Some early electronic-music synthesizers also used this circuit as a voltage-controlled attenuator. Wasn't a stellar performer in that application, either.
73
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Sandbagger
Administrator/The Boss
Posts: 6,245
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Post by Sandbagger on Mar 4, 2017 10:41:32 GMT -5
The two diodes you mention are not meant to be clippers. They are configured as a series-resistance attenuator. The current-source resistor on some radios does not push enough current through the diodes to allow full audio. The AMC and ALC feed into it. Reducing the forward current through the diodes raises their resistance in series with the audio-signal path. Raising the forward-bias diode current reduces the attenuation. We got in the habit of changing the resistor that set the max diode current on some of these radios. Couldn't get full mike audio level without it. Some early electronic-music synthesizers also used this circuit as a voltage-controlled attenuator. Wasn't a stellar performer in that application, either. 73 Actually we're both right. Yes, it is a variable series attenuator, and not a classic shunt clipper. But a diode is not a linear attenuator. If the negative modulation waveform voltage exceeds the bias level setting, and drives it down below the "knee", it will sharply cutoff, or clip. And since clipping results in higher order audio harmonics, that's why they followed it with the low pass filter. Honestly, I don't know why they didn't just employ a typical variable "T"-pad shunt limiter like most other radios do. It would have been far simpler, and probably would have worked better. As it is, by just allowing the AGC circuit around the preamp to limit the modulation level, and bypassing the diode limiter, it can effectively prevent excess of 100%, without clipping distortion. That's the way several older Cybernet SSB chassis took care of modulation control, and it was effective.
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Post by Stranger on Apr 17, 2017 8:56:43 GMT -5
I just picked up the NOS TRC-458 yesterday . Also have a NOS President Grant I picked at a very good price. Couple months ago. Wow, this is a nice looking radio!
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Post by mark4 on May 2, 2017 18:11:31 GMT -5
I too bypass these 2 diodes and install a resistor. I also bypass the LP filters. I have also experimented with changing out the audio amp with a TA7222AP. Mine has stellar audio on AM & SSB. Not edgy sounding.
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Post by astrogoth99 on Nov 25, 2020 13:00:49 GMT -5
That diode mod sounds great. Wish you guys had posted the values and locations for it here.
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