Im looking for AM swing mod for a TRC-457.

[Dave239]

I have a TRC-457 dead keys 4 and swings to 8 on a peak meter, I need to set the dead key at 1 watt and let it swing to its full potential. can anybody give me a rough idea how I might go about this or a link to where I could get instructions on how to do it. Any parts required and so on. Thanks.

 

[Robb]

If the radio was peaked right, it should swing higher than 8w; more like 16w. There are a couple of guys on this forum that know the 457 real well. This radio has the 858 PLL chassis. It can have good AM audio if it is modded in the mic amp circuit - IIRC.

Perhaps forum member unit_399 will see this post and tell you how; he knows the TRC-457 like the back of his own hand.

 

[sonoma]

look at unit399's web site. may have the info you want in it.

http://unit399.wixsite.com/858ssb

 

[Robb]

From unit_399's page:

Locate D23 (next to VR7). In the Realistic sets (TRC449, 457, 458) a 220 ohm resistor is installed in the D23 location. Replace this resistor with a 1N914 diode (observe polarity). Replace C87 (2uf/10v tantalum) with a 10uf/16volt electrolytic. This will make the Realistic modulation limiter circuit the same as on all of the
other 858SSB sets. This simple change really opens up the audio on the Realistic 858s. Your regular contacts
(especially locals) will be amazed at the improvement in your audio. However, do not attempt to defeat the AM modulation limiter, as this will degrade performance. In this case, more is definitely not better.

Lotta great usable info on that page for the 858 chassis owner.

 

[Dave239]

Thanks guys... I've been to that page. That's actually how I found this forum in the first place. I did the mod you mentioned plus the double diode on the limiter (D20 I believe). I have the radio dead keying 4 watts from his suggestions but I need it to dead key from 1 watt up to 12 or 16 or what ever it can. the guy that wants to buy it has a low drive amp but the amp wants lots of swing. if I set the dead key down with the tuning coils, then it doesn't swing that high. Just wondering if there is some way to get the radio to do more swing to save the amp from cooking. Thanks

 

[Dave239]

From unit_399's page:

Locate D23 (next to VR7). In the Realistic sets (TRC449, 457, 458) a 220 ohm resistor is installed in the D23 location. Replace this resistor with a 1N914 diode (observe polarity). Replace C87 (2uf/10v tantalum) with a 10uf/16volt electrolytic. This will make the Realistic modulation limiter circuit the same as on all of the
other 858SSB sets. This simple change really opens up the audio on the Realistic 858s. Your regular contacts
(especially locals) will be amazed at the improvement in your audio. However, do not attempt to defeat the AM modulation limiter, as this will degrade performance. In this case, more is definitely not better.

Lotta great usable info on that page for the 858 chassis owner.

 

[Dave239]

I found this swing mod for another make of radio that puts a resistor and cap in the power supply line to the driver, I'm thinking I might give it a shot. if its the line from the AM AMC. Any thoughts on this mod if it might work for my issue? thanks again.

 

[Robb]

I found this swing mod for another make of radio that puts a resistor and cap in the line to the driver, I'm thinking I might give it a shot. if its the line from the AM AMC. Any thoughts on this mod if it might work for my issue? thanks again.

I could be wrong, but the pic you showed is for an AM only radio that is C class amplifier stage biasing. An SSB driver and final are AB biased, so a swing mod for your 457 will be likely be different.

But even before you go for a swing mod and lowering the dead key; shouldn't you get the modulation in this 457 up to snuff -first? That is what unit_399's mod does.

 

[ExitThirteen]

Honestly, the D858 AM/SSB chassis is not a good board to mod for a "swing" mod, because it uses a form of speech processing. Your best bet is to use an MB8719 AM/SSB chassis for swing mods. Modding for a variable power or a "swing mod" by changing the voltage at the collector of the final will degrade SSB performance, if you use you SSB at all. The audio on the D858 boards are plenty stout, even it tho may not "show" it on the meter. Just lower your deadkey to about 2W at VR8 and let it swing from there.

~Cheers~

 

[Dave239]

Honestly, the D858 AM/SSB chassis is not a good board to mod for a "swing" mod, because it uses a form of speech processing. Your best bet is to use an MB8719 AM/SSB chassis for swing mods. Modding for a variable power or a "swing mod" by changing the voltage at the collector of the final will degrade SSB performance, if you use you SSB at all. The audio on the D858 boards are plenty stout, even it tho may not "show" it on the meter. Just lower your deadkey to about 2W at VR8 and let it swing from there.

~Cheers~

thanks Exit Thirteen I will take your advice. Glad you responded. Take care .

 

[ExitThirteen]

I have tried a few things on the D858 board as far as swing mods go, but none of them seem to provide good results, because of the speech processor circuit. You can disable the speech processor, but then that will also degrade SSB performance. That's why the board is tough to modify for any kind of swing mod.


~Cheers~

 

[Dave239]

ok thanks.. I have a few of these radios so I'm going to play around with your mosfet final mod and the original high power components to see what I like best.

 

[Robb]

Did you peak the transmit coils yet?

 

[Dave239]

Did you peak the transmit coils yet?

yes I did. it does 16 W on sideband.

 

[ExitThirteen]

If you plan on doing my Mosfet mod, you can actually change the 56k resistor to a 100k adjustable pot and set the gate voltage to about 3.9-4V, provided you have a way to transfer heat off the Mosfet properly. Last radio I modded I used a different type of insulator between the Mosfet and the chassis it mounts to, so it thermally transferred heat a little better, so I was able to bump up the bias voltage at the gate up to about 4V. Sounds REAL nice on the air. Good luck.


~Cheers~