Cobra 29 AMC & Hifi

Caps, cap, capacitor, direct inject, bass, lows, hi fi, hifi, hi-fi, high fidelity, mod, mods, modification, upgrade, widen, wide audio, Goldfinger, Cobra, Galaxy, Ranger
479tx
WWPDX MEMBER
Posts: 93
Joined: Thu Jan 10, 2013 2:42 pm

Cobra 29 AMC & Hifi

Postby 479tx » Thu Mar 01, 2018 1:02 pm

Goldfinger,

I know I've read that you prefer keeping the AMC in tact and setting it so it's still limiting just a little (~105% modulation) rather than completely disabling with the hifi mod. Reason being that the mod transformer has bad response below 100hz and severely clipping/overmodulating with the added low frequencies isn't going to sound good at all. Additionally you've pointed out that you can speed up the action of the AMC by changing the value of c71, helping the AMC keep the modulation right at the "sweet spot" instead of slow/sloppy limiting having it be all over the place.

My question is can you do that with the JP36 swing mod? I can't seem to get any forward average power out of the last few 29s I've played with unless I put the cap/resistor mod in place. Most of the time I get a slight backwards twitch on an average reading watt meter unless I install the JP36 mod. I'm looking for clean, but still 100+% modulation and the added low frequencies of the hifi mod.

If your answer is yes, you can still sound clean if you do the JP36 mod, hifi cap mods, and leave the AMC in tact - then would the value of the cap in the JP36 mod affect the tone of the audio?

Thanks for any insight.
User avatar
Goldfinger
Site Maintenance
Posts: 3016
Joined: Sat Jun 30, 2007 12:27 am
Location: Beaver State

Re: Cobra 29 AMC & Hifi

Postby Goldfinger » Thu Mar 01, 2018 4:56 pm

The cap value of the JP36 mod does affect the tone if it's too small, mostly because the radio is responding much lower from the hifi mod. Making it too large makes the RMS swing kinda "whippy" though, so I usually use a 470uF to allow the lows to peak as high as the mids while not being so large it's whippy.
Yes the AMC timing can still be massaged with the hifi mod.
On some radios the AMC control VR4 doesn't have enough range once the dead key is lowered below 2 watts. I change it to a 1K or 2K, whatever I have laying around. Another thing you can try is to change C73 to a 2.2uF so the AMC responds to low frequencies better. (I never tried it yet, only thought about it.)
wwpdx
wwpdx.com
479tx
WWPDX MEMBER
Posts: 93
Joined: Thu Jan 10, 2013 2:42 pm

Re: Cobra 29 AMC & Hifi

Postby 479tx » Thu Mar 01, 2018 6:26 pm

Thanks, Goldfinger!

Do you run the JP36 mod on your wide 29?

I will try the C73 experiment out and let you know what I find.
User avatar
Goldfinger
Site Maintenance
Posts: 3016
Joined: Sat Jun 30, 2007 12:27 am
Location: Beaver State

Re: Cobra 29 AMC & Hifi

Postby Goldfinger » Thu Mar 01, 2018 8:19 pm

I am. I like to tune L14 for maximum peak power on 29/25's always. As such, this radio was doing a little over 5 dead key. My amp required 3 watts to dead key where I wanted it so I dropped it a little.

On the C73 thing, feel free to try higher values if changing to 2.2uF doesn't do much. After all, it's not called experimenting for nothing.. :mrgreen:
wwpdx
wwpdx.com
User avatar
31
WWPDX MEMBER
Posts: 198
Joined: Fri Apr 18, 2008 10:09 pm

Re: Cobra 29 AMC & Hifi

Postby 31 » Wed Mar 07, 2018 12:07 pm

I run the JP36 swing mod on all transformer modulated radios, to control the deadkey a little better. Even on Direct injected stuff. You can do JP36 mod on the new Surface mount Unidens also. I did an 880 that way with a W2IHY.

Return to “rx and tx mods”

Who is online

Users browsing this forum: No registered users and 0 guests