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Radio Shack IRF-510 in a Cobra 29 Impressions

DTB Radio said:
I recommend putting the jumper back in place. If the AMC is intact and working properly, AND adjusted properly, you can crank the mic gain with absolutely zero distortion, yielding better average modulation. You might see a slight dip in the PEP on your meter, but the actual PEP will not have dropped. Change C71 to a 33uf cap, and set the AMC control for about half-way, maybe a bit counterclockwise from halfway, and I think you'll be even happier with the sound.
Click to expand...

DTB,you mention changing C71 to a 33uf cap.Does it matter which way the negative side of the cap is turned?I know on some mods you have to have the stripe facing to the rear of the radio.Thanks
 
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Where is the thread on how to make the "discrete component"? I have a 29 WX ST here I want to put a IRF510 final in, need info. Thanks!
 
443 said:
Where is the thread on how to make the "discrete component"? I have a 29 WX ST here I want to put a IRF510 final in, need info. Thanks!
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If you use the search engine here on the forum and plug in 'irf-520' or 'erf-2030' (same thing) - I'm quite sure you will find more details and info than you might want.
;)
 
DTB Radio said:
I recommend putting the jumper back in place. If the AMC is intact and working properly, AND adjusted properly, you can crank the mic gain with absolutely zero distortion, yielding better average modulation. You might see a slight dip in the PEP on your meter, but the actual PEP will not have dropped. Change C71 to a 33uf cap, and set the AMC control for about half-way, maybe a bit counterclockwise from halfway, and I think you'll be even happier with the sound.
Click to expand...

Thanks DTB, you are the man! Thanks to you and Moleculo, I was able to do some audio mods and then wound up replacing the resistor/cap 'swing mod' at JP36 with the jumper back in place.

I wound up doing the following audio and AMC mods:

- Changed C71 (20 uF 10VDC) electrolytic with a 33 uF 35 VDC
- Installed a 1K resistor in series with D11
- Removed VR4 (a 680k? pot) and replaced with a 1K ohm fixed carbon resistor
- Removed C42 (1000 uF) and replaced with a 2200 uF 16VDC cap. (Moleculo calls for a 3300 uF cap, but my voice is quite bassy and the 2200 worked best for me)... I'll tell ya, this and the next monolythics at C37 and C68 really gave the best audio frequency response for my voice. Talk about 'broadcast quality'..... HOLY COW!
- Changed C37 (0.33 uF monolithic) to a 2.2 uF mono
- Changed C68 (.1 uF mono) to a 2.2 uF. (Note, in another Cobra 29 LTD I merely moved C68 to C37, and added a 2.2 to the C68 spot. Seemed to work equally well)
- Adjusted L17, 23 & 24 for tonal quality under audio and an RF Freq counter.
- After all, I DID indeed wind up adding 68 pF of total capacitance to the rear of C59 (270 pF), for a total capacitance of 338 pF
- Finally, I added a ground jumper from R142 to Audio Transformer to minimize any "squeal" when using a power mike with the dynamike all the way up. (This was essential due the the mods at VR4).

I am now quite pleased with the results. Radio dead keys 3 watts and swings 12. I might try replacing the driver with a 2SC1957 or a 2SC2314 depending on what I have in stock. Peak watts under modulation are currently well over 35, darn near 40. I'm pretty sure that with a lil more 'umpf' from the new driver, I'll probably see both a higher dead key and a maybe the full 40 peak watts on transmit.

I think from now on I might just Mosfet ALL Cobra 29 ST's, NW's etc., all the newer ones (2002 & later) and any made in China, but will stick with the 1969, 2312 or 1307 NPN transistor mods on all OLDER 29 LTD's, GTL's, XLR's, and any made in the Philippines, as they tend to respond real well to NPN bipolars.

Over the next couple of months I'll be ordering a shipment of Eleflow NPN transistors, as I've recently read a Cobra Tech Bulletin which subs them for the 2SC1969's in older 148GTL's. I think the 1969's were rated at 25W dissipation and the Eleflows are good up to 20W, so they should work fine in 148's. But..... I'd like to really try to PUSH them HARD in some older chassis 29's on AM by volting them and seeing what they are capable of, and how long they'll last.

Thanks again Gents... will keep u posted (y)
 

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