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Anyone recognize this Cobra 2000 GTL modification?

A

Alan Anderson

Member
Feb 14, 2020
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I purchased a Cobra 2000 GTL and am trying to remove the mods from the radio - I removed the rotary switch style PLL mod, channels back to normal. Does anyone know what modification this is? Also an idea of what was changed to install this? If I remove it, appears to have no DC to the finals. Any help is appreciated, I know someone has seen this before in either a 148 or a 2000.

Thanks
AlanIMG_1580.jpg
 

Looks for all the world as if it's a booster stage, between L45 and TR39.

I'd guess this was meant to broadband the transmitter, so the power would not drop off so much way above channel 40 or way below channel 1.

Don't remember seeing this trick before.

73
 
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It seems to be in the transmit signal path at C163, it is a 2SC710 running from the 8V transmit rail voltage. I measured the output at the base of TR39 and it was about the normal 86mv. Maybe they were trying to increase the bandwidth. I think I'll just put a 2pf cap back in. I don't want crazy frequency coverage anyway.
 
I've seen this mod, and for you to remove it would be doing yourself and your fellow operators a big favor.

It simply adds an extra stage of amplification to the TX strip.

Instead of 3 stage amp, you inject an extra 4th stage to increase (not necessarily improve) power.

The mod works fine, if you have the proper Bandpass filtering to narrow down your channels versus out of band spurries stuff.

Does not look like that put much in there to "clean it up" - just push more power for those channels.

Because of the mod itself - it tends to "mush up" the AM mode in the peak region (strongest region of channels and frequencies that amp gets tuned for) and makes the operator sound "FM-ish" or pinched up.

I wouldn't be a bit surprised if they put in some type of variable power mod to try and offset that effect. They'd be trying to get their swing back.

Splatters like a banshee when they peak for the funnies - to get away from the typical AM clown crowd. You can hear everything they say because of the images they generate they can't bury it in filtering because it has none right there - it's past the coils that can trim that off. SIGH...at least you knew they were on.

Look up HR2510 Rogerbird site and look for the "Swing mod" it's similar in concept - more current to improve the bandwidth, at a cost of spectral purity in this radio chassis though...

upload_2020-7-17_8-8-9.png

They were just a bloody mess when they drove amps behind it - you didn't have a clean or clear channel for miles around when they operated like that.
 
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They replaced C163 2pf with the mod. I removed it, had to re-tune, but it seems to work fine now. I found last night late where they changed a cap further on, going to look at that tonight. The 2000 was/is a great radio, don't know why people hack on it so much for basically insignificant differences. If you want more power, get it. You aren't going to make a huge difference in a radio with a single final. Get an amp :)

Thanks, I knew someone would have seen this in the past.

Alan
 
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Handy Andy said:
I've seen this mod, and for you to remove it would be doing yourself and your fellow operators a big favor.


Look up HR2510 Rogerbird site and look for the "Swing mod" it's similar in concept - more current to improve the bandwidth, at a cost of spectral purity in this radio chassis though...

View attachment 38867

They were just a bloody mess when they drove amps behind it - you didn't have a clean or clear channel for miles around when they operated like that.
Click to expand...
..
I have a "Swing Mod" on my site???
..
Rogerbird
 
I see you've been busy...thanks for upgrading the site.

The mod refers to C135 R167 and required you to relocate the Tank Trap filter to the Collector leg of the pre-driver 2086. Made the amp stage more Common Emitter style like Uniden would use for their SSB radios.
  • The mod broadbanded the stage, but the Tank trap (Parallel filter) still applied a level of RF attenuation - so the next stage got clean power, but the "Resonance" this filter L132, C134 was not designed to offer the best transfer down to 24 MHz let alone 30MHz and above.
  • Yeah, there was swing with it, the R174 which they should have gone to, would have left the predriver pushing pretty hard into the next stage like it was designed for.
  • They left the R174 part in there and instead adjusted the Predrivers' Emitter leg
upload_2020-10-24_10-21-56.png

From CB Tricks...
hp2510_dia.jpg
The "swing" part was when they did up the variable with it adjusting the 100ohm resistor leg. Similar to CB Tricks offering of the Variable power mod using R174/SWR Cal - only thing they change was the Tap point, to R167 - along with a resistor in-line of 10~15 ohm - never seen it done like this before so we scrambled to find even how the mod ever got there.

So not blaming anyone or anything, just the reversal to Collector side and this variable to R167 changed the outlook on the Radio. So to know the Emitter to Collector swap you had to know the mod.
 
I thought that mod was from Amature Radio today, not Roger.
I remember it was used to increase the drive levels from the pre driver when switching the 477 to a 497?
At first they claimed a high power increase but later came back and refuted the numbers.
I remember doing this years ago on a old Lincoln and decided it was not worth it.

73
Jeff
 
Here is were it came from.



HR2600 Better Hi-Power
Modification
Increase the performance of this popular HF rig.
(Modification adapted from an article by Carl Merrill
73 Amateur Radio Today - April 1993, page 17)

73
Jeff
 
Seems to me it also involved changing Q124 from the 2SC1973 to a NTE number meant for use at 150 MHz. This meant more gain at the lower 27 or 28 MHz frequency. Couldn't see doing that. The 2510 is marginally stable to begin with, and swapping that transistor just seems to invite spurious frequencies.

RF Parts promoted this mod by reprinting it and making the parts available.

Until they were discontinued.

73
 
nomadradio said:
Seems to me it also involved changing Q124 from the 2SC1973 to a NTE number meant for use at 150 MHz. This meant more gain at the lower 27 or 28 MHz frequency. Couldn't see doing that. The 2510 is marginally stable to begin with, and swapping that transistor just seems to invite spurious frequencies.

RF Parts promoted this mod by reprinting it and making the parts available.

Until they were discontinued.

73
Click to expand...
No, that was a different mod,It was not part of the 'Better High Power Mod', by Carl Merrill.
The NTE swap ended up not being recommended because the NTE part was a VHF part and was too noisy in the circuit. Also Carl's mod stated: It works better, and saves the cost of adding an ECG-340...
 
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