I just couldn't resist....
After reading all the hoopla about MOSFETs, I just had to try one in my Cobra 29.
I used to build some serious Cobra 29's using 2SC1969's, 2SC1307's, 2SC2312's, etc.... but it was a LOT of work to properly volt the final in order, get the audio chain and everything up to snuff. It used to take anywhere from 12 to 15 components, and upwards of 2 hours to perform the mods. BUT...(knock on wood), I ALWAYS had satisfied customers (as long as they weren't trying to run the rig on a 3 amp pwr supply!)
Note: (NO... I never clipped D11.... before you even ask)
Anyway.... I just HAD to try the MOSFET mod on my FAVORITE Cobra 29: A Brand New, Still in the Box, 2003 Cobra 29-LTD Classic Harley Davidson Edition. This one was made in CHINA.
Not having an IRF-520 or ERF-2030 available locally, and not wanting to spend more on shipping for a single component than the value of (read: cheapskate)... I went to my local Radio Shack and found they carry the IRF-510 (15W version) for only $1.99. I figured, what-the-heck....
(Yes, I realize the IRF-520 or ERF-2030 are more robust and can dissipate more power, BUT this was purely an experiment.)
So, here is the procedure I used (some of it follows my old 2SC1969 days:
1. Took the radio out of the box, connected it to a 13.8V supply, Wawasse Catalyzer JBC-1000-SM & dummy load, and commenced taking initial readings. Then I hooked it up to a peak reading wattmeter & modulation meter and finished initial tests.
2. Removed radio covers and adjusted AMC for maximum modulation.
3. Removed TR14 (the stock 2078 final) and the following additional components: JP36, R55, R56, C61, & L14'S tuning slug.
4. Fabricated a 'discrete' EN-1230 component part using 1 X 1N4148/1N914 diode + 1 X 470 ohm 1/2 watt carbon resistor + 1 X 1/2 watt 3.3K ohm resistor. Placed same (with positive lead facing toward R123) in place of the removed R56.
5. Installed a 47 ohm 1 watt carbon resistor across a 220 uF 35VDC electrolytic capacitor, with negative facing final in place of the removed JP36.
6. Powered up the radio, turned the dynamike control to MINIMUM, and made note of the dead key wattage.
7. Depowered radio and removed the 33 pF factory cap installed across C62 & replaced it with a 100 pF cap (to see if it would raise my dead key... it DIDN'T....) so I reinstalled the 33 pF cap across C62
8. Experimented with different capacitor settings across C59 (mod calls for 68 pF and I didn't have one.) Settled for a 39 pF Mica which provides approx 309 pF of total capacitance... (higher values reduced my peaks and made my audio with the stock mike sound crappy.) But you can completely skip this addition on many radios, or merely add the called for 68 pF of additional capacitance.
9. Connected the radio (sans covers) back to the test equipment & dummy load and turned the Dynamike control back up to MAX.
10. While saying: 'HEEEEEEEEEEELLLLLLLLLLLLLLOOOOOOOOOOOO, & AUUUUUUUUUUDDDDDDDDDDDIIIIIIIIIOOOOOOOOOOOO, and/or whistling
into the mike, adjusted forward power for maximum via L12, 13, 17 & 20.
11. Checked for harmonics in 54 MHz band with a separate receiver... minimal.
Wanna know if it worked?????
Here's the BEFORE and AFTER RESULTS: (Before was a STOCK radio)
BEFORE: RF Output Deadkey Approx 3.2 watts AFTER: Approx 2.5 watts
BEFORE: RMS Swing Watts Approx 3.5 watts AFTER: Approx 12.5 watts
BEFORE: Percent AM Modulation 80% dynamike at max AFTER: 100+++ Percent... Pins meter on whistles
BEFORE: Peak (PEP) Swing Watts Approx 9.1 PEP AFTER: Almost 40 Watts!!!
I couldn't believe it.... how was a 15W Mosfet (I know, I know, it's cuz of the capacitor mod across JP36) putting out almost 40 Watts??? There must be something wrong with my PEP wattmeter, so I put it on the PEAK meter of my MFJ 962D (low setting) and guess what??? 45 PEAK WATTS on whistles (with the STOCK MIKE and D11 intact!!!)
Can't be..... Can't be.......... this is only a 15W device..... must be bad meters... let's check the 20W setting of the Radio Shack SWR/Power Meter...... PEGS IT. Okay, okay... let's switch to the 200W scale: dang... STILL close to 40 Watts.
Okay, so while I've got the radio open I replace D5, D6 & D24 with NTE583's, connect the antenna & external speaker, and adjust VR1 RX gain to near max... then back off some (to my liking).... and get a radio check.
Locals are ASTONISHED! I'm thinking: "right, right.... I need to hear this thing for myself".... so I bribe my 13 y.o. to get on the rig and I go out in the mobile a few blocks and call her.
All I can say is: "HOLY MOLY!!!" Even with just the factory stock mike it is BOOMING!
So, I do a bunch more tests and experiments (and I did many along the way) and all I can say is WOW!
I looked at the waveform on the Catalyzer and it isn't too bad UNTIL you whistle, or connect a powermike... then it just gets nasty. As long as you keep the modulation as close to 100% (slightly above actually due to the capacitor/resistor mod across JP36) it's not too bad.
So... I immediately ordered a dozen IRF520's to begin experimenting with other rigs. I never even checked the driver transistor to see if it's a 2028 (which I suspect), or a 1957/2314 equivalent. If it's the higher powered 1957 or 2314, it may be possible (if it hasn't been done already) to drive an IRF520 or an ERF-2030 into 50W territory! Most Exports can't even do THAT!
Am I a "believer" of MOSFET's in HF yet? I'd be a fool NOT to... most commercial and HF transceivers made nowadays have MOSFET finals.
Do I think the MOSFET mods are "better" in a Cobra 29 than the old "tried and true 2SC1969 (or other NPN) mods of "yesteryear"??? .... the jury is still out... I'll let you know after I experiment with the IRF520's, ERF-2030's, and a few RD16HHF1's for a while........
At present, I am still inclined to favor the NPN (2SC1969) mod better because you can volt the final for a higher dead key. I have built several 1969 29s' which will deadkey 8-10W and swing to over 20 (RMS) with peaks darn near 50 under full modulation (again, with D11 INTACT!, or else they will squeal like little piglets when the owner hooks up the powermike he just bought at some truck stop).
Perhaps once I get the deadkey up where I want it with the "proper" MOSFET, and not just the RadioShack IRF-510, I can continue to develop a mod for the Cobra 29 that I'd be willing to call a "Competition Ready" radio. Given the 15W power dissipation rating of the Radio Shack device (according to the packaging) it's only a matter of time before I fry this device and by then, the IRF520's will have arrived.
P.S.: My buddy just got a new Galaxy DX98VHP which is (I THINK) comprised of 11 IRF520's (3 in the main body, and 8 in the amplifier section).
Sooner or later, the supply of NPN RF Power Transistors suitable for HF transmitters in the 25-50 W category will dwindle (as in the case of MRF 477, 497, 2SC1969, NTE/ECG Equivalents, etc.....). There is an Asian (Hong Kong?) Company (name escapes me but it reminds me of the American Co. Elecraft)... which mfgs equivalent NPN Power Transistors. There's a guy on eBay in the U.K. who is their sole eBay distributor. Any of you ever try those components???
After reading all the hoopla about MOSFETs, I just had to try one in my Cobra 29.
I used to build some serious Cobra 29's using 2SC1969's, 2SC1307's, 2SC2312's, etc.... but it was a LOT of work to properly volt the final in order, get the audio chain and everything up to snuff. It used to take anywhere from 12 to 15 components, and upwards of 2 hours to perform the mods. BUT...(knock on wood), I ALWAYS had satisfied customers (as long as they weren't trying to run the rig on a 3 amp pwr supply!)
Note: (NO... I never clipped D11.... before you even ask)
Anyway.... I just HAD to try the MOSFET mod on my FAVORITE Cobra 29: A Brand New, Still in the Box, 2003 Cobra 29-LTD Classic Harley Davidson Edition. This one was made in CHINA.
Not having an IRF-520 or ERF-2030 available locally, and not wanting to spend more on shipping for a single component than the value of (read: cheapskate)... I went to my local Radio Shack and found they carry the IRF-510 (15W version) for only $1.99. I figured, what-the-heck....
(Yes, I realize the IRF-520 or ERF-2030 are more robust and can dissipate more power, BUT this was purely an experiment.)
So, here is the procedure I used (some of it follows my old 2SC1969 days:
1. Took the radio out of the box, connected it to a 13.8V supply, Wawasse Catalyzer JBC-1000-SM & dummy load, and commenced taking initial readings. Then I hooked it up to a peak reading wattmeter & modulation meter and finished initial tests.
2. Removed radio covers and adjusted AMC for maximum modulation.
3. Removed TR14 (the stock 2078 final) and the following additional components: JP36, R55, R56, C61, & L14'S tuning slug.
4. Fabricated a 'discrete' EN-1230 component part using 1 X 1N4148/1N914 diode + 1 X 470 ohm 1/2 watt carbon resistor + 1 X 1/2 watt 3.3K ohm resistor. Placed same (with positive lead facing toward R123) in place of the removed R56.
5. Installed a 47 ohm 1 watt carbon resistor across a 220 uF 35VDC electrolytic capacitor, with negative facing final in place of the removed JP36.
6. Powered up the radio, turned the dynamike control to MINIMUM, and made note of the dead key wattage.
7. Depowered radio and removed the 33 pF factory cap installed across C62 & replaced it with a 100 pF cap (to see if it would raise my dead key... it DIDN'T....) so I reinstalled the 33 pF cap across C62
8. Experimented with different capacitor settings across C59 (mod calls for 68 pF and I didn't have one.) Settled for a 39 pF Mica which provides approx 309 pF of total capacitance... (higher values reduced my peaks and made my audio with the stock mike sound crappy.) But you can completely skip this addition on many radios, or merely add the called for 68 pF of additional capacitance.
9. Connected the radio (sans covers) back to the test equipment & dummy load and turned the Dynamike control back up to MAX.
10. While saying: 'HEEEEEEEEEEELLLLLLLLLLLLLLOOOOOOOOOOOO, & AUUUUUUUUUUDDDDDDDDDDDIIIIIIIIIOOOOOOOOOOOO, and/or whistling
into the mike, adjusted forward power for maximum via L12, 13, 17 & 20.11. Checked for harmonics in 54 MHz band with a separate receiver... minimal.
Wanna know if it worked?????
Here's the BEFORE and AFTER RESULTS: (Before was a STOCK radio)
BEFORE: RF Output Deadkey Approx 3.2 watts AFTER: Approx 2.5 watts
BEFORE: RMS Swing Watts Approx 3.5 watts AFTER: Approx 12.5 watts
BEFORE: Percent AM Modulation 80% dynamike at max AFTER: 100+++ Percent... Pins meter on whistles
BEFORE: Peak (PEP) Swing Watts Approx 9.1 PEP AFTER: Almost 40 Watts!!!
I couldn't believe it.... how was a 15W Mosfet (I know, I know, it's cuz of the capacitor mod across JP36) putting out almost 40 Watts??? There must be something wrong with my PEP wattmeter, so I put it on the PEAK meter of my MFJ 962D (low setting) and guess what??? 45 PEAK WATTS on whistles (with the STOCK MIKE and D11 intact!!!)
Can't be..... Can't be.......... this is only a 15W device..... must be bad meters... let's check the 20W setting of the Radio Shack SWR/Power Meter...... PEGS IT. Okay, okay... let's switch to the 200W scale: dang... STILL close to 40 Watts.
Okay, so while I've got the radio open I replace D5, D6 & D24 with NTE583's, connect the antenna & external speaker, and adjust VR1 RX gain to near max... then back off some (to my liking).... and get a radio check.
Locals are ASTONISHED!
I'm thinking: "right, right.... I need to hear this thing for myself".... so I bribe my 13 y.o. to get on the rig and I go out in the mobile a few blocks and call her.All I can say is: "HOLY MOLY!!!" Even with just the factory stock mike it is BOOMING!
So, I do a bunch more tests and experiments (and I did many along the way) and all I can say is WOW!
I looked at the waveform on the Catalyzer and it isn't too bad UNTIL you whistle, or connect a powermike... then it just gets nasty. As long as you keep the modulation as close to 100% (slightly above actually due to the capacitor/resistor mod across JP36) it's not too bad.
So... I immediately ordered a dozen IRF520's to begin experimenting with other rigs. I never even checked the driver transistor to see if it's a 2028 (which I suspect), or a 1957/2314 equivalent. If it's the higher powered 1957 or 2314, it may be possible (if it hasn't been done already) to drive an IRF520 or an ERF-2030 into 50W territory! Most Exports can't even do THAT!
Am I a "believer" of MOSFET's in HF yet? I'd be a fool NOT to... most commercial and HF transceivers made nowadays have MOSFET finals.
Do I think the MOSFET mods are "better" in a Cobra 29 than the old "tried and true 2SC1969 (or other NPN) mods of "yesteryear"??? .... the jury is still out... I'll let you know after I experiment with the IRF520's, ERF-2030's, and a few RD16HHF1's for a while........
At present, I am still inclined to favor the NPN (2SC1969) mod better because you can volt the final for a higher dead key. I have built several 1969 29s' which will deadkey 8-10W and swing to over 20 (RMS) with peaks darn near 50 under full modulation (again, with D11 INTACT!, or else they will squeal like little piglets when the owner hooks up the powermike he just bought at some truck stop).
Perhaps once I get the deadkey up where I want it with the "proper" MOSFET, and not just the RadioShack IRF-510, I can continue to develop a mod for the Cobra 29 that I'd be willing to call a "Competition Ready" radio. Given the 15W power dissipation rating of the Radio Shack device (according to the packaging) it's only a matter of time before I fry this device and by then, the IRF520's will have arrived.
P.S.: My buddy just got a new Galaxy DX98VHP which is (I THINK) comprised of 11 IRF520's (3 in the main body, and 8 in the amplifier section).
Sooner or later, the supply of NPN RF Power Transistors suitable for HF transmitters in the 25-50 W category will dwindle (as in the case of MRF 477, 497, 2SC1969, NTE/ECG Equivalents, etc.....). There is an Asian (Hong Kong?) Company (name escapes me but it reminds me of the American Co. Elecraft)... which mfgs equivalent NPN Power Transistors. There's a guy on eBay in the U.K. who is their sole eBay distributor. Any of you ever try those components???
