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another mosfet fail...

dozerman

hello, its me again
Dec 16, 2013
186
36
38
milan tennessee
just getting back into things after a few years, and decided to tune up my base. ive got a Galaxy 11b, thats been with me for several years. i got bored and had parts just looking at me so i decided to make it mosfet. i still had the cb tricks info on dual final swaps for the 360014b boards, so here goes.

i used 13n10's rather than 520's. never had a lot of luck with 520's. after replacing all the components, i turn it on to set the voltages on the fets. set the gates to 1.9 volts and made sure every thing was kosher. first audio netted about 55 peak. then less with each key up after until i had just enough to produce a reading on the freq counter. ok.. replaced the 13n10's with 520's. same result. i replaced a few of the caps that were in the swap, and can get a 1 watt carrier for a second or two then i get a bad hum that drives the wattage and modulation up. i have good talkback and PA. the variable power no longer works and now when i turn it on, it takes about 3 seconds before everything comes on..

another note, i have about 8 volts on everything surrounding the board. the case, heatsink, antenna jack... any direction would be appreciated
 

Well with the 520's, I had the bias set at 3.50 on all three. The driver got hot any higher. With those I had the "motorboat" sound thru the speaker and had zero output until the oscillation happened. I just swapped those out for fresh 13n10L's. Gates at 1.8 or there about. Got my power and mod back, at 45 peak. But.... something is leaking off. Power will slowly fade on a long dead key. These are staying cool, as is the regs and power supply. The 520's weren't
 
did you use a mylar insulator or a ceramic insulator on each final and driver. this can cause a problem also. and are the bias diodes put in the right direction? the 13N10L mosfets are rather problemish, at least when I tried them they were. the L version will only handle 2.0 volts and the plain 13N10 fets will handle 4.0 volts. I first bought some L versions then threw them out after a couple try's. I can run the 13N10 fets at 3.5 volts and they work fine. make sure the bias diodes are in the right direction. they go in backwards to the bipolar diodes. the original write up for the conversion shows to use the ceramic heat sinks and not the mylar since it causes a capacitance problem with the mosfets.
just some thing to think about.
this is the 6900 board conversion for the mosfets. is this what you used to swap out your bipolr finals.

http://www.cbtricks.com/miscellaneous/fet_papers/mosfet_conv/graphics/6900xxx_ssb_mosfet.pdf
here is the one for the 3600 board.
http://www.cbtricks.com/miscellaneous/fet_papers/mosfet_conv/graphics/3600xxx_ssb_mosfet.pdf
 
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did you use a mylar insulator or a ceramic insulator on each final and driver. this can cause a problem also. and are the bias diodes put in the right direction? the 13N10L mosfets are rather problemish, at least when I tried them they were. the L version will only handle 2.0 volts and the plain 13N10 fets will handle 4.0 volts. I first bought some L versions then threw them out after a couple try's. I can run the 13N10 fets at 3.5 volts and they work fine. make sure the bias diodes are in the right direction. they go in backwards to the bipolar diodes. the original write up for the conversion shows to use the ceramic heat sinks and not the mylar since it causes a capacitance problem with the mosfets.
just some thing to think about.
this is the 6900 board conversion for the mosfets. is this what you used to swap out your bipolr finals.

http://www.cbtricks.com/miscellaneous/fet_papers/mosfet_conv/graphics/6900xxx_ssb_mosfet.pdf
here is the one for the 3600 board.
http://www.cbtricks.com/miscellaneous/fet_papers/mosfet_conv/graphics/3600xxx_ssb_mosfet.pdf
I used ceramic insulators. I've never had much problem with the L's. I've alway subbed them for the 520's. NEVER had luck with them, but it may be the batch. I've still got a bunch of the L's I bought from someone on cbtricks, forget the name. But as I said, I swapped the 520's back out for 13n10's and my problems went away, other than the drifting dead key. I will recheck the caps for leakage in the mod again tomorrow.
 
did you use the 5.6 volt diodes reversed on the finals. some are trying to use the 1n4148 diodes as the bipolar and they do not work.
 
Not to put too fine a point on it, but since the MOSFETs tend to draw half-again more current than the correct parts, did you beef up the power supply to make it half-again larger?

Or just make it grunt until it croaked, maybe?

MOSFET conversions for a mobile radio take advantage of your vehicle's charging system. The extra current you'll need is easily available.

But in a base station, you might need to upgrade the power supply.

The 8-Amp switchmode power supplies sold as "100 Watt" would be a good choice for the upgrade.

73
 
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the best luck I have had with mosfets are the ones from mouser. the last batch I bought were about 43 cents each with 50 pcs. and 8.00 to mail them. have not checked their price in a while since I stopped doing repairs other than my own or a close friends in 2012. I still have plenty for my own use.
 
good catch Nomad, I did not notice it was a base he modded. I did a 142 and had to up the power supply also. it would stumble some when keyed up. new power supply and it worked good. some of the base radios have a problem with a echo added. could be a power supply dropping off peak power.
 
Forgot to mention the zeners, yes sir they are in reverse 5.6. Once I took the 520's out and replaced them last night, 99% of my problems went away. One may have been grounded out, I'm not above making mistakes lol
did you use the 5.6 volt diodes reversed on the finals. some are trying to use the 1n4148 diodes as the bipolar and they do not work.
 
Not to put too fine a point on it, but since the MOSFETs tend to draw half-again more current than the correct parts, did you beef up the power supply to make it half-again larger?

Or just make it grunt until it croaked, maybe?

MOSFET conversions for a mobile radio take advantage of your vehicle's charging system. The extra current you'll need is easily available.

But in a base station, you might need to upgrade the power supply.

The 8-Amp switchmode power supplies sold as "100 Watt" would be a good choice for the upgrade.

73
I've not done anything to the power supply. When I got it originally, I had to replace the 10000 uf cap that feeds the unit but other than that, nothing. I did replace the regs inside the radio with TIP components. As I have better luck with them over the 1012, 827 or 1869's. I noted the voltage on key up before and after and there wasn't an appreciable difference.
 
I think your heat sink might be inadequate. Fading power output for a prolonged keyup sounds like heat saturation.
Other than putting a fan inside the unit to pull heat, I can't do much about the heat sink. I've got a few small fans, I may try one to see what happens. But while I'm keying, I've got my hands on the transistors, and none get hot. Barely warm. The 827 reg gets warm but that's about it. I will look at it more when I get off work but as of midnight last night, I had my power and mod back as well as the variable power. I somehow unsoldered a leg of r238. And the power supply is MUCH cooler. No dimming or waiting to power up.
 
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