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Hello and can I do it?
- Thread starter Dmorgan
- Start date
What type of transistor is in there now? Is it broken?
An old adage come to mind, "If it ain't broke don't fix it!"
An old adage come to mind, "If it ain't broke don't fix it!"
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A friend give the radio and I'm pretty sure the final is bad , been in his truck 3or 4 years running a SWR north of 3.0, he never checked just bought a new radio and threw it in the truck.
I figured I'll play with since it's free
http://www.cbtricks.com/miscellaneo...5/graphics/uniden_bearcat_880_erf2030_mod.pdf
The ERF-2030 and the IRF-520 are interchangable - BTW.
You will also need to make a biasing device to make a MOSFET work in that radio, since the MOSFET is not a direct drop-in replacement:
http://www.cbtricks.com/miscellaneous/fet_papers/mosfet_experiment.htm
The ERF-2030 and the IRF-520 are interchangable - BTW.
You will also need to make a biasing device to make a MOSFET work in that radio, since the MOSFET is not a direct drop-in replacement:
http://www.cbtricks.com/miscellaneous/fet_papers/mosfet_experiment.htm
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I'm hearing an echo of the old motto of the shade-tree mechanic.
"If it ain't broke, keep fixing it until it is".
Does the radio transmit normally now? If it does, what are the odds it will transmit after it has been modified?
If you just have the itch to modify radios, may I suggest that starting with one that has a published schematic diagram can be a lot less frustrating.
Just sayin'.
73
"If it ain't broke, keep fixing it until it is".
Does the radio transmit normally now? If it does, what are the odds it will transmit after it has been modified?
If you just have the itch to modify radios, may I suggest that starting with one that has a published schematic diagram can be a lot less frustrating.
Just sayin'.
73
^^^Parts Changes Needed As Shown^^^
Can't do a direct replacement from bipolar to MOSFET in any radio that had bipolar transistors, or vice versa.
Whoa! Instructions!
Totally missed that.
There are drawbacks to this substitution. To start, that diagram of the "EN369FN", the so-called "companion part" for this mod has an error. If you can't find an original EN369, copying it from this diagram will cause the radio to pop fuses on transmit, until you get disgusted, put in a big fuse and SMOKE is the next result.
There is a post on this forum with the corrected info on that part, but that's just the tip of the iceberg.
The original 2SC2078 provides as much power as the radio can deliver without damage. Putting a hot camshaft in your 4-cylinder front-wheel drive daily driver might make it go faster. But you'll wear out the motor a LOT sooner. Making the final bigger doesn't make the rest of the radio big enough to cope with it.
I would put the same effort into coming up with the funds for an amplifier. If you want to be larger on the channel, that's the way to achieve this.
Even if you get this mod to function smoothly, the power increase won't get you any farther down the road.
An amplifier will.
73
Totally missed that.
There are drawbacks to this substitution. To start, that diagram of the "EN369FN", the so-called "companion part" for this mod has an error. If you can't find an original EN369, copying it from this diagram will cause the radio to pop fuses on transmit, until you get disgusted, put in a big fuse and SMOKE is the next result.
There is a post on this forum with the corrected info on that part, but that's just the tip of the iceberg.
The original 2SC2078 provides as much power as the radio can deliver without damage. Putting a hot camshaft in your 4-cylinder front-wheel drive daily driver might make it go faster. But you'll wear out the motor a LOT sooner. Making the final bigger doesn't make the rest of the radio big enough to cope with it.
I would put the same effort into coming up with the funds for an amplifier. If you want to be larger on the channel, that's the way to achieve this.
Even if you get this mod to function smoothly, the power increase won't get you any farther down the road.
An amplifier will.
73
I have a bunch of irf 520's lying around, just not sure on the companion, can't find any in stock , like I said I have zero dollars in this radio , just thought I'll try and fix it since I have the mosfet's
Hi all. Was playing with mosfets over the weekend when I noticed something a bit strange going on with the information on CBtricks with regards the EN-369FN.
Whilst the information below does contain some maths and Ohms law it does clearly prove that the EN-369FN drawing and in particular the value of R2 is infact not just incorrect, but it's waaaay off!
The example being used is a bog standard Magnum 257 that uses the EN369DR and EN369FN companion parts on the driver and final mosfets. Below is the schematic of the driver and finals mosfets and their biasing resistors and below that the EN-369FN and DR nabbed from CB Tricks .com
Now for the calculations:
Worked driver and final PA bias examples using a Magnum 257 as our example:
All examples are taken with RV11 and RV13 at their midpoint which is 10k and 250k ohm respectively.
Keep in mind that we would expect to bias the driver mosfet at about 3.1 - 3.3 volts and the finals at 3.7 volts.
Driver biasing example - Please refer to red box 1 on the above schematic.
Transmit line voltage is the usual 8 volts DC.
First we calculate the current:
current total = transmit line voltage / (RV13 midpoint + R233 + [R2 of the EN369DR]) =
So current total = 8 / (250000 + 270000 + 320000) = 8 / 840000 = 0.000009523A or 9.523 uA (Micro amps)
Now we can calculate the bias voltage across the EN-369DR
bias voltage across the EN36DR = current total x [R2 of the EN369DR]
So bias voltage = 0.000009523 x 320000 = 3.04 volts.
3.04 volts is a nice driver bias voltage to start with and we can tweak it from there.
Final biasing example - Please refer to red box 2 on the above schematic..
Please note before we start the transmit line voltage is now 7.4 volts as we drop 0.6 volts across transistor Q5
First we calculate the current:
current total = transmit line voltage / (RV11 midpoint + R239 + [R2 of the EN369FN])
So current total = 7.4 / (10000 + 10000 + 230000) = 8 / 250000 = 0.0000296A or 29.6 uA (Micro amps)
Now we can calculate the voltage across the EN-369FN
bias voltage across the EN369FN = current total x [R2 of the EN369FN]
So our bias voltage = 0.0000296 x 230000 = 6.808 volts
Hang on a moment! That’s not good at all, actually this is very BAD. 6.808 volts is almost double what we should be biasing the mosfet at and will destroy it very quickly!
So what’s wrong here? We know full well that the Magnum was never designed to run at 6.8 volts bias, or else they’d be popping finals left right and centre.
The answer to that question is R2 in the EN369FN in the drawing is incorrect.
If we were to decrease value of R2 in the EN369FN to 20k and then redo the maths.
Final biasing example with R2 in the EN369FN at 20k ohms
First we calculate the current:
current total = transmit line voltage / (RV11 midpoint + R239 + [our new R2 of the EN369FN]) =
So current total = 7.4 / (10000 + 10000 + 20000) = 8 / 40000 = 0.000185A or 185 uA (Micro amps)
Now we can calculate the voltage across the EN-369FN
bias voltage across the EN36FN = current total x [our new R2 of the EN369FN]
So bias voltage = 0.000185 x 20000 = 3.7 volts
3.7 volts = A perfect bias!
As you can clearly see from the above simple maths the EN369FN should not be 230k ohms as per the drawing, but should be 20k ohms.
If you use the following drawing edit it will not only work, but work as intended.
Psi
I would make the companion part out of components laying around.
Try tweaking the value of R2, for example use 33K Ohms at R2, or install a variable "trimmer pot" so you can remove it and measure the resistance found once you are happy with the performance of the radio, the resistor should produce about the same bias voltage in place of the trimmer, after you'e found the value you want; use the closest (carbon film?) resistor you have laying around.
Solder in the completed "home made companion part" with the positive lead (striped end "cathode" end of the diode is the hot side) to the gate pin of IRF520N (closest to outside edge of radio) and the negative end to pcb ground.
Remove L204 and R203
IMO 3.7 Volts seems a little hot, I would run it anywhere between 2.7-3.2 Volts @ gate during transmit.
(should run a lot cooler) you'd be able to amplify it either way.. but
The lower the bias voltage at the gate during transmit, the lower the output power. (speaking generally). "Peak-it-out" using L202 + bias voltage
(3.8-4 watts carrier, clean 100% modulation).
I don't know how the MosFet version of the Uniden 880 is configured from the factory.. But i'm confident you could make it work Dmorgan (as long as the only thing it needs is a new final), just keep the solder work neat and tidy, & double check everything.
Oh yeah almost forgot, order some thermal paste if you don't have any! So then you can clean the thermal compound the factory used (if any), and "re-grease" the final. (maybe do all the chassis mounted IC's if you are confident you won't damage anything)
Hope that helps!
Best Regards.
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Umm, okay.
It doesn't transmit.
Is it because of a bad final?
Did you test the old one?
Not the only thing that can fail and shut down the transmit side.
Then again, if you just have the itch to experiment, a radio you won't cry about if it falls on its face is the best choice.
If the stock final is good, doing the MOSFET swap won't make it transmit.
73
It doesn't transmit.
Is it because of a bad final?
Did you test the old one?
Not the only thing that can fail and shut down the transmit side.
Then again, if you just have the itch to experiment, a radio you won't cry about if it falls on its face is the best choice.
If the stock final is good, doing the MOSFET swap won't make it transmit.
73
Thanks nomadradio
That is the most important detail, & one that I left out of my previous post!!
Dmorgan,
You only should experiment on a radio that you know works 100%.
Do you have any way to test the Bi-Polar Junction Transistor (the supposed pre-existing final) also what part number do you see printed on it? It may or may not be failed, if it failed try the mosfet conversion!
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