RCI RT-1 MOSFETs WTF?

[nomadradio]

Ah, okay. So what's wrong with this picture?

This is the tester we use to match Vth every batch of IRF520 and FQP13N10 parts.

Yes, it's a new unused RT1 MOSFET.

And yeah, both the testers like this one that we use show the same result.

My intuitive guess is that the gate-voltage threshold is just a little higher than the max voltage this thing can feed into it.

I gator-clip lead cobbled a test that showed the transistor was functioning okay. No, I didn't try to calibrate the actual Vth. Just wanted to know if it was likely to work in the radio.

Of course, this customer provided the parts and requested we replace the blown finals and driver in this radio. Sure enough, the originals were bad, and they had clobberred the modulator/reg transistor. Changed them and....

Nothing. Nada. Only the weakest signal leaking out into an adjacent radio.

Naturally, the radio has no drive whatever feeding into the driver when you key the mike. Murphy never sleeps.

It's like Roseanne Rosannadanna used to say, "It's always somethin'".

73

 

[nomadradio]

Well, I misspoke about this radio. It was the black radio that was fixed before this one Saturday that needed the modulator transistors. They were okay in this one.

Turns out that setting the bias trimmer pots for zero Volts will get you zero Watts. That's where I had preset the trimmer pots before applying power. Was a long day. Probably best I just turned everything off and went home.

But the issue of setting the bias remains. One trimpot for the predriver and driver both, looks like.

And only one black jumper plug near the center of the pic. Gonna have to work out how this gets done tomorrow.

A schematic would be a big help, especially when one of these radios breaks down the road.

Here's what Gargle found for the radio's circuit-board number.


Your search - "EPT690012D" - did not match any documents.

Not reassuring. Not happy with the data black hole this new radio represents.

73

 

[buickid]

What kind of radio is this, so I don't buy one?

 

[Smokinone]

I can't help you with the issue, but I sure like those little cheap testers. Bought one after messing with audio circuits and played around for days before the tester arrived...and showed me that the data sheet I had was wrong on the transistor leads of the component I was replacing... didn't match the data sheet. Having found that, all went smooth.

Not hijacking, just making an observation.

 

[brandon7861]

Seeing those parts reminds me of a video MikesRadioRepair did. He went on a nice rant about how the max mod is exactly the same on a curve tracer as a tip36 (). And aren't the RT-1's just an IRFZ44N?

 

[brandon7861]

Well, I misspoke about this radio. It was the black radio that was fixed before this one Saturday that needed the modulator transistors. They were okay in this one.

Turns out that setting the bias trimmer pots for zero Volts will get you zero Watts. That's where I had preset the trimmer pots before applying power. Was a long day. Probably best I just turned everything off and went home.

But the issue of setting the bias remains. One trimpot for the predriver and driver both, looks like.

And only one black jumper plug near the center of the pic. Gonna have to work out how this gets done tomorrow.

A schematic would be a big help, especially when one of these radios breaks down the road.

Here's what Gargle found for the radio's circuit-board number.


Your search - "EPT690012D" - did not match any documents.

Not reassuring. Not happy with the data black hole this new radio represents.

73

I don't have the schematic, but I would guess that the pre-driver has a fixed bias, the left pot is for the driver, and the right pot is for the two finals. I would turn each one up individually really slow while feeling the transistors to see which one gets warmer with the excess bias (saves poking around with a probe). Once I knew for sure which pots did what, I would set them both to minimum, then slowly increase the driver pot until the radio's current consumption increases by 50mA and then the other pot (assuming it controls the dual finals) for a further increase of 100mA. Well, that's what I would do for IRF520's, but those are probably better mosfets so maybe 100mA each?? All I know for sure is that most people (and service manuals) say to set mosfet bias by gate voltage, i think those people are nuts! The huge amount of current change with a tiny amount of gate voltage change tells me setting bias via gate voltage is a bad idea...

 

[Eldorado828]

What kind of radio is this, so I don't buy one?

I'm guessing that's an rci-69 base.

 

[Eldorado828]

I'm just a bystander watching what happens and learning from the master magician in this thread.

So little said about the rt1's, I've yet see any information on the proper bias settings. So hopefully that'll come about as this goes on.

I do know, they are actually an IRFZ24NPBF scatched off and labeled as rt1, and...... they seem to pop about as much as a black cat on the fourth of July.

 

[loosecannon]

I don't have the schematic, but I would guess that the pre-driver has a fixed bias, the left pot is for the driver, and the right pot is for the two finals. I would turn each one up individually really slow while feeling the transistors to see which one gets warmer with the excess bias (saves poking around with a probe). Once I knew for sure which pots did what, I would set them both to minimum, then slowly increase the driver pot until the radio's current consumption increases by 50mA and then the other pot (assuming it controls the dual finals) for a further increase of 100mA. Well, that's what I would do for IRF520's, but those are probably better mosfets so maybe 100mA each?? All I know for sure is that most people (and service manuals) say to set mosfet bias by gate voltage, i think those people are nuts! The huge amount of current change with a tiny amount of gate voltage change tells me setting bias via gate voltage is a bad idea...

Brandon i could not agree more!

i, like many, started setting bias on mosfets using voltage like all the manuals suggested until i did one radio where i also looked at quiescent current while setting bias.

holy crap!

watching that current shoot sky high after a few tenths of a voltage increase told me everything i needed to know.
i think the finals were drawing over 1 amp at the suggested voltage level.

i don't even bother with the voltage anymore and just check it once im done because im curious.

if the current is right and the signal looks good then all is well!
LC

 

[Handy Andy]

The huge amount of current change with a tiny amount of gate voltage change tells me setting bias via gate voltage is a bad idea...

... and the sound of a Microphone being dropped ...

'Enuff said!

 

[nomadradio]

So it's a good thing I decided to just knock off at the end of a long Saturday with the 69 base.

A fresh look reveals this black jumper plug. Pull it off, and a spare 2-pin 2.5mm plug is used to put the current meter in line with the two left-hand RF transistors. The plug used for the internal speaker in a Galaxy/Connex/RCI radio works fine. I call them the pre driver and driver. The left-hand trimmer pot controls their gate bias. I set it for 100 mA, on the assumption that this is for two transistors. Seemed to work okay. Didn't waste more time to analyze this.

There is no jumper to break the circuit to the two final transistors. I put the meter on the 10-Amp scale and jumped the DC side of the power switch with it. With the power switch off, the meter now reads the sum total current draw of the radio. Turn the mike gain to zero, mode to USB or LSB, either one. Key the mike. With VR10 set for minimum reading, make a note of it. Might be around 200 mA. Forgot to make a note of what that was on this radio. Advance VR10 slowly. It will make no difference at first, but will abruptly "kick in" around the halfway point more or less. Set VR10 for a reading 100 mA higher than the original minimum reading.

Yeah, this is a "back door" method, but it works. The radio transmits just fine now.

Mystery solved. Setting the bias trimpots for zero voltage was the safer thing to do after replacing all those parts, but that setting also guarantees zero RF power.

At least this way they get set for the desired circuit current.


73

 

[nomadradio]

And aren't the RT-1's just an IRFZ44N?

I have heard that, too. Bought some to play with a while back. I should see if I can find them and see if they baffle the chinesium tester the same way.

73

 

[Handy Andy]

With the power switch off, the meter now reads the sum total current draw of the radio. Turn the mike gain to zero, mode to USB or LSB, either one. Key the mike. With VR10 set for minimum reading, make a note of it. Might be around 200 mA. Forgot to make a note of what that was on this radio. Advance VR10 slowly. It will make no difference at first, but will abruptly "kick in" around the halfway point more or less. Set VR10 for a reading 100 mA higher than the original minimum reading.

Glad to see this.

At least someone deciphered my ramblings....

 

[Spec2079]

Ah, okay. So what's wrong with this picture?

This is the tester we use to match Vth every batch of IRF520 and FQP13N10 parts.

Yes, it's a new unused RT1 MOSFET.

And yeah, both the testers like this one that we use show the same result.

My intuitive guess is that the gate-voltage threshold is just a little higher than the max voltage this thing can feed into it.

I gator-clip lead cobbled a test that showed the transistor was functioning okay. No, I didn't try to calibrate the actual Vth. Just wanted to know if it was likely to work in the radio.

Of course, this customer provided the parts and requested we replace the blown finals and driver in this radio. Sure enough, the originals were bad, and they had clobberred the modulator/reg transistor. Changed them and....

Nothing. Nada. Only the weakest signal leaking out into an adjacent radio.

Naturally, the radio has no drive whatever feeding into the driver when you key the mike. Murphy never sleeps.

It's like Roseanne Rosannadanna used to say"It's always somethin'".

73

This an old post but would like to chime in he has the green lead in the wrong spot needs to be on that empty leg

 

[Cable Guy]

Is that a 9mm snap cap with wires shoved in it? (Plastic cartridge for dry firing a pew pew). If not, what in the world is it?