Connex 3300hp power output problem

[KK4MSA]

Hello everyone,
I have a 3300hp i told a friend id look at for him *sigh*, he blew it up.
All other functions on the radio work, but had no output, this has 3 irf520's in it, i replaced all 3, have power back, but there is a low tone 3-400hz if i had to describe it, i can hear without a speaker attached, and the output is at max no matter where anything is set, the internal adjustments do adjust the output somewhat, so this "tone" or oscillation is modulating the carrier causing heavy current draw.
ive tried it with no mic attached, same thing. Nothing looks burnt around the finals or driver.
Is this something common for this style radio? and has anyone run into something similar after replacing driver and finals? just thought id ask here before i to dig into this thing, in hopes of saving time.

thanks

 

[ExitThirteen]

Sounds like the AM pass regulator transistor has gone fault. That will be located on the right back corner on the side of the chassis, next to the audio IC. Since it's a newer 3300 with mosfets, I'd suspect this will be a 2SB827, although it could be a 2SB817 or a 2SB754. Replace that with a TIP36C and it should restore operation. Let us know how you get on.


~Cheers~

 

[KK4MSA]

Excellent! i had a few sb827's on hand, it was bad,swapped out, restored power, but still have that feedback tone once keyed altho higher in tone now, hihi.
i also adjusted the Driver and final Bias,3.8 driver and 3.6 Final (also when i noticed the tone/feedback was still there), however in doing that adjustment, maybe 4 seconds of carrier that device (827) is getting way to hot, to fast, i am going to check tr50 now. any other components that commonly go bad with this type of failure?
i will say, that device had pretty much NO hs compound on it, and i mean literally a spot, possibly the cause of this whole failure. this unit has not been modded, sure the limiter had been cut, seems common in these types of radios, i just replaced that resistor with a new one as well.
thanks again for your help.

 

[KK4MSA]

tr49,50,53 test good btw.

 

[Handy Andy]

Hmm...

Excellent! i had a few sb827's on hand, it was bad,swapped out, restored power, but still have that feedback tone once keyed altho higher in tone now, hihi.
i also adjusted the Driver and final Bias,3.8 driver and 3.6 Final (also when i noticed the tone/feedback was still there), however in doing that adjustment, maybe 4 seconds of carrier that device (827) is getting way to hot, to fast, i am going to check tr50 now. any other components that commonly go bad with this type of failure?
i will say, that device had pretty much NO hs compound on it, and i mean literally a spot, possibly the cause of this whole failure. this unit has not been modded, sure the limiter had been cut, seems common in these types of radios, i just replaced that resistor with a new one as well.
thanks again for your help.

When you have a "catastrophic failure" that takes out a bunch of stuff - AM Regulator, Final and Driver - many neglect to also replace the Filter cap the output of the AM Regulator to the Driver and Final.

• The low-level Audio noise can stem from several things going on back there.
• These chassis are quite old - based on previous CB designs using Bi-polar as well as Ground tabs that also served as mounting tabs to keep the board in place.
• Because of the age, the audio "thump" or tone can be something as simple as a cap that needs replacement onto a tricky bias pot that got dirty and is showing it's tempest.
• With the above in mind, perhaps some reflow of solder is needed back there as well as old flux clean up if its' really old, or just some fresh solder to offset some of the brittle-ness the lead-free solder (possibly silver or more tin than lead) has. The reheating can do wonders in refreshing the contacts to the side panels and case those mounting lugs did provide.

So the Cap may be on it's way out and ready to fail, so double check your work and if you still have issues - several techs I token ring with offer a possible alternative solution of what HR2510's used to do to keep the MRF477 from overload. They use NFB not on the Driver and Final - but on the Pre-driver - upstream (one stage up) from them. The circuit is a simple 10pF and 330 ohm resistor - the resistor value is typical - but you can use even a higher or larger ohmic value to obtain a power curve you can live with and yet the Driver and Final can withstand.

This little Cap Resistor combo takes some of the Pre-Drivers output energy and re-applies it back to it's Base - to help reduce power but also provides a level of Dynamic Control over the range of power this part produces - It is a Bi-polar part, meaning that it has Admittance and Impedance issues - that the engineers took into consideration but you are using two levels of technology - Bipolar and MOSFET present different kinds of problems for the interface layer between the two. The MOSFET could care less about signal loading - for it has an extremely high input impedance - it doesn't load down the Bipolar - so you'll have to configure the Bi-polars' output to see a load and act accordingly. That is why I suggest the 10pF and 330 ohm resistor - you can try it to see if it keeps the thermal issues down - stabilizes audio and provides better power control and less RFI from this conversion. If it seems to "Clamp" power down too easily - just increase the Resistor to as high as 1K to help with controlling the power curve going to the Driver and Final.

Especially if you use 13N10's instead of IRF520's - the 13's are great and fast acting - but are more prone to failures caused by excessive inputs on their Gate. You run Dual-Finals? Then remind yourself why the IRF's failed - you may want to think about placing some limitations on Audio power swings on this chassis - especially with a Bi-polar to MOSFET conversion - the impedance swing the Bipolar presents to the Gate of the Driver can cause issues with Admittance - not from the MOSFET standpoint, but from the Bi-Polar one stage UP can produce harmonics the Gate in the next stage will not have a problem capturing and reproducing - causing further power problems down the chain from it.

 

[nomadradio]

Does turning the mike gain down to zero eliminate the tone?

73