Need Help Selecting A Better Transistor

[Wire Weasel]

Hi guys, Have a friend with one of RM Italy boxes. Don't have the model # on hand but it's one their typical smaller ones. It's not the little 203, but like the next size up, longer and red in color, 200W range. The preamp transistor failed and I replaced with same for him. Is a BF199. It worked about a month and failed again. I had picked up a Lot of 10 of them so I popped another one in and off he goes. So if this one fails then it is likely apparently that there is something going bad elsewhere in the circuit causing this transistor to fail. - OR, there is the possibility that this part was poorly selected for this part of the circuit and would benefit from a heavier duty one. Has anyone here experienced the same problem and know the fix? Or can anyone suggest a heavier duty replacement for the BF199 that we can try? Thanks !!

 

[Tallman]

Are you sure it is not being over driven? Is it a KL-400? Or perhaps a KL-300?
The 300 is a class "C" amp.

 

[Wire Weasel]

Don't think so .... but what would being overdriven have to do with the preamp?

 

[nomadradio]

Most popular way to pop the preamp is to use the SSB delay.

So long as the relay kicks in immediately, the preamp receives only a very short burst of the radio's output pushed up its 'backside', so to speak. Typically a diode or two is in the circuit to catch this short burst of RF and protect the preamp from the surge current.

The SSB delay capacitor serves to hold the relay closed on sideband, but also delays the "kick-in" time, even when running AM.

Using a preamp while running SSB nearly always clobbers built-in preamps.

73

 

[Robb]

I've often wondered why that happens, seen a couple of amps with popped receive transistors. Thanks for that tip.

 

[Wire Weasel]

Thanks Nomad but not the case here. The op never uses SSB. Also these transistors are failing without even talking. When they fail, they have the effect of killing the receiver altogether .... you're just driving down the road listening and all of the sudden your receiver goes dead like you just reached over and turned the volume down. You start checking around to what is going on and find out it's the amp. Turn the amp OFF and the RX comes back up on the radio ... AND ... this situation actually doesn't even involve using the preamp at all. I've had the same thing happen to a RM 203P of mine. Both in my case and my friends with the different RM model .... neither of us even had the preamp on at the time of the failure, just the amp portion .... and we were not transmitting when the part failed.

 

[Jay Mojave]

Hello All: I have a few pre-amp transistors bite the dust probably like Nomad says. I removed the pre-amp since the radio has a all ready hot receive sensitivity, and installed a -30 dB attenuator allowing to significantly reduce the receive signals at the flip of a switch. Its great for helping Direction Finding local power line noise while stepping out to hit the telephone pole with a sledge hammer to isolate the noisy telephone pole. And long hauls across the country when the boys have all the steam turned on.

Jay in the Great Mojave Desert

 

[Wire Weasel]

Hi Jay .... Good Tip! - Yeah wish this was the case with the RM Italy's but they're using a weird circuit design. With the majority of the pill boxes .... if the preamp transistor fails - you simply lose your preamp and you can keep on trucking. But with these things, it "shunts" the RX pass-through down through the amp. The amp section still works but that doesn't do you any good if you can't hear anything when you're unkeyed. Your only choice is to turn the amp completely OFF. You get your RX back to the radio but you are left amp-less.

 

[nomadradio]

Here's how we fix preamp problems in an amplifier that has this sort of relay for that feature.

Biggest typical reason is that the "preamp off" side of the relay begins to cut out. This will prevent you from operating the radio barefoot, causing the receiver and 'straight' transmit connection to drop out.

Only works if the relay's cover is a separate piece that can be popped off to expose the contacts.

Much easier than removing the whole circuit board to replace the relay with a wire jumper.

73

 

[Jay Mojave]

Hello WW and All: Well that sucks big time! Time for a Mod-O-Matic to fix that piss poor design. Thanks Nomadradio, that's funny but no dough a quick fix.

Using a 8 pill old Messenger M4V Amp while driving under a low branch sheared off my home made 96 inch fiberglass antenna on my truck, and took out a few pills and tuning capacitor. And had just got it working again. So all new pills and if the money went into the pills 8 each, then I installed a bigger and better new relay. The relay has a bigger bypass transit suppression diode, and a diode in series with the 12 volt coil supply. This might help. I also added a diode across the keying transistor. New Antenna, and rebuilt amp and its been hummin and buzzin for several years now. But I don't have a pre-amp as said, the pre-amp switch adds in -30 dB into the receiver. This option also assists me to check someone who may be stretching the truth about his antenna gain and output power.

Have to play radio operator and keep an eye on the pre-amp switch, so that bad radio checks don't send guys out needlessly working on there antennas in the heavy winds and rain out here in the Mojave Desert.

Jay in the Great Mojave Desert

 

[Onelasttime]

I thought most people solve this by eliminating the preamp just bypassing it for good. Not like they do any good anyways and they have been problem prone from day one! Most TS amps the pre-amps fail early on and no one cares. The problem is not the transistor but the circuits design.

You never see pre-amps on really nice tube amps or amateur transistor amps. Usually when you see a pre-amp on any rf amplifer it is a sign that the OEM is pandering trash! RF pre-amps used on linear amplify the noise floor too much to be of any real use.

 

[Wire Weasel]

That would be nice OLT but the RM Italy's aren't designed that way. When this transistor fails the box will no longer pass RX at all so the fix HAS to be made or the box becomes useless.

 

[BBB]

You can open even the sealed miniature relays casings by gently squeezing the top with a pair of pliers until you hear the plastic casing crack. It will usually crack at the base plate where the top is glued to the bottom. You may need to work the pliers around the top of the plastic casing to get it to crack the plastic enough for removal to expose the relays internal components. Be aware that if you are too generous with squeezing the plastic casing you can damage the relays internal components.

It takes some practice so I recommend doing the procedure to a couple of scrap miniature sealed relays first before you try it on your prized amp and/ or have some spare new relays on hand.

I use this method when replacing miniutre relays so as to get to the relay's individual pins for un-soldering them from a double sided PC board. For some reason I just tried it being careful not to damage the relay's internal components out of OCD habit, then once the casing is off just dig in there with the wire cutters and hack to Sh!t out of the old relay to isolate each soldered pin for easy removal... haha.

 

[Tallman]

I thought most people solve this by eliminating the preamp just bypassing it for good. Not like they do any good anyways and they have been problem prone from day one! Most TS amps the pre-amps fail early on and no one cares. The problem is not the transistor but the circuits design.

You never see pre-amps on really nice tube amps or amateur transistor amps. Usually when you see a pre-amp on any rf amplifer it is a sign that the OEM is pandering trash! RF pre-amps used on linear amplify the noise floor too much to be of any real use.

Too true. The receive preamps are noise amplifiers and are not selective at all.
Garbage in and even louder garbage out.

 

[bob85]

rm usually do a none preamp version of their amps using the same board without the extra relay & handful of components so its easy to remove the relay & fit jumpers like the factory.

Its odd that I never come across the preamp transistor blown even when we are close enough to be keying the other guys amp up when we key, its the relays that play up going intermittent oc,