Magnum S9 keeps blowing finals

[James Unit 198]

Magnum S9
Main PCB # 9821R20
CPU PCB # PCBX-0335R3B

The rig has blown 2 set of finals and now on set # 3.

I use the S9 on FM and SSB the power output is keep to 10 watts on FM and about 20 to 30 on SSB and is used as a home base rig.

The SWR is 1.1 and the worst is 1.3.

Has anyone else had this problem? And is there a cure.

James.

[ramblingman]

did you make sure to check the bias at driver and final..after installing new finals

[James Unit 198]

The bias as you call it is set ok. i got the info on the gate voltages from sam at magnum. it uses mosfets and not bipolar trans.

James.

[ramblingman]

i had similiar problem one time with a galaxy dx 2517 i was working on..finals kept blowing..thought to start with maybe i was getting bad batch of finals..but after it came back to me 3 times within a six months period of time..i knew i had to try something else...everytime i did everything by the book,checked and set bias each time..but still darn thing kept blowing finals..even tryed installing fan on back of heat sink..thought this was the problem..still no help...so after a little research i decided maybe the finals were going into self osillation..so i tacked on a r-c feedback circuit..consists of a 330 ohm resistor and a .0047 uf disc cap across the base and collector foils of final...and a 220 ohm resistor and a .0047 uf disc cap across the base and collector foils of the driver...keeping the leads of the resistor and cap as short as possible..sometimes with these higher power transistors this happens..as of the higher the gain of the transistor,the more likely it is to go into self oscillation..and the best way to stablize this is with degeneration or negative feedback..in which some of the output signal is fed back into the input signal out of phase..i also went ahead and changed the negative feedback caps out....then to make a long story short..it must have did the trick because it hasnt returned to me..and i still hear the guy hammering away on it..and its been over a year later...so any way just a thought..might be something to try,when all else fails...good luck with your radio..................ramblingman

[Rick]

Check the ceramic insulator plates known to causes capacitance problems, if they show any sine of silvering change them, also check the transistor which has no ref number but it is glued to the PCB just in front TR43 I have had radios where this has gone short, which causes the finals to run very hot is AM and FM mode. Hope this helps. Let us know how you get on.

There is no such think as negative feedback with this sort of MOSFET as all hi speed Ver MOSFET use a clamp diode from Source to Drain as they are normally used for switching with large current sink.

Using negative feedback in a CB radio is like, well you who know what you are doing know what I am getting at, I do not need to say any more.

[ramblingman]

all i know is/// it fixed the 2517..no longer blows finals

[James Unit 198]

Rick you was right the tran which is glued to the board was short circuit emitter to collector. I have replaced in with BC639 this should be ok has it is 80 volt at 1 amp.

James.

[Rick]

The BC639 will be ok as you only need a transistor which will do 40 volts and 300mA.