Failure Mode in "8 Pill"

[paws264]

What would cause 2 transistors (out of eight) in TS 1600 to blow? Both are on the same board. The other transistors on the other board that shares the combiner is OK.

The 10-ohm` base bias resistors are not burnt, the amp was not driven with more than 30 watts.

The transistors are open from C to E and from B to E

I did in-depth testing of capacitor and resistance values of the components on both boards (nothing out of line)

What would blow 2 transistors on the same board and not the rest?

????

 

[TonyV225]

Are those finals all a matched set date coded the same???? If not and those 2 have the same matched date code they may just have been bad to start with ive seen that before with a few burning out and others being ok and operational its the wonderful world of electronics hell i have even bought brand new parts that were nonfunctional when hooked up.

 

[nomadradio]

Each pair of RF transistors has a 25-ohm 5 Watt feeding bias to the base circuit for that pair. Where that resistor meets two ferrite beads, there will be a 1.6 ohm 1/4 Watt resistor going to ground. If that resistor got clobbered by some previous breakdown, it would cause that pair to draw WAY too much current until it gets replaced.

It's the one answer that I know from experience, anyway.

73

 

[paws264]

Yes, I learned that lesson the hard way with a Texas Star 667.....

On this particular amp, instead of a single 1.6 ohm resistor, I paralleled two 2.7 ohm resistors at the point where the 25 ohm is connected to the ferrite beads; that gives me around 1.3 ohms which should lower the forward bias voltage to a value a little less than stock.

I'm thinking about moving the ferrite beads off the "hot" point and moving them over to ground; from class B to class C. Gonna run it like that for a while to see if it is stable.

I am thinking that over drive might have been the problem as, my driver is swinging over 45 watts (I should think that's still in the safe range, maybe not),. I am also considering removing one transistor from my driver.

I would like to keep it class B but, if I turn the TS-1600 into a class C box and drive it with a sideband biased amplifier will I still put out a quality SSB signal?


.

 

[nomadradio]

Nope. SSB won't sound right unless EVERY stage in line, from the radio on up, has bias.

73