broadcast transmitter Q for tolivac...

[cfz2882]

a couple years ago a local AM station replaced their original
'59 XMTR because they said it started constantly blowing the
output tube-tube that used to last several years before being
replaced because of reduced performance was now blowing out
dead in a week or so.
I have heard high power tubes do not like the filament going
cold when B+ is applied,so possibly intermittant bad connection
in the heater circuit or bad socket possible cause?
Could a "leakey"capacitor blow the tube?
None of the techs who looked at it could find the problem
with the "old skool"transmitter,any other ideas of what may
have been wrong?
BTW the 1959 transmitter was the size of a large refrigerator
while the cheap solid state replacement was the size of a med.
size microwave oven LOL

 

[cadman]

The failure mode of the tube would tell lots. Blown holes in the plate? Open filament? Discoloration inside the envelope?

One thing's for a sure, a transmitter that age should have had a full re-cap a long time ago. If it has good xformers there's no reason it couldn't be repaired. -Cory

 

[tolivac]

Any transmitter I have dealt with is interlocked -you cannot apply the HV supplies unless the tube filaments are on.This will not protect against a tube that has an opened filament even if the fil circuits are on and the filament time delay circuit has cycled.That is another feature standard in tubed transmitters-once the filament,blowers have been activated-a time delay circuit-usually a TD relay.And another thing-in most rigs the filaments won't activate unless the blowers are on and proper airflow is coming from them-and water pumps in our case.I have had tubes with cold filaments out here and the HV turned on without incident-the transmitter won't put out its full power becuase one tube is out-in the case of modulators-the audio will be severly distorted since only one tube of a pair in the push pull modulator is working.Now if the tube fil was on-then failed while the HV was on-the tube will be blown.Just last week in one of our transmitters a modulator tube had a failed filament-then had a grid-cathode short.You could not bias the tube nor get full modulation from the transmitter.Another tube solved the problem.You bias AM modulator stages just like on a Hi-Fi amp or guitar amp.It is a BIG push-pull audio amp.In the case of that transmitter-a 125Kw audio amp.250Kw transmitter.To modulate the 250Kw signal to 100% you need a modulator that develops half the carrier power.-for say a 1Kw broadcast transmitter-500W.
for the transmitter mentioned in cfz2882 example-seems like its a 1Kw AM broadcast transmitter or similar power FM transmitter.I have seen newer BE(Broadcast Electronics)1Kw AM transmitters in a rack mount case the size of a large microwave oven.The transmitter uses a switching power supply.No large transformer.Typical 1Kw AM tube tranmsitters use two tubes in the final RF amp and 2 more in the push pull modulator.usually the same type-833,4-400 are most common for these.For caps-yes these should be recapped-I have done many-for safety-old rigs had LOTS of PCB caps.You had to placard the transmitter if it had PCB parts.Some HV and modulation tr4ansformers had PCB fluid in them.I have had to replace these-and then to celebrate the PCB placards can be removed from the TX.Transmitters built in the late 70's and after had non-PCB parts.The old TX is actually easier to fix then new ones.New transmiters are full of logic and digital circuits instead of relays and tubes.These can be much harder to troubleshoot.

 

[tolivac]

Old trasmitters-another thing to replace-the wire-esp the HV cabling.Visit the neon sign shop for a roll or two of 15Kv sign cable and have at it.Works well for smaller 1kw,5kw AM and FM transmitters.Then go to the electrical supply shop for a small roll of cable for the "primary" wiring for the 220V circuits.I have had cable insulation failers in older transmitters-the wire could be over 50yrs old!