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Harris HT 3.5

C

ChiefEngineer

Anyone have problems with the Harris 3.5 HT series transmitter in the screen supply.

I have replaced a bunch of resistors in the bias and screen supply. The variac was toast as well as the transformer fed by the variac.

In measuring voltage I always have 120 v from side to ground but never have 220 v across the output of the variac.

Transmitter is in a controlled environment and has never had a single problem other than tube replacement.
 
What's the chance you've got a shorted screen bypass cap? It would take the screen voltage to ground and probably do dirt to the screen supply.
 
Used a VOM to check for a short to ground all the way to the tube socket and no short noted. That was my first thought.

Also changed tube to make sure the tube itself wasn't shorted.

If it only shorts under hv there wouldnt be a way to tell though!

In checking voltages I have found voltage on the front panel screen breaker. I also find it all the way to the screen supply.

Given the age of the transmitter I might just replace the caps.

Thanks for the input.
 
There was a field change bulletin that Harris sent out for the HT 3.5 and 7.5 that replaced the screen supply filter capacitors. I remember having to drill holes and such to install them in two transmitters I have - took several hours. Maybe that wasn't done on yours (if it was old enough) and something has gone south.
 
To isolate the problem to either the screen voltage supply or the tube socket and bypass capacitors, you could install a temporary wirewound resistor of sufficient rating on the output of the screen voltage supply. If you get a large voltage drop across the resistor, then the current is going to the socket; if not, your problem probably lies in the supply.

I've ran into many instances where a capacitor would not fail until high voltage was applied.
 
Several years back, a co-worker had one of these transmitters at a station he contracted with. It kept having screen issues and was about to drive us all nuts. Finally found a solder terminal strip that has the solder eyes braded onto a bakelite bar behind the front panel to the right of the top left access door. It was hard to see around the corner but it had broken down and was conducting from one of the terminals to the ground stud. Don’t know if this is the same issue you are having but might be something you want to look for.
Good luck.

w/
 
Amazing. This may be the problem. Same transmitter.

I had a problem in the BIAS supply (a week ago) and after much looking the black burnt marks and the minor explosions with every PLATE ON were the result of a bakelite terminal and 2 lugs (not grounded) that were grounding the bias supply. The terminal strip was secured with 2 screws to the transmitter housing. This provided ground. None of the lugs had ground potential.

Finally saw where the explosions were and replaced a resistor and bias supply was fine.

The only fix was to pull the terminal away from the case and then the explosions stopped.

One might think a terminal strip with no need to ground any of the lugs would be isolated enough to prevent this.

Wouldn't think this would happen twice in the same transmitter??? Was this an issue with the bakelite terminals? Can't believe this happened to someone else. Figured it was a one time fluke.


BTW - Watching SPIKE TV and a chase in Nashville. At least one minute of the huge Blaw Knox profiled next to the car.

What have you heard on the 560 Energy Onix? Up to full power yet or are they on the lower power transmitter.

Thanks for the hint.
 
As I remember, the bakelite was burned and might have been result of plate to screen arc over. I was talking to the folks that have the EO AM 5kw rig yesterday and they say it is working fine now. Seems like the PDM filter was not working before. The high Q phasor was acting like band-pass filter I guess.
w/
 
Several years ago I spent a week working on a 3.5 for a station when their engineer was out of town. After checking everything under the sun we found a bad swapping resistor on the bottom of the socket.

Interesting part of it was - we couldn't find it with the exciter they were using (BE FX-50). We finally found it by hooking up the good ol MX-15, and staarting with the power output on the exciter turned all the way down and slowly increasing drive until "BANG".

The FX50 wouldn't go low enough on power output for us to get the plates on and hold to find the problem. If memory serves right, those were a pain to get to.

Hope it helps.
 
Chief Engineer & I have been taking turns at this one (and we both have been doing this stuff for decades). Let me throw this piece of the puzzle out for comment. Both the original screen transformer and the replacement screen transformer (which are 240V transformers) are/were putting out 900 volts AC with 120V on the primary. That explains two trashed voltmeters & some shorted diodes. The filter caps in this xmtr are rated at 450V & there are two in series. It has a choke input filter. From the 900V cap rating, I'd think that the normal screen voltage would ride in the mid hundreds. Yet, if 240V were applied to this transformer, 1800 volts would be produced. Even with the 0.9 voltage expectation of a choke input filter, that is high enough to do a lot of damage. The 120V was across the 240V and +10 taps, which should have produced the lowest voltage possible. Any thoughts? Makes me wish I still had one of those old TV's with the 5U4 rectifier in it. They made dandy emergency screen supplies...
 
Are you getting the abnormally high secondary voltage without anything connected to the secondary? If so, the only explanation I can think of is shorted primary turns which would change the turns ratio. You could put 120V on the secondary and see what is on the two primary windings.
 
Since I last posted, Chief has determined that the choke is open. The 900V was on the secondary of the transformer with nothing connected...and that was with 1/2 voltage on the primary. The other transformer definitely has a primary to seconday short, but this one shows no shorts. It does however show normal voltage from the CT to one side, but much higher on the other side. That leads one to think that BOTH transformers are bad...what are the odds of that? In any case, Chief has rounded up a xfmr that puts out 300V. That should get the power back up from it's current 15%. Thankfully, this station is in a small market where 15% power does just about as well as 100%. Thanks for your inputs folks...
 
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