Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
Get involved. We want your input!
Apply for Membership and join the conversations about everything related to broadcasting.
After we receive your registration, a moderator will review it. After your registration is approved, you will be permitted to post. If you use a disposable or false email address, your registration will be rejected.
Anyone have any experience with the Harris 2.5 k FM? As simple as this box is, I'm having a hell of a time repairing the raise/lower circuit for the output power.
Would this be a 2.5 K, Circa 1980? If so, the screen adjust control is a variac,
powered by a motor that runs up or down based on the position of the switch. If someone has replaced the variac, it is very possible that the connections thereto are incorrect, hence, it will do strange things when you raise/lower. Check for proper connections and a possible open of this unit. This is especially true if you cannot make power. Check the multimeter and verify the screen voltages are in range. Thanks...JBI
I had the same problem and found that the variac-motor was intermittent / sticky. Harris of course brutally overcharged me for the part, and I knew hunting for the motor on my own would turn something simple into a multi-day project.
This would indeed be a pre-1980 unit...but it does have the wire-wound resister as a power-stat. Could be that I've overlooked something, but I looked everywhere for a varister and couldn't find it. The trouble I'm having is to tune up the transmitter to run proper TPO, I have to detune the loading which results in an arc across the loading capacitor. Not pretty, and they do charge nicely for replacements. When I get the transmitter tuned up properly, it's punching close to 3k. Waaaay above TPO.
I am about to replace the bias supply caps and loading caps. They aren't that expensive and probably could stand to be replaced anyway.
Sounds like too much screen voltage,,,could be shorted variable or maybe a shorted series resistor...again...check at the screen direct and see if it too hi....it almost must be...
I agree that the screen is too high. However, can't control it with the raise/lower function of the transmitter which is making it run wide open. That's one of the reasons I'm replacing the caps and resistors in it. I suspect that over time they have degraded and have allowed the screen to jump. Might have been ok for years running wide open, but has gotten so far out of tolerence that it just can't handle the additional current.
Harris is sending me the parts so we'll see. Right now, I'm on a full-power backup, and not having any trouble with it, fortunately. I had just never heard of a transmitter that couldn't be throttled back from full to almost no power output before. And I'm not sure that this one won't...but according to Harris, it won't go the full range.
If your TPO is too high when properly tuned, tap the plate transformer down. I get 1.12KW with 2600V @ .66A. Another one gets 1.2KW with 3700V @ .445A and yet another one gets 2.1KW with 4300V @ .65A. Tapping the transformer can get you into the sweet spot on the 2.5K in just a few minutes. Another option is to Bias it as hard as you can & reduce the drive...but your efficiency will be better by reducing the plate voltage. As with all transmitters, less plate volts equals lower probability of an arc. Does Brent Lee work for you guys? If so, tell him Bob On The Job said hey...
variac burned and open. replaced. 300 bucks. 2 weeks at low power, SLOW delivery...no backup. bulletin has a fuse in the output because of problems. not originally on schematic or in transmitter but some have them some don't. This one did but it didn't save the variac. WHY?
screen power supply transformer also bad. 600 bucks, two weeks later, none in stock. Station still at lowpower. No screen to tube.
because this seemed unusual I traced 240 from the incoming ac through the screen breaker through the variac to the screen power supply transformer. 240 in, nothing out. Harris confirmed the breaker should blow if shorted, never happened.
Can't believe they don't keep the part in stock.
Can't understand why the station slowly lost power, why the fuse on the output of the variac blew and didn't save the variac and the transformer.
hmmm...now that makes sense. Ever since I took over 8 months ago, the transmitter has been running 4800v @ .650. My TPO is 2300w. I'll retap the transmitter...could be what's causing the whole issue.
mr burns is correct.back in my mw1a(muddywater 1) days, the variac would stick and not make full power.Harris had a mod, basically add a little grease to the contacts to prevent sticking.of course it was always on the worst weather days, it would act up(a engineers dream).the variac caused problems with less than 60days in service which really ticked me off...but overall it was a problem free..Never had to replace a module.
I thought it was a bit high too...but since it has apparently ran that way for years, who was I to question it. I suppose it all changed when I actually tuned it up to specs. Probably why I started having the problems. I got a nice package from Harris today, containing replacment parts. So I'll retap the plate transformer, replace these little parts, and hopefully it'll be grand!
Retapping the plate transformer dropped my plate voltage down to 4300 volts...much better as it's not full scale now. 4300volts x .760amps is giving me right at 2.3kw. Not too bad, although the current is a little high I think. But, according to Harris, it should be golden. Haven't had an arc yet?!?! Been on the air at full power since 2pm this afternoon.
One of my 2.5's that ran 1.2KW TPO at the time would occasionally arc running 4100V with it unloaded to keep the power correct. Dropping the voltage to 3700 & loading it heavier totally eliminated the arcing. Same deal with a CCA 20KW that was running at 14KW. The plate blocker would arc & short about once a year. I dropped the plate voltage from 8000 to 7100 in the early 1980's...that same plate blocker now has almost 25 years on it. I'm a big believer in using the least amount of plate voltage consistent with being able to make 110% power. Good Luck
When you lower the plate voltage, the current must be raised to maintain the correct output power. You mentioned that the output power was capable of going well beyond the licensed power. In this case, tap the transformer so that with everything running full throttle, you get 110% of normal power, This gives you a little breathing room when everyone turns their AC on this summer & drags down the line voltage. Plus, it will put the PA in it's sweet spot.
And yes, you can just reduce the drive to reduce the power. But minor variances in the drive will cause corresponding variances in power output. Tapping the transformer to land the PA in the sweet spot will result in the most effecient and stable output power based on my 40 years of experience.
If you haven't already done so, assuming that your transmitter has the small IPA tube, if you have a 30 watt or higher exciter, you can drive the PA directly & eliminate that stage. One less thing to go wrong and one less tube to ever replace again. If you do that, the PA Grid control will have a direct effect on the exciter's SWR, so tune the PA Grid for minimum reflected power to the exciter, not for maximum PA Grid Current.
Cool...thanks! I have considered driving the PA directly from the exciter. Just haven't done it yet. I do appreciate all the help though. It's running like a track star now.
Just some reference information for you in case it might be of help. I have a Harris FM-2.5H3, which is the predecessor to the 2.5K. They are very similar in design, but I don't have personal experience or knowledge on the 2.5K or know the differences between the two series, other than appearance, so take what you can.
I'm fairly sure the tube complement is the same, with the IPA being a 4X150A and the PA being a 5CX1500A/B. At 2.25kW TPO, PA Ep is about 4,250V and PA Ip is about 0.74A. This yields an efficiency of about 71.5%. With the values you most recently posted, your efficiency is about 70.4%, which isn't far off. Depending on your operating frequency, you could be at about maximum efficiency.
In regards to an earlier post about reducing drive, remember that underdrive kills tubes (and transistors, for that matter). Underdrive usually yields poor efficiency and poor performance, and an amplifier that is no longer saturated can behave very peculiarly - it forces the amplifier to operate in curves that it wasn't designed to operate in. The 2.5H3 operates with about 50W output from the IPA to produce 2 - 2.5kW TPO. Get good drive to the PA, and use screen and plate voltage to control the power output, as you're already doing.
In regards to the screen voltage control, the 2.5H3 doesn't utilize a variac. Instead, a 75W 5kohm rheostat is driven by a small 120V 2RPM motor to control the screen voltage. You might call it a discrete variac on a diet. This circuit is fairly simple and should be easily troubleshooted and repaired at minimal cost. Your 2.5K may use the same type of control. I don't know if you have radomes or deicers, or problems with antenna icing, for that matter. If you do, it will behoove you to have this raise/lower circuit up and running, since these transmitters came before the days of automatic VSWR foldback. By the way, the 2.5H3 runs with a screen voltage of about 450V.
Our 2.5H3 has served us very well over the years, but was relegated to backup service a few years ago when we installed a Z3.5CD in its place. It still runs into the dummy load quite often, and on-air on occasion.
Status
This thread has been closed due to inactivity. You can create a new thread to discuss this topic.
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
