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Harris FM 5K (relay K9)

B

Brian Bowers

Early 1980's version of the 5K (blue box). Anyone familiar with the operation of this relay? Please be aware I only spend 1 day a week at this transmitter site (and it's 50 miles away from me), so I may not have all of the info right away, that you might ask. I might have to get back to you on it.

This relay sometimes doesn't have enough power or strength to pull in, and it doesn't seem to matter how much exciter power is increased. I'm wondering if a capacitor is needed across the coil. One is in a schematic, but has no part number listed. Another schematic doesn't have a cap across the coil. Maybe the relay simply needs to be replaced (good luck right). Transmitter runs OK, when relay is engaged, but sometimes we have to give it a little nudge to engage. Any ideas?
 
I wonder if it's the same contactor that's in a Gates One. If so, I went to a local electric motor rebuilder here (Evans Electric in OKC) and was able to get a new coil part of that contractor from them. Harris sent me the wrong coil voltage so I had to hunt for another option, then go to war with them to get credit with no re-stock fee. That was about 8 years ago.
 
I recently did some work on an old Gates 5k and I was working with the K9 relay. Assuming that it had the same function in the newer transmitters, the relay closes when the IPA is creating sufficient drive to drive the PA tube. If the IPA is too low, the K9 relay won't close. In my case, the IPA socket had some damaged bypass caps. I repaired the IPA tube socket (and a few other problems) and I was able to bring it back to life. So based on your info, I would suspect a weak IPA is causing the relay to unlatch. The K9 relay thinks the IPA drive is too low.

Hope that helps.
 
Lazy J, that is exactly what the K9 does in the FM5K. However, as far as I can tell, the K9 is pre-IPA tube, unless I'm reading that wrong. Which to me, tells me it is more a reflection of exciter drive to the IPA section. Not enough drive, and the relay won't pull in. I'm also measuring less than 1 volt (AC) on this relay coil. To me, that seems extremely low. Otherwise, when we help it pull in, all the numbers are fine on the transmitter. Once pulled in, the relay normally stays there, until a momentary AC outage comes along.
 
Brian Bowers said:
Lazy J, that is exactly what the K9 does in the FM5K. However, as far as I can tell, the K9 is pre-IPA tube, unless I'm reading that wrong. Which to me, tells me it is more a reflection of exciter drive to the IPA section. Not enough drive, and the relay won't pull in. I'm also measuring less than 1 volt (AC) on this relay coil. To me, that seems extremely low. Otherwise, when we help it pull in, all the numbers are fine on the transmitter. Once pulled in, the relay normally stays there, until a momentary AC outage comes along.
Click to expand...

I haven't worked on the 5k, but if the drive is less than 1V and the relay is able to latch, I suspect the relay is working. Hopefully, someone else here will know for sure what is the correct voltage for that relay. If the voltage is low, it is trying to tell you something. There could be an IPA input tuning problem.

In a lot of older designs, bias is completely, or partly, derived from the preceding stage. If the bias disappears, the next stage goes into conduction and starts acting like a diode which consumes a lot of power and converts it to heat. Obviously, it's a bad thing if that happens which is why that relay's in there. Have you pulled the inspection cover off of the IPA to look for damage or loose parts? How's the reflected power from exciter to IPA?
 
Running about 7.5 watts from the exciter, with 0.2 reflected. Manual states to tune the IPA for a dip in Cathode (I don't have the manual in front of me, I don't think it was Grid) current. No problem there. Transmitter makes full power with 7.5 watts out of exciter. Tried increasing the exciter power from 7.5 to 10 watts with really no difference in K9 performance. IPA cavity looks fine too - no blown or fried parts that I can see. Currently, when the relay pulls in (sometimes by itself, other times we need to give it a little nudge), the transmitter runs very close to factory specs, with no problems. IMO, the PA and IPA tune the way they should.
 
Troublesome relay. If it were me, I'd adjust the overload relay to drop out 20% above the normal PA current and bypass the thing. There are a lot of transmitters that don't have those relays and function just fine.
 
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