I repaired a few Quantar repeater with broken exciters. The fault condition is most of the time the same. If you power the Quantar up, all lights come up green and when you key up the repeater the red PA Fault LED turns on immediately. However I also heard reports of an more intermittent fault whit power changing from high to low. I was told that early Firmware on the exciter can cause the same behavior, so keep this in mind. With a known working exciter the station works with no problems whatsoever.
On the status report screen the station gives you the error messages shown in the picture below, no TX forward Power and bad internal SWR.
So time to dig out the big service manual and open up the bad exciter. It looks like the Quantar have seen some moisture due to humidity, in the region of the front LEDs is some corrosion, all solder joints are looking dodgy:
If you have the open exciter lay in front of you the LEDs on the left side, then you will notice two black chip resistors an above you will see a capacitor and an inductor, right hand you will see the final MMIC labeled with A03. The two resistor are the bias resistors for the MMIC, the inductor is an RF choke so the MMIC output don’t see the bias resistors. Nothing fancy going on here.
This is the part of the circuit that we are interested in, L3106, C3109 and U3100. I also checked R3108 and R3109 they where ok in all units that where on my bench, but please check them just to be 100% sure that they are ok.
I found out that the isolation of the inductor was completely gone and the wire went open circuit. If you in luck than this is the only fault you have to deal with, change the inductor check the cap besides and if necessary change it. But unfortunately I had some exciters with not only a broken inductor, the MMIC let the magic smoke out and went bad. In this case you have to change the MMIC, at least where I life the original Motorola part is absolutely unobtainable, so I designed another 50 Ohm MMIC in without any problems.
All Quantars are back in service and running perfect without any errors.
The discussion on an forum came to the conclusion that the black foam behind the vent holes had a low impedance, this in combination with extreme humidity is very likely the cause of the problem. In my opinion we could remove the foam without any negative impact on the performance. After repairing and especially after changing the MMIC, I highly recommend a realignment of the Quantar, only a aligned Quantar is a happy Quantar. If you are working on the repeater it is also a good idea update the firmware to the latest version. There will be a separate article on how to do this.
The parts needed for the repair are:
- L3106 – 220nH +/- 5% SMD case 1008
- C3109 – 100pF +/- 5% 50V SMD case 0805
- U3100 – MMIC 50 Ohm 12dB Motorola part nummer: 4882347W01 (MWA0311T1)
I could not get hands on a original MMIC so i decided to build in a MSA1105 that I had. So I hope this will help somebody, if you have any questions feel free to leave a comment or contact us.