Batboard

My friend has an IFR COM-120B and has been using the unit as a bench service monitor for several years without any problems. Recently, the unit developed a problem as follows: when you depress the GEN Mode switch the unit blows the 2A AC fuse in the power supply. Replacing the fuse the unit starts up going through the initialization and self test routines without any error messages. I can access all the other associated screens, including the Duplexer Mode, and the unit appears to be fine. However, as soon as I press the GEN Mode switch the 2A AC fuse blows. Has anyone had a similar malfunction?

Any insight will be helpful.
Bruno

Check and see if he has the optional amplifier on the gen port. My guess is that there is a fault in that amp, and when switching to the Gen port, the level is above -13dBm, the amp is switched on, and blows the fuse.

Wowbagger - I checked and the optional amplifier and it is not connected. However, I need clarify my previous statement regarding the GEN Mode blowing the fuse - the fuse blew on two previous occasions when I depressed the GEN Mode button, but I have not been able to recreate the same scenario so I cited a premature problem; mea culpa! What really seems to be happening is that the 2A AC fuse is blowing after a 30 to 40 minute warmup period. Fortunately, this PS has a DC connection so I decided to connected a 13VDC power supply to the service monitor to eliminate the AC to DC portion of the power supply. If the DC fuse does not blow and the unit stays powered up then I will assume the problem is in the AC to DC portion of the PS. I will report back the findings within a day or two and let all know how the test made out. Thanks for the assist.

If the unit stays up on DC, then I would check the 300v capacitor across the Vicor power module in the power supply - maybe it has gotten old.

I connected the DC power supply and had the unit running for an hour without any problems. Self Check shows all test passed. I checked the 300V caps with a cap analyzer and also checked ESR - in both cases the caps showed good. Since I have the PS out of the unit I decided to order the DC to DC module (300v to 15v, 750W) which happens to be a Vicor VI-262-EU brick, the AC to DC module is a Powercube UNV300/A (88-230VAC to 300VDC) but that particular brick is virtually unobtanium except via Asia. I also understand that the earlier models used the Vicor VI-AIM AC to DC module which was rated 250W and used the sensing circuit. From what I can see my PS unit is drilled for the Powrcube brick without the sensing circuit so I don't plan on changing anything. I will report back once the DC to DC brick arrives and is installed.

Correction to the previous post: The Vicor VI-262-EU brick is 300V in - 15V out at 250W and the Powercube AC to DC brick is 88-230VAC in - 300V out at 750W.

I've repaired one COM120A and two or three COM120B AC power supplies now that have had the Vicor VI-262-EU modules go bad.

I was lucky enough to find some cheap Vicor rack mount PS cards on eBay that utilized this same module. I bought the cards and pulled the modules, but I understand Vicor will still sell the modules, albeit they are special order and have a substantial lead time. The last time I checked the modules were around $180 each.

Once you swap that module you should be good to go.

Good luck,

Eric
K2CB

I have an entirely different Com-120B generator issue. The Gen output has modulated hum on the signal. When viewing on the spectrum
you see some spurious signals drifting through. Both audio generators are off. I have the PS out and looking at the various voltages with a scope. When viewing the +12EL signal it is clean until the relay pulls in. I isolated this +12EL and booted up. The issue is not the +12EL even though it has lots of ripple. Any thoughts.
Johnny S.

Could you have capacitor plague on the gen board, or the PSU?

Sorry for the delay, life got in the way: Ok, I ramped up this project again, I replaced the Vicor VI-262-EU module - still same results - checked all components, caps (pass ESR test), doides, etc and they all show functional so I focused on the AC to DC module. In this set it is a Powercube UNV300/A and since it is basically difficult to track down I read the Vicor VI-2xx design sheet and opted to install the Vicor VI-AIM-C1 AC/DC module. Note: this is not a direct replacement, but only needs minor modifications so I did the install and eureka the monitor powers up and does not blow the fuses even after a few hours of operation. The power supply also seems to be running cooler than before. But I am not 100% out of the woods yet, the unit powers up nicely, but I can't power off with the front on/off button; I have to reach in the back and power off via the AC switch and then place the AC switch on again to restore applied power. I suspect the front power on/off must be some sort of toggle bistable circuit, but without schematics I might have to live with it the way it is.

Actually, it's just a signal to the power supply - the PSU is supposed to shut down when it sees that signal. Since that shutdown works for both the DC and the AC inputs, I'd guess that you just forgot to connect the line from the front panel to the power supply.

Thanks Wowbagger; I checked a dozen times and I just don't see it - the front panel cables seem in place and correct - This model uses a PCB and resistance push buttons for the front panel and then connects to the interior Audio PCB via a DIP header - no numbers on the header but I did find both contact points for the on/off button and traced to the ribbon connector and all seem to check out OK. I also verified that the 34 position ribbon cable from the motherboard to the front panel has no open traces - all the other functions seem to work and the diagnostics show pass. The molex connection to the power supply is good and I verified the voltages with another COM120B that was available in the shop. Maybe I fat fingered something inside the power supply.

On the Generator hum issue I too suspected a cap issue. I pulled the covers on the Gen module and don't see any electrolytic caps
Some 106 & 206 & 424 SMD caps but ......... I have seen these type caps blow up
No pregnant electrolytic caps that I can see in the PS. I will scope some of the other voltages that go to the Gen and see if that reveals anything.
I will continue to investigate.
Thanks
Johnny

Well, it's worth plugging in a battery (either to the internal connections or to the DC in) and seeing if the off button works for that.

Yes, I connected a 12vdc power source alone and cannot turn off via the front on/off push button. In either case (AC or DC) it powers on as advertised, but have to use the rear AC rocker switch to shut off the unit.

That's odd, because the power switch on the front controls the DC/DC sections that are common between battery, DC, and AC operation. In theory, swapping out the AC/DC brick shouldn't impact that part of the system.

Wowbagger I agree completely - this particular COM120B unit belongs to a small communications service company that I occasionally do work for and volunteered to help out - what I do not know if anyone else had their hands on the unit. Initially the problem in the power supply was blowing fuses after 20 minutes; also the fuses would blow when applying the gen button making me thing the problem was attributed to a fault in that section, but that was my mistake in jumping the gun. The block diagram shows the power on/off signal traveling from the front board thru the mother board to the PS molex connection. I was fortunate that my current shop has a working COM120B and I checked all signals at the PS connection and they both match pin for pin. Just speculating, but I suspect the design may employ some sort of toggle bi stable ckt, perhaps inside the PS, that may be at fault. The shop is thrilled that the unit powers up without blowing fuses and would like to put on the bench ASAP. So I will, reluctantly, return the unit - I just am not comfortable returning a unit that is not 100% even though the shop is satisfied - thank you and all other contributors for their valued experiences.

Wowbagger I am back on the Com120B Generator with the modulated hum. PS volts & waveform look like this is not the issue.
However on the top of the Generator there is a Lock/Unlock LED with no light.
The same is true on the Synthesizer module. I don't know if this is Red/Green or just Green or Red. Anyhow no light.
When viewing the analyzer on the screen in the gen mode there are wandering carriers
around the fundamental. To me this is some thing unlocked.
Thoughts!
Johnny S.

Update: On/off push button problem resolved. Previously, I could use the rubber push button to turn on the unit but I could not turn off the unit via the front panel on/off rubber push button. I took the front panel apart and discovered that the rubber button graphite had resistance in excess of 2k ohms. Don't quote me, but I believe a functional push button resistance should be less than 200 ohms across the surface contact area. To fix the problem I was successful in cleaning and dressing up the contacts with wire glue. End result; resistance across the contacts was measured between 100-200 ohms. The unit now powers on and off with ease.

jstigler wrote:Wowbagger I am back on the Com120B Generator with the modulated hum. PS volts & waveform look like this is not the issue.
However on the top of the Generator there is a Lock/Unlock LED with no light.
The same is true on the Synthesizer module. I don't know if this is Red/Green or just Green or Red. Anyhow no light.
When viewing the analyzer on the screen in the gen mode there are wandering carriers
around the fundamental. To me this is some thing unlocked.
Thoughts!
Johnny S.

You've diagnosed the problem - you have an unlocked LO. IIRC that isn't soft-adjustable - you have to turn a cap on the board to adjust it.

Hi Everyone
I have a COM120B that has developed an issue with the 2nd variable AF generator, I tend to use the GEN 2 for CTCSS and it has become distorted and is noisey, I have removed the 7010-0630-700 card from the digital tray and now it's not variable anymore (of course) but the fixed 1 KHz on GEN2 is clean... has anyone else experienced this or have any ideas ?

Thanks for your help

Bad caps? Check the board and see if you have any leaking parts.

HI Wowbagger

Thanks for the reply... I changed the 10uf cap on the 7010-0630-700 card but that made no difference? would the 47uf and other 10 uf caps on the digital tray be part of the GEN2 circuit ? None of the caps appear leaky

Thanks so much for your help... it's hard with out a circuit.

jstigler wrote:On the Generator hum issue I too suspected a cap issue. I pulled the covers on the Gen module and don't see any electrolytic caps
Some 106 & 206 & 424 SMD caps but ......... I have seen these type caps blow up
No pregnant electrolytic caps that I can see in the PS. I will scope some of the other voltages that go to the Gen and see if that reveals anything.
I will continue to investigate.
Thanks
Johnny

On the gen board are two potentially naughty electrolytic caps - in a small metal shield box near the edge of the board..

they are located in the shield module PLL loop. But beware, they are bipolar electrolytes!

I'm curious if the Vicor VI-262-EU DC-DC converter is a common failure mode for the COM-120B PSU? I have a relatively new (to me) COM-120B that I purchased from eBay. Of course everything worked for about an hour and then the AC went out. The unit powers on fine from the 12VDC input as well as from the internal battery.

I've checked the AC fuses and both have continuity across them. The way the power went out, it seemed like a fuse. On...then immediately off. The power LED on the front doesn't illuminate when AC is applied. It does illuminate when DC is applied to the DC connector on the rear.

Reading through this thread, it sounds like a possibility is the DC-DC converter. This may sound stupid, but I don't have much experience with DC-DC converters. I measure 300VDC (actually around 320VDC) into the DC-DC converter and 0 VDC out. Unless I'm missing something, the DC-DC converter is toast?

I had my DC-DC suddenly blow om my com120c, shut it down at the end of the day and the next morning nothing after some calls to the guy i bought if from and some voltage measurements it was most definitely the converter, i bought a replacement module and a spare, replaced it and it came back to life, sent it to JRY for calibration just recently and its been working great. but i would say if it works on your 12 power i would just replace that module and my bet is like mine it comes back. good luck

yep the Vicor modules are a common failure on the power supplies. Those are the easy failures to fix but the modules are not cheap.

Thanks fellows. I've ordered the part from China for around $70 US. I probably won't see it for a month, so I went ahead and put the COM-120B back together and have been running from DC power. Great piece of test equipment by the way. I'd love to have the SSB filter, but I'm pretty darn happy with this so far. Fingers crossed the RTC never fails. I did order a couple from China, but I don't like the odds that I'll lose my Tracking Generator and Generator Amp options....really the only two I care about.