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Author Topic: Possible design error in FT-990 fan control system?  (Read 1338 times)
GW4FRX
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Posts: 5




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« on: June 07, 2013, 01:36:38 PM »

I mentioned on here recently that the fan motor in my FT-990 failed and was repaired by changing a 1uF 50V capacitor on the motor control PCB. In the course of reassembly and testing it was noted that the fan motor did not always start and run when expected. Investigation suggested an anomaly in the original design. 

The data sheet for the Panasonic FCB34AC12 fan fitted to the FT-990 indicates that the motor voltage range is 10.2-13.8V and that its specified maximum running current is 375mA. In the FT-990 the fan motor is operated by one of two switches, FAN1 in the PA compartment and FAN2 in the PSU. Either switch extends an earth to one side of the fan motor when operated. The other side of the motor is permanently wired to the 13.8V rail via an 18ohm 2W resistor (R9015 on the REG board).

The measured running current for the motor in my example was a little over 300mA and  the starting current was about 500mA. At this latter value about 9V is dropped across the resistor, leaving about 4.8V available for starting. When running, about 5.4V is dropped across the resistor, leaving about 8.4V across the fan motor. Both values are well outside the specified range. It was established during testing with an external 13.8V supply and a selection of low-value resistors that with 18ohms in circuit the motor would only start on one occasion in perhaps four or five. 

It is not clear why the resistor was included unless it was with a view to reducing the operating voltage and hence the RPM and noise level. Clearly it places the motor outside the manufacturers’ specified voltage range and compromises its ability of the motor to start reliably. The stall current of the fan motor is a little over 60mA and presents no threat to the integrity of the 13.8V rail. With a view to reducing the motor voltage to a value slightly below the specified maximum, the resistor was replaced with a 1N5408 diode. This drops about 0.8V in operation and hence leaves about 13V across the motor when running, which has also resulted in a useful improvement in the motor RPM and hence the airflow.

I hope this helps anyone else who might be wondering why the fan in their FT-990 does not always  start.

73 John
GW4FRX

   

 
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KA5IWO
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Posts: 131




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« Reply #1 on: June 07, 2013, 03:22:26 PM »

As old as the ft-990 is now, it may be the fan is just wearing out and has trouble starting up.
Could be dirty bearings, etc...
I had at one time a ft-990 and the ft-1000d and never had an issue with mine, but like any electronic
device with age, things start to break down.
The Ft-1000d, FT-990 and the FT-890 were the best radios on the market at that time before
the DSP radios came around. They do not make them of this build quality anymore.
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KE3WD
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Posts: 5694




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« Reply #2 on: June 07, 2013, 04:52:16 PM »

It is highly unlikely that the engineers who designed the radio did not know how to calculate a simple series voltage drop. 

You rebuilt the fan motor as to that bad cap and you cleaned and relubed the bearings, BUT - there may be shorts in one of the coils somewhere.  A coil "fighting" the other coil(s) can yield readings such as you report. 


73
« Last Edit: June 07, 2013, 04:54:36 PM by KE3WD » Logged
GW4FRX
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Posts: 5




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« Reply #3 on: June 07, 2013, 10:37:55 PM »

>It is highly unlikely that the engineers who designed the radio did not know how to calculate a simple series voltage drop.<

You would think so. But even the best engineers can get things wrong.

>You rebuilt the fan motor as to that bad cap and you cleaned and relubed the bearings, BUT - there may be shorts in one of the coils somewhere.  A coil "fighting" the other coil(s) can yield readings such as you report.<

The manufacturers' data for running current and motor operating voltage is as I quoted. My example is actually well within the specification for running current.

73 John
GW4FRX 


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WX7G
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« Reply #4 on: June 08, 2013, 04:43:16 AM »

This is a case of the original design engineer violating a component spec. These circuits make it into production and eventually a certain lot of components no longer works correctly in the circuit. I have spend many lucrative years of my career redesigning such circuits.
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KE3WD
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Posts: 5694




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« Reply #5 on: June 08, 2013, 04:43:37 AM »

>It is highly unlikely that the engineers who designed the radio did not know how to calculate a simple series voltage drop.<

You would think so. But even the best engineers can get things wrong.

Agreed, but please consider that you are arriving at a conclusion based on one single data point of evidence.  Unless and until we have confirmed observation by a goodly amount of other owners of the same rig reporting the same exact problem, you are basing that conclusion on a statistically insignificant sample.  

You are also basing it on a fan that you have already explained to us that you opened up and replaced at least one component, cleaned and lubed bearings, possibly other things could have happened in disassembly/reassembly that can often go unnoticed, even among the best and most experienced of techs, it happens.  

Quote
The manufacturers' data for running current and motor operating voltage is as I quoted. My example is actually well within the specification for running current.

73 John
GW4FRX 




This is another assumption on your part, in the sense that just because the motor circuit is drawing LESS than the published spec, it must be fine.  

Suggest looking at inrush current draw at startup, but it is not likely to be able to find that in the rig's Service Manual specs.  

I'd likely try comparing inrush to another known good or new fan of same basic type, or better yet, two or three samples.  At the same time I'd also be timing the startup cycle comparisons in an effort to isolate a mean spinup time for such.  

I mentioned earlier that perhaps one of the coils suffered internal shorts.  It also could be a situation where one of the coils is OPEN or suffering from a cold connection, dry joint, etc.  

In any case, I wouldn't blame the design as being at fault simply due to the fact that all observations to this point have been made with only the one user-modified fan unit in place.  Given the total number of units same model and type out there in the world at this point, Occam points the finger at the fan first.  


73
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KE3WD
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Posts: 5694




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« Reply #6 on: June 08, 2013, 04:46:47 AM »

This is a case of the original design engineer violating a component spec. These circuits make it into production and eventually a certain lot of components no longer works correctly in the circuit. I have spend many lucrative years of my career redesigning such circuits.

So have I. 

My only problem with this particular case is that the fan motor was opened and at least one component changed by the owner. 

The reported results are all based upon that one fan motor example and we have no way of knowing what the situation is when a new or at least a fan known to be properly operating in the first place is in that circuit. 

One datapoint based on an altered component...


73
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TANAKASAN
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« Reply #7 on: June 08, 2013, 06:58:04 AM »

I suspect that the resistor is there to cut down the running speed of the fan to reduce noise. If you're feeling brave then remove the resistor and replace it with a 3.3V 5W Zener diode in series, that will give you a nominal 10.5V for your fan from a 13.8V supply.

Tanakasan
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W9IQ
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Posts: 102




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« Reply #8 on: June 09, 2013, 03:50:22 AM »

I think your original hunch that the resistor simply reduces the fan speed, fan volume, and noise is correct. The engineer most likely sized the airflow for the job.

The fan bearings surfaces do wear over time and the moving portions pick up contamination. This often requires more current to get the fan started as it ages. The limiting resistor naturally reduces the available starting current. This may be what you are experiencing.

If you can tolerate more fan noise, you certainly could reduce the series resistor or put in a variable voltage regulator in the circuit to find a balance between acceptable noise and proper starting. Be aware though, that you may simply be delaying the inevitable of having to replace the fan.

A faster running fan also drags more contaminates into the rig or its filters so maintenance will be required at a more frequent interval.

- Glenn DJ0IQ and W9IQ
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WX7G
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Posts: 5920




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« Reply #9 on: June 09, 2013, 05:45:37 AM »

For reliable starting of fans in variable speed circuits electronic control (not a resistor) is used and the fan is always started at maximum voltage to ensure it does not stall. So yes, the fan circuit in the FT-990 is a poor one.
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NN3W
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Posts: 147




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« Reply #10 on: June 09, 2013, 05:39:16 PM »

I had a FT990 fan fail on me.  Bought a replacement from (shudder) RadioMart.  Pretty easy install and replace.  Radio works fine.
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