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Old 10th Feb 2017, 10:41 pm   #1
Karl_M0KRL
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Default Lowe HF-125 resistor identification

Helo to all, just posted this on the EEVblog but with hindsight I thought on here would be a better platform.
I am just in the process of checking my receiver prior to sale and I found a burnt resistor, R6 top right on the keypad interface board. I have looked through the manual and googled for more info but have not been able to identify the correct value for a replacement. If there is any one out there with the same or similar receiver that could just pop the lid and check for me it would be greatly appreciated. The receiver itself appears to be working 100% and the measured value accross the resistor in circuit is around 73ohm. Thanks in advance.. Karl M0KRL
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Old 11th Feb 2017, 12:45 am   #2
Terry_VK5TM
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Default Re: Lowe HF-125 resistor identification

http://radiopics.com/By%20Make/Lowe/Lowe_HF-125.htm

About halfway down the page on the above link is a pic of the board (click to enlarge), it looks to be 560 Ohms.

Terry
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Old 11th Feb 2017, 1:36 am   #3
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Default Re: Lowe HF-125 resistor identification

Yes, looks like 560R
BUT diagonally down right from that resistor is some very dodgy looking soldering! Are they all like that?
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Old 11th Feb 2017, 1:37 am   #4
Karl_M0KRL
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Default Re: Lowe HF-125 resistor identification

Thanks for that, I agree on the 560 Ohm. Very good find.The soldering on mine is nothing like that one thankfully. Karl
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Old 12th Feb 2017, 2:31 am   #5
Terry_VK5TM
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Default Re: Lowe HF-125 resistor identification

I would be concerned at why it burnt up before replacing it.

Terry
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Old 12th Feb 2017, 11:23 am   #6
SiriusHardware
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Default Re: Lowe HF-125 resistor identification

On Lowe receivers the external keypad socket is 2-pole and provides resistor limited power out to the keypad on the same wire as the data from the keypad comes in.

The keypad 'modulates' the voltage on the data / power line by switching another resistance from the data / power line down to 0V, thereby switching the data / supply line voltage between two levels.

A problem on the wiring to the external keypad (ie, short-circuit) might have damaged the receiver's internal limiting resistor.
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Old 13th Feb 2017, 1:20 am   #7
Karl_M0KRL
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Default Re: Lowe HF-125 resistor identification

On further digging it appears that simply plugging in the keyboard plug while the unit is powered can very briefly short the supply line to the keyboard module. The resistor is the weekest link in the chain (thankfully) easy fix and lesson learned. Thankyou to everyone.. Karl
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Old 13th Feb 2017, 11:25 am   #8
ukcol
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Default Re: Lowe HF-125 resistor identification

I agree that R6 looks like 560 ohms but it could be a 56 ohm.

Look at the 3 resistors in the picture below, the one on the right is a cut and paste from another part of the board.

The third ring on R6 doesn't look exactly the same as the brown rings or the black rings on the other two resistors.

I'm just saying there is an outside chance that R6 is a 56 ohms. You may get a clue from measuring the burnt resistor provided it is not open circuit.
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Old 13th Feb 2017, 1:25 pm   #9
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Default Re: Lowe HF-125 resistor identification

What is the maximum supply voltage present within the receiver? Keyboard control sounds like a 5V sort of thing- even a direct short with 560 ohms wouldn't come near to charring it. Even in a 12V situation, 560 ohms would barely dissipate quarter of a watt (oh, alright, it might be nearer 14V, but it would still be hardly stressed!). 56 ohms sounds more likely, really. 73 ohms sounds believable for a toasted "56" ohm film resistor.
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Old 13th Feb 2017, 1:32 pm   #10
ukcol
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Default Re: Lowe HF-125 resistor identification

I had missed that the burnt resistor had been measured at 73 ohms so I agree that it was probably a 56 ohm.
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Old 13th Feb 2017, 2:11 pm   #11
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Default Re: Lowe HF-125 resistor identification

It would be worth fitting Vero pins to the component holes and mounting the replacement resistor on the pins.

Not only will future replacement be easier, the PCB will be protected from damage.

Al
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Old 13th Feb 2017, 7:26 pm   #12
SiriusHardware
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Default Re: Lowe HF-125 resistor identification

Quote:
Originally Posted by turretslug View Post
What is the maximum supply voltage present within the receiver?
Like others, I didn't pick up on the fact that the resistor had been measured at 73R. It would be very rare for a resistor like that to fail 'low value', half burnt ones always go higher than their original value or completely open circuit. The external keypad for my Lowe HF150 derives its supply (5V) by regulating down from the normal voltage on the supply / data line from the radio, so the voltage is at least higher than 5V.

This project for the HF-225 nicely illustrates the hardware method employed.

http://www.veg.nildram.co.uk/hf225.htm
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Old 13th Feb 2017, 8:28 pm   #13
Karl_M0KRL
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Default Re: Lowe HF-125 resistor identification

The voltage at the resistor is just over 10v and the keypad unit has an 8v regulator on its supply. I have replaced the resistor with a 56ohm and all is working perfect.. Karl
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