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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 25th Sep 2007, 10:39 am   #1
Keith
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Default HRO S-meter

Can anyone throw any light on an oversensitive S-meter on an HRO-M. On a reasonable aerial (G5RV at 20ft) it endstops on most signals that I would consider to be S9 or above. I have checked all the resistor values in the bridge, changing those that were significantly out of tolerance. The meter can be zeroed OK as per the handbook at a sensible postion on the adjustment pot. Putting ~100 ohms across the meter brings it back to somewhere near sensible but I feel that this shouldn't be necessary.

Regards,

Keith
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Old 26th Sep 2007, 10:07 am   #2
MichaelR
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Default Re: HRO S-meter

Hi Keith,

Rather than rely on your "hear" can you use a known signal level from a signal generator appropriately injected into the Ae to check the level on the meter.Some of the HRO-M units, S5 was the half deflection position and they were only marked upto that position.My particualr MX has a straightforward 1ma meter movement. I believe meter types varied especially during the war when they fitted what they could get.

Suggestion is only a start to find out if you really do have a problem

regards
Mike
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Old 28th Sep 2007, 5:32 pm   #3
radiomonkey
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Default Re: HRO S-meter

This may seem a silly idea... but have you got the correct meter in the bridge cct, as if adding 100 ohms sorts the problem, could it be that your meter has the ' wrong' internal resistance for the cct as designed. If so, a rearrangement of the bridge R values should sort the problem

HRO M's were made from about 1938 to after WW2 when the ' 5 ' came out and National were known to use ' odd' components - a early ' MX' that I had had the ' wrong' main tunug cap, left over from previous production no doubt

Hope this is of some assistance

Radiomonkey
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Old 29th Sep 2007, 4:09 pm   #4
Keith
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Default Re: HRO S-meter

Thanks for the replies.

Mike - Eventually I will fire up the sig genny to produce the correct 50uV signal but I'm sure that the present arrangement is way too sensitive.

RadioMonkey - It did occur to me me that the wrong meter might be fitted. It looks the part (marked up to S9 at about half FSD) but one possible clue is that it appears to be somewhat under-damped. I shall have to investigate further.

Regards,

Keith
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Old 29th Sep 2007, 10:55 pm   #5
MichaelR
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Default Re: HRO S-meter

Hi Keith


Depending upon the model that you have got the S meter may vary, certainly the early units particularlly with black chassis had an S meter that only showed 5 S units upto mid scale. They were white faced with the national emblem . Later meters had a light Tan coloured background and as I said befoer alot had just 1 ma movements fitted due to shortages during the war.

One further thing to be aware of is that on CW signals the BFO should be turned off when making signal measurments at least according to my manual.

regards
Mike
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