[ICOM] IC-751A meter light modulation

[OZ8AE Joergen]

Dr Walt - W2OKF

Regarding Your problem with Your sync modulation of the light in Your IC-751A.

I have had the same problem with my IC-740 from 1983.

The problem at that time was the 2 fuse holders - inline - in the DC cable connetting the PSU with the transceiver.

When I opened the 2 fuse holders I found one of the 20 amp fuses was green in both ends, giving a small resistance and when drawing up to 15-20 amp for full output of my transceiver then the light started sync with my keying signals - as I am operating CW.

The fuse acted as a small resistor!

I cleaned the fuse and the fuse holder and then I have had absolutely no problem since.


So my suggesting is that You see if You also have a kind of in-line fuse holder in Your set up.
As You did - I messured 13,8 volt DC on the PSU and that voltage only dropped a fraction of a needle when listening and full power.

The next suggestion is with the PS30 as I had that experience 2 days ago - after the PS30 had done a very good job over the last 20 Years.

A friend of mine had the idea that his PS30 was giving overvoltage to his IC-7400.
He just switched the PS30 on and made his messurements. He had between 15 and 16 volt DC.
I took the manual over the PS30 and read the instruction.
They write that with NO load at all - the DC output can go as high as 16 volt DC.
My friend had adjusted to potentiometer but could not detect any change of output voltage.

Therefore I connected a transceiver to the PS30 and made the messurements over + and - on the upper right corner of the board. I had 11,6 volt with the transceiver switched on in RX mode.
As soon as I started to transmit with as low power as 25 watts, the 11,6 volts dropped down to 5 volt DC and the transceiver switched off.

Next step was to switch on the transceiver and then adjust the output voltage to 13,8 volt DC.
I don't know how much I adjusted but only a little.
After that the transceiver was OK in transmit mode with 100 watts output.
Reconnected to an old multimeter and the needle only moved down the wide of the needle when transmitting with full power.

And since that time the transmitter has been working without problem.

I think You are facing the first suggestion I have written.
Try it.

Have a nice weekend

   73 from Virum - Denmark
    OZ8AE Joergen



----- Original meddelelse -----

  Fra: szachara at gulftel.com
  Til: icom at mailman.qth.net
  Dato: Tor, 10. mar 2011 03:52
  Emne: [ICOM] IC-751A Meter Light Modulation

  Good evening... 

  I have an ICOM IC-751A that has started to have a strange symptom. When I am using SSB the meter light modulates in sync with the voice input. I tried it with both the internal PS-35 and an external PS-30 and the result is the same. The PS-30 voltage indication does not vary...Can anyone offer a suggestion as to what might be happening. The transceiver did not exhibit this behavior in the past. 

  Regards,

  Walt, W2OKF

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