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Author Topic: Icom Ic-740 Preamp Switch Noise  (Read 166 times)

AJ4UY

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Icom Ic-740 Preamp Switch Noise
« on: January 12, 2023, 10:13:50 PM »

ICOM model IC-740 Transceiver.
Pre-amp Switch May Cause Audio Distortion.

I was expecting this fault to have been posted, since this is approximately a 25 -30 year old transceiver.
I have not located a similar post with the searches I have performed.

When the Pre-amp switch is selected ON, the Pre-amp function does operate - however, if you wiggle the pushbutton switch
it will create a small amount of Audio distortion. Easily noticed operating and receiving on any band in SSB mode.
The Pre-amp switch only closes to 0.4 to 0.5 ohms. I suspect an Icom or Switch manufacturer defect within the Pre-amp
Pushbutton switch assembly.

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AJ4UY

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Re: Icom Ic-740 Preamp Switch Noise
« Reply #1 on: January 12, 2023, 10:26:38 PM »

Follow up.

In this investigation, I happened to purchase a Icom IC-740 cable bundle ( OEM ), from a seller parting out a old
unit.
This is how the discussion continued . . .
Quote -
" Did you know that the switches you sold to me have a manufacturer defect within their composition ?
I was slightly suspecting that this may be the case. This being the first Icom IC-740 I have seen.
I expect their are several with this defect. However, I wanted a fully operable switch and was
willing to pay your bundled set price, in order to save me the time in matching up a after market
switch, with reasonable and acceptable physical connection characteristics which is easily attachable
to the unit. The sale of the PREAMP and +0.5 switches within your bundle, confirm this suspect.
Both of these switches when in the ON position, close their contacts to 0.4 to 0.5 ohms, never to an
expected 0.0 ohm ( true short ) closure.
I did measure your switches using a Simpson 260 VOM. " End Quote.
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VR2AX

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Re: Icom Ic-740 Preamp Switch Noise
« Reply #2 on: January 13, 2023, 06:19:45 AM »

I have a 751A (actually the AF model, purchased 'new' or NOS in 2003). I noticed when taking it out of storage a couple of years ago that the preamp switch seemed to have developed an issue which was not there when new. This is a different switch from the 740 (had one of those used as well many years ago). The 751 series has a three position toggle switch within the top panel assembly: Preamp on - off - Attenuator. Now the switching function is less 'clean', changing position produces a crackle or static like noise. Fast switching a few times usually clears this. Once settled the noise or distortion disappears. I've assumed some corrosion or other materials change has occurred within the switch, although I've not done any measurements, I plan a thorough clean up and realignment once we complete purchase of a new property, with space for a workshop.

I don't recall if the 740 I had displayed any problem. I vaguely recall the 740 had a reputation of a problem with the band switch becoming intermittent, mine did not seem to be bad in that respect. Have you compared the closed position R value for other similar toggle switches in the 740?
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AJ4UY

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Re: Icom Ic-740 Preamp Switch Noise
« Reply #3 on: January 13, 2023, 06:55:34 AM »

ICOM IC-740
PREAMP Circuit.
You can review this circuit on page 111 of the IC-740 Service Manual.
The function from my understanding is that : A) Preamp switch ON B) Switching BJT transistor Q9 switches from Cutoff
mode to full Saturation mode C) RLY1 picks, placing the Preamp inline D) Pre Amp composed of Q6,Q7 FETS increase the units
Sensitivity from 0.3 uV to 0.15 uV per specifications.
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N8YX

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Re: Icom Ic-740 Preamp Switch Noise
« Reply #4 on: January 13, 2023, 07:12:39 AM »

I have a 751A (actually the AF model, purchased 'new' or NOS in 2003). I noticed when taking it out of storage a couple of years ago that the preamp switch seemed to have developed an issue which was not there when new. This is a different switch from the 740 (had one of those used as well many years ago). The 751 series has a three position toggle switch within the top panel assembly: Preamp on - off - Attenuator. Now the switching function is less 'clean', changing position produces a crackle or static like noise. Fast switching a few times usually clears this. Once settled the noise or distortion disappears. I've assumed some corrosion or other materials change has occurred within the switch, although I've not done any measurements, I plan a thorough clean up and realignment once we complete purchase of a new property, with space for a workshop.
Likewise with two 751As here, and 4x R-71A, 2x R-70...a 745...oxidation is a thing, and DeOxit is the cure.
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AJ4UY

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Re: Icom Ic-740 Preamp Switch Noise
« Reply #5 on: January 13, 2023, 07:37:55 AM »

I can understand switch oxidation. We can be aware of the Relay's, as well.
On the unit mentioned - Initially, I did open the Relay enclosure and adjust the RLY1 contacts slightly /cleaned the
relay contacts in order to achieve a clean Preamp function and clear Audio.

 . . . the IC-740 Preamp pushbutton switch on the front panel was cleaned with CRC QD contact cleaner.
The Simpson 260 VOM still reads a 0.4 to 0.5 ohm open, upon switch contact closure - following the CRC QD contact
cleaning.
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AA7IS

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Re: Icom Ic-740 Preamp Switch Noise
« Reply #6 on: January 13, 2023, 11:55:15 AM »

    When you push the Preamp button does RL-1 switch on? one side will pull to ground when the button is pushed.
    When the preamp button is on it connects Q9 emitter to ground, since the base is sitting at .75 volts, it's turned on and the relay is energized.
    The relay contacts are the most common mode of failure.
    You may find it's switching just fine but actually attenuating somewhat, that's the contacts.
    P.S. that switch contact no matter how noisy will never be heard in the receive audio the switch turns on a relay,
    the relay may have "noisy" contacts effecting the receive but not the switch!

    IT does not matter how crappy the switch contacts are as long as the switch is capable of pulling the emitter of Q9 to ground, if it can't do that then it needs replacing.DE AA7IS

     

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VR2AX

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Re: Icom Ic-740 Preamp Switch Noise
« Reply #7 on: January 14, 2023, 10:08:25 AM »

Good points. To the original poster:

After wiggle of the PRE switch, do you have (clean audio) + (preamp function)?

If yes, what is the next step that produces noise/distortion?
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N1RND

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Re: Icom Ic-740 Preamp Switch Noise
« Reply #8 on: January 17, 2023, 03:36:43 AM »

Don't know if this is any help, but the same problem occurs on the 765 and it is a relay in the tuner unit.
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W1VT

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Re: Icom Ic-740 Preamp Switch Noise
« Reply #9 on: January 17, 2023, 07:15:32 AM »

This is a known issue with relay contacts.  There are two common solutions.
One is to use gold plated contacts to eliminate oxidation.  This works as long as you don't hot switch the contacts with enough RF to burn off the gold.

The other is to run enough current though the relays to burn through the oxidation, typically around 10mA.
This is called wet switching.

https://www.edn.com/wet-and-dry-switching/

A relay designed for dry switching may fail if connected to an antenna that picks up too much RF from a nearby transmitter.
Another thing to consider when answering the question of how closely antennas can be located when connected to radios operating simultaneously.
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AA7IS

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Re: Icom Ic-740 Preamp Switch Noise
« Reply #10 on: January 27, 2023, 06:50:34 AM »

I wonder if this radio got fixed (relay replaced)
Or did it become a pile of parts while focusing on a innocent toggle switch?
« Last Edit: January 27, 2023, 06:52:47 AM by AA7IS »
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AA7IS

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Re: Icom Ic-740 Preamp Switch Noise
« Reply #11 on: January 27, 2023, 08:28:31 AM »

Hopefully AJ4UY will come to understand that the preamp switch is NOT IN THE SIGNAL PATH!!!!
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VR2AX

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Re: Icom Ic-740 Preamp Switch Noise
« Reply #12 on: February 08, 2023, 12:26:37 PM »

This deserves to be written up to avoid misunderstanding or information.
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