[ICOM] IC-746Pro Audio/Receiver Cut Out Issue

[Bert Almemo]

Hi Mark,

When the audio signal drops in the speaker, internal or external, does the
signal strength on the S-meter change too? If so, the T/R relay, I don't
know the RL number, is your best bet. I don't think you have a "weak" 14V.

This is quite a common problem on transceivers with T/R relays. 

73 Bert, VE3NR

 

-----Original Message-----
From: Icom [mailto:icom-bounces at mailman.qth.net] On Behalf Of Mark K3MSB
Sent: Wednesday, November 16, 2016 12:37 PM
To: ICOM Reflector
Subject: Re: [ICOM] IC-746Pro Audio/Receiver Cut Out Issue

Good Afternoon All

Thank you for your replies.

A resistance test on the SEND output show no change when the Transmit
button is enabled.  This indicates RL781 or related components is toast.
This occurred several weeks before the current audio problem manifested
itself.

The External Speaker (and Internal Speaker) are controlled by RL740,  which
may be marginal.

The only common item between RL781 and RL740 is the 14V line,  and I
haven't had the time to trace that through the service manual.

Even if the 14V line is not strong, that would not account for the complete
failure of RL781.   However, a weak capacitor on the 14V line would account
for what we're hearing on the audio.   It would also account for
intermittent operation of RL781 if it were alive.

A problem in my comments is that the schematics show both RL781 and RL 740
in the receive configuration;  If so,  then how does the audio signal
"PAFO" get to the external speaker (or internal speaker) ?     It almost
seems like RL740 needs to be closed in receive and open in transmit,  just
the opposite of RL781.

So, might it be that the radio has two problems - RL781 and a weak 14V line
?

 73 Mark K3MSB
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