EEVblog Electronics Community Forum
Electronics => RF, Microwave, Ham Radio => Topic started by: WB8PFZ on October 16, 2024, 11:56:38 am
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I have a strange issue with a 736 radio. It receives fine on the SSB and CW modes. On FM it receives but the audio output is very week. I have replaced the receiver board with 2 known good ones and the problem remains. Looking at the block diagram I don't see any other place where the audio goes that would cause this issue. Any light would help. Thanks Mike
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Never looked at one before, but I glanced at the block diagram online. It shows that 144 and 440 have two separate RX input paths. Is the FM audio low on both bands?
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Is the AF LPF UNIT F2892101 separate from the RX UNIT F2887101?
SQ SET on pin2 of J05 seems to only connect to the FM side of things on the RX UNIT.
If there is some voltage on this, it might shunt some of the FM audio to ground.
Signal SQ1 on pin1 of J06 seems to indicate FM squelch state.
On connector J12's pin 2 " FM-R", is the voltage the same on FM as it is on the "USB-R", "LSB-R" and "CW-R", when those are chosen?
Those seem to be the unique to FM signals from outside the F2887101 RX UNIT.
SQ1 from J06 seems to be the FM squelch level setting, as it goes to a 10k "B tapper" pot the F2890104 VR A UNIT.
If this is somehow weird, it might be partly squelching the FM audio.
J05 and J12 unsurprisingly seems to go to the F2891000 CNTL UNIT.
On the F2891000 CNTL UNIT.
The FM-R is pulled low by NPN transistor Q40, controlled by MCU pin PB3.
It seems to be a BA1A4P, which is some "digital transistor" from NEC, so it has built in bias resistors.
If this one is weak, the supplied power to the FM side of things might not be as high as it should?
J05's SQ SET seems to go to pin PB5 of the MCU.
Those seem to be the FM controlling things I can find in it.
Assuming the wiring harness and it's connectors are all ok.
For those following along, the schematics can be found at the end of the user manual, which can be found at RigPix:
https://www.rigpix.com/yaesu/ft736r.htm (https://www.rigpix.com/yaesu/ft736r.htm)
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Xrunner, yes it is low on all bands.
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Is the AF LPF UNIT F2892101 separate from the RX UNIT F2887101? It is a separate board mounted on the main board.
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Is the AF LPF UNIT F2892101 separate from the RX UNIT F2887101? It is a separate board mounted on the main board.
It seems to have different inputs for FM audio and SSB/CW audio, so if it was not changed with the RX UNIT, the fault might lie there?
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All 3 RX units have there own AF LPF board. I checked the 5 contacts on J12 to make sure they go low for each mode, usb, lsb, cw and FM. All are good. Also the S meter shows a good reading when receiving just very low audio.
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Something wrong with the FM squelch circuit?
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If something was wrong with the FM squelch circuit would I get low audio or no audio?
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I think I have a good control board I may swap out.
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If something was wrong with the FM squelch circuit would I get low audio or no audio?
I can imagine either scenario occurring in other radios, however I haven't examined the FT-736 schematic to see whether it's possible in this radio.
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I have never had a radio where the squelch controls the radio volume.
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The FM squelch and SSB squelch circuitry is separate and the FM squelch seems to mute the audio by shunting it to ground with some NPN transistor.
My thinking was that if the squelch pot or something else related was not fully operating it could be turning that transistor partially on and shunting some, but not all of the audio to ground.
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Will do some checking. Thanks for the input.
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Mystery solved. i have 2 radios that had the same issue. Thru some probing with the scope I discovered that I lost my audio going from one board to the control board and back. The radio has an optional tone board that needs to see the audio. The one radio that had the board had it installed so the pins did not line up correctly hence the audio never went back to the correct audio section. The other radio was missing the board and had the jumper on the wrong pins in the connector. Stupid stuff but that is what you get with used equipment.
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Nice to hear that it was something simple like that and not some broken parts.
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Yes cheap fix but somewhat frustrating. :)