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Bizarre Marantz 4230 FM Stereo issues

goldswimmerb

goldswimmerb

Failed out of engineering, but look at me now!
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So I've got a Marantz 4230 on the bench at the moment, its got some weird FM issues that I am having trouble tracking down. Basically FM mono works wonderfully, but when it switches to stereo the stereo indicator switches on and off based on the signal strength/signal quality while flashing it is rapidly switching between stereo on and stereo off, trying to trace through the circuit is difficult as the switching causes AC voltage to be induced on the circuit, I thought it may have been the IC so I swapped that out but that made no noticeable change... Any ideas? never seen an FM tuner behave this way.
 
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hofferwood said:
You probably already have seen this, if not, it may help to start.

View attachment 2502332
Click to expand...

So all my voltages before the IC seems to mostly checkout, except for H112, which has already been replaced... The IC is new too. I'm using the model 105 SM because it actually uses the same tuner circuit.

My IC input voltage is about 3v high, which is a bit odd, as the circuit is a 14ishv input that's is supposed to be dropped down to 9.6v by a 100ohm resistor. I'm not seeing the expected pull from the circuit which is odd.
 
Well I'm gonna put mine back on the bench next week (tuning meter is off just a bit) and it bugs the crap outa me.
So I can take some voltage reads for ya, just tell me exactly what you need, and conditions (good signal, no stereo, good sig stereo, no signal and maybe aux. selector volts).
Up to you.
 
hofferwood said:
Well I'm gonna put mine back on the bench next week (tuning meter is off just a bit) and it bugs the crap outa me.
So I can take some voltage reads for ya, just tell me exactly what you need, and conditions (good signal, no stereo, good sig stereo, no signal and maybe aux. selector volts).
Up to you.
Click to expand...

I think I just need the IC voltages since the 4230 manual lacks them. For both stereo and non-stereo, my suspicion is I have a bad L110 coil as it's reading pretty high DC resistance.
 
My voltages in stereo are as follows:
1: 2.78
2: 1.03
3: 0.47 (continuity to gnd)
4: 3.3 fluctuating
5: 1.59
6: 7.68
7: 1.66
8: 7.38
9: 12.69
10: 10.21
11: 10.16
12: 3.8
13: 1.8-2 fluctuates
14: 12.6
15: 1.37
16:2.05
 
hofferwood said:
Well I'm gonna put mine back on the bench next week (tuning meter is off just a bit) and it bugs the crap outa me.
So I can take some voltage reads for ya, just tell me exactly what you need, and conditions (good signal, no stereo, good sig stereo, no signal and maybe aux. selector volts).
Up to you.
Click to expand...


Also let me know the DC resistance between pin 12 and 13 on the IC, mine is showing around 102k ohm which may or may not be correct since I think it's got a resistor in parallel underneath.
 
Last edited:
hofferwood said:
Sorry it took me so long.
My resistance between 12 and 13 is .05 ohm, so I would say your L110 is open.
Click to expand...

No worries, I figured that would be the case. Fingers crossed that a faulty L110 is the issue here so I can get on with restoring the unit.
 
hofferwood said:
Sorry it took me so long.
My resistance between 12 and 13 is .05 ohm, so I would say your L110 is open.
Click to expand...

Just to update, it was in fact an open L110, no clue how on earth that failure occured, but it's working nice and the separation looks good on the scope!
 
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