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Marantz 4240 - yet another FM stereo issue

ocdecio

ocdecio

The proof of the restoring is in the listening.
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I have a 4240 that doesn't get FM stereo no matter what. Signal strength seems good as per the meter, however the muting signal coming from the middle of the IF amplifier to enable stereo in the demux is coming out lower than another working 4240 - working one has 1.55V out of J208 (which connects to the muting pots) and non working has at most 1.05V when a station is strong and tuned it.

I tried the usual - replaced all transistors in the IF amplifier, checked for defective diodes, etc. Before I throw my hand in the air and just replace P200, would there be anything else to try? I am to the point of replacing all 10.7MHz ceramic filters to see if that makes any difference.

Thank you for any light on the matter.
 
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ocdecio said:
I have a 4240 that doesn't get FM stereo no matter what. Signal strength seems good as per the meter, however the muting signal coming from the middle of the IF amplifier to enable stereo in the demux is coming out lower than another working 4240 - working one has 1.55V out of J208 (which connects to the muting pots) and non working has at most 1.05V when a station is strong and tuned it.

I tried the usual - replaced all transistors in the IF amplifier, checked for defective diodes, etc. Before I throw my hand in the air and just replace P200, would there be anything else to try? I am to the point of replacing all 10.7MHz ceramic filters to see if that makes any difference.

Thank you for any light on the matter.
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Did you set the muting per the service manual? If so, I'd check the rear panel muting control, and also R010 to see if you can get the correct voltage.
 

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Ray Gianelli said:
Did you set the muting per the service manual? If so, I'd check the rear panel muting control, and also R010 to see if you can get the correct voltage.
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I basically shorted it, and still wouldn't go past the value as it comes off P200 before the pots.It is about half a volt under the other working 4240 (before the pots).
 
ocdecio said:
I basically shorted it, and still wouldn't go past the value as it comes off P200 before the pots.It is about half a volt under the other working 4240 (before the pots).
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Hmm. Check out H228 and H231, as well as C241 and C244. These are supposed to rectify and filter part of the IF to DC and send it to the base of H308.
 
Thank you. I had checked all diodes and they test OK in circuit (0.3v, open in the other direction - 1N60). I'll check the capacitors. The weird thing is that there is some DC coming out, and the strenght meter shows a strong signal. I have a 26 that I am also scrathing my head without stereo but that one the strength at the meter is basically zero. I replaced all transistors in this board.
 
H308 is sort of the on/off transistor for stereo. Since you have a good working unit, it would be interesting to know the base voltage on H308 of the good unit vs. on the bad unit when tuned to a strong station and centered. If H308 is turned on, you should get stereo. If the DC voltage out of H304 is too high, H307 turns on and H308 turns off, turning off stereo. If the voltage out of H304 is too low, H305 turns off, which makes H306 turn on and H308 turn off - again eliminating stereo. So, it would be interesting to know if H308 is ever turning on in the bad unit. If you have the same voltage on both units at H308 base, maybe it is bad - or maybe one of the transistors involved in turning it on/off.
 
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I replaced all transistors including H308 - H309 - H310. What seems to be happening is the rectified IF signal from the middle of P200 (before the discriminator) is unable to reach the minimum muting level of what seems to be 1.5V in the working unit. It barely gets 1V. I measured before R269 and got 1.7V and before R270 and got 2.7V. For some reason that I can't understand the current is such that the drop at J208 is such that I read 1V at J208. Just for giggles I thought about replacing R270 for a slighlty lower value same with R269 and see if I can force the voltage to reach 1.5V just to see if that turns H308 on.


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Hmmm.. I wonder what kind of voltage you have at the cathodes of H228 and H231 ahead of those two resistors you mentioned.
 
Not sure if I would get a DC voltage I think they are the ones rectifying the signal.
 
I rechecked H308 and H309 they were turning on and off properly. I measured the voltages at the IC, and pin 4 that according to the 105B manual should have zero volts for stereo actually had 9V and it did go to zero when I selected Mono! So there are all conditions for the chip to turn stereo on and do its job. I also see audio coming in on pin 1 which means the muting circuit isn't preventing it.

So.. as it usually is the case... got a brand new set of ICs installed a new one, and sure enough - got stereo.}

Thank you guys for helping out!
 
hofferwood said:
IC-----lol
Glad you got it sorted out!!!
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Amazing how frequent it is the IC. Funny thing is, I had tried with an IC from another board which probably was toast too! I got some and hoped they weren't fake, and got lucky I think.
 
Go figure...probably should have tried that on a couple I gave up on. Adding to list of need to keep in stock.
 
jd3rdcoast said:
Go figure...probably should have tried that on a couple I gave up on. Adding to list of need to keep in stock.
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Not the first time it happened to me (the IC being bad). You can find this one at american microsemiconductors.
 
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