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Fighting with a 2275s FM MPX

goldswimmerb

goldswimmerb

Failed out of engineering, but look at me now!
Subscriber
Hey guys, so I'm working on a 2275 and I noticed that I cannot get an FM stereo signal, I've checked using my FM transmitter I have that it is indeed the FM never switching to stereo, rather than it being the FM Stereo bulb driver being dead.

I've been chasing ghosts for the last hour so I am hoping someone may be able to shed some light on this.

I can have a perfectly tuned FM signal, no noise except the typical high end buzz I get with FM when there's no audio (probably my transmitter) but it never switches to stereo, the tuning meter will be centered and the signal strength meter will be as far as it can go to the right.

When testing voltages I did notice That pin 6 on my HA1156 seems to always be stuck high (14v) even when there is a stereo station strongly tuned. I thought it may be a bad HA1156 but I noticed in some cases when between my "station" I am transmitting and the local radio the unit would break into stereo, though it was tuned to neither station, and pin 6 would drop down to the 1v expected for stereo operation.

I did replace Capacitors on the board to see if that was the issue but it didn't change anything, several caps were wildly out of specification so I thought maybe changing them would help...

I also have a rock solid 19KHz on whatever pin that was, J310 I think

Is it possible that it's just wildly out of alignment? I'd think it wouldn't ever break into stereo if I had a bad FET or Transistor in there.
 
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catrafter said:
I've posted this before, hope it helps.

Tom
Click to expand...
So I have it to a point now where I can get FM stereo from my transmitter, but only on a small window of the FM Muting adjustment. Not sure what that's about. It seems all other stations wind up being mono except for the transmitter which I guess is an improvement.
 
() yes.

Have you carefully aligned the discriminator transformer using the proper tool as indicated in the service manual?

Tom
 
catrafter said:
() yes.

Have you carefully aligned the discriminator transformer using the proper tool as indicated in the service manual?

Tom
Click to expand...

I've got a proper plastic adjustment tool, but no distortion analyzer... I'm able to get it to break into stereo between channels, like it's not getting the 19khz tone when centered but somehow is when it's between my transmitter and the channel it's broadcasting next to
 
I have seen multiple Marantz receivers that had an issue on the small FM discriminator PC board that would cause them to have no MPX stereo. One had been to another shop, that completely recapped the discriminator and IF boards, (without success}. The center tune meter would work SLIGHTLY, but the amount of movement when crossing a station was weak.' and the audio seemed to be slightly lower than it should be. The discriminator alignment would center the meter with no signal, but again the amount of motion was weak. There was also some excessively loud noise when tuning off a station. Because of all the previous work that had been done by another shop on the discriminator board, I just located a used replacement "FM decoder" board from Oaktreevintage, and installed it. After aligning the discriminator, the receiver worked fine, EVEN THE MPX STEREO. I recently worked on another 2275 with the same issue. It came in with the center tune meter off to one side with no signal. I was able to align the discriminator transformer and get the meter centered, but it still had the issues described in the first case. It was virgin had not been violated (not recapped).I spent a good deal of time trying to locate the issue. It appeared to be an issue with the little discriminator transformer (possibly defective internal resonating capacitor). I ultimately found a replacement PC board from another model (a 2245 or something) on Ebay, that was slightly different (connections in different places, and a capacitor for the meter circuit missing), but I was able to modify and install it and again the unit worked perfectly INCLUDING THE MPX STEREO.

I guess the whole point to this post is to let you know that an issue on the FM discriminator board (it is actually a ratio detector) can cause a no MPX stereo issue. I wish I could tell you for sure what the actual defective component on the boards were, but I don't know, as I didn't want to invest any more time in trying to locate it.
 
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The 'discriminator' transformer is the ubiquitous LI140623. I say that because it is used in many models.
Bob, you are correct, the 2275 uses a ratio detector, a variation of the Foster-Seeley discriminator. The Marantz service documents refer to it as a discriminator.

Tom
 
AE Bob said:
I have seen multiple Marantz receivers that had an issue on the small FM discriminator PC board that would cause them to have no MPX stereo. One had been to another shop, that completely recapped the discriminator and IF boards, (without success}. The center tune meter would work SLIGHTLY, but the amount of movement when crossing a station was weak.' and the audio seemed to be slightly lower than it should be. The discriminator alignment would center the meter with no signal, but again the amount of motion was weak. There was also some excessively loud noise when tuning off a station. Because of all the previous work that had been done by another shop on the discriminator board, I just located a used replacement "FM decoder" board from Oaktreevintage, and installed it. After aligning the discriminator, the receiver worked fine, EVEN THE MPX STEREO. I recently worked on another 2275 with the same issue. It came in with the center tune meter off to one side with no signal. I was able to align the discriminator transformer and get the meter centered, but it still had the issues described in the first case. It was virgin had not been violated (not recapped).I spent a good deal of time trying to locate the issue. It appeared to be an issue with the little discriminator transformer (possibly defective internal resonating capacitor). I ultimately found a replacement PC board from another model (a 2245 or something) on Ebay, that was slightly different (connections in different places, and a capacitor for the meter circuit missing), but I was able to modify and install it and again the unit worked perfectly INCLUDING THE MPX STEREO.

I guess the whole point to this post is to let you know that an issue on the FM discriminator board (it is actually a ratio detector) can cause a no MPX stereo issue. I wish I could tell you for sure what the actual defective component on the boards were, but I don't know, as I didn't want to invest any more time in trying to locate it.
Click to expand...
I would try swapping the board, but I hate board swaps, and the only board I have is from a 4300 which looks to have some different components
 
Do you get stereo on regular local FM broadcasters? As in: are you sure your TX is OK?
 
AE Bob said:
I have seen multiple Marantz receivers that had an issue on the small FM discriminator PC board that would cause them to have no MPX stereo. One had been to another shop, that completely recapped the discriminator and IF boards, (without success}. The center tune meter would work SLIGHTLY, but the amount of movement when crossing a station was weak.' and the audio seemed to be slightly lower than it should be. The discriminator alignment would center the meter with no signal, but again the amount of motion was weak. There was also some excessively loud noise when tuning off a station. Because of all the previous work that had been done by another shop on the discriminator board, I just located a used replacement "FM decoder" board from Oaktreevintage, and installed it. After aligning the discriminator, the receiver worked fine, EVEN THE MPX STEREO. I recently worked on another 2275 with the same issue. It came in with the center tune meter off to one side with no signal. I was able to align the discriminator transformer and get the meter centered, but it still had the issues described in the first case. It was virgin had not been violated (not recapped).I spent a good deal of time trying to locate the issue. It appeared to be an issue with the little discriminator transformer (possibly defective internal resonating capacitor). I ultimately found a replacement PC board from another model (a 2245 or something) on Ebay, that was slightly different (connections in different places, and a capacitor for the meter circuit missing), but I was able to modify and install it and again the unit worked perfectly INCLUDING THE MPX STEREO.

I guess the whole point to this post is to let you know that an issue on the FM discriminator board (it is actually a ratio detector) can cause a no MPX stereo issue. I wish I could tell you for sure what the actual defective component on the boards were, but I don't know, as I didn't want to invest any more time in trying to locate it.
Click to expand...
Thanks for this tip!!! You saved me, I had an incredibly small short at C244 that I never would have found without this advice!
 
WOW,! Thank you for replying. After looking to see where C344 is, I realized that the circuitry of the 2275 is different than the two units I was describing in my post. The ones in my post had an "FM Decoder Board" separate from the IF board, where the 2275 has them on the same PC board. I appreciate you describing the resolution to your issue, as I may be able to apply it to the circuitry of the 2230, 2245, etc, as I still have one of the defective PC boards saved and that part of the circuit looks somewhat similar.
 
AE Bob said:
WOW,! Thank you for replying. After looking to see where C344 is, I realized that the circuitry of the 2275 is different than the two units I was describing in my post. The ones in my post had an "FM Decoder Board" separate from the IF board, where the 2275 has them on the same PC board. I appreciate you describing the resolution to your issue, as I may be able to apply it to the circuitry of the 2230, 2245, etc, as I still have one of the defective PC boards saved and that part of the circuit looks somewhat similar.
Click to expand...

I never thought to check the P200 board since I would think that any issues that arise with stereo would be from the MPX board. Wound up finding it by checking voltages at my Transistors on P200 and realized that towards the output of the board (to the MPX decoder) I was getting half the voltage that I was supposed to, traced it back and found a tiny bit of solder that had strayed.
 
The FM demodulation process is fairly complex. The discriminator (or demodulator) has to recover the L+R signal , the pilot signal and the double side band suppressed carrier L-R signal. If the transformer tuning is off for some reason the process falls apart and the MPX decoder doesn't get what it need to decode stereo.

Good job!
 
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