Need Help with Marantz Model 27 FM section

[AngelAT]

Hello AKs,

I managed to find a Model 27 in need of TLC. Managed to make it operate to certain extend. Recapped the power supply and fixed the voltage. Also recapped the pre- and power amps and also the phono section. Also recapped the P200 board. Replaced the speaker terminals and the power chord. Added some bulbs. Aux and Phono operate ok.

Now the problem: I can only hear FM stations when the Muting Off is depressed. This tells me that the signal is very low and cannot be heard without the muting circuit being cut-off. Treating the transistors with freeze does not change anything. Also the Stereo lamp does not engage. Connecting the VM to pins J216 and J610 does nor show change in voltage when tuning the FM.

But FM is not my strength so any help will be appreciated.

Thanks in advance!

 

[Don Lubin]

Since it works with the muting off, I think that there is a good chance that someone turned the muting level pot way up. The pot is at the upper right rear of the receiver near the speaker terminals. If that is not it, II would check is that you have decent signal levels from the antenna by trying it with another receiver.

If the antenna checks out OK, download the service manual from hifiengine.com. The service manual has a section for the FM tuner. I would recommend going through the procedures it specifies.

 

[AngelAT]

Hello Don Lubin,

Thanks for stepping in!
I tried with the pot at the back but it didn't work. Still I used the opportunity to clean it and also the attenuator switch. When I decreased the muting though fm now sounds better. Furthermore, with the attenuator switch to out now the signal strength is way bigger. Also the stereo lamp lights now with strong stations and stereo switch depressed.

I started going through the Fm description, as you advised, in the service manual. Started with H103 and it may be the case hat it is the culprit. when I freeze it the signal level decreases and sound is gone. But the good part is that tinkering with it I now have a bit lower fm reception signal with the muting off in out position. when I depress the muting off switch the sound is audibly stronger and louder, also stereo lamp lights. So H103 is a 2sc829B according to the parts list. Looking at the replacement list I see KSC1674 and 2sc930 as replacements. But I do not have any of them. In another AK thread I read that a fellow mwmber replaces it with KSC1845. Do you that this will do?

 

[Don Lubin]

You know, that freeze spray can make a working circuit not work temporarily if it gets too cold. If you decrease the temperature of a bipolar transistor, the Vbe will go up and Hfe will go down. Vbe should not matter in that circuit but Hfe could, particularly if it is marginal to begin with. Have you removed H103 and tested it? I would first check the collector - emitter leakage current with the base - emitter junction shorted as that tends to be a common problem for that vintage of semiconductors. I've found transistors with that issue in several pieces of my 70s Marantz equipment. The spec says Hfe should be between 60 and 250. I would check that next if the collector - emitter. leakage current is OK. I vaguely recall having to replace a transistor in the tuner section of my Model 24 several decades ago.

The 2SC829B should not be very hard to replace. The specs are very pedestrian compared to what you can get today. I would expect lots of options but I have not looked.

 

[Steven Tate]

If you haven't already done it, I recommend cleaning the big tuning cap using Dr*Audio's instructions here. It's very important to do it exactly as he says and don't use anything that could leave a residue. But any time you have a low signal situation, Cleaning the tuner cap, the attenuator switch, the muting pot, and maybe the Mono switch for good measure is the first step. This is to make sure you have good contacts all the way through the signal path. A little lost signal here and a little there can add up to a weak signal for the IF amp to work with.

KSC1845FTA should be a good replacement for 2SC829B. But I am a fan of the old transistors. I would check everything else first, and if you change it, and it doesn't help - put the old one back in. That way, you don't have the gremlin in the back of your head making you think the spec difference in that transistor are causing some of the problem.

 

[AngelAT]

You know, that freeze spray can make a working circuit not work temporarily if it gets too cold. If you decrease the temperature of a bipolar transistor, the Vbe will go up and Hfe will go down. Vbe should not matter in that circuit but Hfe could, particularly if it is marginal to begin with. Have you removed H103 and tested it? I would first check the collector - emitter leakage current with the base - emitter junction shorted as that tends to be a common problem for that vintage of semiconductors. I've found transistors with that issue in several pieces of my 70s Marantz equipment. The spec says Hfe should be between 60 and 250. I would check that next if the collector - emitter. leakage current is OK. I vaguely recall having to replace a transistor in the tuner section of my Model 24 several decades ago.

The 2SC829B should not be very hard to replace. The specs are very pedestrian compared to what you can get today. I would expect lots of options but I have not looked.

So I took out this transistor and replaced it with ksc1845. This didn't solve the problems. The original tested ok, so I just returned it back.

 

[AngelAT]

If you haven't already done it, I recommend cleaning the big tuning cap using Dr*Audio's instructions here. It's very important to do it exactly as he says and don't use anything that could leave a residue. But any time you have a low signal situation, Cleaning the tuner cap, the attenuator switch, the muting pot, and maybe the Mono switch for good measure is the first step. This is to make sure you have good contacts all the way through the signal path. A little lost signal here and a little there can add up to a weak signal for the IF amp to work with.

KSC1845FTA should be a good replacement for 2SC829B. But I am a fan of the old transistors. I would check everything else first, and if you change it, and it doesn't help - put the old one back in. That way, you don't have the gremlin in the back of your head making you think the spec difference in that transistor are causing some of the problem.

Thanks Steven!
Will do as you advise and will come back.
I have cleaned switches but not the tuning cap.

 

[AngelAT]

A quick update:

I did the tuner cleaning procedure and now it's way better!
Still I have only 3-4 stations that I get well and with stereo lamp. These are the strongest ones with signal strength over 50-60%. But they are the same level with/without the Muting off.
With the Muting off depressed I get way more stations.

 

[Don Lubin]

The muting should not affect the signal level so that behavior is correct. The muting level pot on the back determines how strong the signal needs to be not to be muted. If you set the muting level low you will hear more stations with the muting on.

 

[PE9ZZ]

It could be that the tuner front end alignment is off but you will need specialist equipment to fix that. It is possible to use off-air signals but that is tricky. What antenna do you use?

 

[AngelAT]

The muting should not affect the signal level so that behavior is correct. The muting level pot on the back determines how strong the signal needs to be not to be muted. If you set the muting level low you will hear more stations with the muting on.

As I said, I do not know much about tuners. Thanks for your support, guys! This is the first such issue I manage to, more or less, solve.
Still I don't get it - how can I have more stations and better signal quality with the Muting off in out position...

 

[AngelAT]

It could be that the tuner front end alignment is off but you will need specialist equipment to fix that. It is possible to use off-air signals but that is tricky. What antenna do you use?

Unfortunately, I do not have any tuner alignment equipment.
I use a 'T'- shaped 300 Ohm antenna. With that I get good signal - some of the stations are 60-70% signal strength.
What off-air signals do you mean? I am willing to learn, so if there is a thread or sth similar, it will be interesting for me.

 

[Don Lubin]

As I said, I do not know much about tuners. Thanks for your support, guys! This is the first such issue I manage to, more or less, solve.
Still I don't get it - how can I have more stations and better signal quality with the Muting off in out position...

Muting is used so that you don't have to hear weak stations. The muting level determines how strong the station has to be before you can hear them. It has no affect on the signal quality. If you crank the muting level pot to the "Min" position, you will get the maximum number of stations when muting is turned on. However, you may find that many marginal stations get through that you would rather not be bothered with. Adjust it to a level where you can hear the acceptably strong stations and not the weak and noisy ones. If you want to hear everything, turn muting off.

 

[PE9ZZ]

What off-air signals do you mean? I am willing to learn, so if there is a thread or sth similar, it will be interesting for me.

FM alignment using off-air signals (ie the ones you get from your dipole) can be used but since the signals may not be very stable it is hard to use them that way. Simple FM signal generator and multimeter are the bare essentials to do it right but if you make sure your signals are as stable as possible (get your dipole antenna far away from yourself) this can be done. Follow sections 4.3.1 and 4.3.2 in the service manual but use local stations near 90 and 106 MHz instead. If you are careful you may be able to get good results this way. Make sure to measure the signal levels with your multimeter before you start. Connect it across the tuning meter.

 

[AngelAT]

Muting is used so that you don't have to hear weak stations. The muting level determines how strong the station has to be before you can hear them. It has no affect on the signal quality. If you crank the muting level pot to the "Min" position, you will get the maximum number of stations when muting is turned on. However, you may find that many marginal stations get through that you would rather not be bothered with. Adjust it to a level where you can hear the acceptably strong stations and not the weak and noisy ones. If you want to hear everything, turn muting off.

Thanks Don!

 

[AngelAT]

FM alignment using off-air signals (ie the ones you get from your dipole) can be used but since the signals may not be very stable it is hard to use them that way. Simple FM signal generator and multimeter are the bare essentials to do it right but if you make sure your signals are as stable as possible (get your dipole antenna far away from yourself) this can be done. Follow sections 4.3.1 and 4.3.2 in the service manual but use local stations near 90 and 106 MHz instead. If you are careful you may be able to get good results this way. Make sure to measure the signal levels with your multimeter before you start. Connect it across the tuning meter.

Thanks PE9ZZ!
I will try this a bit further in time as I now started working on a Tandberg TR2040 with a noisy channel.

 

[Don Lubin]

I just noticed from the photo that you have the later version of the Model 27. I have the old version which does not have the "Hi Filter" button and just a round red light for the FM stereo indicator. Not sure if there were any other differences but I would not be surprised.

 

[AngelAT]

Yes, Don
Mine is the seven button version. As far as I saw online there are some differences. Also at the boards.

 

[Don Lubin]

I bought mine new in 1970. Not sure when the later model came out.

 

[AngelAT]

I bought mine new in 1970. Not sure when the later model came out.

Niiice!!!
I was born round these years...
I have no idea when the seven-button model 27 came around - probably short after the six-button.
So far I was a 22xx model fan, but lately I managed to find a Model 18 with WC and was really impressed. So now I have this Model 27 and also bought two incomplete Model 26s and will try to make one full Model 26.
But this is a project for a bit further in time...