Scott am/fm synthezized Tuner needs help

[PGR]

I have a model MS100 60hz 300W tuner that has been acting funny and I need some assistance to help me diagnose it and to also find sources and schematic background. Ya its old 35 years anyway but produces beautiful clear sound plus it has lots of potential sound sources.

I have just replaced the CAps on the main board - about five of them and the receiver started to work again and what I thought was very well. I got almost all the way thru a favourite record (vinyl) say 20 minutes and the old problem popped up again - I had actually forgotten about it. BUT there it was again.

Just listening to good sound very comfortable volume level and BAMM! the volume doubles or triples all of a sudden...uncomfortable levels if the volume was set so you could have listened from across the room. It is like a capacitor finally woke up and started doing its job, it extremely annoying ...leaves you running to the stereo to turn it down before the neighbours start banging on the door.

OK so I need help to figure out what is doing this and how to repair it. I am not a trained technician but I have soldering abilities and can follow directions I also have equipment including a scope.

Would like to figure out what is happening here . thanks for your replies in advance.

PGR.

 

[MrBoat]

Does it have a remote?

 

[PGR]

To be honest I have never used the remote but it does have the remote type screen on the face of the receiver. The remote it self does not exist.

 

[MrBoat]

To be honest I have never used the remote but it does have the remote type screen on the face of the receiver. The remote it self does not exist.

Try covering the little remote receptor window. Perhaps other light, such as fluorescents are interfering with it somehow. I have a Scott amp as well. It was turning on and off by itself. I never was sure if covering that sensor fixed it or it fixed itself.

Just a thought is all.

 

[PGR]

I'm prepared to try anything Black tape I have! But I really think it is something like a diode or cap letting go . Any other suggestions and I will follow up on them.
Thanks MrBoat

 

[TLS Guy]

You can get another one working for $35.

As far as yours is concerned I can get a free service manual schematic. They are available on sites for purchase.

As to your unit, I suspect a power supply issue, as it affects both channels. My hunch is the louder state is the correct state and that there is a section not being supplied the correct voltage on the quieter mode. That unit is new enough that there are probably solid state voltage regulators. I would suspect something do do with voltage regulation.

 

[PGR]

Are you kidding me TLS GUY. you have to be doing some commercial work ? Not what I need for my listening pleasure but real fund to look at.
Is there a chance I could get a schematic from you on this unit so I could begin to trace back the voltage drop ...I agree that type of problem is what I have and I also agree that the louder sound is the correct sound it is more clear with stronger treble tones to it. I think the coil is feeding a set of four diodes , two per amp side and I have as you say a voltage lose on one side But I am really not sure what is suppose to be happening here .
Do you or anyone else know how to test or measure this voltage regulator?

 

[TLS Guy]

Are you kidding me TLS GUY. you have to be doing some commercial work ? Not what I need for my listening pleasure but real fund to look at.
Is there a chance I could get a schematic from you on this unit so I could begin to trace back the voltage drop ...I agree that type of problem is what I have and I also agree that the louder sound is the correct sound it is more clear with stronger treble tones to it. I think the coil is feeding a set of four diodes , two per amp side and I have as you say a voltage lose on one side But I am really not sure what is suppose to be happening here .
Do you or anyone else know how to test or measure this voltage regulator?

I'm sorry, I left I vital word out of my post. I can not get a free service manual or schematic for your unit. If you do search though you will find service manuals for purchase. I do not have a manual or schematic for your unit.

Once you have a manual it should show correct voltages. You will need to measure the voltage to various sections.

Now you have to work on units live and there are high voltages. To make it safer to work on equipment a Variac is strongly recommended.

The four diodes are a bridge rectifier.

I suspect your problem is in the regulation part of the circuit.

My rig is largely for the pleasure of my wife and I, family and friends. I do do some archiving for clients, and mastering. Engineers do come here from time to time to check a mix.

 

[PGR]

TlS Guy
Thanks for the diagram. Starting to make some sense I have replaced the caps on both sides of the stereo system, so the 470's and the 0.1 uf but until I did some reading on the voltage regulator I had no idea what it was for. I also checked out how to test the little thing so good to go there. What I hope to see is one of these is not delivering properly. I don't even know what the voltage should be coming out.. I Do remember that the flow on the board is printed as 35V and the caps purchased were for 35 V as well so the out put will be less than that. Would it be correct to say if the measured voltage is not the same going into the regulator then the up steam diodes are messed up?

 

[TLS Guy]

TlS Guy
Thanks for the diagram. Starting to make some sense I have replaced the caps on both sides of the stereo system, so the 470's and the 0.1 uf but until I did some reading on the voltage regulator I had no idea what it was for. I also checked out how to test the little thing so good to go there. What I hope to see is one of these is not delivering properly. I don't even know what the voltage should be coming out.. I Do remember that the flow on the board is printed as 35V and the caps purchased were for 35 V as well so the out put will be less than that. Would it be correct to say if the measured voltage is not the same going into the regulator then the up steam diodes are messed up?

You need to pony up and buy a service manual on line.

You do not have experience with this. We really do not recommend individuals without experience work on live equipment. This unit will have to be tested live.

Given the date I suspect the power amp section is unregulated. If the voltage was low at the output stage then it would distort and sound awful. I suspect that the voltage gain stages are regulated and the voltages are usually of the order of a volt, sometimes +5/-5 volts.

Now you can do lot of damage testing a unit like this with a standard multimeter. This type of service requires an expensive amplified high impedance meter of the order of 2 megohms.

Honestly this unit is not really worth the cost of service. If you want to get it working you are going to have to find an experienced service tech. These days because of the throw away society, and the fact that manufacturers no longer issue schematics and service manuals and do everything possible to frustrate independent service techs and DIY guys like me, service techs are far and few between. There are less everyday.

If that unit I directed you to on eBay is still available is by far your cheapest solution.