Repair an Old Radio

[dark_hawk]

Hi,

I have an old Sony ICF-J40 Radio.

The radio had some problems from abuse and bad storage conditions but I was able to fix most of it's problem expect for a slight static/distorted sound on FM.
Here is the schismatic and the layout of the board.


My question is, is there a way to fix that distorted/static sound on the FM? Which compartments should I check/replace/add that will help lower that static noise.

Link to the service manual
http://www.radioscanner.ru/files/download/file18235/sony_icf-j40_service_manual.pdf
Thanks

[bob91343]

While I understand your question, a good answer would be very lengthy.  If you have distortion on FM, the best approach is to find out why.  You need an FM generator and look at the detector output with an oscilloscope.  If it's clean, the problem isn't in the tuner section.

If it's distorted, make sure the IF stages are doing their job, and see if perhaps the discriminator is misaligned or has a bad component, etc.  So try a gentle alignment while tuning the signal generator or the radio slightly to see if you can make the audio look better.

How distorted is it?  Does it depend on signal strength?

[CatalinaWOW]

There is an implicit answer to part of the debugging.  He states that the radio is fine except for some distortion on FM.  Implying the AM section is working fine and also the amplifier.  So the problem has to be in the FM tuner/demodulator.  And I agree with the simplified answer on how to track such problems.

[trevatxtal]

Forgive me for quoting the obvious Check the battery voltage first when on AM then when on FM.
Back in the 70/80s the most common complaint is just as above, High internal resistance in the battery the cause.

[Tomorokoshi]

1. Can you take a guess at the frequency components of the static and distorted sound? With the battery problem mentioned above, it might be a low-frequency warble. How would you characterize the static? 100 Hz? 1 kHz? 10 kHz? Constant or in bursts? Does it change with tapping it?

2. Can you get pictures of both sides?

[dark_hawk]

While I understand your question, a good answer would be very lengthy.  If you have distortion on FM, the best approach is to find out why.  You need an FM generator and look at the detector output with an oscilloscope.  If it's clean, the problem isn't in the tuner section.

If it's distorted, make sure the IF stages are doing their job, and see if perhaps the discriminator is misaligned or has a bad component, etc.  So try a gentle alignment while tuning the signal generator or the radio slightly to see if you can make the audio look better.

How distorted is it?  Does it depend on signal strength?

Thank you for the reply.

That is way above my head in respect to knowledge and equipment.
The best way I can describe the distortion is that if there is silence there is something like a continuous hum in the background and if there is music it sounds a bit distorted on the high notes.

There is an implicit answer to part of the debugging.  He states that the radio is fine except for some distortion on FM.  Implying the AM section is working fine and also the amplifier.  So the problem has to be in the FM tuner/demodulator.  And I agree with the simplified answer on how to track such problems.

Thanks for the reply.

AM sounds fine and I think the amplifier is also fine.

Forgive me for quoting the obvious Check the battery voltage first when on AM then when on FM.
Back in the 70/80s the most common complaint is just as above, High internal resistance in the battery the cause.

Thanks for the reply.
The batteries are fresh Sony D batteries. I also run it from my bench power supply when I'm working on the board, so I don't think the battery is the issue.

1. Can you take a guess at the frequency components of the static and distorted sound? With the battery problem mentioned above, it might be a low-frequency warble. How would you characterize the static? 100 Hz? 1 kHz? 10 kHz? Constant or in bursts? Does it change with tapping it?

2. Can you get pictures of both sides?

Thanks for the reply.

1- If I had to guess about the frequency of the hum/static I think it's in 100hz-1Khz range and it does not change with tapping it.

2- Pictures of the board:

[Tomorokoshi]

Is there a noticeable difference while switching slowly between FM and AM? Can you change the nature of the problem by pushing or twisting the band selection switch?

...
The radio had some problems from abuse and bad storage conditions...
...

Possible things to look at:

1. Dirty contacts in the big band selection switch. It looks very difficult to remove, which would likely damage the board anyway. However, a good electronics spray cleaner may be able to get in from the sides or through the slider access. Spray it in and work the switch back and forth to clear off the contacts. Different cleaners work for different problems. Make sure any cleaners are safe for plastics.

2. Silver tarnish on connections. For instance, notice VR1 (Tone) and VR2 (Fine Tune). The black is Silver Sulfide (Ag₂S). If that built up on the wiper, they could be at a significantly higher resistance than the tuned value. It's possible this could be more of a problem with FM than AM.

[shakalnokturn]

To add to Tomorokoshi's good ideas, you also have 2-3 solders on the CXA I.C. that look suspicious, maybe worth checking the ones under the shielding too.

[bob91343]

While I respect all the sincere replies to your initial request, I note that you claim lack of expertise in signal tracing, etc.  With this in mind, it appears that repairing this radio may indeed be a challenge beyond your expertise, and if you are successful it may just be luck.

Perhaps you have a friend who has the equipment and experience to help you along.  If you were local, we could set up the radio on my work bench and see what is the problem.

I recently helped a friend in a similar situation.  We tracked down an open resistor in the detector circuit of a 70 year old communications receiver.  He couldn't be happier with the result.  And no way could I have found the problem without the help of my test equipment.

[tkamiya]

Looking at circuit diagram, it appears AM/FM has its own section up until CF2 (Ceramic Filter 2).  While AM and short wave signal path is discrete, FM is most entirely done by the IC.  My approach would be to replace two capacitors under top B+ (I cannot read the label), and C16 and C17, c22 and c32.

AND, reflow the solder on pin 5 and 6.  They look to be not completely filled.  It may be held by one side only, making susceptible to cracking with temperature cycle.

Radio of this era, I am very familiar with.... 

Can I hear what your FM sounds like?

[dark_hawk]

While I respect all the sincere replies to your initial request, I note that you claim lack of expertise in signal tracing, etc.  With this in mind, it appears that repairing this radio may indeed be a challenge beyond your expertise, and if you are successful it may just be luck.

Perhaps you have a friend who has the equipment and experience to help you along.  If you were local, we could set up the radio on my work bench and see what is the problem.

I recently helped a friend in a similar situation.  We tracked down an open resistor in the detector circuit of a 70 year old communications receiver.  He couldn't be happier with the result.  And no way could I have found the problem without the help of my test equipment.

Thanks for the reply. I would be content with luck at the moment to be honest.

Unfortunately my field of work is far from electronics and I don't know anyone with that kind of equipment.

Looking at circuit diagram, it appears AM/FM has its own section up until CF2 (Ceramic Filter 2).  While AM and short wave signal path is discrete, FM is most entirely done by the IC.  My approach would be to replace two capacitors under top B+ (I cannot read the label), and C16 and C17, c22 and c32.

AND, reflow the solder on pin 5 and 6.  They look to be not completely filled.  It may be held by one side only, making susceptible to cracking with temperature cycle.

Radio of this era, I am very familiar with.... 

Can I hear what your FM sounds like?

Thanks a lot of the reply.

Could you please point me to those two caps above B+?
Direct link to the service manual:
http://www.radioscanner.ru/files/download/file18235/sony_icf-j40_service_manual.pdf

I will replace all the mentioned caps and reflow the pins.

[tkamiya]

Look at top right.  Locate B+.  There are two caps below that.

[bob91343]

In the middle of the diagram there are two blocks for which there isn't good information.  FM IF DET and FM Discriminator.  There is the place that likely is causing distortion.  However, there is no way to work on those blocks without further info.

Perhaps they are ICs.  If so, I would consider replacing them.  But first, an alignment is in order.  A misalignment will indeed cause distortion.  Again unfortunately, a proper allignment requires equipment.  If you try to make adjustments without equipment, you risk making the problem worse.  So if you do attempt it, don't use a heavy hand.  Turn the adjustments only a little to see if it helps.  Then put it back where it was.

Those two capacitors mentioned above operate in all modes so won't cause distortion only on FM.

[Cyberdragon]

In the middle of the diagram there are two blocks for which there isn't good information.  FM IF DET and FM Discriminator.  There is the place that likely is causing distortion.  However, there is no way to work on those blocks without further info.

Perhaps they are ICs.  If so, I would consider replacing them.  But first, an alignment is in order.  A misalignment will indeed cause distortion.  Again unfortunately, a proper allignment requires equipment.  If you try to make adjustments without equipment, you risk making the problem worse.  So if you do attempt it, don't use a heavy hand.  Turn the adjustments only a little to see if it helps.  Then put it back where it was.

Those two capacitors mentioned above operate in all modes so won't cause distortion only on FM.

No, the whole thing is an integrated circuit with support components. If the main IC failed it's probably terminal (probably cost as much as the whole thing for a new one).

FM is not like AM, it's extremely sensitive and "tweaking" randomly often just ruins it.

There is not much at all in the FM chain, it's mostly in the IC. The only adjustments in the FM front end are CT2 and L7, which I would advise not touching without a signal generator. There's also CT1 and L5, the oscillator, which could definitely cause the problem, but again, beware.

As far as passives the only other things are CF2 and CF3.

[tkamiya]

Those two capacitors mentioned above operate in all modes so won't cause distortion only on FM.

The circuit has two distinct signal path.  One path is discrete and the other integrated.  It is entirely possible caps are loading power and the voltage is dragged down below IC's requirement.  (but not the discrete)  Besides....  what's two more?

[Cyberdragon]

Those two capacitors mentioned above operate in all modes so won't cause distortion only on FM.

The circuit has two distinct signal path.  One path is discrete and the other integrated.  It is entirely possible caps are loading power and the voltage is dragged down below IC's requirement.  (but not the discrete)  Besides....  what's two more?

No, again, read the schematic. The entire radio is just an IC with a few passive support components. There is no "discrete section". The only two transistors outside the IC are the band selector control and the "tone amp". So those capacitors will effect the entire operation of the set.

In fact C16 and C17 are just for the AM AGC. C32 is just the audio amps noise filter (hence NF, you'd hear that on AM to) and C22 is the same for ripple at the audio section.

Recap it if you want, but I tell you that won't fix it.

EDIT: A reflow might help though, there are definately bad joints.

[mslr]

Please also think of just an interference with some equipment nearby. E.g. an electronic ballast. Just move the radio to a different place or move and turn it. You may hear the distortion changing. Maybe you‘re hunting a ghost... Especially look for all the ground connections of the shieldings. Some of this connections are only established in the original housing.


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[dark_hawk]

Thanks a lot for all the replies.

I ended up replacing the caps of the whole board. Replacing the caps made the problem worse, or it might have been placebo, can't really tell without proper test equipment.
I measured all the de soldered caps and found they were fine in regard to physical shape, capacity and ESR, those caps were of higher quality/lower ESR than the caps I replaced them with.
I then re soldered all the original caps back in, and suddenly the static almost disappeared. There is still a tiny low frequency hum in the background, but not like before and you won't even notice it unless you are looking for it.
My guess would be one of the caps had a dodgy connection.