What are the 'actual' effects of old electrolytics in a vintage radio?

[Crystaleyes]

Originally Posted by stj

a cap that reads higher uF than it's marking usually indicates it's internally failing.
the internal electrical leaking fools the cap meter.

Is there a threshold with that higher µF? Like 10% or 20% for example?

And could using 2200µF instead of 1000µF on the output caps cause any problems? Specifically, would it affect the output volume?
Gonna put a 1000uF on one channel and find out...

[Per Hansson]

Capacitors in olden times where often rated -10 +50% or even -10 to +75% on capacitance so there is "some" error margin

[stj]

i have *never* found a cap that was legitamatly over-spec.
plenty under, but i suspect the factory grades them and the good ones go to specific customers!!!

get a component tester that tells you the leakage.

[keeney123]

Originally Posted by Crystaleyes

Is there a threshold with that higher µF? Like 10% or 20% for example?

And could using 2200µF instead of 1000µF on the output caps cause any problems? Specifically, would it affect the output volume?
Gonna put a 1000uF on one channel and find out...

Are these caps +Electrolytic caps? If + Caps I would be concern with ESR values. A Larger Cap will also have a larger Xc.

Here is a link on Xc

https://www.calculatorsconversion.co...mula-examples/

It would be interesting to find out how good are the specifications of your stereo from a specification sheet.

Here would be a good place to learn some of this.

https://training.ti.com/signal-noise...o-applications

[Crystaleyes]

All of the electrolytics were replaced after several had burst and others had a ridiculously high ESR or voltage loss.

Two other ceramic caps had blown (literally).


One question I do have puzzling my little mind is;

There are two 1Ω resistors (R106 + R206) following the bridge rectifier which the supply for most of the unit passes through. One for the amplifier right channel and record deck input, and the other for the left channel and radio supply. The resistors in there now are 2 Watts and are likely not the originals, and I imagine they are acting as fuses.
Now the whole unit is 31 Watts (as per manual) so should these resistors not be a higher wattage rating? After all, every transistor in the radio and amplifier sections would consume a total of much more than 4 watts.

The supply here is 127v which would give around 0.25A at 31 Watts, so why don't I just swap the two 1Ω resistors for 0.3A fuses?

Please feel free to enlighten me if I am missing something.

The full schematic is attached below.

[keeney123]

Originally Posted by Crystaleyes

All of the electrolytics were replaced after several had burst and others had a ridiculously high ESR or voltage loss.

Two other ceramic caps had blown (literally).


One question I do have puzzling my little mind is;

There are two 1Ω resistors (R106 + R206) following the bridge rectifier which the supply for most of the unit passes through. One for the amplifier right channel and record deck input, and the other for the left channel and radio supply. The resistors in there now are 2 Watts and are likely not the originals, and I imagine they are acting as fuses.
Now the whole unit is 31 Watts (as per manual) so should these resistors not be a higher wattage rating? After all, every transistor in the radio and amplifier sections would consume a total of much more than 4 watts.

The supply here is 127v which would give around 0.25A at 31 Watts, so why don't I just swap the two 1Ω resistors for 0.3A fuses?

Please feel free to enlighten me if I am missing something.

The full schematic is attached below.

Usually, a wattage rating for a stereo is for the output channels. It can be written in various forms. It can mean 35 watts per channel or 35 watts for all channels. The voltage in the speaker drivers/transistors is 24 Volts Dc. The maximum voltage going to the speakers will be somewhat less, transistor drop. Usually when on calculates this you would use what is called a load line. That is simply a graph of current virus voltage. The line would be drawn from maximum current that the circuit can handle to maximum voltage applied. Then what happens is the audio frequency signal will deviate this DC load line. This how one amplifies a small input signal. By creating a frequency switch that controls the DC output voltage. For tubes or transistors, it would be the same procedure.
It also seems that one of your resistors go to another part of the circuit. I believe it goes to a Zener diode. The Spanish symbols are a little hard to understand. Do you have tunable coils in the FM circuit? Also, you will want to blow all the dust out of the tunable air Capacitors. This stereo seems of have Automatic Frequency Control or AFC.

[Per Hansson]

Originally Posted by Crystaleyes

One question I do have puzzling my little mind is;

There are two 1Ω resistors (R106 + R206) following the bridge rectifier which the supply for most of the unit passes through. One for the amplifier right channel and record deck input, and the other for the left channel and radio supply. The resistors in there now are 2 Watts and are likely not the originals, and I imagine they are acting as fuses.
Now the whole unit is 31 Watts (as per manual) so should these resistors not be a higher wattage rating? After all, every transistor in the radio and amplifier sections would consume a total of much more than 4 watts.

The supply here is 127v which would give around 0.25A at 31 Watts, so why don't I just swap the two 1Ω resistors for 0.3A fuses?

Please feel free to enlighten me if I am missing something.

The full schematic is attached below.

A resistor only needs to be rated for the voltage that is across it, an example:

At 127V x 0.25A you have ~ 32W as you say.
Ohms law says: V = I x R (0.25A x 1Ω = 0.25V)
So at that wattage your resistor is only dropping 0.25V
And 0.25V across a 1Ω resistor is only 0.0625W so it will barely get warm.

Now this is a little off because the voltage after the bridge rectifier is 127V x √2 = 180V but of course this has a negliable effect if you redo the math...

Also I'm not sure what the voltage actually is after the transformer, the math is only correct if it is a 1:1 transformer and it likely is not.
However even if I'm off by a factor of 10 a regular 0.25W resistor will still be fine
And considering this is not a vacuum-tube based device the voltage is likely to be much lower so that means the resistors have even less than what I calculated to dissipate...

The purpose of the resistor is probably not to act like a fuse but a inrush current limiter.
If you replace it with a fuse it will not limit the inrush any longer so I would not recommend that.

[keeney123]

Originally Posted by Per Hansson

A resistor only needs to be rated for the voltage that is across it, an example:

At 127V x 0.25A you have ~ 32W as you say.
Ohms law says: V = I x R (0.25A x 1Ω = 0.25V)
So at that wattage your resistor is only dropping 0.25V
And 0.25V across a 1Ω resistor is only 0.0625W so it will barely get warm.

Now this is a little off because the voltage after the bridge rectifier is 127V x √2 = 180V but of course this has a negliable effect if you redo the math...

Also I'm not sure what the voltage actually is after the transformer, the math is only correct if it is a 1:1 transformer and it likely is not.
However even if I'm off by a factor of 10 a regular 0.25W resistor will still be fine
And considering this is not a vacuum-tube based device the voltage is likely to be much lower so that means the resistors have even less than what I calculated to dissipate...

The purpose of the resistor is probably not to act like a fuse but a inrush current limiter.
If you replace it with a fuse it will not limit the inrush any longer so I would not recommend that.

If you look on the schematic that Crystaleyes provides you will see the two resistors he is talking about are connected after the rectification and they are connected to the 24 volts that powers the output transistors. On the schematic it even lists the voltage as 24.4 volts. Sorry, that is 25.4 volts. Was going from memory of schematic.

[Topcat]

Originally Posted by Per Hansson

The easy one is "mains hum" (50/60Hz) heard through the speakers when the big filtering capacitors are failing.
You can also have failure of frequency filters like in my post here, then it may make really really bad noises as lti said above:
https://www.badcaps.net/forum/showthread.php?t=93004

That's what I've always experienced. I rebuilt an old Emerson 'lifetimer' radio from 1963-ish with the usual 60Hz hum.....I don't know why I wasted my time on it....but the clock worked, so I went ahead and fixed it... The main cap was a dual value oddball...had to improvise to fix it.

[Crystaleyes]

Originally Posted by keeney123

If you look on the schematic that Crystaleyes provides you will see the two resistors he is talking about are connected after the rectification and they are connected to the 24 volts that powers the output transistors.

And if you look just under those resistors you will see two arrows pointing to a circle and a star. These are to indicate that they are supplying power to where those symbols reappear in the other stages of the receiver.

[Crystaleyes]

Originally Posted by keeney123

Usually, a wattage rating for a stereo is for the output channels. It can be written in various forms. It can mean 35 watts per channel or 35 watts for all channels. The voltage in the speaker drivers/transistors is 24 Volts Dc. The maximum voltage going to the speakers will be somewhat less, transistor drop. Usually when one calculates this you would use what is called a load line. That is simply a graph of current virus voltage. The line would be drawn from maximum current that the circuit can handle to maximum voltage applied. Then what happens is the audio frequency signal will deviate this DC load line. This is how one amplifies a small input signal. By creating a frequency switch that controls the DC output voltage. For tubes or transistors, it would be the same procedure.

Nicely explained. Cheers

It also seems that one of your resistors go to another part of the circuit. I believe it goes to a Zener diode. The Spanish symbols are a little hard to understand. Do you have tunable coils in the FM circuit? Also, you will want to blow all the dust out of the tunable air Capacitors. This stereo seems of have Automatic Frequency Control or AFC.

Well actually, the radio section is still dead. I haven't really examined it, but the voltage isn't getting to the FM board for sure.
The turntable and tape inputs are both now working so I figured that my friend (who owns it) could just use the tape input connected to their device if they want the radio?

The main reason that I haven't attempted the radio is that there is a board missing from the middle of the unit, which I think is the decoder. The plan is to try to find another (but complete) radio and harvest the missing board.

The important thing is that the amp section is now working.

[Crystaleyes]

Originally Posted by Per Hansson

A resistor only needs to be rated for the voltage that is across it, an example:

At 127V x 0.25A you have ~ 32W as you say.
Ohms law says: V = I x R (0.25A x 1Ω = 0.25V)
So at that wattage your resistor is only dropping 0.25V
And 0.25V across a 1Ω resistor is only 0.0625W so it will barely get warm.


The purpose of the resistor is probably not to act like a fuse but a inrush current limiter.
If you replace it with a fuse it will not limit the inrush any longer so I would not recommend that.

Thank you for the explanation.

In the end, I just left them as per the schematic and all is working fine,

[keeney123]

In the day FM was very important to the customer. It had so much reduced noise to AM. Of course, that was in the day music was making leaps and bounds of change. It was interesting to listen to the radio because not only the music was interesting but, the radio DJ was also interesting. This was in the rock and roll era. Stil people who listen to Classical also found interest in listening to music and the DJs. This was a much different format and may be able to find it here and there. I suppose the money then went into the digital format along with the industry. Still, I find vinyl records with a good dual turntable with a good class AB stereo amplifier so much better than anything on the market today and I am not alone. I really do not know why one would need a decoder for a FM radio. Perhaps it is just kind of a selector board for listening to mono FM or Stereo FM and whether to use an internal antenna or an external antenna. My belief is that the OM stands for Option Modulation and CO stands for Control Option it this schematic as I do not see any Optical in this system.

[Crystaleyes]

To be honest, I am not 'sure' that the missing board is a decoder.

I had said that, as the schematic download came with three pages and one of them has a separate circuit that is labelled 'Decodificador' (decoder), and from photos I have found of complete radios (and empty holes in the PCB), there is definitely a board missing which sits between the radio and amplifier sections.

Totally agree with you regarding the vinyl and decent amp for sound quality, although this unit is a family inheritance so my friend isn't seeking top quality sound, just that it works!

As for the AM / FM switchover, I had long thought it was because AM travelled further and that the Military wanted the AM frequencies for themselves, but that's for another thread.

Do you have any idea as to the kind of voltages should be present in order for the radio section to work?

[keeney123]

Originally Posted by Crystaleyes

To be honest, I am not 'sure' that the missing board is a decoder.

I had said that, as the schematic download came with three pages and one of them has a separate circuit that is labelled 'Decodificador' (decoder), and from photos I have found of complete radios (and empty holes in the PCB), there is definitely a board missing which sits between the radio and amplifier sections.

Totally agree with you regarding the vinyl and decent amp for sound quality, although this unit is a family inheritance so my friend isn't seeking top quality sound, just that it works!

As for the AM / FM switchover, I had long thought it was because AM travelled further and that the Military wanted the AM frequencies for themselves, but that's for another thread.

Do you have any idea as to the kind of voltages should be present in order for the radio section to work?

It has been 47years since I study Fm Radio. For the Transmission and Receiving voltage it will be small for where the radio is. There will be a lot of tuned circuits. This is the from end of the FM radio. There is a carrier frequency for both AM and FM that is fixed. I cannot remember the exact frequency for Fm. I believe it is somewhere up at 460 MHz. I believe all the FM channel ride on this frequency that is transmitted. This 460MHz then is stripped out at the intermittent Frequency stage. From what I remember by feeding the 460 MHz back into itself. Then you would have a different MHz tuning for the different channels this is where you would be tuning to a particular channel. These channels are separated by a certain amount so as not to interfere with the adjacent channels. The frequency modulation is done by controlling the frequency of one channel. Changes in frequency of that channel is the audio intelligence riding on the channel. This frequency change is then striped from the channel to create the audio signal that powers the amplifier. Unlike Amplitude Modulation Frequency modulation is very stable because it is not dependent to the amplitude of the signal for audio intelligence riding on top of. The varying of frequency even thou it is line of sight is much clearer because obstructions to not interfere with audio signal like they do this Amplitude. This is what I can remember, and I would make sure you check my old memory.
In modulation in radio, it is done with inductors and capacitors. The energy that flows back and forth between these components create and oscillation. When one has peak power of the oscillation it is called the quiescent point.
The military uses Phase modulation. In Vietnam they use phase modulation with match tunnel diodes. The diodes were made at the same time so they could only reproduce a match pair. This was done so no one other than the operator and receiver could tune into the frequency. They were equipped with a destruct button so they would not fall into enemy hands.

[R_J]

Originally Posted by Crystaleyes

To be honest, I am not 'sure' that the missing board is a decoder.

I had said that, as the schematic download came with three pages and one of them has a separate circuit that is labelled 'Decodificador' (decoder), and from photos I have found of complete radios (and empty holes in the PCB), there is definitely a board missing which sits between the radio and amplifier sections.

Totally agree with you regarding the vinyl and decent amp for sound quality, although this unit is a family inheritance so my friend isn't seeking top quality sound, just that it works!

As for the AM / FM switchover, I had long thought it was because AM travelled further and that the Military wanted the AM frequencies for themselves, but that's for another thread.

Do you have any idea as to the kind of voltages should be present in order for the radio section to work?

The missing decoder will likely be for the FM Stereo, without it you will only receive FM mono.
I would start by cleaning the switches with a cleaner like deoxit, The voltage for the radio section is from the * off R106, which goes to R44 and GR11 (zener) If I read it correctly BZY 94/C10 it will be a 10 volt zener. When FM is selected 10 volts should be at point X which connects to C95 (M75) and this powers the FM front end.
The AM section seems to be powered all the time and they likely just switch the audio from the am (OM) or fm sections, they will also switch different coils for the AM (OM) section to receive the different bands

[keeney123]

Thanks RJ for the help.

[Crystaleyes]

Originally Posted by R_J

The missing decoder will likely be for the FM Stereo, without it you will only receive FM mono.
I would start by cleaning the switches with a cleaner like deoxit, The voltage for the radio section is from the * off R106, which goes to R44 and GR11 (zener) If I read it correctly BZY 94/C10 it will be a 10 volt zener. When FM is selected 10 volts should be at point X which connects to C95 (M75) and this powers the FM front end.
The AM section seems to be powered all the time and they likely just switch the audio from the am (OM) or fm sections, they will also switch different coils for the AM (OM) section to receive the different bands

Excuse the delayed response, but thanks for that info.

I didn't actually get back onto that receiver although I have picked up a 'donor' which has the missing board in question, so when I get on it, I'll fill in th edetails here.

Cheers