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 Post subject: Sylvania RM-300X
PostPosted: Jul Mon 29, 2019 5:45 am 
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Location: Warner Robins, GA
I have one and was wondering if anyone here who has one or a RM-300R would be able to check the audio gain from the AM or FM audio at the function selector switch to the volume control with the bass and treble controls set to flat.

I disconnected the loudness taps on mine a few years back as I didn't like that feature and it changed the tone controls some and might have caused an increase in gain as the volume control seems overly sensitive.

Would like to compare it to what I get for gain.


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 Post subject: Re: Sylvania RM-300X
PostPosted: Jul Mon 29, 2019 1:13 pm 
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Location: Utica, NY 13502 (USA)
The "loudness" circuit usually consists of a resistor in series with a capacitor connected between the volume control's loudness tap and ground. This forms a low-pass filter giving the effect of an increase in bass response at low volume levels. Removing the circuit will make the control seem more sensitive until the control's wiper reaches the area of the loudness tap. Shorting out the associated capacitor will remove the bass increase but still maintain the control's original action at low volume levels.

Some loudness circuits are part of a negative feedback loop. The circuit will need to be analyzed to see where modifications can be made.

Dave


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 Post subject: Re: Sylvania RM-300X
PostPosted: Jul Mon 29, 2019 3:52 pm 
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Oh ok I see now.

So I should have left the resistors in place then and just shorted the caps.

Here's the stock amp schematic.

Attachment:
Amp 1 original.png
Amp 1 original.png [ 97.62 KiB | Viewed 437 times ]


Attachment:
Amp 2 original.png
Amp 2 original.png [ 123.9 KiB | Viewed 437 times ]


Here's how it is now.

Attachment:
Amp 1.png
Amp 1.png [ 80.17 KiB | Viewed 437 times ]


Attachment:
Amp 2.jpg
Amp 2.jpg [ 163.5 KiB | Viewed 437 times ]


I want to say the bass and treble controls were boost/cut with the loudness taps connected and are now cut only with the loudness taps not connected, but I cannot remember back that many years ago as it wasn't too long after I got the radio that I removed the taps.

EDIT:

Here's the equivalent resistance of the volume control if the caps are bypassed.

Attachment:
Volume control resistance.png
Volume control resistance.png [ 2.65 KiB | Viewed 432 times ]


The total resistance will now be 229.788K.

C51 I either upped to .68uF or 1uF (cannot remember, but will look when I pull the chassis to try adding the resistors back) and if it is .68uF I will need to up it to 2uF to eliminate any chance of phase shift.

That might also affect the bass control and make it start rolling off the bass at a higher frequency given that the impedance seen by frequencies below what the caps attenuated would have been around 500K.

Perhaps the treble control might be affected as well.

It acts just like there's too much gain and I see why. With the first audio transistor seeing a load of 500K with the bass and treble controls full CW, there would be more output being fed to the volume control.

Now I can try adding those resistors and see how it affects the sound.

If it negatively affects the sound, I will need to add a series resistor to drop the signal somewhat, but I don't see any place I can add a resistor without potentially affecting the sound in some way shape or form.

If I add a resistor between the AM/FM selector switch and Q10, that will raise the impedance and may throw off the FM de-emphasis filter and could also cause more of a Miller capacitance issue.

If I add a resistor before the tone controls, that may affect the treble control.

If I add a resistor after the tone controls that may affect the tone controls.

The only place I can see adding a resistor is in series with C55 given with how Q11 and 12 are wired, Miller capacitance won't be an issue.


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 Post subject: Re: Sylvania RM-300X
PostPosted: Jul Mon 29, 2019 8:46 pm 
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Location: Utica, NY 13502 (USA)
Just short C54 and C55. That volume pot has 2 loudness taps. No other caps are involved in the loudness compensation circuit. Some loudness compensation circuits also boost treble at low volume settings. This circuit doesn't do that. You are certainly free to make any other changes that you wish but shorting C54 and C55 will make the volume control adjustment range less abrupt at low volume settings without introducing any tonal alterations.

Dave


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 Post subject: Re: Sylvania RM-300X
PostPosted: Jul Mon 29, 2019 9:16 pm 
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I'll try that and see how it affects the sound.

I got metal film resistors of the proper values so I will connect those between the taps and ground.


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 Post subject: Re: Sylvania RM-300X
PostPosted: Jul Mon 29, 2019 9:23 pm 
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Location: Utica, NY 13502 (USA)
Great!

Dave


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 Post subject: Re: Sylvania RM-300X
PostPosted: Jul Mon 29, 2019 9:34 pm 
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Much easier to try those resistors than to trace out the PC board to find where the carbon comp resistors on the board are given the amount of wiring there is along with the compactness of the design.

EDIT:

Will try the resistors in a bit.

Going to use a RF generator to provide an AM signal to the radio then modulate it to full modulation and see how much signal is at the input to the first audio stage.

I can then feed a frequency of that level to the input I added to the radio.

That way I have a proper signal to see just how sensitive the volume control is.

I'll then turn up the volume control until right before the point of distortion.

Will compare the gain with and without the resistors and will also see how the bass control, is affected by the resistors given the lower impedance.

I did notice something weird though.

Feeding a 400Hz signal to the external input, I noticed that when I turn the treble control CCW, the 400Hz signal drops some then comes back up to the level it was at, although it now has a phase shift.


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 Post subject: Re: Sylvania RM-300X
PostPosted: Jul Tue 30, 2019 7:52 pm 
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Well instead of using a modulated RF generator I just fed an audio signal at a level to where the first audio stage was right before the point of distortion.

The resistors helped a great deal.

With an output of 2.5Vpp 400Hz at the wiper of the volume control with the bass control full CCW I get the following.

Resistors installed I get 1.47Vpp

No resistors installed volume control adjusted to 2.5Vpp I get 1.96Vpp so it changed the operation of the bass control somewhat. Will test it tonight and see how well it all works.

EDIT:

Tested it and the volume control is much less sensitive, but I don't like how high in frequency the bass control cuts the audio level.

Otherwise things are ok and the audio sounds good.

I may put another cap of the same value in parallel with the existing bass control cap to see how it affects the frequency at which the bass starts to be reduced and how it affects the range of the bass control.


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 Post subject: Re: Sylvania RM-300X
PostPosted: Aug Fri 02, 2019 4:43 am 
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I like how the volume control works now, but with the audio output I have which is buffered by an OP-AMP to drive a balanced/unbalanced to balanced/unbalanced transformer to properly interface with a full range plate amp with balanced input using 1/2 the primary to step up the voltage some, it isn't enough to drive the amp properly without the input and master level controls set to 3/4 CW.

So given the issue with the bass control not being right as well I'm thinking of removing the resistors and use a single series resistor after the coupling cap to the second audio stage. That way I get more output for the plate amp while the radio's amp isn't driven as hard.

It will have the same effect as the resistors I think, but will provide more output signal.


Unless there's a better way to do it.

EDIT:

Here's what I did.

I set the input level to right before the point of distortion in the first stage as that's what I did when I initially tested the volume control with the taps connected.

I then set the volume control to where the audio was right before the point of distortion which was at the second tap.

I disconnected the resistors without moving the volume control any.

It took a 1 meg series resistor to drop the level down enough to where it just was before the point of distortion.

Wanting to keep the resistance about the same so the bass control would be right I tried a 400K resistor in series with the wiper of the volume control and at the other end of the resistor I put a 400K resistor to ground. That worked about the same as the 1 meg resistor.

I then calculated the resistance of the existing circuit and it was 260K without the two resistors on the loudness taps.

I calculated the reistance with the resistors added and it is 278K now.

So the load seen by the first audio stage now varies between 500K and 278K based on where the volume control is set.

Will test it later today and see how well it works.

That should give me more signal to the external audio output.

I raised the B+ to the OP-AMP to 21Vdc so it could handle the maximum audio level the first audio stage puts out without distortion.

EDIT 2:

I like the increased signal available to the OP-AMP driving the audio output but I noticed two things.

1. There's a slight bit of audio present in the internal speaker with with volume control full CCW which might be stray coupling as it has no bass and very little midrange.

2. The volume control is still sensitive, but not as much as before.

So it looks like I'll go back to using the resistors and just give the OP-AMP a gain of 2 along with trying a higher value of cap for the bass control.

What I'll do is set the volume control at a certain voltage at 100Hz and see the voltage level I get with the bass control full CCW. I will then select a cap that gives me the same voltage with the volume control resistors in place after setting the voltage at the volume control to what it was before adding the resistors. I did notice with the resistors the audio voltage was a good bit lower though.

Will solve all my issues.


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 Post subject: Re: Sylvania RM-300X
PostPosted: Aug Fri 09, 2019 1:24 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20205
Location: Warner Robins, GA
Using the resistors and giving the OP-AMP a gain of 2 seems to work good.


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