Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Sep Fri 21, 2018 3:11 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 8 posts ] 
Author Message
 Post subject: RCA K-80 detector circuit mod
PostPosted: Feb Thu 15, 2018 3:06 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 19036
Location: Warner Robins, GA
I have an RCA K-80 console which has the RC-415A chassis that has the volume control in the detector circuit which puts DC on the volume control.

So far in the nearly 20-25 years I've had the console the control has given no noticeable trouble and has never been scratchy.

Having done some amp mods a few years ago to make the console use 6V6 tubes after replacing the original power transformer which got a bit too warm for my liking the audio gain was higher, but the volume control wasn't giving any trouble so I thought as the background noise from my PC fans and the window unit AC was quite noisy.

I moved the console into the new to me doublewide and noticed that with my two tube broadcaster in the same room I could hear audio with the control full CCW since the room is much quieter than my building is and also the audio gain was too high.

Fixed the gain by using a 6SN7 as the first audio tube and phase splitter which also reduced the potential for audio to leak from the detector diode to the first audio tube which tubes like the 6SQ7 would sometimes do.

Still had the volume control issue.

Sometimes the sound would be lacking in bass and a little louder at full CCW and turning the radio on and off solved it sometimes, but occasionally it came back.

Could the fact that DC is on the volume control be causing some of the issue?

If so would it be best to install a 250K resistor in place of the 250K volume control and cap couple to the tuner position of the input switch?

If that will work can I still use the 250K control or will that reduce the audio level and/or cause any other issues such as loading the detector too much?

Here's the original circuit.

Attachment:
RCA original.jpg
RCA original.jpg [ 51.1 KiB | Viewed 596 times ]


Here's the modified circuit.

Attachment:
RCA modified.jpg
RCA modified.jpg [ 51.8 KiB | Viewed 596 times ]


Is there any real benefit to removing the DC from the volume control since it has worked for so long as is?

I do plan on removing the volume control, taking it to work and seeing if I can fix the issue with it. Think it only goes down to something like around 120 ohms at full CCW measuring from the wiper terminal to ground and from prior experience with a Zenith radio that incorporated the feedback into the volume control I know 120 ohms is enough to let a little audio through.

Also as I recall I could short the wiper terminal directly to the volume control terminal that is grounded and the console went totally silent far as audio was concerned.


Top
 Profile  
 
 Post subject: Re: RCA K-80 detector circuit mod
PostPosted: Feb Tue 20, 2018 3:44 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 19036
Location: Warner Robins, GA
In dealing with the receiver over the last few days I do believe there is indeed too much audio gain even with the mod I did using the 6SN7 so I will be installing the mod to the detector circuit to remove the DC from the volume control.

As far as the detector goes the DC voltage will be the same.

I know the impedance will be lower given it will now see 125K instead of 250K.

What I don't know is how that will affect the audio quality. I'm sure it will reduce the level slightly, but that is ok.


Top
 Profile  
 
 Post subject: Re: RCA K-80 detector circuit mod
PostPosted: Feb Tue 20, 2018 4:35 pm 
Member
User avatar

Joined: Jan Fri 02, 2009 11:32 pm
Posts: 533
Location: Equinunk PA 18417
That .1uf should be increased to at least 10uf, otherwise you may notice low frequency roll-off at lower settings of the volume control. The cap forms a 1st order high pass filter with the volume control. For instance, for a low volume setting with tap at 2k from ground, a .1uf cap will reduce frequencies below 1kHz, around -3db at 800hz, -6db and 400, -9 at 200, -12 at 100hz. Different settings of the volume control will produce different results. Fletcher-Munson would not be happy! Is the volume control wirewound?

_________________
-Richard Fairbanks


Top
 Profile  
 
 Post subject: Re: RCA K-80 detector circuit mod
PostPosted: Feb Tue 20, 2018 4:42 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2191
Location: Saskatoon
TubeRadio, I think you need to take a closer look at the schematic. I had a look at the one on NostalgiaAir, and it doesn't look like what you've drawn. I don't see any DC on the volume control certainly not on the wiper, because it has a DC blocking capacitor.


Top
 Profile  
 
 Post subject: Re: RCA K-80 detector circuit mod
PostPosted: Feb Tue 20, 2018 5:08 pm 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 23352
Location: Annapolis, MD
I see only the AGC DC voltage going through the switch to the top of the VC. Hard to see why that would be an issue.

_________________
-Mark http://pixellany.com

"It's always something". --Gilda Radner (1946 - 1989)


Top
 Profile  
 
 Post subject: Re: RCA K-80 detector circuit mod
PostPosted: Feb Tue 20, 2018 7:00 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 19036
Location: Warner Robins, GA
richfair wrote:
That .1uf should be increased to at least 10uf, otherwise you may notice low frequency roll-off at lower settings of the volume control. The cap forms a 1st order high pass filter with the volume control. For instance, for a low volume setting with tap at 2k from ground, a .1uf cap will reduce frequencies below 1kHz, around -3db at 800hz, -6db and 400, -9 at 200, -12 at 100hz. Different settings of the volume control will produce different results. Fletcher-Munson would not be happy! Is the volume control wirewound?


Good eye Rich.

The schematic is actually drawn in error.

The wiper should be going to the .0047uF capacitor.


BobWeaver wrote:
TubeRadio, I think you need to take a closer look at the schematic. I had a look at the one on NostalgiaAir, and it doesn't look like what you've drawn. I don't see any DC on the volume control certainly not on the wiper, because it has a DC blocking capacitor.


The RC-415C chassis uses a dc blocking capacitor and a 2 meg volume control.

Mine as stated in the first post uses the RC-415A chassis which uses a 250K volume control directly in the detector circuit. It's not very easy to see where it connects on the riders schematic though as it takes tracing wires and figuring out how the switch is shown on the schematic and what direction it should turn.

http://www.nostalgiaair.org/pagesbymode ... 015513.pdf

Page 6.


Here's the corrected schematic.

Attachment:
RCA modified 2.jpg
RCA modified 2.jpg [ 52.05 KiB | Viewed 511 times ]



Looking at the original schematic I see two problems with what I am thinking of doing.

1. The 22K resistor which is in the IF transformer and is where the audio signal comes from that goes to the source switch.

As it is now for a 1 Vac input I get 919 mVac across the volume control.

If I mod the detector like I planned the impedance would be halved so for the same 1Vac input I would get 850mVac across the volume control.

The problem with that is this.

The service data doesn't list any gain data of any sort so I don't know what the gain of the stock amp circuit would have been and I sure don't know what the gain of the modified amp is nor do I know what the gain should be. I want it so that on a stronger station I get audio distortion around 3/4 CW rotation of the volume control.

2. The addition of a coupling cap which to the AGC line will look like this

Attachment:
circuit.png
circuit.png [ 5.21 KiB | Viewed 511 times ]


That would muck with the AGC time constant wouldn't it?

Here's the schematic showing the detector circuit

Attachment:
RCA.png
RCA.png [ 166.83 KiB | Viewed 511 times ]


Top
 Profile  
 
 Post subject: Re: RCA K-80 detector circuit mod
PostPosted: Feb Wed 21, 2018 2:29 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2191
Location: Saskatoon
Temporarily disconnect the volume control wiper lead and ground it. Or else, turn the volume control all the way CCW and connect a jumper from the wiper to a good nearby signal ground. If you no longer pick up any audio, then the volume control is at fault. This can happen when the ground end of volume pot has a bad connection. It could be either a bad solder connection (but I assume you've already checked that) or the rivet connecting the resistance element is making poor contact.

If you're still picking up stray signals with the input grounded, then the audio stage is picking up and detecting the stray RF. This is likely due to the modifications that you made to the circuit. This is the risk that you run when you take a well designed circuit and try to "improve" it.


Top
 Profile  
 
 Post subject: Re: RCA K-80 detector circuit mod
PostPosted: Feb Wed 21, 2018 3:44 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 19036
Location: Warner Robins, GA
When I do a mod I always think it through thoroughly to see if it will actually improve performance or correct a deficiency and if I am unsure I will ask here on the forums.

That said when I installed the 6V6 tubes the console did work good with a good bit of extra gain. Think at the time which was maybe around 10 years ago the 6F6 tubes were a bit expensive and didn't want to spend the money. At some point I realized one operating point for the 6V6 tubes could produce 10 watts of audio so I said why not and proceeded to mod the amp to where I could get closer to 10 watts. Never really used the console much so I didn't pay much attention to the audio gain issue I had created which was further added to by making the second 6SQ7 (was a 6SF5 originally) a self split phase inverter which is better than the original phase splitter circuit RCA used. The extra B+ current helped bring B+ down closer to where it should be as I had used a power transformer from an amp that put out a little higher B+ voltage. I also installed a different RCA speaker with a little higher field coil resistance than stock (original working speaker suffered at my attempts to make the console HI-FI maybe 14 years ago) which helped with the B+ drop.

One other reason for going with the 6SN7 is it solved the potential issue some tubes like the 6SQ7 have of the audio being coupled from the diode itself to the triode. The primary purpose though was a much needed reduction in audio gain.

The audio was silenced when I grounded the volume control wiper.

I know it's the control as no matter what I have done I can get resistance with the control full CCW. Usually around 1.5K, but when I turn it full CW then back I get around 150 ohms plus the resistance element shows a good bit of wear at full CCW.

Think it may have always been a problem, but didn't notice it until I moved the console into a mostly quiet room.


Decided to try the detector mod and the added coupling cap didn't affect the AVC action at all and thinking more about it I realized the circuit of the detector is likely low impedance so the equivalent circuit I posted would not have much effect.

The audio without the mod was 2.457 Vrms at 400Hz.

With the mod it is 1.756 Vrms

And when I turned the volume control up I could still get distortion at around or slightly above 3/4 CW rotation of the volume control and that was without the part 15 transmitter being received its strongest as the eye was open maybe a slight bit and AVC was -11 Vdc.

The lower audio signal may completely eliminate the volume control issue for now until it gets worn enough to where I need to have Mark Oppat fix it.

The extra audio gain may be beneficial if I decide to connect an external input to the console which if I did it would be a record player that can handle 78s given many replacement ceramic cartridges don't quite put out the same audio signal level as a lot of original cartridges did that would have been around when this console was made.

So I will do a test receiving my part 15 transmitter being fed some music and see what I get.



EDIT:

Tested the console and so far so good.

Receives AM stations quite well when connected to the antenna I have run outside.

Receives my two tube transmitter quite good as well.

No audio distortion at all when listening at a normal volume level, but I'm sure will still distort at a little above 3/4 CW rotation of the volume control.

Also the audio that was present with the volume control full CCW has been further reduced with the lower output from the detector which is due to the 22K resistor dropping more audio voltage.

Now to update the main amp schematic with the detector mod plus fix the volume control wiring error on that schematic.

One benefit of this mod is I am no longer limited to using a 250K volume control which will make it easier to find a suitable replacement should it ever need to be replaced.

The .1uF coupling cap I originally selected was so there was no roloff of the bass as the roloff doesn't happen until the second audio stage using the .0047uF coupling cap. Plus using that value will lessen the phase shift at the lower audio frequencies. A .22uF cap got used as that was what I had on hand which is even better.

The .0047uF cap at the second audio stage starts slowly rolling off the bass at 70Hz and is maybe 2-3 dB down at 40Hz.

The .01uF caps for the power amp stage start rolling the bass off at 70Hz and is 2-3 dB down at 40Hz.

Makes it sound like a typical console radio.

Here's the full amp schematic.

Attachment:
RCA K-80 amp.png
RCA K-80 amp.png [ 54.48 KiB | Viewed 488 times ]


Top
 Profile  
 
Post New Topic Post Reply  [ 8 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: Dare4444, gtb81 and 1 guest



Search for:
Jump to:  
























Privacy Policy :: Powered by phpBB