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 Post subject: Sonora RCU 208 Restoration Help
PostPosted: Feb Sun 17, 2013 8:53 pm 
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Joined: Apr Sat 03, 2010 1:14 am
Posts: 39
Location: Virginia
I have a Sonora RCU 208 that I purchased at the Antiques Radio Show at Renninger's Market. It was advertised as able to power on but when I checked it, I was unable to get it to power up. Since then I have:
- Cleaned the variable capacitor
- Checked and replaced tubes as necessary
- Replaced all wax and electrolytic capacitors
- Checked resistors and replaced one
- Did continuity check on the switch which was o.k.
- This radio has no power transformation and I rewired it according to the AA5 safety instructions I found on-line

After all that work, it still does not power on, i.e. the has changed. The dial lamp does not light either. This is about the extent of my restoration skills and was wondering what else needs to be investigated and how to do it.


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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Feb Sun 17, 2013 9:04 pm 
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Tube base information here. http://www.nj7p.org/Tube.php.

If set is getting power and tubes are not lighting, one of them has open filament.


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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Feb Sun 17, 2013 9:36 pm 
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Location: Virginia
So even if the Tubes test good, there can still be an open filament?


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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Feb Sun 17, 2013 10:31 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 36069
Location: Livermore, CA
Hi

Tube filaments are wired in series along with a 33 ohm resistor (R8) & switch. If the resistor, tube, switch or line cord is bad no tubes light. A 35Z5 has a tapped filament. Some testers only operate part of the filament. This tube could still be bad and pass on a tube tester.

http://www.nostalgiaair.org/PagesByMode ... 018524.pdf

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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Feb Sun 17, 2013 10:57 pm 
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Joined: Apr Sat 03, 2010 1:14 am
Posts: 39
Location: Virginia
Thanks for the quick all replies! You have given me some more troubleshooting work to do Norm, thanks.

- I did test R8 (33 Ohm) but left both ends connected so I'll unsolder one end and test it again to be sure (took a guess nothing else connected in parallel to it).
- The switch has good continuity and I do measure ~120v between where the line from the swithc connects to the 35Z5 tube and the ground so I assume the line cord is good?
- I have another 35Z5 tube that I'll swap out and see if that makes a difference. All tubes were checked on a Simpson tester.


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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Feb Tue 19, 2013 4:34 am 
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Joined: Apr Sat 03, 2010 1:14 am
Posts: 39
Location: Virginia
Good news, I was able to get the set to power on, including the dial lamp, by swapping out
tubes from a working radio. Thanks for pointing out the bad filament.

However, I'm unable to recieve any stations. I get mostly noise by trying to tune the radio. Sometime there is also a hum. All caps have been replaced except the Mica cap.


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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Feb Tue 19, 2013 4:36 pm 
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Posts: 36069
Location: Livermore, CA
Do you have around 100 volts on pin #3 of 35L6? What voltage do you find on pin #6 and #8 of audio 12SK7. Both should be positive with pin #8 being higher.

Touch center terminal on the volume control. This should give hum in the speaker.

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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Feb Thu 21, 2013 1:28 am 
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Location: Virginia
I measured 116 vDC on pin #3 of 35L6 & 10vDC on pin #6 and 27.5 vDC on #8 of audio 12SK7.

The radio has a constant Buzz/Hum to it. I did replace both of the 40MFD electrolytic capacitors. The volume control has two terminals (no center terminal that I saw) and the Hum does go up slightly when I touch it with a screwdriver.


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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Feb Thu 21, 2013 2:56 am 
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Location: Livermore, CA
Voltages are close enough. Both on 12SK7 are a little lower than the schematic but not enough to cause hum.

What voltage do you measure on pin #6 of 12SQ7. This one should be positive around 50 volts. Does the amount of hum change with volume turned down? If you have another 35L6 or 50L6 try it in place of 35L6.

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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Feb Thu 21, 2013 3:57 am 
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Joined: Apr Sat 03, 2010 1:14 am
Posts: 39
Location: Virginia
I measured +60.5 vDC on pin #6 of 12SQ7. I hooked up the antenna this time (the back is in bad shape) and was able to pull in 1 am station however it is very raspy and LOTS of static.

The volume control seems to have no effect, it doen't seem to raise/lower the volume when turned nor effect the hum which is now mostly drowned out by the static. I looked for an opening to spray the volume control but didn't see one.

I also tried two other 35L6s, one from a known working set, and I couldn't tell the difference.


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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Feb Thu 21, 2013 7:10 am 
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Location: Livermore, CA
Not sure which is worse, hum or static. Shouldn't have much of either.

Hum is usually caused by electrolytic filter caps. Are they at least 40 mf @ 160 volts and new, not NOS (New Old Stock).

Disconnect one end of cap C7. It connects from plate of the 12SQ7 to ground. This cap can be responsible for static sound.

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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Feb Fri 22, 2013 1:18 am 
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PhantomF4 wrote:
The volume control has two terminals (no center terminal that I saw)

That would then be a problem as well, since the print shows R4, the VC, with it's center wiper connected to C4, which then connects to the 12SQ7 input grid.

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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Feb Fri 22, 2013 3:07 am 
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Joined: Apr Sat 03, 2010 1:14 am
Posts: 39
Location: Virginia
First, this discussion has been very beneficial to me. My restoration skills thus far have been limited to replacing capacitors and changing tubes. Doing the suggested checks and trying to see what you are referring to in the schematic has been educational, thanks to everyone for their time.

Now, maybe I can get a pass on a newbie mistake. I rechecked my new electrolytic capacitors that I installed and noticed one of them was 16MFD/160V as opposed to 40 MFD. The other capacitor is the correct size (40MFD/160V). Could that be causing the hum? I'll order another 40 MFD and install it but it could be a week or two to get one.

Fifties: I see the R4 & C4 connections your referred to in the schematic. Is Pin #2 the input grid for the 12SQ7 tube then? I don't see how you knew it was connected to the center wiper. I see two terminals on the inside face of the switch that has the hot line from the cord and the other side going to the 35Z5GT tube. There are other terminals on the chassis side of the volume control. Are those what you were referring to? I'm also thinking the switch needs to be sprayed but can't see a port.

Norm, I now see from the schematic the voltage readings you had me check. I wasn't exactly sure what the numbers were besides the pins. What if the voltages you had me measure were not correct? I assume I would back trace whatever was upstream (resistors, capacitors, tubes) from them and replace them.

Hope to report back after I get that 40MFD capacitor installed.


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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Feb Fri 22, 2013 3:25 am 
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Location: Livermore, CA
That should take care of hum. Order 47 mf @ 160 volts if you have a hard time getting one marked 40 mf.

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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Feb Fri 22, 2013 4:06 am 
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PhantomF4 wrote:

Fifties: I see the R4 & C4 connections your referred to in the schematic. Is Pin #2 the input grid for the 12SQ7 tube then? I don't see how you knew it was connected to the center wiper.

The arrow pointing toward the resistor symbol in the print signifies the center wiper. The other side of it is connected to C4, and that cap then goes to the input grid of the 1st audio detector, the 12SQ7. I'm not sure which pin, but if you Google that tube's basing diagram, you can learn.

PhantomF4 wrote:
I see two terminals on the inside face of the switch that has the hot line from the cord and the other side going to the 35Z5GT tube. There are other terminals on the chassis side of the volume control. Are those what you were referring to?

What you see are the switch lugs, the switch being piggybacked on the volume control. If you look further in, you should see the three VC solder lugs.

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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Mar Sun 03, 2013 2:25 am 
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Location: Virginia
Installed a new 40MFD capacitor. The radio still has a hum though but not quite as pronounced. I also played around with the antenna leads and was able to reduce the static and tune in the stronger stations. Defintely an improvement but I'm not happy with it yet.

- Still have a hum after recapping
- The band switch has no eftect. When switching bands, the A.M. station continues to play regardless of the band switch position
- It doesn't consistently tune in the stations, I have to either play with the dial and/or the antenna to get a station tuned.


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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Mar Sun 03, 2013 3:14 am 
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Joined: Nov Sat 27, 2010 6:15 pm
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Sounds like you need to work to clean up bandswitch contacts, controls, and check the windings in the front end. I use deoxit-5 spray and let it dry overnight. Other opinions abound. If there is any evidence of hacking, or deliberate defeating of shortwave bands, it must be set back correctlly if you care to listen to other bands. If the set was mine, I would probably replace each and every paper and electrolytic capacitor and all carbon resistors - makes a huge difference in performance and long term reliability, but costs very little to do.


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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Mar Sun 03, 2013 3:19 am 
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PhantomF4 wrote:
I also played around with the antenna leads and was able to reduce the static and tune in the stronger stations.

What exactly did you do?

PhantomF4 wrote:
- Still have a hum after recapping

Put the set on it's back, so the underside of the chassis is accessible, plug it in, turn it on, and tune to a quiet spot on the dial, i.e., no stations, so you just hear the hum, and increase the volume.
Keeping one hand on your lap, and using a non-metallic probe, like a plastic twiddle stick, or wooden chopstick, gently try moving wiring and components slightly, to see if you can affect the volume of the hum.
When the set was originally wired, there was a specific position for everything, and in recapping, you very likely moved things around.
PhantomF4 wrote:
- The band switch has no eftect. When switching bands, the A.M. station continues to play regardless of the band switch position

There is no band switch on this set; the center control is for tone.
PhantomF4 wrote:
- It doesn't consistently tune in the stations, I have to either play with the dial and/or the antenna to get a station tuned.

You'll have to elaborate a little on this; what happens when you adjust the tuner to the frequency of a station in your area?

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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Mar Tue 05, 2013 2:50 am 
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Joined: Apr Sat 03, 2010 1:14 am
Posts: 39
Location: Virginia
I did try moving connections around but it didn't effect the hum. Both 40MFD capacitors have voltage readings so I know those are working. I did compare the hum to other non-capped and re-capped radios that I have and this radio is a little worse but not sure I can make it much better. There also tends to be some rasp/scratchiness when tuning in the stations.

It does pull in many of the local stations which I'm happy about. The antenna leads are not soldered to the antenna so I have to hold the leads and play with the tuner to get stations. The stations of course come in much stronger when I'm touching the leads. Is there any other way to enhance the reception? Maybe a different antenna? I have a Motorola 5X11U with a "high efficiency loop" that has no trouble pulling in the local stations.

Good catch on the lack of a band switch. The glass dial also has a meter scale and that threw me off.


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 Post subject: Re: Sonora RCU 208 Restoration Help
PostPosted: Mar Tue 05, 2013 8:34 am 
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PhantomF4 wrote:
I did try moving connections around but it didn't effect the hum. Both 40MFD capacitors have voltage readings so I know those are working. I did compare the hum to other non-capped and re-capped radios that I have and this radio is a little worse but not sure I can make it much better. There also tends to be some rasp/scratchiness when tuning in the stations.

If the filter caps had no voltage, the set wouldn't work at all. You might try an old trick, reversing the AC plug in the wall receptacle.

PhantomF4 wrote:
This radio has no power transformation and I rewired it according to the AA5 safety instructions I found on-line

I just noticed this. What exactly did you do? This may have some effect on the hum.

AFA scratchiness when tuning, it sounds like one or more of the rotor plates are touching the stators at certain points.

With the set unplugged, rotate the tuner slowly, looking to see if there are places where the plates touch. You can gently bend them straight, as necessary.

PhantomF4 wrote:
It does pull in many of the local stations which I'm happy about. The antenna leads are not soldered to the antenna so I have to hold the leads and play with the tuner to get stations. The stations of course come in much stronger when I'm touching the leads. Is there any other way to enhance the reception? Maybe a different antenna? I have a Motorola 5X11U with a "high efficiency loop" that has no trouble pulling in the local stations.

Are you saying that the set's loop antenna leads are not soldered to the circuit? If so, you need to do that.
The signal is stronger when you touch the leads because of your body's capacitance to ground, which is normal for most any AM radio.
Usually, the antenna that comes with the particular radio is matched to it, so it's the best one to use. Using an external loop along with it, such as a Selec-A-Tenna, can help quite a bit. They are merely placed behind the antenna, and the RF is inductively coupled to the set's loop, increasing the gain and lowering the noise floor.

PhantomF4 wrote:
Good catch on the lack of a band switch. The glass dial also has a meter scale and that threw me off.

Most of the older sets did have two scales; Kilocycles, and meters (I think just to make the dial look more serious).

You need to pay more attention to the service info, if you are going to continue fixing old sets.
The tone control was clearly marked in the pictorial diagram, and there was no band switch showing in the print, at the front end part of the circuit, where it would be placed. :wink:

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