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 Post subject: Saba 80 US (No sound)
PostPosted: Jan Sat 27, 2018 7:34 pm 
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Joined: Feb Mon 13, 2017 10:46 pm
Posts: 158
Location: Atlanta GA
Hello,

Been a while since I worked on one of these German radios from the early 60's so hopefuly somebody here can get me pointed in the right direction so I do not waste too much time chasing the wrong thing. This radio is extremely clean inside and out. It powers up and I get a faint hum out of the speaker when I put my ear to the speaker grill. The green tuning eye works as it should when I turn the tuning dial to known stations. Of course I do not get any audible confirmation from the speaker but clearly the radio is tuning into FM stations accross the dial since the green eye closes fully as I go from station to station. So, assuming the switches and volume pot are clean what should I look for first to determine why I am not getting sound from the speaker?

Thanks
BB


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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Sat 27, 2018 9:50 pm 
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Location: San Jose, CA USA
Doing detailed troubleshooting of the audio section stage-by-stage is not hard, but a few quick things to look for when you first open it up:

1. Audio tubes glowing?

2. Output transformer blown? (A common problem if the radio was operated with original paper plate coupling cap from 1st audio to output tube)

3. Output cathode resistor blown? (same cause as above)

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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Sat 27, 2018 10:42 pm 
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Joined: Feb Mon 13, 2017 10:46 pm
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Location: Atlanta GA
Yes, all tubes are glowing. If I am getting a slight hum from the speaker doesn't that mean that the output transformer should still be good? Seems like if the output transformer was blown then I should get no sound what so ever?


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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Sat 27, 2018 10:59 pm 
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Location: San Jose, CA USA
You're right. If you have a faint hum in the speaker, then all three of the suggestions I mentioned above wouldn't apply.

Try a "buzz" test by turning up the volume and touching the center terminal of the volume control (either with your finger, or if you don't feel safe doing that, use a piece of wire a couple feet long). If you get a buzz, then the audio amplifier section is working at least to some extent, and I would focus on whether audio is getting through the switching network between the FM detector and the 1st audio tube.

If you don't get a buzz on the volume control, then try touching a piece of wire on grid #1 of the output tube. Here you should get at least a faint hum. If you do get a hum here but not on the volume control, focus on the circuitry around the 1st audio tube. If you don't get even a faint buzz here, focus on the output tube circuitry, checking voltages on the tube, looking for bad components, etc.

Since this radio is probably old enough to have paper capacitors, it's likely that you'll need to do a full recap of the radio. If some of these quick tests don't lead to a diagnosis, you may want to start with a recap of the power supply and audio amplifier sections for starters and see if you can get the audio amplifier section working. The tuner sections won't have too many more paper caps, but there may be a few that you should also replace.

By the way, you mentioned "assuming the switches are clean...". The function selector switches on German radios in general are highly prone to dirty contacts. One of the most likely failure points in the radio. So unless you have good reason to assume the switches are making good contact, you need to check them and clean them, and verify that they are making good contact.

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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Sat 27, 2018 11:19 pm 
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Joined: Feb Mon 13, 2017 10:46 pm
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Location: Atlanta GA
Tom,

Great info. Do you think before I go any further with powering this radio up I should go ahead and replace the electrolytic caps? Of course I will also replace all the paper caps. This lack of audio output from the speaker really surprised me on this radio since it still looks so new. I do understand that those "switches" on these German radios can cause this problem and will first clean them once I get the chassis out of the case. I will also see if I get any audio from other sources like the tape input and the SW bands.

Thanks for your help,
BB


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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Sat 27, 2018 11:33 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
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Location: Near Brandon, Iowa
bluebellyyankee wrote:
Tom,

Great info. Do you think before I go any further with powering this radio up I should go ahead and replace the electrolytic caps? Of course I will also replace all the paper caps. This lack of audio output from the speaker really surprised me on this radio since it still looks so new. I do understand that those "switches" on these German radios can cause this problem and will first clean them once I get the chassis out of the case. I will also see if I get any audio from other sources like the tape input and the SW bands.

Thanks for your help,
BB

Replace the electrolytic filter caps before you power up the set again. Resist the urge to replace any of the paper/foil caps until you get the set working. And test the audio stage tubes; the EABC80 or EL95 tube can fail and cause the problem that you are describing (apparent reception as indicated by the magic eye tube but no audio output).

If still no audio after vetting the tubes, look for a coupling cap in the audio stages that has gone open.


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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Sun 28, 2018 12:01 am 
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Location: Mountains of Mourne. Ireland.
https://www.radiomuseum.org/r/saba_80us80_u.html

Your radio is the export version of the German (domestic) model Königsfeld 14
https://www.radiomuseum.org/r/saba_koenigsfeld_14.html

Königsfeld 14 is the same as Villingen 12
https://www.radiomuseum.org/r/saba_villingen_12.html

Villingen 12 schematic here...
https://www.doctsf.com/grandlivre/fiche ... 691&ori=ps

Greg.

image: http://www.saba.pytalhost.com/1964/0003.jpg


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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Sun 28, 2018 3:09 pm 
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Joined: Feb Mon 13, 2017 10:46 pm
Posts: 158
Location: Atlanta GA
This morning I removed the chassis and promptly found this little fuse that is mounted on this transformer was corroded/crusted pretty badly. Touched both ends with an ohm meter and no continuity. So I removed the fuse, cleaned the ends off and tested it again and still no continuity. So, I assume that is the output transformer that the fuse was mounted to and this is the reason why I am not getting sound from the speaker? The schematic shows it to be rated at 0.035A. Besides vibration from the radio being shipped, what would the probable cause for this fuse to blow? Looking into the fuse with a magnifying glass I see no burn mark, only the tiniest filament is visible. Any idea where to get a fuse this small?

Thanks,
BB

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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Sun 28, 2018 3:38 pm 
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Location: Mountains of Mourne. Ireland.
Image

this 0.035A fuse is discussed here on the Saba-forum,
with one poster suggesting to bridge it!.. Best to ask lorenz200w or Tom... :)
http://saba-forum.dl2jas.com/index.php/ ... Sicherung/

(sketchy) translated page...
https://translate.google.com/translate? ... cherung%2F

Greg.

Edit: Components...
which ones to replace, and which ones to leave alone.

Image
5. Erofol II (not in the picture).

http://saba-forum.dl2jas.com/index.php/ ... gsfeld-14/

https://translate.google.com/translate? ... feld-14%2F


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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Sun 28, 2018 8:50 pm 
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Location: San Jose, CA USA
That's a very low current fuse with a very fine wire. It's possible it blew in the past to protect something in the output stage, or it might simply have corroded over the years because of how fine the wire is.

As discussed in the German forum, most German radios don't have a fuse at this point in the circuit, so it is not unreasonable to simply bridge it. However, before doing that, I would make sure all paper caps in that part of the circuit are replaced.

If you can find a replacement fuse with such a small current rating, that would be ideal, but OK to bridge it, as long as you keep your eyes, ears, and nose on alert when you power it up to make sure you don't have too much current flowing in the output stage. Checking voltage on R439 on the cathode of the output stage is another way to make sure current is correct. The correct voltage is partially cut off on that snippet of the schematic above, but I'm pretty sure it simply says 6 volts. That would be 18 mA, which is well within a reasonable current range to protect with a 35 mA fuse. The next possible value would be 16 volts, which would be 48 mA, so that can't be it -- the fuse would blow. A check in a tube manual for correct bias on an EL95 suggests that about 9 V would be correct grid bias. So it is possible that the schematic value is actual 8.6 or 9.6 V. Anyway, that's the ballpark you want to see for the cathode voltage. Make sure the resistance of R439 is close to its correct value of 330 ohms.

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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Sun 28, 2018 9:17 pm 
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Location: Near Brandon, Iowa
Since you earlier in the thread reported getting a humming from the speaker, the fuse was probably OK until you disturbed it. It likely failed mechanically due to cap-end corrosion and was not electrically "popped".

This is one of the last years for Saba tube radios. I have restored many earlier Sabas, and none of them have had this fuse. More than other radio makes I've seen, 50s and 60s Sabas seemed to be susceptible to output transformer primary burnout when the tiny bypass cap between the output tube plate and ground shorts out, which it seems to do more frequently than one would expect. This overcurrents the OT primary and burns it out. I'd guess that this fuse was added as a belated attempt by the Saba engineers to protect their output transformers from "sudden burnout syndrome" in the event of cap failure.

You can temporarily bypass the fuse with a jumper wire, but I recommend that you first replace the plate bypass cap. Also replace the coupling cap from the plate of the first audio amp tube (EABC80) to the grid of the EL95 tube, as suggested earlier in the thread. If this latter cap is leaky it can also overcurrent the output transformer primary (along with the output tube).

Small-value Buss-style fuses are sold for use in test equipment.


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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Sun 28, 2018 9:48 pm 
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Location: Atlanta GA
So this time I powered up the radio without that fuse in place and I actually still get the very slight hum through the speaker. So there is a slight hum whether or not the fuse is inserted or removed. Keep in mind it is a very slight hum that you have to put your ear up to the speaker to detect it. I checked resistor number 439 and it does read exactly 330 ohms however I am not removing it from the circuit to test it but since I get 330 with it still installed on the circuitboard I think it’s OK.

Clarify for me what the risk would be if I temporarily bridged this fuse just to power the radio up again to see if I get some sound through the speaker.

Any chance you could tell me what number the bypass and coupling capacitors are on the schematic that you referred to that I should replace off of the EABC 80 tube?

Is there a easy quick way to bench test this output transformer that this fuse was bad on?


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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Sun 28, 2018 11:36 pm 
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Since part of the B+ flows through the output transformer in a hum bucking arrangement, it makes sense that you still get a slight hum even with the fuse blown.

For a very short test (a few tens of seconds), it's OK to bridge the fuse and try it.

The most important paper cap to replace to protect the output tube is C415 (22 nF or .022 uF) on the schematic for the Koenigsfeld at Radiomuseum.

If you replace this capacitor, there should be minimal risk of blowing something up if you bridge the fuse.

You'll want to replace all electrolytic and paper caps in this set fairly soon, but for brief testing, simply replacing C415 helps reduce the risk a lot.

Quick easy bench test for the output transformer is simply to measure the resistance of both the primary and secondary windings. The primary should measure a couple hundred ohms, and the secondaries a few ohms each. This output transformer is unusual in that it has a separate secondary winding as part of the bass tone control feedback circuit.

Another quick test of the output transformer and speaker is to touch a battery across the primary winding with the set turned off. You should get a click in the speaker if speaker and transformer are good.

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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Sun 28, 2018 11:57 pm 
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Joined: Feb Mon 13, 2017 10:46 pm
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Tom

I have a new 20000 pf or .020 microfarad capacitor leftover from a recent overhaul of another radio. Any problem with reducing the original (#415) capacitor value from .022 down to .020? I also found one paper 470 picofarad (#424) and removed it. It is reading 730 picofarads on my DMM. I know that a DMM is not the best way to check capacitors but since it reads so much higher than 470 do you think I would be okay to re-install it just to do a quick test of the radio once I install this new .020 capacitor in place of the original .022 capacitor?

Thanks
BB


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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Mon 29, 2018 12:29 am 
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Using the .02 uF capacitor is fine to replace the .022 uF. Make sure the replacement has the same or higher voltage rating as the original.

The 470 pF cap is a bit of a concern. The reason why caps may read high on a DMM is because they have resistive leakage in parallel with their capacitance, which the meter interprets as additional capacitance. Since that cap has the full B+ on it, leakage can be a bit of a risk, particularly if it leads to a short. But the risk here is lower than for the .022 uF cap mentioned above. So it's OK to test briefly with the 470 pF cap left in place for now.

However, if you find that with the fuse replaced and the .022 uF cap replaced the radio appears to work just fine, resist the temptation to simply leave it that way and use the radio for prolonged periods of time. Trouble is brewing...

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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Mon 29, 2018 12:53 am 
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Joined: Feb Mon 13, 2017 10:46 pm
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Location: Atlanta GA
Ok, just finished installing the new capacitor and re-installed the old 470pf. Bridged the fuse block and sure enough I now have sound from the speaker. So my problem was only related to the bad fuse. Unplugged it for now and will replace all the electrolytic caps and that paper cap tomorrow. Check out this picture of this capacitor, why the 3rd leg coming off of it? Should I replace it?

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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Mon 29, 2018 1:03 am 
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bluebellyyankee wrote: Check out this picture of this capacitor, why the 3rd leg coming off of it? Should I replace it?
No... follow the bottom (english) link underneath the (components) photo that I posted earlier.

5. Erofol II (not in the picture).

Greg.


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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Mon 29, 2018 1:12 am 
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I agree with Greg. One type of capacitor commonly used in German radios is polystyrene. They have clear plastic encapsulation and the aluminum foil layers are visible inside. They are reliable and generally do not need to be replaced.

The particular type shown in your picture is not so easy to identify, since it has an opaque label layer that looks yellowish. Most polystyrene caps are simply clear plastic with shiny aluminum visible inside, with the values printed directly on the clear plastic. Those are easily identified every time. The type you have here requires a little more careful observation of the construction or the name like "Erofol" (short for "Ero" (brand name) and "Folie" (foil).

The third wire is probably a ground for a shield.

Sometimes a radio has 3-wire paper caps that need to be replaced. The shielding can be provided by wrapping insulated grounded wire around a normal tubular cap. I've also just used a heat gun to remove the capacitor from the shell, leaving the grounded foil and cardboard shell, into which you can insert the new smaller tubular cap, while using the original shield. Anyway, this last paragraph does not apply to the capacitor you have here.

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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Mon 29, 2018 2:16 am 
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Greetings to the Forum:

I am somewhat curious.... removing the plate voltage from a screen grid tube without also removing the screen voltage usually results in damage to the screen structure in the tube.

Is the screen circuit (labeled +3 in the schematic) so designed as to limit the screen current to safe values?

If not, is the audio PA tube in this radio so designed as to tolerate this sort of abuse?

If neither of the above is true, what are the chances that the audio PA tube is now defective, even though it appears to be working somewhat?

Regards,

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Jim T.
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 Post subject: Re: Saba 80 US (No sound)
PostPosted: Jan Mon 29, 2018 2:19 am 
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I agree that the output tube may be damaged if it is operated for a long time without plate current. Once this radio is recapped, checking tube element voltages should give some clue as to whether this tube has been damaged already (either by recent tests, or something long ago if the fuse was already open when this radio was retired).

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