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 Post subject: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Thu 08, 2018 3:52 pm 
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Greetings,

I am about to do a recap job on 2 of my own tube radios. I'm just about to place my order for the caps but there are a few issues that I need help clarifying.

---------
There is a resistor (the bottom of photo in yellow shrink wrap) that is shorted with a wire. The solder joints look original. I can't imagine the function of a resistor that is shorted in parallel. This resistor is soldered to the solder lugs of a 4-button switch whose function is to select the tone, for Jazz, orchestra, speech... There are also 3 caps and one more resistor in that same part of the circuit.

My questions: What do I make of this resistor being shorted? Do I do anything about it or ignore it? I cannot find a schematic for this radio, BTW.

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Is it necessary to clean the dust particles between the blades of a tuning cap? If so, how? Here is a picture of the condition this cap is in.

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Please see that there seems to be a tiny metal particle between two blades. I will vacuum it out, but if there are any other metal particles that I can't reach, how badly can that kind of short damage the circuit?

--------

There are some caps that I am unable to identify, in terms of value and also in terms of composition.

I found this video https://www.youtube.com/watch?v=67M7fsbLUIU&t=709s where the host shows some caps that are similar in appearance, but I'm not sure they are the same. In the video, the host explains that those are polystyrene caps and that they are very stable and dependable, unless they've been overheated. If those are in fact polystyrene caps, do I need to replace them? If they are not and do need replacing I need help interpreting their values. Here is how they are marked.

70/10
125V


20/10
125V


500/10
500V

...and so on...

The make is RUWEL.

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Also, is that larger cap, marked 4000pF also the same kind of cap?

---------

I read on http://www.justradios.com that new film caps do not have a polarity. However, in this video https://www.youtube.com/watch?v=BnR_DLd1PDI&t=2335s the host says that new film caps do in fact have a polarity and should be installed with outer shield on the negative.

Which is true?

--------

Lastly, thank you all in advance for helping me out.


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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Thu 08, 2018 4:02 pm 
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Adinol wrote:
I cannot find a schematic for this radio, BTW.
Maybe we can. What are the brands and models you are working with?

.


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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Thu 08, 2018 4:07 pm 
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The film caps do not really have a polarity, but in some very high end audio circuits, I believe, the orientation of the caps does matter. In this case, as in most tube radios, you do not need to worry about the orientation of the capacitors in your radio.

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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Thu 08, 2018 5:16 pm 
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Location: Dallas, TX
If that part is shorted out by a wire it isn't in the circuit. Maybe the switch is bad and the wire bypassed the problem. I doubt it is stock, some repairmen are very good at soldering.

I would try a soft brush (paint) and perhaps compressed air to clean the variable cap. A really good cleaning would require you to remove the tuning cap and wash it, but you can probably get by with \out that.

The caps with the clear case are polystyrene alright. Very good, shouldn't need replacing. They melt very easy however, be very careful with soldering iron around them.

In some high sensitivity audio circuits or RF circuits the outer foil may matter but in almost all circuits it doesn't matter.

You should really get a schematic.

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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Thu 08, 2018 5:50 pm 
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Thank you for your replies.

So, those silver caps are polystyrene, after all. I will not replace them, however, for future reference and also because I might have to replace a damaged one I would still like to understand these labels.

70/10
125V


20/10
125V


500/10
500V


30/20
125V


The radio in question is a Tefifon T574. I was able to get a schematic for a T573 which is a similar radio. The difference is that the T573 does not have those tone switches, so there's no way to interpret the shorted resistor form that schematic. Also the two T573 schematics that I got from two different sources are both such low rez that it is impossible to read any values. I've been searching the web for quite a long time and was unable to fins a T574 schematic or even a readable T573 one.

But if you guys have a source I was unable to find I would surely appreciate it.

Many thanks...


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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Thu 08, 2018 6:12 pm 
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You will need to replace the electrolytic capacitors in the set and those are usually mounted on top of the chassis in an aluminum can and will have markings on the side. In European sets usually there are two in each can and almost always 50uF\. You can replace these by getting two individual 47uF, 450 volt rated capaitors.

I usually use either a paint brush or compressed air to clean the tuning condenser. The bearings on either end will benefit from a few drops of oil.


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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Thu 08, 2018 9:15 pm 
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Joined: May Thu 14, 2015 4:15 pm
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Location: Dallas, TX
70/10----------------- 70pf/ 10 %
125V---------------- 125 volt


20/10------------------ 20 pf/ 10 %
125V------------------- 125 volt etc.


500/10------------------ 500pf/ 10%
500V


30/20-------------------- 30pf/ 20 %
125V

One other thing about the polystyrene type is that they are kind of large considering there value.
They have very low leakage and are nearly as stable as the mica type.

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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Thu 08, 2018 9:54 pm 
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Notimetolooz wrote:
70/10----------------- 70pf/ 10 %...

...One other thing about the polystyrene type is that they are kind of large considering there value.
They have very low leakage and are nearly as stable as the mica type.


Thank you for this information, Notimetolooz. It is very helpful.


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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Fri 09, 2018 8:22 am 
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Greetings to Adinol and the Forum:

Here's a trick you can use to get rid of metal particles in the tuning capacitor. The best, of course it to wash it out, but if the metal particles persist after your best efforts, you can usually clear them with the following technique. (NOTE! This trick is a bit dangerous... especially if you do not have an isolation transformer. Use at your own risk!) Remove the tuning capacitor from the radio if possible or at least remove all wiring to the stators (be careful to note where everything goes so you can put it back). If you don't have a dim-bulb tester, now is the time to build one. Connect the dim bulb tester to the tuning capacitor using a suicide cord. Plug the dim-bulb tester into your isolation transformer. Using only one hand (insulating glove if you have such; stand on a rubber mat), rotate the tuning capacitor through its full range of motion. The light will light and the capacitor will sputter as the metal particles are burned away. When the capacitor can be rotated throughout its range without the dim-bulb lamp illuminating, disconnect from the isolation transformer, move the suicide cord to the other capacitor section and repeat.

Use all possible precautions to avoid a current path through you. This process can be dangerous and I am not responsible for any consequences associated with its use... I merely point out that I have seen this technique used and it works. You assume all risk associated with this procedure.

Regards,

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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Fri 09, 2018 2:26 pm 
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Interesting procedure, Jim. Does sound like a do-at-your-own-risk kind of procedure.

I was able to blow out some junk with my compressor at lower setting. I think I'll first desolder the tuning cap and test for continuity, as I'm rotating. Some of the outside fins were bent outwards and I gently straightened them. I gotta make sure they aren't contacting the stator fins.

I have some other questions...


I'm thinking of retrofitting two radios with fans and LED strips behind the dial glass. The radios are not for sale so I'm not worried about things not being 100% original. Are those kinds of retrofits something that people ever do?

Also, the antenna connection on the back of radios has an earth terminal. Would I connect that straight into mains earth?

Thanks...


PS - I placed the order for the caps, last night. Can hardly wait....


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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Fri 09, 2018 3:55 pm 
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Location: Grand Chute, Wisconsin
Adinol wrote:
Some of the outside fins were bent outwards and I gently straightened them.
They may have been bent on purpose. On many variable capacitors the end plate is designed to be bendable to correct dial alignment problems. See an example of bent tabs on the end plates below. Notice how the outside plates have slots in them? That's to allow bending of the plates.

Image

.


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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Sat 10, 2018 2:30 am 
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wiscojim wrote:
They may have been bent on purpose. On many variable capacitors the end plate is designed to be bendable to correct dial alignment problems...


Arghhh!!! So did I just mess up this radio?

Any way I can bend them back to the correct compensated position?


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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Sat 10, 2018 2:38 am 
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Adinol wrote:
Some of the outside fins were bent outwards and I gently straightened them.
If the end plates are bent, they should be left alone.
That's particularly true if the plates are segmented with slots in them as shown in your photo.

These adjustments were provided to improve tracking accuracy across the band.
If they were made at the factory, they should not be changed.
Your photo definitely DOES show factory adjustment.

- Leigh

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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Sat 10, 2018 2:57 am 
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Yes, I realize now that I made yet another beginner mistake. Those bent plates were in fact at the end where there are segments.

Is there any way that I might be able to fix this?

Thanks...


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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Sat 10, 2018 3:03 am 
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Adinol wrote:
Is there any way that I might be able to fix this?
I wouldn't worry about it.

The adjustments affect the middle- and high-frequency portions of the band. Once you get the low end of the tuning set accurately, the cap can be adjusted to improve the tracking.

This is a fine adjustment, only made in expensive sets because it took technicians' time on the assembly line, thus dding to the production cost.

Leave it as is and evaluate the tracking after you get the set working and aligned. If you think an adjustment would improve the set, use the photo as a guide.

- Leigh

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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Tue 13, 2018 4:29 am 
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I checked a European forum and did a search for your radio for any threads. There was one there from 2012 with the OP asking for a schematic for the 574 and getting one from one of the members. An interesting point is that he mentioned also seeing a 5K resistor that had a jumper across it for the solo tone control. Here is the link to the forum website. Just join the forum and try asking for a schematic. Someone there might have one. Here's the link. https://www.dampfradioforum.de/index.ph ... 17b19b8c0c

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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Tue 13, 2018 7:48 am 
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Adinol wrote:

I have some other questions...


I'm thinking of retrofitting two radios with fans and LED strips behind the dial glass. The radios are not for sale so I'm not worried about things not being 100% original. Are those kinds of retrofits something that people ever do?


They do make direct LED bulb for replacements these existing incandescent lamps. Fans would only place additional stress on the power supply and add noise to the radio; these original designs have sufficient ventilation already.

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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Tue 13, 2018 3:05 pm 
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Copiertech wrote:
I checked a European forum and did a search for your radio for any threads. There was one there from 2012 with the OP asking for a schematic for the 574 and getting one from one of the members. An interesting point is that he mentioned also seeing a 5K resistor that had a jumper across it for the solo tone control. Here is the link to the forum website. Just join the forum and try asking for a schematic. Someone there might have one. Here's the link. https://www.dampfradioforum.de/index.ph ... 17b19b8c0c


It's really nice of you to look into this beyond this forum. Thank you.

I looked a bit more at the jumper, as I am recapping. That jumper is very likely to be factory work, as it is the one continuous wire going through 3 solder points. Here's an image.

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I now have a theory. Please tell me what you think.

All the switches were soldered like this at the assembly line. The last technicians that would do the fine tuning would decide whether to cut the jumper or leave it, based on whatever parameters they measured. So, on some units this jumper is cut and on some it is intact. Does that sound like a possibility?

If it's not to difficult I'll draw up a schematic of this switch and post it here.

Speaking of schematics, I would definitely be interested in obtaining one for the T574 model. The forum you mentioned is all in German. How do international folks manage? Do people just use Google translator or is there a translating feature on that forum that I do not see? Also, is it in any way bad etiquette to join a forum only to ask for one favor?

I also looked more into the radiomuseum.org site, where I got the low rez version of the T573 schematic. Members might be able to obtain a much better schematic (and more info from the community). However, they have some strict rules that you have to disclose your real name, birth date, real photo etc. Although I know they are good guys I have serious reservations about agreeing to those terms. Simply because any database can be hacked and I don't like the idea of my private info being there. Are any of you members there and what do you guys think about that?

AJJ wrote:
They do make direct LED bulb for replacements these existing incandescent lamps.


Are those some kid of LED's with the same light bulb sockets? I have not seen any such LED's on the sites I've been studying. Is there a link? Or a technical name I can Google? Thanks.

AJJ wrote:
Fans would only place additional stress on the power supply and add noise to the radio; these original designs have sufficient ventilation already.


I was actually thinking more of installing a separate 12VDC power supply (probably a salvaged wallwart).

From what I've been able to read online, these Tefifon units actually had a heating issue. This is why they moved one tube all the way to the left side of the chassis and now there's an unused tube slot in the center. That tube was damaging the plastic cassette tray and also heating up the plastic tapes that were stored inside that tray. I was thinking that even with that tube moved to the side there might be more heat cooking up the cassettes, on hot days and I was thinking that a small fan sucking the heat up might be a good idea. This is all a learning process for me and I might place a fan there and measure the temperature with and without the fan, to see what difference it makes.

Phew! long post...

Thank you all...


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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Tue 13, 2018 3:56 pm 
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Google translate will automatically translate the forum threads. Use right click to start it. It should be alright to ask for the schematic. That forum focuses on European radios and some of its members were engineers who were part of the design teams for Grundig, Saba, Telefunken and other companies.

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 Post subject: Re: Need Newbie Clarifications on Tube Radio Restoration
PostPosted: Feb Tue 13, 2018 5:58 pm 
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My ability to decipher handwriting is not perfect. I think this handwritten note implies the T573 is sufficient for servicing the T574 (schematic downloaded from radiomuseum).

https://www.radiomuseum.org/r/tefi_tefi ... ht571.html

Also, this source provides the same schematic for both the T573 and T574. Perhaps the difference in model number mainly reflects the year they were manufactured.

http://nvhr.nl/gfgf/schema.asp?Merk=Tef ... mitted=yes

It appears LED lamps are mainly marketed towards pinball machine restorers. Many (not all) radio dial lamps are #44/#47/#51 (bayonet base), #46 (screw base). You need to identify the type that's in your set. Do an online search for "pinball led."

As for dealing with schematics written in different languages...component symbols and values are drawn mostly the same way; that should be sufficient for simple restoration work. You may still need to translate a few words, but that shouldn't be too burdensome.

The "heat" problem you noted appears to be an issue with the placement of storage space, not impacting performance of electronics. Maybe there's a way to put a thin sheet of styrofoam under the plastic tray?


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