general-el: Problem of reception ;GE F70 (F 70)

Problems with General Electric F70 (1937) 1 Can somebody help solving this problem? I have restaured this radio (superheterodyne, 7 tubes, 3 bands). It worked well initially and suddenly stopped working. The radio tunes to a - only a few- frequencies bu only a BRRR noise is heard on the loudspeaker. - All the tubes have been checked and seem ok. - Just in case I replaced the converter (6A8). No improvement - Surprisingly, Injecting a signal with a signal generator (RF/AF)(Heathkit RF1) at the antenna, the AF is clearly heard at the loudspeaker. Changing the RF in the generator the radio tunes well at all frequencies in the 3 bands and AF is clearly heard. - IF is 465 kHz. Injecting this frequency at the antenna, AF is heard at the loudspeaker AT ALL positions of the tuning condenser. I do not understand why the radio seems to work well with the signal genarator and not with the external stations. The reception here is strong. Thanks in advance for your help León Shanghai, PR China

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Thanks Mario for the quick answer. Yes, the antenna worked well. Also here (Shanghai) the reception is very strong for some 10 AM radio stations. Muy agradecido. León

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Dear Leon
Are you permanently now in Shanghai? If so, I should change your profile because a member can not change country.

To this problem:

1
Did you inject the signal at the same point (socket) with the same type of plug? Sometimes there is an automatic switch which the plug operates ... Is that the case?
You might measure with an ohmmetre if the signal can follow the path.

2
If that path is really OK: What signal strength do you inject and what sort of antenna do you plug in or connect?

If you answer those points, somebody might e able to help you better. We have first to understand your situation fully.

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Dear Ernst, Yes, I will be in Shanghai for the next 3-5 years. You are right, I tried to change the profile but I could not change the country: I was going to send you an e-mail asking how to do it. My new address is 201107 Shanghai, PR China. E-mail stays the same To your questions: 1- Yes, it was at the same socket. I checked if the signal was reaching the grid of the converter tube and it did. So did the signal from the oscillator. 2- The antenna was a simple wire 4m long. The RF output of the generator is 3V but at tried different levels of attenuation down to some 20% and the AF (400Hz) was always clearly heard at the loudspeaker. With external stations I can hear only a change of noise to something like BRRR at the wavelelengths I know the strongets local stations are. Let's see if somebody can help
Best regards León

Change 4 May 2008 by EE:
I have taken out your address details and telephone numbers. Neverput this into a forum because you might get spammed by mail or telephone. But in the profile neither guests nor spiders can see your entry!

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Are you sure you are not having an interference? Try a different power outlet just in case. Also try the radio during day time when less fluorescent lights are on.

Two things to clean  and check: tube sockets and bandswitch. I use emery paper on the tube pins and then contact cleaner on the tube sockets and bandswitch. You can use an owl to re-spring the tube sockets, the point is to make 100% sure that there are no false contacts.

Good luck

Mario

 

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Hello Gentlemen.

I think, MARIOs first idea "Noise" from Outside ist the right way.

 

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Hello Gentlemen. I hope you can unterstand my englisch!
Text:

This device has on the input terminal of the current supply, two condensers from the mains to Mounting (Chassis) Are supposed to keep away Noise Signals from the Radio, and bring it back to EARTH (Ground) from the mains.
That  has only a right function,  if an EARTH Wire  (Ground) is connected at the radio.

That is not the case, if the troubles are led onto the chassis as interference current. From there they reach the side of the antenna coil which one is due to the chassis. From there the Noise stream goes through the coil in the direction of antenna connector and leaves the radio over the natural capacity of the antenna (4 meters wire) and flows for the environment (Building ground) back to the Noise Generator (Computer, Neon lamps etc.) They in the building or outside can stand.

This noise stream is recognized as antenna signal or station and expressed in the loudspeaker.
For the Input circuit is the same effect when the Generator is in series with the Earth or Antenna wire connected. Is really always high frequency that one the direction changes. (AC)
In a closed circuit, the generator can be at every place positioned.

When a signal generator is connected to A and E, through his mains connection with one (2 or 3 point) is carried with or without protective ground (Ground) the Noise current flows back to Power Line (Phase) or Line Earth (neutral) before they can produce an interference current on Antenna Input, that otherwise will receive as a signal through the antenna coil.

Important!! In this case, the antenna capacity of the 4 meters is missing against EARTH. The Noise Current can not crossing the Antenna coil.

 

Hans M. Knoll

Attachments:

• Noise Current (60 KB)

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Dear Leon
As you can see on post 5, I changed your address in your profile.

I always admire how Mr. Knoll helps our members.
Only here active members can understand which big effort it is to look up the schematics, take copies, write into the copy, scan it, process it to the right size etc. and publish in a foreign language. And this all in a certain age!

His answer  is surely the solution - I would make a bet.

By the way:
Since you posted this via your chosen langauge, Spanish (but in English), your question has (still) the flag for Spanish and therefore Mr. Knoll did not see it  at all (he has probably opted in German and English - most German will even only have opted in German, I presume - but he is sometimes caring for English too) - until I told him about your problem. I asked him if he could opt in (just for a few minutes) Spanish too, as a reading language in the forum.
You might have to edit your first post and change flag - will you please?

For anybody:
We have such good options as a member for languages but please select your language in the forum editor if you post in an other language than your main language - or change your language flag completely for that time.

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Dear Mario and Hans, It looks like you were absolutely right and the origin of the problem was some kind of source of noise, altthough I am not completely sure what it was. I tried to improve the allignment of the radio and I succeded in receiving stations in the SW band. AM was still a problem with the same noise covering the external signal. Taking the radio outside and trying different location around the house I fpund finally a spot where the reception was ok. I think the problem could originate in a power transformer some 50m from the house. In addition I realized that we have steel mosquito nets in the windows, this could explain why the reception was better in SW than in AM. Sorry I have not replied before but I have been travelling all week. I really approeciate your help. Hans, I will study in more details your answer and try to bring more light to this problem. Thanks for the time you took. Enrst, Thanks for conveying the question to Hans. In the future I will list under the relevant language, Best regards León

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hello Leon.

you wrote:

Hans, I will study in more details your answer and try to bring more light to this problem.

Yes i  see, you have much to do in your new location.

It is only important , when  this jobb sometime is finish,  to give your results to anybody here.

Have o good time there, regards hans

 

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Hi Hans, I am a beginner in this job and this is only a weekend hobby for me ( Sure this will be the case for many of our members). Sometimes I finally make things work but I do not necessarily understand why! I believe the problem is due to some source of interference that is permanent. I can reproduce the problem by taking the radio back to the initial location. Clearly it is not due to the power line but to some exyternal inteference in the MW. I also believe that the signal strength of the signal generator (coonected directly to the antenna terminal) was able to overcome the problem. Not so the weaker RF signal received from the stations. I am not sure I will be able to bring more clarity...but I will try. Thanks again Best regards León

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Hello Leon.

To thanks for your representation from your view. It is left only to you, something and how much time and knowledge or experience you can bring in here.

It is a question here only of showing to you and to other readers for Mr. Erb and me, can one and wants to help one you,       nothing else.

Best regards, Hans

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Dear Mr.Leon:

If receiver will suddenly stopped working, it may be local oscillator stop working, this will due to

(a): old tube low cathode electron emission efficiency. change new tube 6AK8 if cannot improve then increase tube filament voltage, please check filament voltage must be 6.3V at least.

(b): AC power line voltage too low, or variation out of tolerance, please check ShangHai power line voltage? If suddenly stop working, this means local oscillation ceased due to weak positive feedback from coils for oscillation, you cannot change the tuirn ratio of coils to increase positive feedback till oscillation. then increase power line voltage (same as filament), more thermal electrons will continued oscillation.

(c): use higher mu converter tube for substituting 6A8 converter tube.

(d): If need injecting signal to start oscillation, these status means local oscillator just beyond the critical point of oscillating, it just need more thermal electron to trigger to oscillation or need high and stable ac power supply!

(e): Please check AC power line voltage again, I have been visit some city in China and measuring from 260 to 185 V !!!

This Is my experience about it, for your reference only, hope can help you solve problems of reception!

 

Besr Regards

Don Chuang

 

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