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 Post subject: Common Signal Generator Problem?
PostPosted: Oct Fri 16, 2020 10:42 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1302
Location: Bloomington, IL USA
I have recapped two or three signal generators over the years. Following the recap, all tube voltages were in spec yet in all three cases there was no output on any bands. I just finished an EICO 315 Signal Generator and it has the same problem. Am I missing something obvious on all these signal generators?
Thanks,
Joe


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 Post subject: Re: Common Signal Generator Problem?
PostPosted: Oct Fri 16, 2020 10:47 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Albuquerque, NM 87123
Yes.

If re-capping was an end-all solution, EVERYONE would be a technician.


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 Post subject: Re: Common Signal Generator Problem?
PostPosted: Oct Fri 16, 2020 10:48 pm 
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Location: Sunbury, Ohio 43074
I suppose the first question is, did they work before you started changing parts? If so, then it stands to reason that something you did is the cause of the problem.

If you decided to re-cap before applying power, then all you can do now is systematic troubleshooting to figure out what's gone wrong. First verify the power supply voltages.

Normally, switches and pots can easily cause a "no output" condition. Without exception this older gear will need the switches and pots cleaned. Sometimes, the tube sockets, and tube pins, as well. You can 'test' this by exercising every switch and pot dozens of times, and removing/reinserting each tube a few times carefully.

If this doesn't clear it up, it's time to start troubleshooting, and each instrument will be a little different.

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 Post subject: Re: Common Signal Generator Problem?
PostPosted: Oct Fri 16, 2020 10:59 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 9987
Location: Long Island NY
Were you able to get any of the other generators working? If so, what did it take to fix them?

The other question is, how are you verifying the generators work (or not)? They don't put out a lot of voltage so if you are trying to measure them with a meter, a frequency counter, or even some oscilloscopes there may not be enough signal there to see anything.

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"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


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 Post subject: Re: Common Signal Generator Problem?
PostPosted: Oct Fri 16, 2020 11:16 pm 
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Location: Montvale NJ, 07645
Are you tying all the capacitors to their original points? Maybe a lead dressing issue? I don't see any real difficulty in troubleshooting one of these if you can fix a radio.


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 Post subject: Re: Common Signal Generator Problem?
PostPosted: Oct Fri 16, 2020 11:23 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1302
Location: Bloomington, IL USA
The latest one (EICO) had been recapped before I started except for the filter can which I recapped. All was very neatly done before me. I used two separate frequency counters, NADA and then used my TEK 475A scope, and NADA. The schematic for the EICO doesn't have any tube voltages, but all the plate and cathode voltages are reasonable in my opinion. I have sprayed and exercised all switches and controls. I guess my final attempt will be to check all the coils for continuity.
Anyone have a EICO 315 schematic with voltages noted on it?


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 Post subject: Re: Common Signal Generator Problem?
PostPosted: Oct Fri 16, 2020 11:45 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1659
First look at the tuning capacitor. If it's shorted due to bent plates or foreign shavings that would cause it.


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 Post subject: Re: Common Signal Generator Problem?
PostPosted: Oct Fri 16, 2020 11:45 pm 
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The assembly manual for the 315 is at this link and includes resistance and voltage readings.
http://bama.edebris.com/download/eico/315/315-const.djvu


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 Post subject: Re: Common Signal Generator Problem?
PostPosted: Oct Sat 17, 2020 12:19 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Bloomington, IL USA
Thanks for the info. I noted that all of that info says the oscillator is a 6SL7 but mine has a 7 volt Loctal in it. Must be a different model.


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 Post subject: Re: Common Signal Generator Problem?
PostPosted: Oct Sat 17, 2020 12:43 am 
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The 6SL7 is the output cathode follower and the audio oscillator, not the RF oscillator. The 6C4 is the RF oscillator.

Blown resistors in the output attenuator are a common failure, often caused by accidentally connecting the output to B+ without a coupling capacitor.

Have you checked all the resistors?

Do you have a VTVM? If so, what's the voltage at the grid of V1, the 6C4?

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 Post subject: Re: Common Signal Generator Problem?
PostPosted: Oct Sat 17, 2020 1:11 am 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island NY
The Eico 315 went through at least a few production runs. Some have 6SL7s, but at least one run was made with a functionally equivalent Loktal tube. Certain brands of Loktals had pin corrosion problems so you may want to check, clean, and re-check the tube and socket pins. They use half of the tube as an output cathode follower so if is not making good contact with its socket there won’t be any output.

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"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


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 Post subject: Re: Common Signal Generator Problem?
PostPosted: Oct Sat 17, 2020 2:21 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1302
Location: Bloomington, IL USA
I will do these checks tomorrow on Sat and report back. (attenuator resistors, tube pins and sockets, and grid voltage of oscillator tube.)


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 Post subject: Re: Common Signal Generator Problem?
PostPosted: Oct Sat 17, 2020 3:23 pm 
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Joined: Jun Sun 23, 2013 9:03 pm
Posts: 483
Location: Canandaigua, NY
When you spray, don't get any on tuning caps as the resulting film will throw them off. If you use flux remover, watch out for polystyrene caps, as it can cause both short and long term damage.


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 Post subject: Re: Common Signal Generator Problem?
PostPosted: Oct Sun 18, 2020 1:05 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1302
Location: Bloomington, IL USA
Well,
I found a bad solder joint on the grid of the RF oscillator tube. And to make things more difficult the test lead had an intermittent open. Works great now.
Thanks for all your help.


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 Post subject: Re: Common Signal Generator Problem?
PostPosted: Oct Sun 18, 2020 4:01 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3900
Location: Lexington, KY USA
So to address the original question: yes, not working is a common signal generator problem, at least with old ones. It is a common problem for old electronic equipment in general.

It's likely each of your generators has some different set of reasons for not working. Wanna start on the next one?

Ted


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 Post subject: Re: Common Signal Generator Problem?
PostPosted: Oct Mon 19, 2020 3:39 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 9438
Location: Beaver Falls, PA. USA
If you are dealing with kit-built equipment, loose and poorly soldered connections are common, as well as misalignment; don't assume that the unit ever worked properly.

I just finished up on a Heathkit IM-1104 multimeter with a nasty intermittent in the ohms circuit, caused by cold solder joints. As a bonus, the AC volts calibration was way off.

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