Trouble shooting with a scope

4/17/2010 1:12:09 PMBruce Walters

I have an Emerson with standard broadcast and short wave bands. The amplifier section seems to work fine but the tuner is not finding stations. No obvious open coils/trnsformers, shorted caps etc. I put a signal generator on it and I try to trace the signal with a scope but the scope creates so much noise that it is useless. Is there a trick to using a scope to trace a generator signal through a receiver? I have learned enough to work through most problems, but the tuners are still very challenging for me, so I am trying to make this one my "learning" experience.

4/17/2010 4:09:15 PMEdd

Sir Walter . . . . . .

Any chance that the Emerson would be a DM-331 ? , or if not, what is its model #, so that its schematic could be researched.

Otherwise, in the interim, U should be able to connect up the scopes input to a connection in the audio stream and then inject a 455 Khz, 400/1Kc tone modulated signal into the grid of the IF stage and confirm its amplified presence on the sco0pe display and then move forward with signal injection to the mixer stage.

If that succeeds all the way to the front end, then it just might be a local oscillator presence problem.

73's de Edd

:I have an Emerson with standard broadcast and short wave bands. The amplifier section seems to work fine but the tuner is not finding stations. No obvious open coils/trnsformers, shorted caps etc. I put a signal generator on it and I try to trace the signal with a scope but the scope creates so much noise that it is useless. Is there a trick to using a scope to trace a generator signal through a receiver? I have learned enough to work through most problems, but the tuners are still very challenging for me, so I am trying to make this one my "learning" experience.
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4/17/2010 7:52:06 PMBob Masse

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:Sir Walter . . . . . .
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:Any chance that the Emerson would be a DM-331 ? , or if not, what is its model #, so that its schematic could be researched.
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:Otherwise, in the interim, U should be able to connect up the scopes input to a connection in the audio stream and then inject a 455 Khz, 400/1Kc tone modulated signal into the grid of the IF stage and confirm its amplified presence on the sco0pe display and then move forward with signal injection to the mixer stage.
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:If that succeeds all the way to the front end, then it just might be a local oscillator presence problem.
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:73's de Edd
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::I have an Emerson with standard broadcast and short wave bands. The amplifier section seems to work fine but the tuner is not finding stations. No obvious open coils/trnsformers, shorted caps etc. I put a signal generator on it and I try to trace the signal with a scope but the scope creates so much noise that it is useless. Is there a trick to using a scope to trace a generator signal through a receiver? I have learned enough to work through most problems, but the tuners are still very challenging for me, so I am trying to make this one my "learning" experience.
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: Hi! Bruce,
Check your email. I sent you Sylvania's "How to Service Radios with an Oscilloscope". GoodLuck!
Bob Masse
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4/17/2010 9:28:05 PMScott

Bob,
Is this book available on the internet somewhere?

:: Hi! Bruce,
:Check your email. I sent you Sylvania's "How to Service Radios with an Oscilloscope". GoodLuck!
:Bob Masse
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4/18/2010 12:01:52 PMBob Masse

:Bob,
:Is this book available on the internet somewhere?
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: Hi! Scott,
If you scroll down and find the topic " Free
Information Sites" and click on Site #12 and
scroll down to near the bottom you will find both
"Servicing Radio and Television with a Vacuum Tube
Volt Meter" and "How to Sevice Radios with an
Oscilloscope" by Sylvania. Hope it helps.
Bob Masse
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4/18/2010 4:23:51 PMScott

::Bob,
::Is this book available on the internet somewhere?
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:: Hi! Scott,
: If you scroll down and find the topic " Free
: Information Sites" and click on Site #12 and
: scroll down to near the bottom you will find both
: "Servicing Radio and Television with a Vacuum Tube
: Volt Meter" and "How to Sevice Radios with an
: Oscilloscope" by Sylvania. Hope it helps.
: Bob Masse
Thanks Bob....downloaded both and appreciate your help on finding them. Good stuff.

4/17/2010 8:22:22 PMBruce Walters

:I have an Emerson with standard broadcast and short wave bands. The amplifier section seems to work fine but the tuner is not finding stations. No obvious open coils/trnsformers, shorted caps etc. I put a signal generator on it and I try to trace the signal with a scope but the scope creates so much noise that it is useless. Is there a trick to using a scope to trace a generator signal through a receiver? I have learned enough to work through most problems, but the tuners are still very challenging for me, so I am trying to make this one my "learning" experience.
:

4/17/2010 8:30:40 PMBruce Walters

Hi Edd,

Your so reliable it's amazing, but where did you buy your ouiji board? It is indeed the 331! I discovered that the signal generator I had is a great paper weight and nothing more, so I bought a new one and I can see the carrier wave go though the tuner. I don't get any actual radio signal at the second transformer, but at least I can start to understand what's hapening. I printed out your thoughts and I will diappear into my workbench for a few hours and see if I can learn something new. Bob sent me a great book on scoping radios, so I may actually be able to learn something from this experience. Once I get this tuner mastered, I will consider myself a novice radio fixer, until then I am only a fumbler. Thanks again Edd.

::I have an Emerson with standard broadcast and short wave bands. The amplifier section seems to work fine but the tuner is not finding stations. No obvious open coils/trnsformers, shorted caps etc. I put a signal generator on it and I try to trace the signal with a scope but the scope creates so much noise that it is useless. Is there a trick to using a scope to trace a generator signal through a receiver? I have learned enough to work through most problems, but the tuners are still very challenging for me, so I am trying to make this one my "learning" experience.
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4/17/2010 11:42:07 PMScope, Smope

Who needs a scope? These old sets can be fixed with a screwdriver and a wet finger.

:I have an Emerson with standard broadcast and short wave bands. The amplifier section seems to work fine but the tuner is not finding stations. No obvious open coils/trnsformers, shorted caps etc. I put a signal generator on it and I try to trace the signal with a scope but the scope creates so much noise that it is useless. Is there a trick to using a scope to trace a generator signal through a receiver? I have learned enough to work through most problems, but the tuners are still very challenging for me, so I am trying to make this one my "learning" experience.
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4/18/2010 1:08:58 PMBob Masse

:Who needs a scope? These old sets can be fixed with a screwdriver and a wet finger.
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:Who needs a scope? Who needs a screw driver? You can use a chisel or an old kitchen knife. Some people like to learn all they can and take the servicing aspect to higher degree. It makes the hobby much more enjoyable for them. I am of those people and yes I will be using my scope in radio servicing.
BoB Masse
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4/18/2010 1:20:54 PMTerry Decker

::Who needs a scope? These old sets can be fixed with a screwdriver and a wet finger.
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::Who needs a scope? Who needs a screw driver? You can use a chisel or an old kitchen knife. Some people like to learn all they can and take the servicing aspect to higher degree. It makes the hobby much more enjoyable for them. I am of those people and yes I will be using my scope in radio servicing.
:BoB Masse
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Anyway Bruce-
Yeah, it sounds like the oscillator isn't working. Check the voltages around that tube, and the 1st IF. I'm sure Edd or someone can tell you what they should be. My vote is for the tube, even though it might test good. Also, even though you said the coils test good, I'd go over them again.
PS-Don't use a wet finger unless you keep one hand in your pocket. (No, not THAT pocket)
Terry

4/18/2010 5:25:44 PMNo Scope Necessary

Good, someone with a sense of humor. This is a hobby, it isn't "rocket science" and it's supposed to be fun. These old sets are simple. You just need some basic knowledge of what the circuits do and how they do it. Some internet research will do the trick and you'll have more fun than you should be allowed by law.

Now as for checking an LO, the easiest way is to use a second radio to look for the signal. Set the second set to a clear frequency that is near the top of the band, such as 1500KHZ, then tune the repair set to a frequency that is 1500 less the IF frequency. 1500-455=1045, for example. You should be able to hear the repair set's LO on the set tuned to 1500 if it is running. The dials are not exact so you'll need to rock the repair set's tunig knob back and forth to find it. Try this out with two working radios first and you'll soon see how easy it is to determine if an LO is working.

Learn how these sets work and you may not even ned to wet the finger.

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:Ha ha, funny stuff-
:Anyway Bruce-
:Yeah, it sounds like the oscillator isn't working. Check the voltages around that tube, and the 1st IF. I'm sure Edd or someone can tell you what they should be. My vote is for the tube, even though it might test good. Also, even though you said the coils test good, I'd go over them again.
:PS-Don't use a wet finger unless you keep one hand in your pocket. (No, not THAT pocket)
:Terry
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4/18/2010 10:21:40 PMHave A Scope Use It

Yes, you can repair most old radios with a simple V.O.M. or the old time V.T.V.M. If you do have a scope, and leaned how to use it, along with how a radio works. Why not use it. Peak an IF on the scope, see how close you got with just the audio tone. Alignment of a Hallicrafters SX model would be almost imposable without one. Not your average AA5 common pooh-dunk radio.

4/19/2010 12:44:46 PMBruce Walters

There has been some great humor and great discussion on here. I suspect that the problem is due to someone's mistake, mine or the previous owner. This thing was a disaster when I got it. But, I will use some of the ideas presented and see what I can do. Thanks for the answers and the fun. I agree that the wet finger fixes 90%, but when the beasts have been miswired, the wet finger doesn't work to well. Besides, I can get the bejimminies knocked out of me with a scope too. Just grab the chasis with one hand and touch the scope with the other elbow, try it some time,it really gets your attention.

:Yes, you can repair most old radios with a simple V.O.M. or the old time V.T.V.M. If you do have a scope, and leaned how to use it, along with how a radio works. Why not use it. Peak an IF on the scope, see how close you got with just the audio tone. Alignment of a Hallicrafters SX model would be almost imposable without one. Not your average AA5 common pooh-dunk radio.
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4/19/2010 1:23:17 PMThomas Dermody

If you are going to work on AC/DC radios, be sure to either use an isolation transformer on the radio, or to unground the scope. A shock is possible even between AC-only radios and the scope, due to capacitive coupling through the power transformer, plus any extra RF to line coupling capacitors, so be careful here, too.

T.

4/19/2010 8:55:59 PMTerry Decker

:I have an Emerson with standard broadcast and short wave bands. The amplifier section seems to work fine but the tuner is not finding stations. No obvious open coils/trnsformers, shorted caps etc. I put a signal generator on it and I try to trace the signal with a scope but the scope creates so much noise that it is useless. Is there a trick to using a scope to trace a generator signal through a receiver? I have learned enough to work through most problems, but the tuners are still very challenging for me, so I am trying to make this one my "learning" experience.
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So- try it. It's obvious that something is wrong in the osc. section, so I'd concentrate on that.
Good luck-
Terry