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 Post subject: Help with RCA WR-50B signal / sweep generator
PostPosted: Oct Tue 20, 2009 12:26 am 
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I'm the proud owner of a new (50 year old?) RCA WR-50B signal/sweep generator. I checked the two 12AT7 tubes. They test good and seem to be getting plate voltage & filament voltage. It looks good inside. The two 40uF x 150V electrolytes are in a "Can", but I actually tried to measure their capacitance in the circuit. They both came out almost the same, each about, 44 uF, 10% high. The other CAPS look new like orange dips but dark red. They look like they are original manufacture.

I tried to "hear it" with a short wave radio that can tune 100Khz to 29.9Mhz. Nothing. I connected it to a scope, nothing. I made a jury rig RF demodulator with what I had and still nothing. Here is the schematics. Any hints where to start?

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PostPosted: Oct Tue 20, 2009 10:53 pm 
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You should see band A, B and maybe C when feeding directly into the scope, without a demod probe. You don't say what kind of scope you're using.

You may see something using a very narrow sweep modulation or at the modulation in/out jack without a demod probe. That will confirm the audio oscillator is working.

Have you made the basic troubleshooting tests like B+, plate voltages, burned resistors, etc?


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 Post subject:
PostPosted: Oct Tue 20, 2009 11:28 pm 
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Philco Don wrote:
You should see band A, B and maybe C when feeding directly into the scope, without a demod probe. You don't say what kind of scope you're using.

You may see something using a very narrow sweep modulation or at the modulation in/out jack without a demod probe. That will confirm the audio oscillator is working.

Have you made the basic troubleshooting tests like B+, burned resistors, etc?

Hi Mr Philco Don. I am using a Tek 2225 with 1x/10x probe. I used the 1x probe with a 22pF capacitor also. The demodulator was totally a joke, jury rig I made up on the spot with a bread board. It did nothing. I'm hunting for a real RF probe or going to make a proper one.

Here are the plate voltages (even I see they are wrong) :wink:

Filament voltages are all 6 volts (in parallel) OK

V1A VFO
pin #6 - 134 (should be 60V) Very High
pin #7 - 0.0 (should be -0.4) Non-existent
V1B Cathode follower
pin #1 - 143 (should be 150V) OK

V2A Crystal Osc
pin #1 35 (should be 130V) Very Low
V2B Audio Osc
pin #6 135 (should be 100V) High

Per schematics voltages where measured with a VTVM and settings:

    Band = F @ 12MC
    RF = Hi
    Mod = Int
    VFO =On
    %Mod = Max. Cw
    RF Attn = Max. Cw
    No Xtal in socket.

I'm pretty sure I found the pins #1 and #6 on both tubes. The chassis is stampted/labeled with the tube, i.e. V1 or V2. I measured to chassis ground? May be I should measure to cathode not ground? The audio modulation is working, % modulation works, close to 400 Hz as advertised. It's a sine wave, not perfect, with a little wave or distortion along slopes of the trace.

I tried RF again, connecting inside the unit (bypassing the cable and .001 uf Cap) and got a 60Hz trace (100 microvolts?). Controls had no effect, except audio could be superimposed on this. Better do some more checking. Suggestions? Thanks


Last edited by gmcjetpilot on Oct Wed 21, 2009 2:49 am, edited 1 time in total.

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 Post subject:
PostPosted: Oct Wed 21, 2009 1:39 am 
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Location: SW WA state
Have you cleaned your switch contacts yet? That would be another good thing to check.
Your high plate voltage on the VFO bothers me. If it should be 60 volts, then something is horribly wrong. Either the tube is drawing zero current and you're thus seeing full B+ on it (no voltage drop across the 47K resistor ), or the resistor has shorted, or the cap across the plate resistor has shorted.
With 0 volts on the grid, it would draw more current than when it has -0.4 volts on it.
What does the cathode section of the VFO tube (Pin 8?) read?
It looks like it should go through switch section S1C... If that has a bad contact, the tube wouldn't work. Your cathode should be at or close to ground potential. Check that....
I have two of the units at the shop...

-Tom


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 Post subject:
PostPosted: Oct Wed 21, 2009 2:24 am 
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Tom Herman wrote:
Have you cleaned your switch contacts yet? That would be another good thing to check.
Your high plate voltage on the VFO bothers me. If it should be 60 volts, then something is horribly wrong. Either the tube is drawing zero current and you're thus seeing full B+ on it (no voltage drop across the 47K resistor ), or the resistor has shorted, or the cap across the plate resistor has shorted.
With 0 volts on the grid, it would draw more current than when it has -0.4 volts on it.
What does the cathode section of the VFO tube (Pin 8?) read?
It looks like it should go through switch section S1C... If that has a bad contact, the tube wouldn't work. Your cathode should be at or close to ground potential. Check that....
I have two of the units at the shop...

-Tom

FUNNY YOU SAID THAT. I just cleaned the tube sockets, tube pins, all the contacts, both pots and the four slide switch................... IT WORKS GREAT..... I put it on my Freq counter and it is easy to set and seems stable, dials a little off. My SW/MW radio picks up the RF both carrier wave and modulated. Yea! :D

When I first got started on repairing/restoring tube radios way back in 2009, :roll: , all I knew how to do was clean tube pins & sockets, contacts and pots, measure a few voltages. Than I got some test equip and that's all I wanted to do, hook it up to the scope (which I'm a rookie at) and skip that cleaning stuff. Lesson learned, basics first!

By the way the traces look nice and the sweeps (455kc and 10.7kc) look very interesting and correct! The only funny thing I noticed, are the lowest two bands A (85-200kc) and B (200-550kc) have sine waves that are slightly flat at peak. Depending on how I set the scope, I can make them look normal. I might have had the WR50B at lower power. Not sure that matters or that I was using the scope properly. The waves on the other bands look proper.

I need to go back and now check the voltages again, as soon as I stop doing a victory lap while patting my self on my back. I'll give an update when I get time. Thanks Gents!!!


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 Post subject:
PostPosted: Dec Tue 22, 2009 8:39 am 
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Location: Glendale, California
yeah, my new one have the same quirkes. Both A and B bands have slightly flattened peaks. More pronounced on the A band, but still not bad.

Probably normal for these generators.

-John


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 Post subject:
PostPosted: Dec Tue 22, 2009 1:16 pm 
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The 12AT7's are a high conductance tube. They need to be in good shape and reasonably well balanced between each other in this generator to get the cleanest waveforms.

It is normal for the modulated patterns to be a bit flat at the lower carrier frequencies of this generator. You can improve the lower band waveforms and also dramatically increase the percentage you can modulate by swapping to lower conductance 12AU7 tubes. But, the expense will be losing output on most of the upper "F" band which I found to be not the greatest thing anyway. Although it will generate up to and a bit beyond the rated 40 MHz, it starts getting ugly over about 25-30 MHz to the point I wouldn't really use it there.

The only reason the WR50B manual describes the demodulator probe and some other BS for using it with a scope is all date related. The WR50's are a radio/TV service man generator. In their time, most service grade oscilloscopes had VERY low bandwidths (if you had a 5 or 10 MHz scope you were 'da man!) and they weren't able to display everything the WR50's were capable of producing. A modern scope of 50 MHz and above will display all the WR50's capabilities and then some.



Mark


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 Post subject:
PostPosted: Dec Sat 26, 2009 12:15 am 
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MarkPalmer wrote:
The 12AT7's are a high conductance tube. They need to be in good shape and reasonably well balanced between each other in this generator to get the cleanest waveforms.

It is normal for the modulated patterns to be a bit flat at the lower carrier frequencies of this generator. You can improve the lower band waveforms and also dramatically increase the percentage you can modulate by swapping to lower conductance 12AU7 tubes. But, the expense will be losing output on most of the upper "F" band which I found to be not the greatest thing anyway. Although it will generate up to and a bit beyond the rated 40 MHz, it starts getting ugly over about 25-30 MHz to the point I wouldn't really use it there.

The only reason the WR50B manual describes the demodulator probe and some other BS for using it with a scope is all date related. The WR50's are a radio/TV service man generator. In their time, most service grade oscilloscopes had VERY low bandwidths (if you had a 5 or 10 MHz scope you were 'da man!) and they weren't able to display everything the WR50's were capable of producing. A modern scope of 50 MHz and above will display all the WR50's capabilities and then some. Mark


I have not checked all freqs but my WV-50B puts out a nice wave form, in the 455khz and 10.7Mhz, which is all I need. Of courser FM band freqs would be nice but this will not go there.

I AM STILL CONFUSED ABOUT THE BANDPASS FILTER DEMODULATOR.... Not arguing with you but part of the reason may be the level of technology of scopes but the bandpass filter/demodulator/detector needs it the display marker properly (do to pre-injection)

"For a another reason, there is a compatibility problem here too. A wide-band scope will not show an oscillator-type marker because the marker appears for the full length of the trace, rather than just being a short pip. You need to use a narrow-bandwidth scope, or use a low-pass filter to convert your wide-band scope to narrow-band. It appears the the 368 originally had a probe that included the lp filter. Do you have that probe? It is called the "broadband detector" in the 368 manual.(see bottom left paragraph on page 6 of the manual)." (Al ak "toobs")

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Tube: AM/FM Zeniths, RCA, TrueTone table tops; Transistor: Kaito KA1103, TenTec RX320D, Pioneer SX780


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 Post subject:
PostPosted: Dec Sun 27, 2009 12:54 am 
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gmcjetpilot wrote; "Of courser FM band freqs would be nice but this will not go there."

Wouldn't do you any good on an FM radio if it did as this sig gen only has AM--no FM. Some of the service grade sig gens will go up to 200 MHz or more but again, only with AM--no FM.
Rick


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 Post subject:
PostPosted: Dec Sun 27, 2009 4:30 am 
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Yes, the WR50's modulate AM only.

You won't be able to get a crystal marker to display on a modern scope as they show it in the WR50 manual, but you can confirm it is operating by inserting the crystal while displaying the modulated waveform- it will become a bit blurry. Then switch the VFO to off to observe the marker crystal output on the scope.

The 455 kHz sweep output is nice on the WR50's, but be aware not every tube AM receiver uses a 455 kHz IF.

Mark


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 Post subject:
PostPosted: Dec Sun 27, 2009 5:40 am 
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MarkPalmer wrote:
Yes, the WR50's modulate AM only.

You won't be able to get a crystal marker to display on a modern scope as they show it in the WR50 manual, but you can confirm it is operating by inserting the crystal while displaying the modulated waveform- it will become a bit blurry. Then switch the VFO to off to observe the marker crystal output on the scope.

The 455 kHz sweep output is nice on the WR50's, but be aware not every tube AM receiver uses a 455 kHz IF.

Mark
I have already tried it on both FM and AM, looked at the sweep responce traces with 10.7Mhz and 455kHz, they look nice. The problem is my 10.7mhz crystal is off from the peak and something is screwed up. The crystal is fine. I think it is the phase shift issue which distorts the trace. The PIP is not too bad just not in phase.

Talking to Al "Toobs", he came up with a plan to use a phase shifter..... using X and Y (and scope on X-Y). Feed the response into X-VERT and the 60Hz phase shift circuit into Y-HORZ. Reading the manual it seems that is what it needs to work with a modern scope.

I don't have a 455Khz crystal but ordered some 455Khz ceramic resonators. If they don't work they are cheap. The real deal crystals are hard to find for cheap or at all. The response is nice however. Thanks for the heads-up on the IF freq. All my radios are 455Khz. I like the scope/sweep method, even for AM. For the challenge I want to perfect the technique. I am getting close. I put it on the side for awhile but going to get back at it.

I've three sweeps, RCA WR-50B, Eico 368, Wavetek 1801a. I have one straight Eico 330 (solid state) for an extra marker on the Eico. The Wavetek needs an external marker Gen. The internal ones are all too high 5Mhz and 50Mhz Harmonic. Too bad. I guess the Wavetek was made for CATV. However the Wavetek 1801a does go below 1Mhz, it will sweep 455Khz, even though its spec says 1-755Mhz, the dial goes down to "0" and produces a CW freq below 1Mhz.

_________________
Tube: AM/FM Zeniths, RCA, TrueTone table tops; Transistor: Kaito KA1103, TenTec RX320D, Pioneer SX780


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 Post subject:
PostPosted: Dec Tue 29, 2009 12:20 am 
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Its been a while since I used one for sweeping but I recall the sweep to be very wide. In other words, anything in the 450-470 kc range would easily be covered.


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 Post subject:
PostPosted: Dec Tue 29, 2009 7:19 am 
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Jack Shirley wrote:
Its been a while since I used one for sweeping but I recall the sweep to be very wide. In other words, anything in the 450-470 kc range would easily be covered.


That is a good point. The sweep is 10% of the value or 45Khz, so it might go even more than 433-477Khz. From manual:

Sweep Output:
455 kc, center-frequency
10.7 Mc, center-frequency
Sweep width approximately center frequency.

Common AM radio receiver IF Freqs: 450 kHz, 455 kHz, 460 kHz, 465 kHz, 470 kHz, 475 kHz, 480 kHz

The bandwidth of AM is about 12Khz total, 6Khz each side, However if you go with 12Khz each side that should be enough.
So that means center freq of 450, 455, 460 and may be 465Khz +/- 12Khz might be possible possible.
Good luck finding a 450, 460, 465Khz marker crystal. It might get ugly on the edge of the sweep?
Forget about a 470 and 480Khz, but I bet you can tweek the center freq if you wanted to?
I am pretty sure they settled on 455Khz fairly early one in the USA. Not sure about foreign radios or Pre-WarII.

_________________
Tube: AM/FM Zeniths, RCA, TrueTone table tops; Transistor: Kaito KA1103, TenTec RX320D, Pioneer SX780


Last edited by gmcjetpilot on Jan Thu 21, 2010 4:30 am, edited 1 time in total.

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PostPosted: Dec Tue 29, 2009 2:58 pm 
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Yes, 455 was the 'norm' at the time it came out.
It will go even wider than the +/- 10% but it starts rolling off so you have to factor that in. Not all that drastically though so you can use it over quite a wide range.

Like MarkPalmer was saying about the crystal, I don't think its intended to be a marker in this unit. I did something similar simply by calibrating my setup on the scope face with a crayon marker using the sig gen itself (or maybe it was a 2nd sig gen? - I forget).


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PostPosted: Dec Tue 29, 2009 4:56 pm 
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The sweep on the WR50's is wide, and I used it quite a bit for aligning before I got a frequency counter. I seem to get better results on tube broadcast radio IF's by peak aligning with a straight signal set to what the chassis calls for. I recently peaked a Howard radio made around 1940 that had an IF of 468 kHz, of all frequencies to pick that would be on the outer edge of the WR50's 455 kHz sweep range. The good thing with those sets is that the alignment doesn't have to be an exacting science, (at least not on the broadcast band) and the radio will still play well. With about all of the solid state ham/shortwave sets I work on the alignments call for a specific frequency for each procedure and you want the signal to be as close to that mark as possible rather than being on a sweep where you don't know for sure what frequency the sweep starts and ends at and have no real good way of measuring it.

The WR50 doesn't even have to be very stable when in the sweep mode, and it's good for broadcast receiver alignment in that respect. When going up in the shortwave bands or where things get more selective, you rely more on it being stable on a straight carrier frequency. Modulation level also has to be carefully watched on a scope with the WR-50 or any other older type generator where it is not calibrated in any way and isn't rated to go much beyond 30%. The WR-50's attenuation is also nothing more than an uncalibrated pot, so care needs to be taken there also to keep the output low enough to not overload the receiver while still watching that the modulated output signal remains clean and is on frequency.

The WR-50's were an inexpensive signal generator intended for the radio repairman. They were one of the lesser of the evils because their output was relatively clean and stable, and the 455 kHz sweep output allowed the repairman to hook the generator up to the receiver's detector and peak the IF transformers for adequate performance in 5 minutes or less. They attempted to give the WR-50 a few other "selling points" with some sweep/marker capability, but it wasn't useful for TV with no FM output and much better sweep/marker generators were available for that purpose.

Mark


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PostPosted: Jan Thu 21, 2010 3:39 am 
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On my WV-50B, in A and B range:
(B) 85-200kHz
(C) 200-550kHz

The sine wave is distorted on the scope, very rounded on the bottom and flat on the top. It is not symmetric 4 units above zero and 3 below (0.2 v/div).

In the higher freqs the sine wave looks good but still not symmetric, and has some some "noise" at the peaks, the trace gets a little thicker. (modulation is off)

Bad Caps or just the nature of the beast?
How does a distorted sine wave affect radio alignment?

_________________
Tube: AM/FM Zeniths, RCA, TrueTone table tops; Transistor: Kaito KA1103, TenTec RX320D, Pioneer SX780


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PostPosted: Jan Thu 21, 2010 3:19 pm 
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Your WR50B sounds about par for the course. I re-capped mine a few years ago and it does the same thing with slightly top-flattened sine waves on the A and B bands, the other bands looking better.

There is probably little or no effect on alignments with the slight distortion of the sine waves on the lower bands. I only say this because short of having used it myself for that purpose, (mainly on AM broadcast radios) there were quite a few tube generators made prior to the WR-50B that were a whole lot worse, producing heavily distorted output that looked nothing like a sine wave at all.

I think RCA took the two 12AT7 design and tried to press the limits of it on the lower bands and top "F" band. I tried 12AU7's in place of the AT7's, and the lower band waves cleaned up, especially when modulation was added. (And it would even modulate above 50%) The consiquence was losing most of the upper "F" band on the generator, I think it only generated up to about 12-15 MHz.
Now that I no longer use my WR-50B for any working purposes, I might take the time and experiment with it more when I get some time.

Mark


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PostPosted: Jan Thu 21, 2010 6:22 pm 
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MarkPalmer wrote:
Your WR50B sounds about par for the course.

snip <insert awsome good info>

The consiquence was losing most of the upper "F" band on the generator, I think it only generated up to about 12-15 MHz.
Now that I no longer use my WR-50B for any working purposes, I might take the time and experiment with it more when I get some time. Mark


Thanks Mark, this is why Antique Radio Forum members are the best on the Web, of any forum of any topic. Always never cease to be amazed at the quality and depth of the replies. :D

Despite all that the RCA WR-50B is a cool little machine.

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Tube: AM/FM Zeniths, RCA, TrueTone table tops; Transistor: Kaito KA1103, TenTec RX320D, Pioneer SX780


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