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 Post subject: Help! What's happing to my RCA SG Kit, RF output.
PostPosted: Mar Thu 05, 2020 6:49 pm 
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Joined: Aug Tue 13, 2019 4:11 pm
Posts: 49
I just finished replace paper and electrolytic capacitors and resistors on this RCA Signal Generator Kit (model nt9k), per 2 parts instructions on this link.
http://www.nt9k.com/rcasignalgenkit.html
The RF pot on this is not original when I got it. Rather it was replaced by Borne multi turn 5K pot and a separated on/off switch.
To check the RF output, I hook it up to my EICO 147a signal tracer RF input to listen for a signal tone.
Nothing, no tone regardless of RF pot, until I switch the AF Out/Int Mod on and turn the AF pot up a bit then I can hear the tone.
If I crank up the RF output, the tone get lower. The inversion would happen if the lower the RF pot.
What's going on here? Please help me to diagnose, fix this problem.
I have double and triple check on components, connection wires etc..


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 Post subject: Re: Help! What's happing to my RCA SG Kit, RF output.
PostPosted: Mar Thu 05, 2020 7:25 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1218
This is normal. You won't hear anything on a signal tracer if the signal isn't modulated. Get an oscilloscope and you will discover what is actually happening.


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 Post subject: Re: Help! What's happing to my RCA SG Kit, RF output.
PostPosted: Mar Thu 05, 2020 7:42 pm 
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Joined: Aug Tue 13, 2019 4:11 pm
Posts: 49
Thanks Bob, That's good to hear (or not hearing in this case). No o-scope for me yet.
So can I test the RF output by place it near the working AM radio per many threads suggested here?
Do I need to calibrate it before I try out the radio alignment process? Should I go ahead and replace the Selenium Rectifier?


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 Post subject: Re: Help! What's happing to my RCA SG Kit, RF output.
PostPosted: Mar Thu 05, 2020 9:15 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1218
Your questions have multiple answers, depending on one's outlook.

You don't need calibration for ordinary alignment. In fact, I did most of my alignments with no generator at all, just the broadcast stations received. Their frequencies are very accurate.

The important calibration, lacking on the inexpensive generators, is signal amplitude. You need a decent unit to have that. The frequency calibration ought to be good if you are doing critical alignments of complex radios, but for most work it's not needed.

Replacing the selenium rectifier is a matter of choice. If it's working, leave it alone, but have some diodes in the queue in case you have to replace it.

Yes you can 'tune in' the generator with a radio. You may have to fish around to find the signal, depending on calibration. You will also find multiple responses if you explore. There will be the image in the receiver as well as its intermediate frequency and the generator's harmonics. Use a short piece of wire as an antenna, plugged into the generator output jack. The signal you seek is on the same frequency, both generator and receiver dials. It will be the strongest one, and you can control its strength with the output amplitude knob on the generator. You will also hear heterodynes when the frequency is also occupied by a radio station.


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 Post subject: Re: Help! What's happing to my RCA SG Kit, RF output.
PostPosted: Mar Thu 05, 2020 9:43 pm 
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Joined: Aug Tue 13, 2019 4:11 pm
Posts: 49
Thanks again Bob for answer all question. Will try it out. I am just trying do some simple AA5 radio alignment, to familiar myself with the process. Nothing fancy.


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 Post subject: Re: Help! What's happing to my RCA SG Kit, RF output.
PostPosted: Mar Fri 06, 2020 1:29 am 
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Joined: Feb Sun 17, 2008 11:36 pm
Posts: 1774
Location: Dayton, Ohio
Some multi-turn pots are often inductive and are unsuitable for use in r.f. circuits.

Charlie


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 Post subject: Re: Help! What's happing to my RCA SG Kit, RF output.
PostPosted: Apr Mon 06, 2020 4:53 pm 
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Joined: Aug Tue 13, 2019 4:11 pm
Posts: 49
I believe this unit is not working at all. So going back at it to do some checking.
The power transformer on this unit has two black wires label 117v/60 cycle (primary) and on the other side, 2 red wires label 125v/15ma dc, 2 green wires label 6.3v/0.6A.
The resistant value on two reds (Secondary) is about 286ohm and about 76ohm on the blacks (Primary). I took the voltage reading with the TF out of the unit, on the bare red secondary wires, it read about 150v ac. Is this normal or is it broken?


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 Post subject: Re: Help! What's happing to my RCA SG Kit, RF output.
PostPosted: Apr Mon 06, 2020 7:20 pm 
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Joined: Aug Tue 13, 2019 4:11 pm
Posts: 49
Also I want to check the wiring per instruction/schematic for the trimpots. On the instruction pg10 (attached pdf) it said pin 1 of the pot go ground.
When I read the schematic (also attached pdf), I thought, it always read pin 1 as top, 2-middle and 3-bottom.
Per this schematic, would pin 3 be grounded instead of pin 1?


Attachments:
rca_signal_generator_kit_pg10.pdf [208.9 KiB]
Downloaded 8 times
RCA_Schematic.jpg.pdf [97.22 KiB]
Downloaded 9 times
potentiometer-pinout.png
potentiometer-pinout.png [ 28.8 KiB | Viewed 304 times ]
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 Post subject: Re: Help! What's happing to my RCA SG Kit, RF output.
PostPosted: Apr Mon 06, 2020 8:23 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 9476
Location: Long Island NY
A potentiometer like this consists of a semi-circle of carbon on a phenolic wafer, with a movable contact attached to the shaft. The movable contact is in the middle (terminal 2). When you are in front of the instrument turning the knob, terminal 1 is on your left and terminal 3 is on the right.

In conventional usage we want things to go down as we move the knob counter-clockwise, and to go up as we turn the knob clockwise. So the terminal on the left--1--is the low side (ground or low side of the circuit) and the terminal on the right--3--is the high side where the most signal exists. The movable contact is the output. When turned counter-clockwise it moves towards terminal 1, the low side, which reduces the signal level on terminal 2. When moved clockwise towards terminal 3 there's less resistance so more signal appears on terminal 2.

Now if we look at the assembly instructions for the kit, they show it the same way, keeping in mind that you are viewing the backs of the pots so terminals 1 and 3 are flipped compared to when you were looking from the knob side. Your notes on the schematic are reversed as you have the #3 terminals connected to ground which is the low side of the circuit. If you actually hooked it up this way it would work, except you'd have to turn the knobs down in order to turn the signals up!

The resistance readings of your power transformer look reasonable. Why do you think there is a problem with it? Look at the tube--does the heater light up when you plug it in and turn it on? Since somebody evidently changed one of the pots and the on/off switch, maybe there is a problem with that?

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

Thomas A. Edison


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 Post subject: Re: Help! What's happing to my RCA SG Kit, RF output.
PostPosted: Apr Mon 06, 2020 9:23 pm 
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Joined: Aug Tue 13, 2019 4:11 pm
Posts: 49
The reason I believe this unit is not working, because when I plug this to the signal tracer in AF modulated mode, there is no different in the tone, regardless of which band and frequency I select.
Knowing pin 2 as output, I thought pin1 normally would be input and 3 would be grounded. I did swap the pin1 and pin3 around and back make no difference. Swap another tube make no difference, and yes the indicator bulb and the tube all light up.
I have been looking at this unit and check and double check for a while. It's simple enough, but yet it driving me nuts.
I think I am going to rewire almost everything except the coils, and follow instructions to make sure it work. The only thing caught me off guard, between the wiring and the schematic for the 2 pots.
Also, I don't have a 5k pot with on/off switch for the RF on hand, so I am going to use the 10k linear pot with on/off switch.


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 Post subject: Re: Help! What's happing to my RCA SG Kit, RF output.
PostPosted: Apr Mon 06, 2020 9:49 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 9476
Location: Long Island NY
This is not an audio frequency generator except in the very limited sense that it has a 400-Hz tone for modulating the RF carrier. The 400-Hz tone is available as an external output for testing the audio stages in radios, but the only control you have over it is amplitude. The audio frequency will always be 400 Hz regardless of the band you are on, or where the main tuning dial is set.

If you want an audio oscillator or generator that has variable frequency and perhaps more output and a better waveform, there are lots and lots of them available. What you have is an RF signal generator with built-in modulation, not a full-fledged audio signal generator. No matter how much you rewire it, it will never be a full audio signal generator.

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

Thomas A. Edison


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 Post subject: Re: Help! What's happing to my RCA SG Kit, RF output.
PostPosted: Apr Mon 06, 2020 10:10 pm 
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Joined: Aug Tue 13, 2019 4:11 pm
Posts: 49
Understood now. It's just that, couple things per instructions at the end to verify the unit operate normal. It say measure to ac voltage of the AF then the RF output, for 0 to 10vac and 0-15vac respectively, as you vary the control from minimum to maximum, doesn't check out for me. The AF always stay about 15vac and less then 1vac for the RF.
I watched some youtube where people use the RF signal generator to do radio alignment and I thought you could hear different tones on different band/frequency, so I thought that what's I expect.
I haven't use this unit to do radio alignment yet so I want to check it out. I have some success with AM alignment by listening so far.
The FM alignment is the one I really want to use this unit for.


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 Post subject: Re: Help! What's happing to my RCA SG Kit, RF output.
PostPosted: Apr Tue 07, 2020 12:13 am 
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Joined: Mar Sat 03, 2007 12:53 am
Posts: 795
If you turn the modulation switch on internal and set the RF frequency to 1000 Khz and adjust the RF output to maximum, you can put the output cable near an AM radio and if you slowly tune the radio around 1000 Khz, you should hear an audio tone from the radio. If so then the basic functions of signal generator are working.


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 Post subject: Re: Help! What's happing to my RCA SG Kit, RF output.
PostPosted: Apr Tue 07, 2020 11:45 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 486
MyLastWord wrote:
I have some success with AM alignment by listening so far.
The FM alignment is the one I really want to use this unit for.


I'm a little baffled by that remark. Your generator appears to be a simple type with an amplitude modulated carrier, and by the look of its dial goes to 50MHz, making it fine to assist in the alignment of AM radios of medium wave and short wave comms radios of up to 30MHz.

To align an FM radio you require a much more sophisticated generator that goes above 100MHz, a "typical FM radio" tunes in the 90MHz region.

In addition the generator must have precise frequency modulation and sweep functions if you have any hope of setting up the ratio detector or discriminator properly and it requires a scope too. With a primitive generator, you would only make the FM radio worse off if you attempted adjustments.

If you want a really good and relatively inexpensive generator that will align AM & FM radios I would suggest the Philips PM5326, it also has a digital frequency counter (unlike an analog dial) there is no ambiguity in the frequency output.


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