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 Post subject: O-Scope and Sig-Gen
PostPosted: Mar Fri 15, 2019 6:39 pm 
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Joined: Sep Sat 06, 2008 6:17 pm
Posts: 996
Location: Central Pa, 17044
Hi all, I have a dumb question...
I have a Sencore SC-61 waveform analyzer and a Leader
LSG -16 signal generator. My question is, when aligning a
AM radio, I know how to align it with the 455kc tone and doing
the high and low ends of the dial. Now the Sencore has a frequency
read out up to 100Mcs. Now can I use this instrument as a signal
counter? I have read all kinds of info online about radio alignment,
I use my faithful Triplett on the speaker voice coils to get the best
voltage signal...I use the AC scale, I see some guys use the DC scale???

Thanks for any information,
Gary


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 Post subject: Re: O-Scope and Sig-Gen
PostPosted: Mar Fri 15, 2019 7:04 pm 
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Gary T. Lane wrote:
I have read all kinds of info online about radio alignment,
I use my faithful Triplett on the speaker voice coils to get the best
voltage signal...I use the AC scale, I see some guys use the DC scale???

As an alternative to using a VOM on AC across the voice coil, or the VOM "Output" connection (with function set to AC) across the primary of the output transformer, some folks prefer to put a VTVM on the AGC line and monitor the AGC DC voltage. Using the AGC voltage instead of the AF output voltage to the speaker allows you to perform an alignment with the volume turned all the way down.

You must use a VTVM or a DMM with 10 MegOhm input impedance to monitor the AGC line. A VOM would load it down too much.

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 Post subject: Re: O-Scope and Sig-Gen
PostPosted: Mar Fri 15, 2019 9:22 pm 
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Posts: 20278
Location: Utica, NY 13502 (USA)
Alternatively, you can use your scope to monitor the detector output. Whatever method you employ, use the weakest signal possible from the RF generator when aligning a radio. This means that you will be continually reducing the output of the RF generator as the radio comes into alignment.

Dave


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 Post subject: Re: O-Scope and Sig-Gen
PostPosted: Mar Fri 15, 2019 9:55 pm 
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Gary T. Lane wrote:
Hi all, I have a dumb question...
I have a Sencore SC-61 waveform analyzer and a Leader
LSG -16 signal generator. My question is, when aligning a
AM radio, I know how to align it with the 455kc tone and doing
the high and low ends of the dial. Now the Sencore has a frequency
read out up to 100Mcs. Now can I use this instrument as a signal
counter?


You can use the frequency counter in the SC-61 to set the signal to the right frequency, but not as an output indicator. The scope could certainly be used for an output indicator, but I like seeing the needle move on a VTVM.

I don't see an LSG-16 from Leader in a quick search, so I don't know what it is capable of. An LSG-216 has a digital readout and looks like a decent generator, so if that is the case, you need nothing more than the signal generator and an output indicator.


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 Post subject: Re: O-Scope and Sig-Gen
PostPosted: Mar Sat 16, 2019 12:04 am 
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Dave Doughty wrote:
Whatever method you employ, use the weakest signal possible from the RF generator when aligning a radio. This means that you will be continually reducing the output of the RF generator as the radio comes into alignment.
This is the whole reason for locking the AGC (aka AVC) line with a battery across the integrating capacitor, 1.5 or 3 volts, positive to ground.

With the batter in place, the AVC does not affect the set's gain, so you just adjust for maximum negative AVC voltage and you're done. No fiddling with signal levels.

- Leigh

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 Post subject: Re: O-Scope and Sig-Gen
PostPosted: Mar Sat 16, 2019 12:12 am 
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Location: Houston, TX
Gary T. Lane wrote:
Now the Sencore has a frequency read out up to 100Mcs. Now can I use this instrument as a signal
counter?

I see there's a YT video on that generator: https://youtu.be/yc1927uuSSk

It can push 100mV out, so the SC61 can absolutely view that. Since the scope's best sensitivity is 50mV/div with the 10x probe, you'd need to use the direct probe DP226 for max input sensitivity of 5mV/div. to get a good look at the waveform, but the low bandwidth of 455kHz shouldn't pose a problem. You'd run into problems with FM, since the direct probe has a max bandwidth of 15MHz.

Remember to properly terminate the interconnect cable to prevent ringing (important with FM, even though the direct probe's resistance wire dampens it somewhat).


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 Post subject: Re: O-Scope and Sig-Gen
PostPosted: Mar Sat 16, 2019 2:59 pm 
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Joined: Sep Sat 06, 2008 6:17 pm
Posts: 996
Location: Central Pa, 17044
Yes,that's the sig-gen I have.

Thanks guys for the information,

Gary


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 Post subject: Re: O-Scope and Sig-Gen
PostPosted: Mar Thu 21, 2019 3:09 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1382
Location: Arlington, VA USA
Leigh wrote:
Dave Doughty wrote:
Whatever method you employ, use the weakest signal possible from the RF generator when aligning a radio. This means that you will be continually reducing the output of the RF generator as the radio comes into alignment.
This is the whole reason for locking the AGC (aka AVC) line with a battery across the integrating capacitor, 1.5 or 3 volts, positive to ground.

With the batter in place, the AVC does not affect the set's gain, so you just adjust for maximum negative AVC voltage and you're done. No fiddling with signal levels.

- Leigh


Leigh,

How do you handle radios with multiple AVC gain stages? Use multiple batteries? For example, I believe this Philco has two gain stages that are rather independent.
http://www.nostalgiaair.org/pagesbymode ... 013395.pdf

One created by R31 and C12. The other from C11? Not quite sure what R27 is for? I like your method but have run into this situation a few times.


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 Post subject: Re: O-Scope and Sig-Gen
PostPosted: Mar Thu 21, 2019 3:38 pm 
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Joined: Jul Mon 26, 2010 8:30 pm
Posts: 25707
Location: Annapolis, MD
This one is making my head hurt.....

There are indeed 2 AGC circuits---one for the front end RF stage, and the other for the mixer and IF stages. The latter uses the second diode for "clamping" the AGC so it cannot go positive. It looks like the 2 circuits have different time constants.

The other interesting thing is the use of the suppressor grid for AGC

One thing I do not understand:
In the common AGC circuit, one side of the IF secondary goes to the diode, and the OTHER side goes to the volume control and the AGC line. I'm not clear on the ramifications of taking one of the AGC signals from the diode side---wouldn't the DC voltage be the same at either location?

perhaps with more caffeine......;)

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-Mark
"Measure voltage, but THINK current." --anon.


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