Hi all. I've finally got to the stage of performing an IF and RF alignment on my STC 536 radio.
Back in 2017 I built the Silicon Chip IF alignment generator and finally had a project to use it on.
I loosely coupled the output of it to 6A8 anode and powered up the generator and set it to 455kHz.
As soon as I turned on AM modulation on the generator things went pear shape.
No measurements made any sense and adjusting IF cores made no meaningful change.
I then took a look at the gen output on my CRO and could immediately see why.
With modulation off I got a nice carrier at the right frequency but as soon as mod was turned on the output became a series of gated square wave carrier bursts at the modulation frequency.
I pulled the generator apart to check my work but everything was correct.
Has anyone had any experience with this generator.
If it worked correctly it would have been a very handy device but it's only use at the moment is as an RF generator without modulation.

Which project is that? The Touch Screen DDS Signal Generator (April 2017)?

Possibly the kit I put together described in 'special projects' "SC DDS Superhet alinement?

Thats the one I use as a general purpose sweep generator using a CRO as the indicator.

Fred.

Searching on their misspelled 'alinement', leads to this thread here on VR where Fred reviews that kit, concluding 'This kit gets the Fred Lever “Seal Of Approval” just for setting superhet IF coils let alone anything else':

https://vintage-radio.com.au/default.asp?f=12&th=56

Yes it's the touch screen one mounted in a small jiffy box.
It works fine for a clean carrier output but when "AM 1kHz" is selected it produces a square wave gated carrier rather than a sine wave modulated carrier.

Photo uploaded to Post 5.

IIRC, David Tipton (member Tippy on here) uses a similar sig gen in his YouTube videos:

DavidTipton101/videos">https://www.youtube.com/DavidTipton101/videos

Thanks GTC. I've watched all David Tiptons video's.
He does wonderful restorations and I have seen him use this generator at times.
I'll have to go back over some of those video's to see how he uses it.
I have a Philips PM 5324 RF generator also which will certainly do the job but I liked the idea of having a small gen which takes up far less bench space.

You should never use modulation when you are doing alignment. This applies with any generator.

Modulation is only there to help you fault-find dead stages!

Your scope shot seems to indicate that the modulation is w-a-a-y over 100%! Is there a tweaky to adjust it? Maybe a wrong value resistor.....

Irext,
I tend to use the SC unit as a sweep gen simply because it is so handy to touch set the frequency ect.
I don't use it as the authors suggest, I like to display the response of a tuning coil or IFT on a CRO screen.
Yes, I know that does not give you all the tech information but I like to see the actual shape of the response not what a program thinks it is.
Thats old fashioned but that's how I work, by feel.
I'll get into the workshop this week sometime and view what it in your post #5.
I'm just snowed under at the moment.
Fred.

One needs modulation to get sound if you are aligning by ear. I usually use it as I am using an oscilloscope and it will show up distortion.

One should not calibrate the Signal Generator with modulation turned on. The electronic one may take its feed of of the actual oscillator and should not need such a procedure, if set correctly when built. The old entry level ones with mechanical dials are a wholly different ball game.

What brings a lot unstuck is cramming too much signal down its throat activating the AGC (where fitted) or overloading the front end with a TRF. Marcus & Levy quote a mere 50uV into the modulator grid of a superhet. That's why its volume up & signal strength down. Activate the AGC and you will never get it right.

Alignment can be a bit of a black art.

Know the snowed under feeling, slowed down by a Wireless set 19 as noted MK III. As noted, but now its gone I can work on reducing the line, a series of wet days means that one can run a double shift. You can get a lot done that way as the parasites on the phones are greatly reduced.

Six today just on Solar panels. Two fake Telstra Bills & more, its out of control.

Marc

I finally got back into the workshop and set my SC unit to the same settings as in post #5.
My unit shows much the same output except the modulation is not as deep.
Irext's has about 90% from top down, mine has about 50%.
What difference that makes I don't understand.
Remember I don't use the "alignment " function at all I just use it as a sweeper with a sperate CRO as the indicator.

Thanks everyone for all the helpful info.
I have now been able to adjust the I.F's with ,as you say, modulation turned off using my CRO at the detector input.
I initially had trouble with the radio bursting into oscillation just as the I.F' cores approached a peak in adjustment.
It turned out to be the shield on the 6U7 not making good contact between each half of the shield.
Connecting an earthed clip lead to the shield proved this.
I went to work with some emery paper and cleaned the groove where the spring steel clip that holds the two halves together seats and cleaned the clip as well.
80+ years of oxidation wasn't helping at all.
The two I.F. cores peaked up nicely now and the radio is performing well.
Just the RF section to do now once the dial glass is back in place.
I have restrung the two dial loops (one from the dial shaft to the tuning gang and the other to the dial pointer) which now works smoothly
Identifying all the relevant tweakers will be the next challenge.

I have mentioned it before above: One does not normally hook the signal generator to the Plate.one always uses a series blocking cap on a generator that has none. The input is into the Signal grid which on 6A8. 6J8 and several others is the top cap.

Failure to block DC wrecks the bias on the grid. The plate will not amplify but its normally in a resonant circuit, which will be substantially thrown off frequency by any coupling to it.

Marc

I second Marc, the best way to inject a signal without upsetting any circuits is to feed into a grid circuit preceding the first IF stage and loosely coupled.
You cannot hook any outside circuit that has reactance to any IF grid or plate directly, you will detune the circuit you hook to.
A DC blocking cap is a must, don't rely on any internal cap in the sweep generator especially if its a solid state unit.
A loose coupling can be a clip or a probe clamped to the outside of wire insulation or a grid wire.

On a non RF stage set you can even hook to the aerial wire lead and turn the generator up untill you get response.
Even though the tuning coils are not resonated at 455kc enough signal will leak through the convertor to drive the IF.
I just park the tuning as low as possible say 550Kc.
It is an advantage to shut the local oscillator down by shunting a coil in a way that does not damage anything.
Then you dont have conflicting signals from the local oscillator.

All common sense.
Fred.