Frequency Generator Modification for Radio Repair

To fix this 1946 Arvin AM radio that belonged to Vonna’s father, will involve some component replacements. When that is done, there may need to be adjustment of the radio oscillation circuits.

To adjust the oscillator circuits I need to provide a precise radio frequency (RF) carrier to the radio, with an amplitude-modulated signal on that carrier. Preferably of 400 Hz signal on a 455 kHZ carrier.

Modern units that do this for radio repair can cost many hundreds of dollars to over a thousand, or even more.  Retro units, while cool and sorta plentiful, are still costly, and by this time need repair too. 

Some cheap frequency generators are digitally creating the signal and don’t have a way to introduce a signal modulated onto the carrier.

So what about this little unit which is new and much less expensive? It’s an analog generator with a nice carrier signal and you can modulate a signal on top.

The problem is, if you look at the dial, it is not very accurate. In order to do the repair right, you want that 455 kHz Intermediate Frequency right on.  The dial is just not accurate enough. I need to add a digital frequency counter to set the generator accurately.

So I bought some kits for very little, and had a hoot building them while on Corona-lockdown. 

The unit with the three knobs is a frequency generator that is not for creating the carrier, it for creating a signal, more on that later in another project.

It is wired to a Frequency Counter I built and is also being read by my oscilloscope in the background. 

Problem is the output of the generator is too low for the counter to read it at higher frequencies.

If you zoom in on the scope you can see the generator is already clipping before outputting 0.5V peak to peak. So the counter needs a strong signal. When I hook it to the Lodestar RF generator, the counter can’t read it unless at full volume (attenuation). 

Finally, the problem statement:  need to get a strong signal out of the RF generator for a counter that will work even when the attenuation is turned to minimal output, so that I can precisely set the RF output frequency.

Here is the schematic for the frequency generator. The RF output used for sending a signal to a radio under test is on the far right. That signal needs to be turned down so that it is barely readable. So if you follow the circuit back to the left, there is an attenuator circuit. I want to grab the signal for the counter before the attenuation. So I want to grab it just as it leaves Q3 before R9.  Incidentally, on the bottom is the power supply in the frequency generator. After the transformer there are two diodes on either side of the trans secondary with a center tap ground. It measures about 25VDC.

So the plan is to take the RF signal off after the transistor Q3, with a 0.01microfarad coupling cap. That will go to the frequency counter. (Actually, I will end up tapping in on the other side of R9 by the potentiometer for ease of access.)

The frequency counter requires about 9VDC to operate and I don’t want to mess with a battery. So I will power it directly from the power supply in the generator. To get the voltage right, I will use a DC-DC buck converter.

The DC-DC buck converter. Here shown stepping down a 9V battery to 5V regulated. This was less than $2.

So let’s void the warranty and crack this open.

Here, after probing around, I found the three leads for the potentiometer. The the one on the right is on the other side of R9 on the schematic and a lot easier to get to, so I will tap the signal there.

Here I tapped onto the pad with a cap and terminated to a terminal tie strip mounted to a ground tap on the circuit board. 

Did a lot of testing every step to be sure it was all working.

Here is the power supply board top. Notice the two rectifying diodes next to each other by the white connector with the blue and red leads.  This is where I will source the 25-26VDC out of the power supply. This will be used to drive the frequency counter via the buck converter.

The other side of the power supply board. The arrow points to three pads next to each other. The one I want is in the center.

So here I mounted the buck converter on stand-offs on the left, and tied in the input from the center tab, after the diodes, and grabbed a negative off the ground rail.

This so cool. On the right is the 25VDC from the power supply going to the little buck converter. I am tweaking a small variable resister that varies the converters internal oscillation circuit, resulting in a lower output voltage. Here I tweaked it to 9VDC out as you can see.

Built another frequency counter, and then wired the whole thing up and tested it. Works great.

In the back of the case, I added a BNC connector and a power outlet jack.

Now wired to the back of the cabinet.

A final check.

Labeled the outlets.

So here it is finished. The counter is set on top attached to a box. It connects for signal via a BNC connector to the back of the generator, tapping into the pre-attenuated RF signal. The counter is powered by plugging into the back of the generator sourcing 9VDC provided by the generator’s power supply via the little buck converter. Here is shows the generator putting out precisely 20 MHZ.

And finally, here it is putting out precisely 455 kHz needed to tune the Intermediate Frequency for a superheterodyne AM radio from the 1946 timeframe.

I credit xraytony on youtube for knowing how to do this. It was a fun project. 

And now to use it…

See more of Mark’s projects here.

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