[Mondial]

Honestly Eric, you really don't need a sweep generator for a proper alignment of a 40's or 50's FM radio. The manufacturer's alignment instructions usually assumed the service tech did not have a sweep generator and usually gave a procedure using a common AM generator.

That said, a sweep generator can allow you to optimize the IF bandpass and discriminator curve for optimum performance, but practically speaking you would never notice the difference in actually listening to the radio. The sweep generator with an oscilloscope will allow you to actually see the performance of the IF and detector, and as an educational tool it can be very valuable in understanding their operation.

If you are going to get a sweep generator, get the best quality you can afford. The Eico's and alike were cheap when new and had performance to match. Today for almost the same price, you can buy an older HP which cost thousands when new and now can be had for pennies on the dollar. ( the HP 8601A cost $2250 in the 1970's, about the price of a new compact car at the time ! )

[Eric Adams]

Thanks guys. I think Mondial nailed it. If I can learn more by using one than I'm going to go for it. I put a bid in on the 8601A that Mondial told me about, so we'll see what happens. Then we'll talk about probes.

[Eric Adams]

I won the auction for the HP 8601A for $125.00. I have a question about the cables needed. I have the BNC to BNC cables to hook up my frequency counter and scope. I also have a cable with BNC to alligator clips that I use for my RF generator but it doesn't have the 50 ohm resistor. Should it have a 50 ohm resistor across it for all RF outputs, my RF generator as well as the sweep generator when used as an RF generator?
I've been reading the manual and watching videos on You Tube and I think I'm getting an idea how to use it. This really is on of the fun parts of this hobby!

[Mondial]

Yes the RF cable should be terminated in 50 ohms with any generator, especially at VHF frequencies. It does not matter much at AM broacast freqs as the wavelengths are hundreds of meters long. But at 100 MHz, a wavelength is only 3 meters, and a 1/4 wavelength cable (only a few feet) which is unterminated will act as a resonant circuit causing the output to vary wildly as the freq is changed. So you really want the cable terminated in its characteristic impedance at VHF for accurate measurements.

[Eric Adams]

OK, I'll get to work adding resistors to my cables. Once it arrives and I check it out I'll let you know how it's doing.

Thanks again for your help.

[Eric Adams]

Mondial, it has arrived. Now, for a few questions: I've owned some AM/FM radios in the past that have had instructions for an alignment with a sweeper and scope. Now, of course, the 3 FM radios I have only have the instructions with an output meter. I tried to hook things up and don't get the pattern on the scope that I'm looking for. I'm pretty sure I know the proper settings on the sweeper.
My basic question: Is there a standard place to connect the generator and scope on all radios no matter what type? Is there somewhere that I can read about this to get a good understanding? I've been watching videos on you Tube and one guy was using the equipment. The problem was that he started out by pointing and saying, "I've got my sweep generator connected down there and I have my scope connected down here".
So that's where I'm at. BTW, I used the generator on CW and it works fine.

Thanks

[Mondial]

Every radio will be different, Some have a ratio detector, others have a discriminator and the 48-482 has a quadrature detector. Each will require a different connection for the scope.

To sweep the IF the best place to connect the sweeper is the grid of the mixer tube, for the detector the grid of the limiter or last IF stage.

Let me know the models you are working on and a link to the schematics, and I can give you a better idea of the connections.

Glad you received the generator in one piece and it works. You can check the sweep function by connecting your counter to the aux output and using the manual sweep function. For example set the generator to symmetrical sweep, freq to 5 MHz and sweep width to 1 MHz. When you turn the manual knob from end to end, the freq should vary from 4.5 to 5.5 MHz. Switching to slow sweep should do the same thing automatically but you need to set the sweep slow enough so you can follow the variation on the counter.

[Eric Adams]

The sweep works just as you said. Now for the fun stuff. As a matter of fact, my 48-482 is sitting on the bench right now. http://www.nostalgiaair.org/Resources/637/M0013637.htm
I've never been happy with the FM so I guess this would be a place to learn. If you could tell me why you hook it up the way you do then I think I'd have a better chance of learning what to do with any radio.

Thanks again,

[BrendaAnnD]

Eric,

Don't expect a lot of performance on a 40's FM receiver. Compared to today's radios, they're virtually deaf

[Eric Adams]

I found that out Brenda, thanks. I'm just trying to get them as best as possible and I really enjoy working with this equipment and learning more. It's funny, but I feel like I'm just scratching the surface and I want to get in as deep as I can.

[Mondial]

Actually I was quite surprised at the performance of the 48-482 on FM. It is much better than any other 40's FM receiver I have tested. It does have 3 IF stages, an RF stage, a triode mixer, and the quadrature detector so maybe it should not be so surprising.

If you have low sensitivity, check the 33K plate resistor of the 7W7 RF stage. It was open on mine. Philco underrated this resistor at 1/2 watt as it dissipates significantly more than that and therefore fails regularly. I replaced it with a 2W metal oxide resistor. The 100K screen resistor is also marginal and usually increases in value.

The first thing I would try is to sweep the Fm1000 detector. Connect the generator set to 9.1 MHz, 1MHz sym sweep to the grid of the 7H7 3rd IF, marked point "A" on the schematic. Use the fastest sweep rate range of the generator. Connect the AC coupled scope probe to the junction of C304 and C305 in the FM detector section. Point is marked "B" on the schematic. As you increase the signal level you should notice the detector lock in and see a sloped waveform with noise at each end. You can decrease the sweep width with the sweep vernier to get a better expanded view. Then adjust the detector slug on top for the most symmetrical waveform with the straightest , most linear slope on the center portion. This will be the optimal adjustment for the detector.

[Eric Adams]

I have a Philco 49-905 that I just finished that has much better FM than my 48-482 so I guess I do have some issues. I'll check the resistors first. One issue I have is that when I tune in an FM station on the 48-482, I can rock the tuner and get the station at 2 places about 1/8 of an inch apart. I was hoping the correct alignment would cure that. One more thing, are these instructions to adjust all IFs or just the detector?
I'll keep you posted on my progress.

Thanks

[Mondial]

If you are getting each station at two places on the dial then there definitely is a problem with the alignment or the IF circuit itself. My 48-482 tunes stations with a single wide bandpass response.

The directions I gave are for adjustment of the FM detector only. If it is aligned and working properly then you can proceed to the IF. Since you are injecting signal into the last IF, you are bypassing the previous stages which may not be working correctly. When setting the generator, have the counter connected and check the center freq with zero sweep width to insure it is right on 9.1 MHz.

[Eric Adams]

I got it! Wow, it really made a difference. I'll check the other IFs now that I think I have an idea where to hook them up.
BTW both of the resistors you mentioned were replaced during the rebuild.
I'll let you know if I run into any other issues. I'm going to dive into some of my other radios also.

[Mondial]

Now that you have the detector aligned, you can proceed to check the IF bandpass response. If you have a dual trace scope, you can leave one probe on the FM detector output and place the second probe on the AGC output of the diode detector (7C6) . That way you can see the IF response superimposed on the detector curve. They should line up so that the IF response is in the center of the linear portion of the detector slope. If the IF response is flat topped with steep sides and centered on the detector reponse, then you are all done!

You will need to connect a 47K resistor in series with the scope probe when connecting to the diode detector ( junction of R417 and R412), to prevent loading of the RF signal present there.