All, I’m working through my first alignment and I’ve got a problem. The radio is and RCA RC-521 and has been completely re-capped. The amplifier sounds great when I run my MP3 player through it but the radio on broadcast band only tunes in the higher frequencies(>1000kc). Well, since I’ve got all the equipment to do an alignment I thought I’d give it a try. Step 3 calls for setting the signal generator to 1500kc, set the dial to 1500kc and adjust the trimmer C25 for maximum voltage. This is where I have a problem. Carefully turning the C25 trimmer screw there is only one very hard to find spot allows for any voltage to show up on my scope and everywhere else throughout its range there is zero voltage. There is no way to peak the voltage since a thousandth of a turn in any direction and the voltage drops to zero. So I’m pretty sure the 5-50pf adjustment cap is bad. Cleaning wasn’t successful. I thought I’d buy a replacement for this mica trimmer cap but I have no idea of its voltage requirements. Can anyone out there suggest a replacement and would this trimmer cause me to lose the lower frequencies on the broadcast band (I do pick up some very weak stations that seem to be duplicates of the higher frequency stations)?
Thanks,
Keith

HI KEITH:

C25 IS A VERY SHARP TUNING CAP.
GO TO STEP 3 FIRST AND ADJUST L5 FIRST.TURN ON S.G MODULATION AND SET TO 30%.THEN DO STEP 4.
YOU WILL HAVE TO GO BACK AND FORTH UNTIL THE 600 KC&1500KC LINE UP IN THEIR RESPECTIVE MARKS.

PATIENCE!!!

WALTER-W2WIQ

One problem that can cause the observed symptom (only receiving part of the band) arises if
the local oscillator (LO) is adjusted on the wrong side of the station.
There is usually enough adjustment range to permit this error during alignment.

The LO should operate on the high side of stations in the broadcast band.
This can be checked with a digital radio close to the RCA.
Tune the RCA to mid-band (1000 kc) and the digital radio to 1455kc ( = station + IF ).
You should hear the local oscillator in the digital radio. If not, rock the RCA tuning back and forth a bit.

If you can't find the LO, it may be operating on the low side of the station.
Tune the digital radio to 545kc ( = station - IF ) and look for the LO signal down there.
If you find it, it's on the wrong side. This could cause the C25 adjustment to be critical, because
the oscillator is not designed to work at such low frequencies.

Why do you assume the trimmer is bad? Did you disassemble it?
Some adjustments are quite critical, particularly oscillator frequency. That does not indicate a malfunction.

How accurate is the signal generator frequency?

The adjustments need not be done at the exact frequencies shown in the instructions. Close is good enough.

Do you have a strong local station at 1400kc or above? If so, tune the radio to receive it.
Adjust C25 while tuning the radio until the radio dial indicates the station frequency.

Repeat using a station at the low end and adjusting coil L5.

- Leigh

Thanks for all the advice, as I said this is my first alignment. My Craftsman DMM measures frequency and I'm using that to calibrate my generator but Leigh gave me another idea. I've got a Talking house transmitter that can broadcast between 550 -1700 kHz. Can I set my Talking house transmitter to 1500kc set the RCA dial scale to 1500 and tune C25 to pick up the broadcast, then set the transmitter to 600kc move the dial scale to 600 and set L5 to pick up the broadcast? If I do this do I need to turn the volume down or up. I'm going to sleep on this and hit it again tomorrow night. I'll try the digital radio trick tomorrow night.
Thanks again for lots of great advice.
Keith

Hi Keith,

The talking house transmitter will work as long as you can set its frequency accurately.
The signal level might be higher than desirable, but it's certainly satisfactory for setting the RCA on frequency.
The radio volume does not affect these adjustments. Set it at a comfortable level.

Unfortunately there is some interaction between L5 and C25.

The best way to preset the adjustments if they're way off is to close C25 snugly (not tight), then open it 2 turns.
For this re-alignment it's important to start with the lower frequency (L5) then do the higher one.

Tune the transmitter to 600 kc and set L5 for the proper LO frequency (above the station frequency).
Use the digital receiver to confirm that the LO is operating above the station at (600 kc + 455 kc).
Retune the receiver while adjusting L5 to set the radio dial to the proper position.

Tune the transmitter to 1500 kc and adjust C25 for proper injection.
As before, adjust the radio tuning and C25 to set the dial to the proper position.
You won't be able to find that LO frequency on the digital receiver (it's too high), but you can check
to make sure the LO is not operating below the station at (1500 kc - 455 kc).

Go back and forth adjusting L5 and C25 a few times since there is some interaction.
Then check the tuning around mid-band. The dial should be reasonably accurate.

- Leigh

I just did this test and I get a load high pitch squeal when the digital radio is tuned to 1450kc and the radio dial is around 1000kc so it looks like I passed this test. I'll work on this some more tonight.

Keith,

Is there a legible schematic available online? NostalgiaAir does not seem to list this model.

If a radio does not work, the chances are that alignment will not solve the problem. The exception is cases where the golden screwdriver has been at work. In most cases, an alignment will make a working radio work better. So repair first, then align.

A very common reason for radios being completely dead over part of the tuning range is a bent plate in the tuning capacitor. If any gang of the tuning cap shorts out, it will kill your reception. Sometimes the short only appears as the capacitor is tuned to the lower part of the band.

A hyper-critical alignment adjustment does not sound right. Perhaps your trimmer really is bad. The fault is usually a short between plates, rather than the capacitance changing suddenly.

You can check both the tuning capacitor and trimmers with an ohmmeter, though you will probably have to disconnect the associated coil first.

Mica compression trimmers can usually be repaired. The mica can be replaced, if it is broken.

Keep us posted about how you are doing.

Ted

Here's a link to the schematic on Nostalgiairehttp://www.nostalgiaair.org/PagesByModel/917/M0015917.pdf They had it listed under V-205. I'm making some interesting observations but i don't know what they mean. I'll report back soon after I do a bit of experimentation.
Keith

I have a question - where are you measuring the output with your 'scope?

All I just can't seem to get C25 to adjust. Either I have voltage or I don't and there's no apparent adjustment. I think I need a recommendation on a modern replacement. But here are some other observations. I moved the Talking house transmitter to within 10 feet of the radio and I used it to transmit at frequencies between 600kc and 1400kc. I can pick up the lower frequency signals but they are very weak and as I increase the broadcast frequency the signals come in stronger. With just an antenna everything above 1000kc comes in strong. We've got a local Army Corps of Engineers station that broadcasts at 1610kc that I can pick up at 1620kc and very weak at around 700kc and again around 500kc (I don't think this should be happening). Using Leigh's digital radio trick my digital radio can pick up the oscillations at around 455kc above where the dial is set on the rca. So I can pick up the oscillations at when the rca dial is at 1200 and the digital radio is at around 1700, when the rca dial is at 900 I can pick up the oscillation in the digital radio at around 1300kc but the sound is much weaker than at the higher frequencies. I checked the tuning condensor for shorts throughout it's range and it seems fine. I have a signal generator and a scope but to be honest I'm not sure if I'm using them correctly. However, my DMM is reading very close to the reading on the signal generator scale and I am picking up a sign wave on my scope that I can increase or decrease in voltage during most of the alignment steps. I'm attaching the scope where it is shown in the manual. When I turn up the radio volume I can hear a high pitch frequency that gets louder as I increase the voltage on the trimmers so I think I'm using my scope right. I think I'd like to try this procedure again with a new C25 trimmer (5-50pf). Can someone suggest a modern replacement for this. Will a ceramic trimmer work?
Thanks,
Keith

Hi Keith,

This bothers me, because it tends to contradict some of the other observations.

You'll always find a spurious response on the other side of the injection frequency, removed from the station frequency
by two times the IF, in this case 910kc. The receiver front-end attenuates these spurious responses.

A bit of circuit theory...
Whenever you apply two frequencies to a mixer, you get four frequencies out:
The two original frequencies and their sum and difference.
In a receiver, the difference is at the Intermediate Frequency (IF). The other three output signals are ignored.

So if you have a station at 1000kc and the LO is at 1455kc, the difference is 455kc.
However, if the station is at 1000kc and the LO is at 545kc, the difference is also 455kc.
This is why the LO can be mis-adjusted below the station and still hear the station.

If you're listening to a station on 1610kc your LO can be on 1610+455=2065 or 1610-455=1155. Either will work.
If your LO is at 1155 it will also receive a signal at 1155-455=760kc.
This is the symptom you're reporting, and it suggests the LO is on the low side, but that contradicts your other tests.

I'm confused.

- Leigh

Keith,

Thanks for the link to the schematic!

It looks as if C25 is the local oscillator trimmer for the MW broadcast band.

Try listening for the LO signal on your second radio when you adjust C25. What happens to the signal heard in the other set? Does the oscillator die, or does it shift in frequency? You might also try using your scope for this; probe the 6SA7 cathode.

I suspect that part of your problem could be a bad coil or bandswitch. Have you verified continuity through the switch and coils?

Ted

With your 'scope (oscillograph?) connected to the point shown, you will only have the DC AVC voltage and the audio from the input signal. The double diode 6H6 has shunted half the RF to ground and the capacitor C43 has got rid of the RF - all you have left is the audio riding on a DC AVC voltage. If you are not injecting a modulated signal into the early stage of the radio, there's no audio where your 'scope is connected.

Given, this is my first alignment I need to go back to the basics. I've got my signal generator attached to the point shown on the image below when adjusting L5 and C25. The instructions say to attach it to the grid of the 6SA7 tube. Hopefully I've got that right. I spent about 4 hours on this after I did the local oscillator test using a digital radio so it's possible that now it's adjusted on the wrong side. To be honest I'm not exactly sure how one could do that but I'm sure I'll know by the time I get this thing finished. I won't be able to hit it again until this evening. It's amazing how fast time goes when you dive into a project. I went into my workshop last night and 4 hours disappeared instantly.
Thanks,
Keith

!#@#@#%, it wasn't until I blew up the schematic to show how my generator was hooked up that I could see that this tube has more than one grid. I simply found the big letter "G" on 6SA7 schematic and hooked it up. After all the instructions just said to attach to the 6SA7 grid and with my aging eyes I could only see one"G". Is this my problem?
If it is I apologize for the inconvenience I caused everyone but and the same time thanks because I'm learning a lot and I'll never make that mistake again.
Keith

Try connecting the signal generator to the antenna terminals and see if that trimmer cap now works.

Keith,

For adjusting the local oscillator you want the "G" grid on pin 8 of the tube, not the "SG" or "OG" grids.

You certainly do not want to connect to the "OMG" grid!

So the red arrow in your 4:43 pm post points to the correct point in the circuit. I don't see where there might be a mistake.

The antenna terminals should work OK for this, as well.

For this adjustment, the signal generator level should be pretty small; keep the DC magnitude of the AVC line under one volt.

Did your IFs line up OK, in steps 1 and 2?

Using the second radio, you can easily determine if the local oscillator is close to where it should be. If the second radio only tunes the MW broadcast band, check with the RCA tuned to 600kHz and 1100kHz. If the second radio tunes to 1.955 or 1.960MHz, check the RCA tuned to 1500, or even clear to the highest mark on the dial.

Did you check to see what the local oscillator is doing as you turn C25? Use that second radio. You can make the check with the RCA tuned lower, if need be, in case the second radio does not tune above the MW band. Have you looked at the local oscillator signal with your scope? You can probe the cathode of the 6SA7. A 10X probe would be good here, to minimize loading.

Ted

Keith,

When you adjust C25 is it tight or does it turn feely?

Trimmers are normally an integral part of the tuning capacitor. Finding a replacement for those is difficult.

If yours is a separate component then you can use a compression mica (still being made) or a rotary ceramic.

- Leigh

Fortunately, C25 is a compression mica and is a separate component. it turns freely but has no real adjustment. Adjustment is either no voltage or voltage. It's very difficult to adjust it to a position where i can see voltage on my scope. No fine tuning here. It spite of that I spent another 2 hours aligning this radio while hooking up my signal generator to the antenna. I can now tune into my transmitter at all frequencies. For some reason I pick up the local 1610AM station very loud at around 700 and again around 570 but the transmitter for this local station is within view of my house (I don't pick this up on my other radios). Can anyone recommend a supplier for a 5-50pf mica compression trimmer? Voltage rating? There is an identical compression capacitor C23 that is used to adjust the "B" band, is there any reason I can't swap these? I don't think there is anything to hear on that band anyway. Next I need to get this out of my basement and back into the cabinet to see how it does with the big internal loop antenna.
Thanks,
Keith

Does the signal stop as you tighten the screw?
That would indicate that the capacitor is shorting out.

Did you disassemble the cap? I think you mentioned that earlier in the thread.

----

Picking up the 1610 station at 700 is most likely an image, assuming your LO is operating on the high side.
The LO would be at 700 + 455 i= 1155. Mixing 1155 with 1610 yields an IF of 455 kc.

Images are a normal aspect of the way superheterodyne receivers work. The purpose of the tuned circuits at the receiver front end is to attenuate the images while allowing the desired stations to get through.

- Leigh