Adjusting radio strength meter (moved from MPX thread)

[thornev]

At Z5 -- On the side of that transformer with three terminals, one of the outside terminals will have a 1.5K resistor (brown-green-red-silver) connected to it. The side of the resistor that is NOT connected to the transformer is Test Point #3.

At Z5 -- On the side of that transformer with three terminals, the middle terminal connects to a 270Ω resistor (red-vilot-brown-silver). The other side of this resistor connects to one lead of a small RF choke coil (looks like a mid size resistor with wire coiled around its body, and a dab of paint on each end to keep the ends of the coil from coming lose). The other lead of the choke coil (not connected to the resistor) represents Test Point #4.

Dave

Dave - In the attached picture I've marked what I think are test points 3 and 4. Am I correct? And just so I don't blow up my receiver, I attach the red lead to the test point, and the black lead to a chassis ground, right?

 

[larryderouin]

Thornev: An easier way to locate Test point 3 is find the 8uf 50V axial electrolytic (it should have been replaced with a 10uf 50v axial). The NEGATIVE SIDE of the 8(10)uf cap is TP 3. It looks like you might have TP4 correct. But to make absolutely sure, can you zoom in on the area closer and straight down with another pic? This one blurs too much at 200% zoom.

 

[thornev]

Thanks, larry. Is this picture better? TP3 is pointing to the bottom of the resistor. TP4 is pointing to the right side of the resistor wrapped in a copper coil.

 

[larryderouin]

To answer your question to gadget about VTVM's.... VTVM (vacuum tube Volt Meter) is a Multimeter that uses Vacuum tubes and has a fairly large Analog meter. It's a lot better for doing things like this as it 1.) has an impedance that won't load down the circuits, 2.) measuring is not segmented like on a Digital with it's sampling rate and changing numbers......while the DVM is changing numbers at a rate of one every second or so, the VTVM is pretty much seamless. Plus you can do a + or - ZERO scale from the middle of the scale which is extremely hard to do with a DVM, if at all.

You had TP 4 right. But as I said in the previous post I was unsure of TP3. The pic now is good. TP 3 is REMARKED.
Here's the corrected pic.



You had TP 3 on R107which is a 1K (Brn Blk Red). R118 is a 1.5K is BRN GRN RED which connects to the 8(10)uf lytic. On the 500c/800c TP3 is always on the NEGATIVE side of the 8(10)uf axial electrolytic cap

 

[thornev]

I completed the IF transformer adjustments per Dave's instructions. Turning the top of Z5 was very difficult. It seemed very tight but I was able to turn it and get the DMM to almost zero. I believe the FM stereo was slightly better when I followed the KM-60 instructions. The strength meter is slightly less now than it was. And there is one station that I was able to get in stereo before that I can't get in stereo now. I don't know what I could have done wrong. The differences were tuning the IF transformers backward this time, 5-1, rather than forward, 1-5 before, and I didn't use the test points before. Maybe the crappy weather is affecting reception. I'll give it a few days. The meter still won't go above 4, but it doesn't matter. I'm getting strong signals.

Thanks to Dave and Larry (and anyone else for your 1 or 2 posts) for all your help. I'm really enjoying my 500-C.

 

[dcgillespie]

Thorne -- Weather and windy days can definitely play into reception capability. The direction you go in adjusting the transformers won't have any effect on the accuracy of the adjustments, as long as a weak signal was used throughout the adjustment process. The KM-60 speaks of using a weak signal as well.

So what is the highest reading you are getting on a strong station now? And, did all the adjustments react sharply? Or were any very broad in effect? Also, were the alignment tools you used plastic? If any metal tools were used, they distort the setting after being removed from the transformer -- which is why they should never be used for alignment purposes.

Dave

 

[thornev]

Dave - It's snowy and grey today so that doesn't help. I had to use the "local" lugs on the antenna posts because with the distant lugs I couldn't get a weak signal. I guess the 2 previous attempts at this (the 1st with KM-60 and the 2nd with your instructions) did a pretty good job. Today was attempt #3 using the two 47K resistors and the two test points.

The highest reading I get now is 4. Most of the adjustments reacted immediately. There was one which did not react immediately, I forget which one. I used all plastic tools except for the top of one, I think it was Z4 (or Z3) because it's not hex; it's a wide rectangle and none of the plastic tools fit. I had to use a small flat screwdriver which is not magnetic. So maybe that one adjustment was problematic. I'm happy with the result so I don't mean to complain. I was just reporting my observations.

Thorne

PS - One other thing... when I dial in a station so that it's as clear as possible, the strength meter is sometimes below its highest reading.

 

[dcgillespie]

If it is only a "sometimes" thing then that is often due to an FM phenomenon called "Multipath", where in because FM signals are basically line-of-sight in nature, the antenna can receive the same signal at two slightly different times. To the receiver, this represents two of the same signal, but slightly out of phase with each other, causing what you observe -- once again, showing why the antenna is so important in receiving good FM MPX Stereo.

It honestly sounds like you've done about as good a job as can be done without precision alignment equipment, and particularly if you are getting signal strength readings of 4 now. When you started, it was producing readings of what, 2.5? And with the touchy-tuning, ringing- sound stereo? I've got to believe your receiver is performing well, delivering waaaay better FM and FM MPX Stereo performance than when you first started this trek!

Dave

 

[thornev]

You're absolutely right, Dave. Thanks again for all your help. Thorne