Dynaco FM-3 Antenna question

[Mike Stamper]

I recently procured an old FM-3, and did the usual restoration: New film and electrolytic capacitors, checked & replaced tubes, checked and replaced a few resistors, checked voltages on every tube, aligned the IF, discriminator, stereo, and front end. I am pretty pleased with the results except for two things. First, the antenna and mixer coil adjustments don't have any effect. I am getting stations low on the dial, but I am unable to affect the gain of these stations by adjusting L1 and L3. The antenna and mixer trimmer capacitors DO have an effect, and I am able to peak them on higher frequency stations. Second, attaching a dipole antenna to the 300-ohm connectors yields poor reception on all frequencies. However, attaching a long wire antenna to the rightmost (rear view) antenna screw yields very good reception. If I have to live with this, I'm OK because the tuner works very well with this configuration. However, I still think something is wrong, but I can't figure out what it is. I am out of ideas and would appreciate advice. One thing I did was to ground the chassis using a three-wire plug. I did this because I was feeling a little residual AC when I touched the metal chassis or cover. Antenna transformer L1 is grounded to the chassis. Could this be a factor?

 

[cq.ab0nf]

I have one of these, and I believe they were kits. Mine works OK, but it's not yet restored.

The factory alignment procedure assumes you do not have test equipment and even discourages the use of it. Is this the procedure you followed? It calls for adjustments using broadcast stations. Is it possible these stations near 88 are strong enough to engage the limiter, which might be why L1/L3 have no affect when adjusting?

This being a kit, it's possible that it has some wiring mistakes?

 

[Mike Stamper]

"Is it possible these stations near 88 are strong enough to engage the limiter, which might be why L1/L3 have no affect when adjusting?" I deliberately picked weak stations when adjusting L1 and L3. No effect at all. I got better results on the IF and discriminator adjustments using test equipment (signal generator) than with broadcast stations. I doubt that a "wiring mistake" would affect the RF front end. I checked the voltages at the two tubes, and they are OK, as are all of the other tubes. I am hoping that others have had this problem and can comment.

 

[audiotemp]

Second, attaching a dipole antenna to the 300-ohm connectors yields poor reception on all frequencies. However, attaching a long wire antenna to the rightmost (rear view) antenna screw yields very good reception.

Does attaching your long wire antenna to the leftmost antenna screw terminal produce the same result as connecting it to the rightmost screw terminal?

 

[Mike Stamper]

"Does attaching your long wire antenna to the leftmost antenna screw terminal produce the same result as connecting it to the rightmost screw terminal?" Correct. Same effect.

 

[PE9ZZ]

Correct. Same effect.

Suggests something is very wrong with L1 (and L3). Easiest to check is continuity between 2&4 (L1) and 1&2 (L3). They MUST peak at the low end, if nothing happens their inductance is either way off for some reason or they have a broken wire. Which for L3 means V2 won't be biased properly. Maybe the core is broken or not the right type so they never achieve resonance at the right frequency.

 

[Mike Stamper]

I will check L1 and L3 as you suggest. Here is a question that I bet no one knows. What are the nominal inductance ranges of L1 and L3? I have a meter that measures inductance. It would be great to verify that these coils are in fact adjustable within their expected range.

 

[PE9ZZ]

There inductors are too low in value to measure reliably. You must first calibrate your test setup and only then you will be able to measure something useful. Typically the combination of C1A and -B would be around 25 pF for 88 MHz. This needs an inductance of 131 nH to resonate.

 

[Mike Stamper]

Sounds like the best I can do is check L1 and L3 for continuity.

 

[PE9ZZ]

It is certainly easiest.

 

[Bob@FM]

I had a problem with an FM3 in that area described. It was actually difficult to see, but the inductor was not making continuity to the board trace - it had broken the copper trace right at the leg solder point. My recall was the solder blob was still there, but underneath it, the trace was broken. Thus making it invisible to eyeball inspection. Was open when tested for continuity to the trace though.
It was the inductor that sits right at the edge of the board - had to use a small wire to make the connection from L leg across the trace. Made a huge difference in my case. All the board grounds on that one also need to be checked...