[Tyler DeArmond]

Hello,

This is my first post on audiocircle. I've found lots of helpful stuff here! Hopefully I can get some guidance on my FM3.

I've got several FM3's in various conditions. I'm trying to get through the alignment process but I'm stuck on the discriminator stage. I'm using a Fluke DVM which shouldn't load the circuit unduly, I think.

I can get through step 24A (I only peak at about -8v), but when I attempt the next step of adjusting for zero I am unable to get a zero reading. I'm getting low voltages in the 0.5-1.5 range. The schematic seems to show some parts that must be inside this transformer can? I don't see the diodes associated with this stage on the board.

Any help would be greatly appreciated!

[dB Cooper]

The FM3 was designed to be aligned without a meter. The very first resouce you need to consult is:

http://www.avahifi.com/index.php?option=com_content&id=185&Itemid=181

Another resource for things like a power supply board that replaces the impossible-to-find original and de-emphasis boards to replace the originals that had incorrect de-emphasis:
http://quadesl.com/refurb/refurb_dynaFM3.html

Another must-check FM3 resource is this guy, who has a lot of upgrade parts up to and including a complete monolithic motherboard that replaces all original boards and eliminates almost all point-to-point wiring:
http://home.comcast.net/~netminer/FM3ZONE.html

And lastly you can find schematics and manuals here:
http://home.indy.net/~gregdunn/dynaco/

Frank Van Alstine and the gentleman who operates FM3 Zone are good resources on this vintage classic. Good luck.

[Tyler DeArmond]

Right, I'm familiar with those various resources. I've installed the FM3zone boards and upgraded the other parts mentioned by those articles. In fact, I actually had the pleasure of going to Frank Van Alstine's place to watch him do a couple of FM3 alignments, so I thought this would go more smoothly than it is.

The unit was working just fine until I tried to align it. 

I found Bill Thomas' comments (on this website) on how to set the T1 transformer to 10.7mhz. I tried this using an old RF generator and a frequency counter. I ended up having to set the sig gen with only the frequency counter attached and then attach the sig gen to only T1 through a 100pf cap. The only one of that value I have is a dipped mica. Not sure if that's a problem.

A glitch I ran into in setting T1 was that my frequency counter won't maintain a reading at the lowest settings on the sig gen. And the FM3 requires very nearly the lowest voltage output from the sig gen. So, I'm trusting that the signal is still at 10.7 even when the sig gen is barely on voltage-wise. The sig gen is an old Paco E-200. When viewed with my little 100mhz digital scope I can also read the 10.7mhz signal from the Paco but it too looses the signal as I get low on the voltage range. So, I can't actually verify the signal voltage and frequency at the needed level of operation of the sig gen.

So, back to the point, I did get T1 to give a peak reading on what I assumed was 10.7mhz but could not verify during the work or otherwise. I could only verify it by increasing the output of the sig gen and measuring it with only one device attached at a time.

 I was fully aware that I could be messing things up when I started in on this, the goal for me is to learn how to properly set the 10.7mhz signal and get through the alignment successfully on my own.

Thanks for any thoughts on how I might get a more confident lock on the 10.7 T1 deal. Or other thoughts on what I'm up against if perhaps I do have the IF stages aligned correctly.

I have a bunch of FM3's so I'm willing to experiment a bit to for the sake of learning this process. Do I just need a better sig gen or a more sensitive way to get a read on that signal at low voltages?

[Tyler DeArmond]

To be a bit more clear, my concern is that the sig gen is outputting noise or inaccurately at the very low end of its range of output and that I have aligned my T1 to that noise rather than to 10.7.

But on the up side, prior to adjusting T1, I was reading 10.9mhz across the later stages of the IF section and now it does in fact read 10.7mhz in those later stages. When taking those readings I'm using the frequency counter in the AC connected mode. No external cap in circuit.

That seemed like progress at the time.

[Lefty052347]

http://www.avahifi.com/index.php?option=com_content&view=article&id=185&Itemid=181

This is an Audio Basics article concerning the FM3 that many have found useful.

Regards,
Dean

[Tyler DeArmond]

Thanks. Yes. I have read that one.

[Tyler DeArmond]

So, I finally just started working through the entire voltage chart and
correcting things until I was getting pretty much the voltages called for. Not
all of them are perfect, but mostly pretty good.

Got to that discriminator stage and still couldn't get through the alignment
deal. So, I replaced all related caps and resistors and still no luck. So, I
swapped out the T5 disc. transformer. That did the trick! Weirdly, both the bad
T5 and the replacement measured the same as far as I could tell in terms of
locating diodes and caps by measuring across the pins. So, it must be another
cap inside or something else that's not making correct contact.

Whatever. It's working now and I'm going to just enjoy it a while before
finishing the rest of the alignment on it. It's sounding really good!